This is gdb.info, produced by makeinfo version 6.3 from gdb.texinfo. Copyright (C) 1988-2017 Free Software Foundation, Inc. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with the Invariant Sections being "Free Software" and "Free Software Needs Free Documentation", with the Front-Cover Texts being "A GNU Manual," and with the Back-Cover Texts as in (a) below. (a) The FSF's Back-Cover Text is: "You are free to copy and modify this GNU Manual. Buying copies from GNU Press supports the FSF in developing GNU and promoting software freedom." INFO-DIR-SECTION Software development START-INFO-DIR-ENTRY * Gdb: (gdb). The GNU debugger. * gdbserver: (gdb) Server. The GNU debugging server. END-INFO-DIR-ENTRY This file documents the GNU debugger GDB. This is the Tenth Edition, of 'Debugging with GDB: the GNU Source-Level Debugger' for GDB (GDB) Version 8.0.1. Copyright (C) 1988-2017 Free Software Foundation, Inc. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with the Invariant Sections being "Free Software" and "Free Software Needs Free Documentation", with the Front-Cover Texts being "A GNU Manual," and with the Back-Cover Texts as in (a) below. (a) The FSF's Back-Cover Text is: "You are free to copy and modify this GNU Manual. Buying copies from GNU Press supports the FSF in developing GNU and promoting software freedom."  File: gdb.info, Node: Altering, Next: GDB Files, Prev: Symbols, Up: Top 17 Altering Execution ********************* Once you think you have found an error in your program, you might want to find out for certain whether correcting the apparent error would lead to correct results in the rest of the run. You can find the answer by experiment, using the GDB features for altering execution of the program. For example, you can store new values into variables or memory locations, give your program a signal, restart it at a different address, or even return prematurely from a function. * Menu: * Assignment:: Assignment to variables * Jumping:: Continuing at a different address * Signaling:: Giving your program a signal * Returning:: Returning from a function * Calling:: Calling your program's functions * Patching:: Patching your program * Compiling and Injecting Code:: Compiling and injecting code in GDB  File: gdb.info, Node: Assignment, Next: Jumping, Up: Altering 17.1 Assignment to Variables ============================ To alter the value of a variable, evaluate an assignment expression. *Note Expressions: Expressions. For example, print x=4 stores the value 4 into the variable 'x', and then prints the value of the assignment expression (which is 4). *Note Using GDB with Different Languages: Languages, for more information on operators in supported languages. If you are not interested in seeing the value of the assignment, use the 'set' command instead of the 'print' command. 'set' is really the same as 'print' except that the expression's value is not printed and is not put in the value history (*note Value History: Value History.). The expression is evaluated only for its effects. If the beginning of the argument string of the 'set' command appears identical to a 'set' subcommand, use the 'set variable' command instead of just 'set'. This command is identical to 'set' except for its lack of subcommands. For example, if your program has a variable 'width', you get an error if you try to set a new value with just 'set width=13', because GDB has the command 'set width': (gdb) whatis width type = double (gdb) p width $4 = 13 (gdb) set width=47 Invalid syntax in expression. The invalid expression, of course, is '=47'. In order to actually set the program's variable 'width', use (gdb) set var width=47 Because the 'set' command has many subcommands that can conflict with the names of program variables, it is a good idea to use the 'set variable' command instead of just 'set'. For example, if your program has a variable 'g', you run into problems if you try to set a new value with just 'set g=4', because GDB has the command 'set gnutarget', abbreviated 'set g': (gdb) whatis g type = double (gdb) p g $1 = 1 (gdb) set g=4 (gdb) p g $2 = 1 (gdb) r The program being debugged has been started already. Start it from the beginning? (y or n) y Starting program: /home/smith/cc_progs/a.out "/home/smith/cc_progs/a.out": can't open to read symbols: Invalid bfd target. (gdb) show g The current BFD target is "=4". The program variable 'g' did not change, and you silently set the 'gnutarget' to an invalid value. In order to set the variable 'g', use (gdb) set var g=4 GDB allows more implicit conversions in assignments than C; you can freely store an integer value into a pointer variable or vice versa, and you can convert any structure to any other structure that is the same length or shorter. To store values into arbitrary places in memory, use the '{...}' construct to generate a value of specified type at a specified address (*note Expressions: Expressions.). For example, '{int}0x83040' refers to memory location '0x83040' as an integer (which implies a certain size and representation in memory), and set {int}0x83040 = 4 stores the value 4 into that memory location.  File: gdb.info, Node: Jumping, Next: Signaling, Prev: Assignment, Up: Altering 17.2 Continuing at a Different Address ====================================== Ordinarily, when you continue your program, you do so at the place where it stopped, with the 'continue' command. You can instead continue at an address of your own choosing, with the following commands: 'jump LOCATION' 'j LOCATION' Resume execution at LOCATION. Execution stops again immediately if there is a breakpoint there. *Note Specify Location::, for a description of the different forms of LOCATION. It is common practice to use the 'tbreak' command in conjunction with 'jump'. *Note Setting Breakpoints: Set Breaks. The 'jump' command does not change the current stack frame, or the stack pointer, or the contents of any memory location or any register other than the program counter. If LOCATION is in a different function from the one currently executing, the results may be bizarre if the two functions expect different patterns of arguments or of local variables. For this reason, the 'jump' command requests confirmation if the specified line is not in the function currently executing. However, even bizarre results are predictable if you are well acquainted with the machine-language code of your program. On many systems, you can get much the same effect as the 'jump' command by storing a new value into the register '$pc'. The difference is that this does not start your program running; it only changes the address of where it _will_ run when you continue. For example, set $pc = 0x485 makes the next 'continue' command or stepping command execute at address '0x485', rather than at the address where your program stopped. *Note Continuing and Stepping: Continuing and Stepping. The most common occasion to use the 'jump' command is to back up--perhaps with more breakpoints set--over a portion of a program that has already executed, in order to examine its execution in more detail.  File: gdb.info, Node: Signaling, Next: Returning, Prev: Jumping, Up: Altering 17.3 Giving your Program a Signal ================================= 'signal SIGNAL' Resume execution where your program is stopped, but immediately give it the signal SIGNAL. The SIGNAL can be the name or the number of a signal. For example, on many systems 'signal 2' and 'signal SIGINT' are both ways of sending an interrupt signal. Alternatively, if SIGNAL is zero, continue execution without giving a signal. This is useful when your program stopped on account of a signal and would ordinarily see the signal when resumed with the 'continue' command; 'signal 0' causes it to resume without a signal. _Note:_ When resuming a multi-threaded program, SIGNAL is delivered to the currently selected thread, not the thread that last reported a stop. This includes the situation where a thread was stopped due to a signal. So if you want to continue execution suppressing the signal that stopped a thread, you should select that same thread before issuing the 'signal 0' command. If you issue the 'signal 0' command with another thread as the selected one, GDB detects that and asks for confirmation. Invoking the 'signal' command is not the same as invoking the 'kill' utility from the shell. Sending a signal with 'kill' causes GDB to decide what to do with the signal depending on the signal handling tables (*note Signals::). The 'signal' command passes the signal directly to your program. 'signal' does not repeat when you press a second time after executing the command. 'queue-signal SIGNAL' Queue SIGNAL to be delivered immediately to the current thread when execution of the thread resumes. The SIGNAL can be the name or the number of a signal. For example, on many systems 'signal 2' and 'signal SIGINT' are both ways of sending an interrupt signal. The handling of the signal must be set to pass the signal to the program, otherwise GDB will report an error. You can control the handling of signals from GDB with the 'handle' command (*note Signals::). Alternatively, if SIGNAL is zero, any currently queued signal for the current thread is discarded and when execution resumes no signal will be delivered. This is useful when your program stopped on account of a signal and would ordinarily see the signal when resumed with the 'continue' command. This command differs from the 'signal' command in that the signal is just queued, execution is not resumed. And 'queue-signal' cannot be used to pass a signal whose handling state has been set to 'nopass' (*note Signals::). *Note stepping into signal handlers::, for information on how stepping commands behave when the thread has a signal queued.  File: gdb.info, Node: Returning, Next: Calling, Prev: Signaling, Up: Altering 17.4 Returning from a Function ============================== 'return' 'return EXPRESSION' You can cancel execution of a function call with the 'return' command. If you give an EXPRESSION argument, its value is used as the function's return value. When you use 'return', GDB discards the selected stack frame (and all frames within it). You can think of this as making the discarded frame return prematurely. If you wish to specify a value to be returned, give that value as the argument to 'return'. This pops the selected stack frame (*note Selecting a Frame: Selection.), and any other frames inside of it, leaving its caller as the innermost remaining frame. That frame becomes selected. The specified value is stored in the registers used for returning values of functions. The 'return' command does not resume execution; it leaves the program stopped in the state that would exist if the function had just returned. In contrast, the 'finish' command (*note Continuing and Stepping: Continuing and Stepping.) resumes execution until the selected stack frame returns naturally. GDB needs to know how the EXPRESSION argument should be set for the inferior. The concrete registers assignment depends on the OS ABI and the type being returned by the selected stack frame. For example it is common for OS ABI to return floating point values in FPU registers while integer values in CPU registers. Still some ABIs return even floating point values in CPU registers. Larger integer widths (such as 'long long int') also have specific placement rules. GDB already knows the OS ABI from its current target so it needs to find out also the type being returned to make the assignment into the right register(s). Normally, the selected stack frame has debug info. GDB will always use the debug info instead of the implicit type of EXPRESSION when the debug info is available. For example, if you type 'return -1', and the function in the current stack frame is declared to return a 'long long int', GDB transparently converts the implicit 'int' value of -1 into a 'long long int': Breakpoint 1, func () at gdb.base/return-nodebug.c:29 29 return 31; (gdb) return -1 Make func return now? (y or n) y #0 0x004004f6 in main () at gdb.base/return-nodebug.c:43 43 printf ("result=%lld\n", func ()); (gdb) However, if the selected stack frame does not have a debug info, e.g., if the function was compiled without debug info, GDB has to find out the type to return from user. Specifying a different type by mistake may set the value in different inferior registers than the caller code expects. For example, typing 'return -1' with its implicit type 'int' would set only a part of a 'long long int' result for a debug info less function (on 32-bit architectures). Therefore the user is required to specify the return type by an appropriate cast explicitly: Breakpoint 2, 0x0040050b in func () (gdb) return -1 Return value type not available for selected stack frame. Please use an explicit cast of the value to return. (gdb) return (long long int) -1 Make selected stack frame return now? (y or n) y #0 0x00400526 in main () (gdb)  File: gdb.info, Node: Calling, Next: Patching, Prev: Returning, Up: Altering 17.5 Calling Program Functions ============================== 'print EXPR' Evaluate the expression EXPR and display the resulting value. The expression may include calls to functions in the program being debugged. 'call EXPR' Evaluate the expression EXPR without displaying 'void' returned values. You can use this variant of the 'print' command if you want to execute a function from your program that does not return anything (a.k.a. "a void function"), but without cluttering the output with 'void' returned values that GDB will otherwise print. If the result is not void, it is printed and saved in the value history. It is possible for the function you call via the 'print' or 'call' command to generate a signal (e.g., if there's a bug in the function, or if you passed it incorrect arguments). What happens in that case is controlled by the 'set unwindonsignal' command. Similarly, with a C++ program it is possible for the function you call via the 'print' or 'call' command to generate an exception that is not handled due to the constraints of the dummy frame. In this case, any exception that is raised in the frame, but has an out-of-frame exception handler will not be found. GDB builds a dummy-frame for the inferior function call, and the unwinder cannot seek for exception handlers outside of this dummy-frame. What happens in that case is controlled by the 'set unwind-on-terminating-exception' command. 'set unwindonsignal' Set unwinding of the stack if a signal is received while in a function that GDB called in the program being debugged. If set to on, GDB unwinds the stack it created for the call and restores the context to what it was before the call. If set to off (the default), GDB stops in the frame where the signal was received. 'show unwindonsignal' Show the current setting of stack unwinding in the functions called by GDB. 'set unwind-on-terminating-exception' Set unwinding of the stack if a C++ exception is raised, but left unhandled while in a function that GDB called in the program being debugged. If set to on (the default), GDB unwinds the stack it created for the call and restores the context to what it was before the call. If set to off, GDB the exception is delivered to the default C++ exception handler and the inferior terminated. 'show unwind-on-terminating-exception' Show the current setting of stack unwinding in the functions called by GDB. Sometimes, a function you wish to call is actually a "weak alias" for another function. In such case, GDB might not pick up the type information, including the types of the function arguments, which causes GDB to call the inferior function incorrectly. As a result, the called function will function erroneously and may even crash. A solution to that is to use the name of the aliased function instead.  File: gdb.info, Node: Patching, Next: Compiling and Injecting Code, Prev: Calling, Up: Altering 17.6 Patching Programs ====================== By default, GDB opens the file containing your program's executable code (or the corefile) read-only. This prevents accidental alterations to machine code; but it also prevents you from intentionally patching your program's binary. If you'd like to be able to patch the binary, you can specify that explicitly with the 'set write' command. For example, you might want to turn on internal debugging flags, or even to make emergency repairs. 'set write on' 'set write off' If you specify 'set write on', GDB opens executable and core files for both reading and writing; if you specify 'set write off' (the default), GDB opens them read-only. If you have already loaded a file, you must load it again (using the 'exec-file' or 'core-file' command) after changing 'set write', for your new setting to take effect. 'show write' Display whether executable files and core files are opened for writing as well as reading.  File: gdb.info, Node: Compiling and Injecting Code, Prev: Patching, Up: Altering 17.7 Compiling and injecting code in GDB ======================================== GDB supports on-demand compilation and code injection into programs running under GDB. GCC 5.0 or higher built with 'libcc1.so' must be installed for this functionality to be enabled. This functionality is implemented with the following commands. 'compile code SOURCE-CODE' 'compile code -raw -- SOURCE-CODE' Compile SOURCE-CODE with the compiler language found as the current language in GDB (*note Languages::). If compilation and injection is not supported with the current language specified in GDB, or the compiler does not support this feature, an error message will be printed. If SOURCE-CODE compiles and links successfully, GDB will load the object-code emitted, and execute it within the context of the currently selected inferior. It is important to note that the compiled code is executed immediately. After execution, the compiled code is removed from GDB and any new types or variables you have defined will be deleted. The command allows you to specify SOURCE-CODE in two ways. The simplest method is to provide a single line of code to the command. E.g.: compile code printf ("hello world\n"); If you specify options on the command line as well as source code, they may conflict. The '--' delimiter can be used to separate options from actual source code. E.g.: compile code -r -- printf ("hello world\n"); Alternatively you can enter source code as multiple lines of text. To enter this mode, invoke the 'compile code' command without any text following the command. This will start the multiple-line editor and allow you to type as many lines of source code as required. When you have completed typing, enter 'end' on its own line to exit the editor. compile code >printf ("hello\n"); >printf ("world\n"); >end Specifying '-raw', prohibits GDB from wrapping the provided SOURCE-CODE in a callable scope. In this case, you must specify the entry point of the code by defining a function named '_gdb_expr_'. The '-raw' code cannot access variables of the inferior. Using '-raw' option may be needed for example when SOURCE-CODE requires '#include' lines which may conflict with inferior symbols otherwise. 'compile file FILENAME' 'compile file -raw FILENAME' Like 'compile code', but take the source code from FILENAME. compile file /home/user/example.c 'compile print EXPR' 'compile print /F EXPR' Compile and execute EXPR with the compiler language found as the current language in GDB (*note Languages::). By default the value of EXPR is printed in a format appropriate to its data type; you can choose a different format by specifying '/F', where F is a letter specifying the format; see *note Output Formats: Output Formats. 'compile print' 'compile print /F' Alternatively you can enter the expression (source code producing it) as multiple lines of text. To enter this mode, invoke the 'compile print' command without any text following the command. This will start the multiple-line editor. The process of compiling and injecting the code can be inspected using: 'set debug compile' Turns on or off display of GDB process of compiling and injecting the code. The default is off. 'show debug compile' Displays the current state of displaying GDB process of compiling and injecting the code. 17.7.1 Compilation options for the 'compile' command ---------------------------------------------------- GDB needs to specify the right compilation options for the code to be injected, in part to make its ABI compatible with the inferior and in part to make the injected code compatible with GDB's injecting process. The options used, in increasing precedence: target architecture and OS options ('gdbarch') These options depend on target processor type and target operating system, usually they specify at least 32-bit ('-m32') or 64-bit ('-m64') compilation option. compilation options recorded in the target GCC (since version 4.7) stores the options used for compilation into 'DW_AT_producer' part of DWARF debugging information according to the GCC option '-grecord-gcc-switches'. One has to explicitly specify '-g' during inferior compilation otherwise GCC produces no DWARF. This feature is only relevant for platforms where '-g' produces DWARF by default, otherwise one may try to enforce DWARF by using '-gdwarf-4'. compilation options set by 'set compile-args' You can override compilation options using the following command: 'set compile-args' Set compilation options used for compiling and injecting code with the 'compile' commands. These options override any conflicting ones from the target architecture and/or options stored during inferior compilation. 'show compile-args' Displays the current state of compilation options override. This does not show all the options actually used during compilation, use *note set debug compile:: for that. 17.7.2 Caveats when using the 'compile' command ----------------------------------------------- There are a few caveats to keep in mind when using the 'compile' command. As the caveats are different per language, the table below highlights specific issues on a per language basis. C code examples and caveats When the language in GDB is set to 'C', the compiler will attempt to compile the source code with a 'C' compiler. The source code provided to the 'compile' command will have much the same access to variables and types as it normally would if it were part of the program currently being debugged in GDB. Below is a sample program that forms the basis of the examples that follow. This program has been compiled and loaded into GDB, much like any other normal debugging session. void function1 (void) { int i = 42; printf ("function 1\n"); } void function2 (void) { int j = 12; function1 (); } int main(void) { int k = 6; int *p; function2 (); return 0; } For the purposes of the examples in this section, the program above has been compiled, loaded into GDB, stopped at the function 'main', and GDB is awaiting input from the user. To access variables and types for any program in GDB, the program must be compiled and packaged with debug information. The 'compile' command is not an exception to this rule. Without debug information, you can still use the 'compile' command, but you will be very limited in what variables and types you can access. So with that in mind, the example above has been compiled with debug information enabled. The 'compile' command will have access to all variables and types (except those that may have been optimized out). Currently, as GDB has stopped the program in the 'main' function, the 'compile' command would have access to the variable 'k'. You could invoke the 'compile' command and type some source code to set the value of 'k'. You can also read it, or do anything with that variable you would normally do in 'C'. Be aware that changes to inferior variables in the 'compile' command are persistent. In the following example: compile code k = 3; the variable 'k' is now 3. It will retain that value until something else in the example program changes it, or another 'compile' command changes it. Normal scope and access rules apply to source code compiled and injected by the 'compile' command. In the example, the variables 'j' and 'k' are not accessible yet, because the program is currently stopped in the 'main' function, where these variables are not in scope. Therefore, the following command compile code j = 3; will result in a compilation error message. Once the program is continued, execution will bring these variables in scope, and they will become accessible; then the code you specify via the 'compile' command will be able to access them. You can create variables and types with the 'compile' command as part of your source code. Variables and types that are created as part of the 'compile' command are not visible to the rest of the program for the duration of its run. This example is valid: compile code int ff = 5; printf ("ff is %d\n", ff); However, if you were to type the following into GDB after that command has completed: compile code printf ("ff is %d\n'', ff); a compiler error would be raised as the variable 'ff' no longer exists. Object code generated and injected by the 'compile' command is removed when its execution ends. Caution is advised when assigning to program variables values of variables created by the code submitted to the 'compile' command. This example is valid: compile code int ff = 5; k = ff; The value of the variable 'ff' is assigned to 'k'. The variable 'k' does not require the existence of 'ff' to maintain the value it has been assigned. However, pointers require particular care in assignment. If the source code compiled with the 'compile' command changed the address of a pointer in the example program, perhaps to a variable created in the 'compile' command, that pointer would point to an invalid location when the command exits. The following example would likely cause issues with your debugged program: compile code int ff = 5; p = &ff; In this example, 'p' would point to 'ff' when the 'compile' command is executing the source code provided to it. However, as variables in the (example) program persist with their assigned values, the variable 'p' would point to an invalid location when the command exists. A general rule should be followed in that you should either assign 'NULL' to any assigned pointers, or restore a valid location to the pointer before the command exits. Similar caution must be exercised with any structs, unions, and typedefs defined in 'compile' command. Types defined in the 'compile' command will no longer be available in the next 'compile' command. Therefore, if you cast a variable to a type defined in the 'compile' command, care must be taken to ensure that any future need to resolve the type can be achieved. (gdb) compile code static struct a { int a; } v = { 42 }; argv = &v; (gdb) compile code printf ("%d\n", ((struct a *) argv)->a); gdb command line:1:36: error: dereferencing pointer to incomplete type ‘struct a’ Compilation failed. (gdb) compile code struct a { int a; }; printf ("%d\n", ((struct a *) argv)->a); 42 Variables that have been optimized away by the compiler are not accessible to the code submitted to the 'compile' command. Access to those variables will generate a compiler error which GDB will print to the console. 17.7.3 Compiler search for the 'compile' command ------------------------------------------------ GDB needs to find GCC for the inferior being debugged which may not be obvious for remote targets of different architecture than where GDB is running. Environment variable 'PATH' ('PATH' from shell that executed GDB, not the one set by GDB command 'set environment'). *Note Environment::. 'PATH' on GDB host is searched for GCC binary matching the target architecture and operating system. Specifically 'PATH' is searched for binaries matching regular expression 'ARCH(-[^-]*)?-OS-gcc' according to the inferior target being debugged. ARCH is processor name -- multiarch is supported, so for example both 'i386' and 'x86_64' targets look for pattern '(x86_64|i.86)' and both 's390' and 's390x' targets look for pattern 's390x?'. OS is currently supported only for pattern 'linux(-gnu)?'.  File: gdb.info, Node: GDB Files, Next: Targets, Prev: Altering, Up: Top 18 GDB Files ************ GDB needs to know the file name of the program to be debugged, both in order to read its symbol table and in order to start your program. To debug a core dump of a previous run, you must also tell GDB the name of the core dump file. * Menu: * Files:: Commands to specify files * File Caching:: Information about GDB's file caching * Separate Debug Files:: Debugging information in separate files * MiniDebugInfo:: Debugging information in a special section * Index Files:: Index files speed up GDB * Symbol Errors:: Errors reading symbol files * Data Files:: GDB data files  File: gdb.info, Node: Files, Next: File Caching, Up: GDB Files 18.1 Commands to Specify Files ============================== You may want to specify executable and core dump file names. The usual way to do this is at start-up time, using the arguments to GDB's start-up commands (*note Getting In and Out of GDB: Invocation.). Occasionally it is necessary to change to a different file during a GDB session. Or you may run GDB and forget to specify a file you want to use. Or you are debugging a remote target via 'gdbserver' (*note file: Server.). In these situations the GDB commands to specify new files are useful. 'file FILENAME' Use FILENAME as the program to be debugged. It is read for its symbols and for the contents of pure memory. It is also the program executed when you use the 'run' command. If you do not specify a directory and the file is not found in the GDB working directory, GDB uses the environment variable 'PATH' as a list of directories to search, just as the shell does when looking for a program to run. You can change the value of this variable, for both GDB and your program, using the 'path' command. You can load unlinked object '.o' files into GDB using the 'file' command. You will not be able to "run" an object file, but you can disassemble functions and inspect variables. Also, if the underlying BFD functionality supports it, you could use 'gdb -write' to patch object files using this technique. Note that GDB can neither interpret nor modify relocations in this case, so branches and some initialized variables will appear to go to the wrong place. But this feature is still handy from time to time. 'file' 'file' with no argument makes GDB discard any information it has on both executable file and the symbol table. 'exec-file [ FILENAME ]' Specify that the program to be run (but not the symbol table) is found in FILENAME. GDB searches the environment variable 'PATH' if necessary to locate your program. Omitting FILENAME means to discard information on the executable file. 'symbol-file [ FILENAME ]' Read symbol table information from file FILENAME. 'PATH' is searched when necessary. Use the 'file' command to get both symbol table and program to run from the same file. 'symbol-file' with no argument clears out GDB information on your program's symbol table. The 'symbol-file' command causes GDB to forget the contents of some breakpoints and auto-display expressions. This is because they may contain pointers to the internal data recording symbols and data types, which are part of the old symbol table data being discarded inside GDB. 'symbol-file' does not repeat if you press again after executing it once. When GDB is configured for a particular environment, it understands debugging information in whatever format is the standard generated for that environment; you may use either a GNU compiler, or other compilers that adhere to the local conventions. Best results are usually obtained from GNU compilers; for example, using 'GCC' you can generate debugging information for optimized code. For most kinds of object files, with the exception of old SVR3 systems using COFF, the 'symbol-file' command does not normally read the symbol table in full right away. Instead, it scans the symbol table quickly to find which source files and which symbols are present. The details are read later, one source file at a time, as they are needed. The purpose of this two-stage reading strategy is to make GDB start up faster. For the most part, it is invisible except for occasional pauses while the symbol table details for a particular source file are being read. (The 'set verbose' command can turn these pauses into messages if desired. *Note Optional Warnings and Messages: Messages/Warnings.) We have not implemented the two-stage strategy for COFF yet. When the symbol table is stored in COFF format, 'symbol-file' reads the symbol table data in full right away. Note that "stabs-in-COFF" still does the two-stage strategy, since the debug info is actually in stabs format. 'symbol-file [ -readnow ] FILENAME' 'file [ -readnow ] FILENAME' You can override the GDB two-stage strategy for reading symbol tables by using the '-readnow' option with any of the commands that load symbol table information, if you want to be sure GDB has the entire symbol table available. 'core-file [FILENAME]' 'core' Specify the whereabouts of a core dump file to be used as the "contents of memory". Traditionally, core files contain only some parts of the address space of the process that generated them; GDB can access the executable file itself for other parts. 'core-file' with no argument specifies that no core file is to be used. Note that the core file is ignored when your program is actually running under GDB. So, if you have been running your program and you wish to debug a core file instead, you must kill the subprocess in which the program is running. To do this, use the 'kill' command (*note Killing the Child Process: Kill Process.). 'add-symbol-file FILENAME ADDRESS' 'add-symbol-file FILENAME ADDRESS [ -readnow ]' 'add-symbol-file FILENAME ADDRESS -s SECTION ADDRESS ...' The 'add-symbol-file' command reads additional symbol table information from the file FILENAME. You would use this command when FILENAME has been dynamically loaded (by some other means) into the program that is running. The ADDRESS should give the memory address at which the file has been loaded; GDB cannot figure this out for itself. You can additionally specify an arbitrary number of '-s SECTION ADDRESS' pairs, to give an explicit section name and base address for that section. You can specify any ADDRESS as an expression. The symbol table of the file FILENAME is added to the symbol table originally read with the 'symbol-file' command. You can use the 'add-symbol-file' command any number of times; the new symbol data thus read is kept in addition to the old. Changes can be reverted using the command 'remove-symbol-file'. Although FILENAME is typically a shared library file, an executable file, or some other object file which has been fully relocated for loading into a process, you can also load symbolic information from relocatable '.o' files, as long as: * the file's symbolic information refers only to linker symbols defined in that file, not to symbols defined by other object files, * every section the file's symbolic information refers to has actually been loaded into the inferior, as it appears in the file, and * you can determine the address at which every section was loaded, and provide these to the 'add-symbol-file' command. Some embedded operating systems, like Sun Chorus and VxWorks, can load relocatable files into an already running program; such systems typically make the requirements above easy to meet. However, it's important to recognize that many native systems use complex link procedures ('.linkonce' section factoring and C++ constructor table assembly, for example) that make the requirements difficult to meet. In general, one cannot assume that using 'add-symbol-file' to read a relocatable object file's symbolic information will have the same effect as linking the relocatable object file into the program in the normal way. 'add-symbol-file' does not repeat if you press after using it. 'remove-symbol-file FILENAME' 'remove-symbol-file -a ADDRESS' Remove a symbol file added via the 'add-symbol-file' command. The file to remove can be identified by its FILENAME or by an ADDRESS that lies within the boundaries of this symbol file in memory. Example: (gdb) add-symbol-file /home/user/gdb/mylib.so 0x7ffff7ff9480 add symbol table from file "/home/user/gdb/mylib.so" at .text_addr = 0x7ffff7ff9480 (y or n) y Reading symbols from /home/user/gdb/mylib.so...done. (gdb) remove-symbol-file -a 0x7ffff7ff9480 Remove symbol table from file "/home/user/gdb/mylib.so"? (y or n) y (gdb) 'remove-symbol-file' does not repeat if you press after using it. 'add-symbol-file-from-memory ADDRESS' Load symbols from the given ADDRESS in a dynamically loaded object file whose image is mapped directly into the inferior's memory. For example, the Linux kernel maps a 'syscall DSO' into each process's address space; this DSO provides kernel-specific code for some system calls. The argument can be any expression whose evaluation yields the address of the file's shared object file header. For this command to work, you must have used 'symbol-file' or 'exec-file' commands in advance. 'section SECTION ADDR' The 'section' command changes the base address of the named SECTION of the exec file to ADDR. This can be used if the exec file does not contain section addresses, (such as in the 'a.out' format), or when the addresses specified in the file itself are wrong. Each section must be changed separately. The 'info files' command, described below, lists all the sections and their addresses. 'info files' 'info target' 'info files' and 'info target' are synonymous; both print the current target (*note Specifying a Debugging Target: Targets.), including the names of the executable and core dump files currently in use by GDB, and the files from which symbols were loaded. The command 'help target' lists all possible targets rather than current ones. 'maint info sections' Another command that can give you extra information about program sections is 'maint info sections'. In addition to the section information displayed by 'info files', this command displays the flags and file offset of each section in the executable and core dump files. In addition, 'maint info sections' provides the following command options (which may be arbitrarily combined): 'ALLOBJ' Display sections for all loaded object files, including shared libraries. 'SECTIONS' Display info only for named SECTIONS. 'SECTION-FLAGS' Display info only for sections for which SECTION-FLAGS are true. The section flags that GDB currently knows about are: 'ALLOC' Section will have space allocated in the process when loaded. Set for all sections except those containing debug information. 'LOAD' Section will be loaded from the file into the child process memory. Set for pre-initialized code and data, clear for '.bss' sections. 'RELOC' Section needs to be relocated before loading. 'READONLY' Section cannot be modified by the child process. 'CODE' Section contains executable code only. 'DATA' Section contains data only (no executable code). 'ROM' Section will reside in ROM. 'CONSTRUCTOR' Section contains data for constructor/destructor lists. 'HAS_CONTENTS' Section is not empty. 'NEVER_LOAD' An instruction to the linker to not output the section. 'COFF_SHARED_LIBRARY' A notification to the linker that the section contains COFF shared library information. 'IS_COMMON' Section contains common symbols. 'set trust-readonly-sections on' Tell GDB that readonly sections in your object file really are read-only (i.e. that their contents will not change). In that case, GDB can fetch values from these sections out of the object file, rather than from the target program. For some targets (notably embedded ones), this can be a significant enhancement to debugging performance. The default is off. 'set trust-readonly-sections off' Tell GDB not to trust readonly sections. This means that the contents of the section might change while the program is running, and must therefore be fetched from the target when needed. 'show trust-readonly-sections' Show the current setting of trusting readonly sections. All file-specifying commands allow both absolute and relative file names as arguments. GDB always converts the file name to an absolute file name and remembers it that way. GDB supports GNU/Linux, MS-Windows, SunOS, Darwin/Mach-O, SVr4, IBM RS/6000 AIX, QNX Neutrino, FDPIC (FR-V), and DSBT (TIC6X) shared libraries. On MS-Windows GDB must be linked with the Expat library to support shared libraries. *Note Expat::. GDB automatically loads symbol definitions from shared libraries when you use the 'run' command, or when you examine a core file. (Before you issue the 'run' command, GDB does not understand references to a function in a shared library, however--unless you are debugging a core file). There are times, however, when you may wish to not automatically load symbol definitions from shared libraries, such as when they are particularly large or there are many of them. To control the automatic loading of shared library symbols, use the commands: 'set auto-solib-add MODE' If MODE is 'on', symbols from all shared object libraries will be loaded automatically when the inferior begins execution, you attach to an independently started inferior, or when the dynamic linker informs GDB that a new library has been loaded. If MODE is 'off', symbols must be loaded manually, using the 'sharedlibrary' command. The default value is 'on'. If your program uses lots of shared libraries with debug info that takes large amounts of memory, you can decrease the GDB memory footprint by preventing it from automatically loading the symbols from shared libraries. To that end, type 'set auto-solib-add off' before running the inferior, then load each library whose debug symbols you do need with 'sharedlibrary REGEXP', where REGEXP is a regular expression that matches the libraries whose symbols you want to be loaded. 'show auto-solib-add' Display the current autoloading mode. To explicitly load shared library symbols, use the 'sharedlibrary' command: 'info share REGEX' 'info sharedlibrary REGEX' Print the names of the shared libraries which are currently loaded that match REGEX. If REGEX is omitted then print all shared libraries that are loaded. 'info dll REGEX' This is an alias of 'info sharedlibrary'. 'sharedlibrary REGEX' 'share REGEX' Load shared object library symbols for files matching a Unix regular expression. As with files loaded automatically, it only loads shared libraries required by your program for a core file or after typing 'run'. If REGEX is omitted all shared libraries required by your program are loaded. 'nosharedlibrary' Unload all shared object library symbols. This discards all symbols that have been loaded from all shared libraries. Symbols from shared libraries that were loaded by explicit user requests are not discarded. Sometimes you may wish that GDB stops and gives you control when any of shared library events happen. The best way to do this is to use 'catch load' and 'catch unload' (*note Set Catchpoints::). GDB also supports the the 'set stop-on-solib-events' command for this. This command exists for historical reasons. It is less useful than setting a catchpoint, because it does not allow for conditions or commands as a catchpoint does. 'set stop-on-solib-events' This command controls whether GDB should give you control when the dynamic linker notifies it about some shared library event. The most common event of interest is loading or unloading of a new shared library. 'show stop-on-solib-events' Show whether GDB stops and gives you control when shared library events happen. Shared libraries are also supported in many cross or remote debugging configurations. GDB needs to have access to the target's libraries; this can be accomplished either by providing copies of the libraries on the host system, or by asking GDB to automatically retrieve the libraries from the target. If copies of the target libraries are provided, they need to be the same as the target libraries, although the copies on the target can be stripped as long as the copies on the host are not. For remote debugging, you need to tell GDB where the target libraries are, so that it can load the correct copies--otherwise, it may try to load the host's libraries. GDB has two variables to specify the search directories for target libraries. 'set sysroot PATH' Use PATH as the system root for the program being debugged. Any absolute shared library paths will be prefixed with PATH; many runtime loaders store the absolute paths to the shared library in the target program's memory. When starting processes remotely, and when attaching to already-running processes (local or remote), their executable filenames will be prefixed with PATH if reported to GDB as absolute by the operating system. If you use 'set sysroot' to find executables and shared libraries, they need to be laid out in the same way that they are on the target, with e.g. a '/bin', '/lib' and '/usr/lib' hierarchy under PATH. If PATH starts with the sequence 'target:' and the target system is remote then GDB will retrieve the target binaries from the remote system. This is only supported when using a remote target that supports the 'remote get' command (*note Sending files to a remote system: File Transfer.). The part of PATH following the initial 'target:' (if present) is used as system root prefix on the remote file system. If PATH starts with the sequence 'remote:' this is converted to the sequence 'target:' by 'set sysroot'(1). If you want to specify a local system root using a directory that happens to be named 'target:' or 'remote:', you need to use some equivalent variant of the name like './target:'. For targets with an MS-DOS based filesystem, such as MS-Windows and SymbianOS, GDB tries prefixing a few variants of the target absolute file name with PATH. But first, on Unix hosts, GDB converts all backslash directory separators into forward slashes, because the backslash is not a directory separator on Unix: c:\foo\bar.dll => c:/foo/bar.dll Then, GDB attempts prefixing the target file name with PATH, and looks for the resulting file name in the host file system: c:/foo/bar.dll => /path/to/sysroot/c:/foo/bar.dll If that does not find the binary, GDB tries removing the ':' character from the drive spec, both for convenience, and, for the case of the host file system not supporting file names with colons: c:/foo/bar.dll => /path/to/sysroot/c/foo/bar.dll This makes it possible to have a system root that mirrors a target with more than one drive. E.g., you may want to setup your local copies of the target system shared libraries like so (note 'c' vs 'z'): /path/to/sysroot/c/sys/bin/foo.dll /path/to/sysroot/c/sys/bin/bar.dll /path/to/sysroot/z/sys/bin/bar.dll and point the system root at '/path/to/sysroot', so that GDB can find the correct copies of both 'c:\sys\bin\foo.dll', and 'z:\sys\bin\bar.dll'. If that still does not find the binary, GDB tries removing the whole drive spec from the target file name: c:/foo/bar.dll => /path/to/sysroot/foo/bar.dll This last lookup makes it possible to not care about the drive name, if you don't want or need to. The 'set solib-absolute-prefix' command is an alias for 'set sysroot'. You can set the default system root by using the configure-time '--with-sysroot' option. If the system root is inside GDB's configured binary prefix (set with '--prefix' or '--exec-prefix'), then the default system root will be updated automatically if the installed GDB is moved to a new location. 'show sysroot' Display the current executable and shared library prefix. 'set solib-search-path PATH' If this variable is set, PATH is a colon-separated list of directories to search for shared libraries. 'solib-search-path' is used after 'sysroot' fails to locate the library, or if the path to the library is relative instead of absolute. If you want to use 'solib-search-path' instead of 'sysroot', be sure to set 'sysroot' to a nonexistent directory to prevent GDB from finding your host's libraries. 'sysroot' is preferred; setting it to a nonexistent directory may interfere with automatic loading of shared library symbols. 'show solib-search-path' Display the current shared library search path. 'set target-file-system-kind KIND' Set assumed file system kind for target reported file names. Shared library file names as reported by the target system may not make sense as is on the system GDB is running on. For example, when remote debugging a target that has MS-DOS based file system semantics, from a Unix host, the target may be reporting to GDB a list of loaded shared libraries with file names such as 'c:\Windows\kernel32.dll'. On Unix hosts, there's no concept of drive letters, so the 'c:\' prefix is not normally understood as indicating an absolute file name, and neither is the backslash normally considered a directory separator character. In that case, the native file system would interpret this whole absolute file name as a relative file name with no directory components. This would make it impossible to point GDB at a copy of the remote target's shared libraries on the host using 'set sysroot', and impractical with 'set solib-search-path'. Setting 'target-file-system-kind' to 'dos-based' tells GDB to interpret such file names similarly to how the target would, and to map them to file names valid on GDB's native file system semantics. The value of KIND can be '"auto"', in addition to one of the supported file system kinds. In that case, GDB tries to determine the appropriate file system variant based on the current target's operating system (*note Configuring the Current ABI: ABI.). The supported file system settings are: 'unix' Instruct GDB to assume the target file system is of Unix kind. Only file names starting the forward slash ('/') character are considered absolute, and the directory separator character is also the forward slash. 'dos-based' Instruct GDB to assume the target file system is DOS based. File names starting with either a forward slash, or a drive letter followed by a colon (e.g., 'c:'), are considered absolute, and both the slash ('/') and the backslash ('\\') characters are considered directory separators. 'auto' Instruct GDB to use the file system kind associated with the target operating system (*note Configuring the Current ABI: ABI.). This is the default. When processing file names provided by the user, GDB frequently needs to compare them to the file names recorded in the program's debug info. Normally, GDB compares just the "base names" of the files as strings, which is reasonably fast even for very large programs. (The base name of a file is the last portion of its name, after stripping all the leading directories.) This shortcut in comparison is based upon the assumption that files cannot have more than one base name. This is usually true, but references to files that use symlinks or similar filesystem facilities violate that assumption. If your program records files using such facilities, or if you provide file names to GDB using symlinks etc., you can set 'basenames-may-differ' to 'true' to instruct GDB to completely canonicalize each pair of file names it needs to compare. This will make file-name comparisons accurate, but at a price of a significant slowdown. 'set basenames-may-differ' Set whether a source file may have multiple base names. 'show basenames-may-differ' Show whether a source file may have multiple base names. ---------- Footnotes ---------- (1) Historically the functionality to retrieve binaries from the remote system was provided by prefixing PATH with 'remote:'  File: gdb.info, Node: File Caching, Next: Separate Debug Files, Prev: Files, Up: GDB Files 18.2 File Caching ================= To speed up file loading, and reduce memory usage, GDB will reuse the 'bfd' objects used to track open files. *Note BFD: (bfd)Top. The following commands allow visibility and control of the caching behavior. 'maint info bfds' This prints information about each 'bfd' object that is known to GDB. 'maint set bfd-sharing' 'maint show bfd-sharing' Control whether 'bfd' objects can be shared. When sharing is enabled GDB reuses already open 'bfd' objects rather than reopening the same file. Turning sharing off does not cause already shared 'bfd' objects to be unshared, but all future files that are opened will create a new 'bfd' object. Similarly, re-enabling sharing does not cause multiple existing 'bfd' objects to be collapsed into a single shared 'bfd' object. 'set debug bfd-cache LEVEL' Turns on debugging of the bfd cache, setting the level to LEVEL. 'show debug bfd-cache' Show the current debugging level of the bfd cache.  File: gdb.info, Node: Separate Debug Files, Next: MiniDebugInfo, Prev: File Caching, Up: GDB Files 18.3 Debugging Information in Separate Files ============================================ GDB allows you to put a program's debugging information in a file separate from the executable itself, in a way that allows GDB to find and load the debugging information automatically. Since debugging information can be very large--sometimes larger than the executable code itself--some systems distribute debugging information for their executables in separate files, which users can install only when they need to debug a problem. GDB supports two ways of specifying the separate debug info file: * The executable contains a "debug link" that specifies the name of the separate debug info file. The separate debug file's name is usually 'EXECUTABLE.debug', where EXECUTABLE is the name of the corresponding executable file without leading directories (e.g., 'ls.debug' for '/usr/bin/ls'). In addition, the debug link specifies a 32-bit "Cyclic Redundancy Check" (CRC) checksum for the debug file, which GDB uses to validate that the executable and the debug file came from the same build. * The executable contains a "build ID", a unique bit string that is also present in the corresponding debug info file. (This is supported only on some operating systems, when using the ELF or PE file formats for binary files and the GNU Binutils.) For more details about this feature, see the description of the '--build-id' command-line option in *note Command Line Options: (ld.info)Options. The debug info file's name is not specified explicitly by the build ID, but can be computed from the build ID, see below. Depending on the way the debug info file is specified, GDB uses two different methods of looking for the debug file: * For the "debug link" method, GDB looks up the named file in the directory of the executable file, then in a subdirectory of that directory named '.debug', and finally under each one of the global debug directories, in a subdirectory whose name is identical to the leading directories of the executable's absolute file name. * For the "build ID" method, GDB looks in the '.build-id' subdirectory of each one of the global debug directories for a file named 'NN/NNNNNNNN.debug', where NN are the first 2 hex characters of the build ID bit string, and NNNNNNNN are the rest of the bit string. (Real build ID strings are 32 or more hex characters, not 10.) So, for example, suppose you ask GDB to debug '/usr/bin/ls', which has a debug link that specifies the file 'ls.debug', and a build ID whose value in hex is 'abcdef1234'. If the list of the global debug directories includes '/usr/lib/debug', then GDB will look for the following debug information files, in the indicated order: - '/usr/lib/debug/.build-id/ab/cdef1234.debug' - '/usr/bin/ls.debug' - '/usr/bin/.debug/ls.debug' - '/usr/lib/debug/usr/bin/ls.debug'. Global debugging info directories default to what is set by GDB configure option '--with-separate-debug-dir'. During GDB run you can also set the global debugging info directories, and view the list GDB is currently using. 'set debug-file-directory DIRECTORIES' Set the directories which GDB searches for separate debugging information files to DIRECTORY. Multiple path components can be set concatenating them by a path separator. 'show debug-file-directory' Show the directories GDB searches for separate debugging information files. A debug link is a special section of the executable file named '.gnu_debuglink'. The section must contain: * A filename, with any leading directory components removed, followed by a zero byte, * zero to three bytes of padding, as needed to reach the next four-byte boundary within the section, and * a four-byte CRC checksum, stored in the same endianness used for the executable file itself. The checksum is computed on the debugging information file's full contents by the function given below, passing zero as the CRC argument. Any executable file format can carry a debug link, as long as it can contain a section named '.gnu_debuglink' with the contents described above. The build ID is a special section in the executable file (and in other ELF binary files that GDB may consider). This section is often named '.note.gnu.build-id', but that name is not mandatory. It contains unique identification for the built files--the ID remains the same across multiple builds of the same build tree. The default algorithm SHA1 produces 160 bits (40 hexadecimal characters) of the content for the build ID string. The same section with an identical value is present in the original built binary with symbols, in its stripped variant, and in the separate debugging information file. The debugging information file itself should be an ordinary executable, containing a full set of linker symbols, sections, and debugging information. The sections of the debugging information file should have the same names, addresses, and sizes as the original file, but they need not contain any data--much like a '.bss' section in an ordinary executable. The GNU binary utilities (Binutils) package includes the 'objcopy' utility that can produce the separated executable / debugging information file pairs using the following commands: objcopy --only-keep-debug foo foo.debug strip -g foo These commands remove the debugging information from the executable file 'foo' and place it in the file 'foo.debug'. You can use the first, second or both methods to link the two files: * The debug link method needs the following additional command to also leave behind a debug link in 'foo': objcopy --add-gnu-debuglink=foo.debug foo Ulrich Drepper's 'elfutils' package, starting with version 0.53, contains a version of the 'strip' command such that the command 'strip foo -f foo.debug' has the same functionality as the two 'objcopy' commands and the 'ln -s' command above, together. * Build ID gets embedded into the main executable using 'ld --build-id' or the GCC counterpart 'gcc -Wl,--build-id'. Build ID support plus compatibility fixes for debug files separation are present in GNU binary utilities (Binutils) package since version 2.18. The CRC used in '.gnu_debuglink' is the CRC-32 defined in IEEE 802.3 using the polynomial: x^{32} + x^{26} + x^{23} + x^{22} + x^{16} + x^{12} + x^{11} + x^{10} + x^8 + x^7 + x^5 + x^4 + x^2 + x + 1 The function is computed byte at a time, taking the least significant bit of each byte first. The initial pattern '0xffffffff' is used, to ensure leading zeros affect the CRC and the final result is inverted to ensure trailing zeros also affect the CRC. _Note:_ This is the same CRC polynomial as used in handling the "Remote Serial Protocol" 'qCRC' packet (*note qCRC packet::). However in the case of the Remote Serial Protocol, the CRC is computed _most_ significant bit first, and the result is not inverted, so trailing zeros have no effect on the CRC value. To complete the description, we show below the code of the function which produces the CRC used in '.gnu_debuglink'. Inverting the initially supplied 'crc' argument means that an initial call to this function passing in zero will start computing the CRC using '0xffffffff'. unsigned long gnu_debuglink_crc32 (unsigned long crc, unsigned char *buf, size_t len) { static const unsigned long crc32_table[256] = { 0x00000000, 0x77073096, 0xee0e612c, 0x990951ba, 0x076dc419, 0x706af48f, 0xe963a535, 0x9e6495a3, 0x0edb8832, 0x79dcb8a4, 0xe0d5e91e, 0x97d2d988, 0x09b64c2b, 0x7eb17cbd, 0xe7b82d07, 0x90bf1d91, 0x1db71064, 0x6ab020f2, 0xf3b97148, 0x84be41de, 0x1adad47d, 0x6ddde4eb, 0xf4d4b551, 0x83d385c7, 0x136c9856, 0x646ba8c0, 0xfd62f97a, 0x8a65c9ec, 0x14015c4f, 0x63066cd9, 0xfa0f3d63, 0x8d080df5, 0x3b6e20c8, 0x4c69105e, 0xd56041e4, 0xa2677172, 0x3c03e4d1, 0x4b04d447, 0xd20d85fd, 0xa50ab56b, 0x35b5a8fa, 0x42b2986c, 0xdbbbc9d6, 0xacbcf940, 0x32d86ce3, 0x45df5c75, 0xdcd60dcf, 0xabd13d59, 0x26d930ac, 0x51de003a, 0xc8d75180, 0xbfd06116, 0x21b4f4b5, 0x56b3c423, 0xcfba9599, 0xb8bda50f, 0x2802b89e, 0x5f058808, 0xc60cd9b2, 0xb10be924, 0x2f6f7c87, 0x58684c11, 0xc1611dab, 0xb6662d3d, 0x76dc4190, 0x01db7106, 0x98d220bc, 0xefd5102a, 0x71b18589, 0x06b6b51f, 0x9fbfe4a5, 0xe8b8d433, 0x7807c9a2, 0x0f00f934, 0x9609a88e, 0xe10e9818, 0x7f6a0dbb, 0x086d3d2d, 0x91646c97, 0xe6635c01, 0x6b6b51f4, 0x1c6c6162, 0x856530d8, 0xf262004e, 0x6c0695ed, 0x1b01a57b, 0x8208f4c1, 0xf50fc457, 0x65b0d9c6, 0x12b7e950, 0x8bbeb8ea, 0xfcb9887c, 0x62dd1ddf, 0x15da2d49, 0x8cd37cf3, 0xfbd44c65, 0x4db26158, 0x3ab551ce, 0xa3bc0074, 0xd4bb30e2, 0x4adfa541, 0x3dd895d7, 0xa4d1c46d, 0xd3d6f4fb, 0x4369e96a, 0x346ed9fc, 0xad678846, 0xda60b8d0, 0x44042d73, 0x33031de5, 0xaa0a4c5f, 0xdd0d7cc9, 0x5005713c, 0x270241aa, 0xbe0b1010, 0xc90c2086, 0x5768b525, 0x206f85b3, 0xb966d409, 0xce61e49f, 0x5edef90e, 0x29d9c998, 0xb0d09822, 0xc7d7a8b4, 0x59b33d17, 0x2eb40d81, 0xb7bd5c3b, 0xc0ba6cad, 0xedb88320, 0x9abfb3b6, 0x03b6e20c, 0x74b1d29a, 0xead54739, 0x9dd277af, 0x04db2615, 0x73dc1683, 0xe3630b12, 0x94643b84, 0x0d6d6a3e, 0x7a6a5aa8, 0xe40ecf0b, 0x9309ff9d, 0x0a00ae27, 0x7d079eb1, 0xf00f9344, 0x8708a3d2, 0x1e01f268, 0x6906c2fe, 0xf762575d, 0x806567cb, 0x196c3671, 0x6e6b06e7, 0xfed41b76, 0x89d32be0, 0x10da7a5a, 0x67dd4acc, 0xf9b9df6f, 0x8ebeeff9, 0x17b7be43, 0x60b08ed5, 0xd6d6a3e8, 0xa1d1937e, 0x38d8c2c4, 0x4fdff252, 0xd1bb67f1, 0xa6bc5767, 0x3fb506dd, 0x48b2364b, 0xd80d2bda, 0xaf0a1b4c, 0x36034af6, 0x41047a60, 0xdf60efc3, 0xa867df55, 0x316e8eef, 0x4669be79, 0xcb61b38c, 0xbc66831a, 0x256fd2a0, 0x5268e236, 0xcc0c7795, 0xbb0b4703, 0x220216b9, 0x5505262f, 0xc5ba3bbe, 0xb2bd0b28, 0x2bb45a92, 0x5cb36a04, 0xc2d7ffa7, 0xb5d0cf31, 0x2cd99e8b, 0x5bdeae1d, 0x9b64c2b0, 0xec63f226, 0x756aa39c, 0x026d930a, 0x9c0906a9, 0xeb0e363f, 0x72076785, 0x05005713, 0x95bf4a82, 0xe2b87a14, 0x7bb12bae, 0x0cb61b38, 0x92d28e9b, 0xe5d5be0d, 0x7cdcefb7, 0x0bdbdf21, 0x86d3d2d4, 0xf1d4e242, 0x68ddb3f8, 0x1fda836e, 0x81be16cd, 0xf6b9265b, 0x6fb077e1, 0x18b74777, 0x88085ae6, 0xff0f6a70, 0x66063bca, 0x11010b5c, 0x8f659eff, 0xf862ae69, 0x616bffd3, 0x166ccf45, 0xa00ae278, 0xd70dd2ee, 0x4e048354, 0x3903b3c2, 0xa7672661, 0xd06016f7, 0x4969474d, 0x3e6e77db, 0xaed16a4a, 0xd9d65adc, 0x40df0b66, 0x37d83bf0, 0xa9bcae53, 0xdebb9ec5, 0x47b2cf7f, 0x30b5ffe9, 0xbdbdf21c, 0xcabac28a, 0x53b39330, 0x24b4a3a6, 0xbad03605, 0xcdd70693, 0x54de5729, 0x23d967bf, 0xb3667a2e, 0xc4614ab8, 0x5d681b02, 0x2a6f2b94, 0xb40bbe37, 0xc30c8ea1, 0x5a05df1b, 0x2d02ef8d }; unsigned char *end; crc = ~crc & 0xffffffff; for (end = buf + len; buf < end; ++buf) crc = crc32_table[(crc ^ *buf) & 0xff] ^ (crc >> 8); return ~crc & 0xffffffff; } This computation does not apply to the "build ID" method.  File: gdb.info, Node: MiniDebugInfo, Next: Index Files, Prev: Separate Debug Files, Up: GDB Files 18.4 Debugging information in a special section =============================================== Some systems ship pre-built executables and libraries that have a special '.gnu_debugdata' section. This feature is called "MiniDebugInfo". This section holds an LZMA-compressed object and is used to supply extra symbols for backtraces. The intent of this section is to provide extra minimal debugging information for use in simple backtraces. It is not intended to be a replacement for full separate debugging information (*note Separate Debug Files::). The example below shows the intended use; however, GDB does not currently put restrictions on what sort of debugging information might be included in the section. GDB has support for this extension. If the section exists, then it is used provided that no other source of debugging information can be found, and that GDB was configured with LZMA support. This section can be easily created using 'objcopy' and other standard utilities: # Extract the dynamic symbols from the main binary, there is no need # to also have these in the normal symbol table. nm -D BINARY --format=posix --defined-only \ | awk '{ print $1 }' | sort > dynsyms # Extract all the text (i.e. function) symbols from the debuginfo. # (Note that we actually also accept "D" symbols, for the benefit # of platforms like PowerPC64 that use function descriptors.) nm BINARY --format=posix --defined-only \ | awk '{ if ($2 == "T" || $2 == "t" || $2 == "D") print $1 }' \ | sort > funcsyms # Keep all the function symbols not already in the dynamic symbol # table. comm -13 dynsyms funcsyms > keep_symbols # Separate full debug info into debug binary. objcopy --only-keep-debug BINARY debug # Copy the full debuginfo, keeping only a minimal set of symbols and # removing some unnecessary sections. objcopy -S --remove-section .gdb_index --remove-section .comment \ --keep-symbols=keep_symbols debug mini_debuginfo # Drop the full debug info from the original binary. strip --strip-all -R .comment BINARY # Inject the compressed data into the .gnu_debugdata section of the # original binary. xz mini_debuginfo objcopy --add-section .gnu_debugdata=mini_debuginfo.xz BINARY  File: gdb.info, Node: Index Files, Next: Symbol Errors, Prev: MiniDebugInfo, Up: GDB Files 18.5 Index Files Speed Up GDB ============================= When GDB finds a symbol file, it scans the symbols in the file in order to construct an internal symbol table. This lets most GDB operations work quickly--at the cost of a delay early on. For large programs, this delay can be quite lengthy, so GDB provides a way to build an index, which speeds up startup. The index is stored as a section in the symbol file. GDB can write the index to a file, then you can put it into the symbol file using 'objcopy'. To create an index file, use the 'save gdb-index' command: 'save gdb-index DIRECTORY' Create an index file for each symbol file currently known by GDB. Each file is named after its corresponding symbol file, with '.gdb-index' appended, and is written into the given DIRECTORY. Once you have created an index file you can merge it into your symbol file, here named 'symfile', using 'objcopy': $ objcopy --add-section .gdb_index=symfile.gdb-index \ --set-section-flags .gdb_index=readonly symfile symfile GDB will normally ignore older versions of '.gdb_index' sections that have been deprecated. Usually they are deprecated because they are missing a new feature or have performance issues. To tell GDB to use a deprecated index section anyway specify 'set use-deprecated-index-sections on'. The default is 'off'. This can speed up startup, but may result in some functionality being lost. *Note Index Section Format::. _Warning:_ Setting 'use-deprecated-index-sections' to 'on' must be done before gdb reads the file. The following will not work: $ gdb -ex "set use-deprecated-index-sections on" Instead you must do, for example, $ gdb -iex "set use-deprecated-index-sections on" There are currently some limitation on indices. They only work when for DWARF debugging information, not stabs. And, they do not currently work for programs using Ada.  File: gdb.info, Node: Symbol Errors, Next: Data Files, Prev: Index Files, Up: GDB Files 18.6 Errors Reading Symbol Files ================================ While reading a symbol file, GDB occasionally encounters problems, such as symbol types it does not recognize, or known bugs in compiler output. By default, GDB does not notify you of such problems, since they are relatively common and primarily of interest to people debugging compilers. If you are interested in seeing information about ill-constructed symbol tables, you can either ask GDB to print only one message about each such type of problem, no matter how many times the problem occurs; or you can ask GDB to print more messages, to see how many times the problems occur, with the 'set complaints' command (*note Optional Warnings and Messages: Messages/Warnings.). The messages currently printed, and their meanings, include: 'inner block not inside outer block in SYMBOL' The symbol information shows where symbol scopes begin and end (such as at the start of a function or a block of statements). This error indicates that an inner scope block is not fully contained in its outer scope blocks. GDB circumvents the problem by treating the inner block as if it had the same scope as the outer block. In the error message, SYMBOL may be shown as "'(don't know)'" if the outer block is not a function. 'block at ADDRESS out of order' The symbol information for symbol scope blocks should occur in order of increasing addresses. This error indicates that it does not do so. GDB does not circumvent this problem, and has trouble locating symbols in the source file whose symbols it is reading. (You can often determine what source file is affected by specifying 'set verbose on'. *Note Optional Warnings and Messages: Messages/Warnings.) 'bad block start address patched' The symbol information for a symbol scope block has a start address smaller than the address of the preceding source line. This is known to occur in the SunOS 4.1.1 (and earlier) C compiler. GDB circumvents the problem by treating the symbol scope block as starting on the previous source line. 'bad string table offset in symbol N' Symbol number N contains a pointer into the string table which is larger than the size of the string table. GDB circumvents the problem by considering the symbol to have the name 'foo', which may cause other problems if many symbols end up with this name. 'unknown symbol type 0xNN' The symbol information contains new data types that GDB does not yet know how to read. '0xNN' is the symbol type of the uncomprehended information, in hexadecimal. GDB circumvents the error by ignoring this symbol information. This usually allows you to debug your program, though certain symbols are not accessible. If you encounter such a problem and feel like debugging it, you can debug 'gdb' with itself, breakpoint on 'complain', then go up to the function 'read_dbx_symtab' and examine '*bufp' to see the symbol. 'stub type has NULL name' GDB could not find the full definition for a struct or class. 'const/volatile indicator missing (ok if using g++ v1.x), got...' The symbol information for a C++ member function is missing some information that recent versions of the compiler should have output for it. 'info mismatch between compiler and debugger' GDB could not parse a type specification output by the compiler.  File: gdb.info, Node: Data Files, Prev: Symbol Errors, Up: GDB Files 18.7 GDB Data Files =================== GDB will sometimes read an auxiliary data file. These files are kept in a directory known as the "data directory". You can set the data directory's name, and view the name GDB is currently using. 'set data-directory DIRECTORY' Set the directory which GDB searches for auxiliary data files to DIRECTORY. 'show data-directory' Show the directory GDB searches for auxiliary data files. You can set the default data directory by using the configure-time '--with-gdb-datadir' option. If the data directory is inside GDB's configured binary prefix (set with '--prefix' or '--exec-prefix'), then the default data directory will be updated automatically if the installed GDB is moved to a new location. The data directory may also be specified with the '--data-directory' command line option. *Note Mode Options::.  File: gdb.info, Node: Targets, Next: Remote Debugging, Prev: GDB Files, Up: Top 19 Specifying a Debugging Target ******************************** A "target" is the execution environment occupied by your program. Often, GDB runs in the same host environment as your program; in that case, the debugging target is specified as a side effect when you use the 'file' or 'core' commands. When you need more flexibility--for example, running GDB on a physically separate host, or controlling a standalone system over a serial port or a realtime system over a TCP/IP connection--you can use the 'target' command to specify one of the target types configured for GDB (*note Commands for Managing Targets: Target Commands.). It is possible to build GDB for several different "target architectures". When GDB is built like that, you can choose one of the available architectures with the 'set architecture' command. 'set architecture ARCH' This command sets the current target architecture to ARCH. The value of ARCH can be '"auto"', in addition to one of the supported architectures. 'show architecture' Show the current target architecture. 'set processor' 'processor' These are alias commands for, respectively, 'set architecture' and 'show architecture'. * Menu: * Active Targets:: Active targets * Target Commands:: Commands for managing targets * Byte Order:: Choosing target byte order  File: gdb.info, Node: Active Targets, Next: Target Commands, Up: Targets 19.1 Active Targets =================== There are multiple classes of targets such as: processes, executable files or recording sessions. Core files belong to the process class, making core file and process mutually exclusive. Otherwise, GDB can work concurrently on multiple active targets, one in each class. This allows you to (for example) start a process and inspect its activity, while still having access to the executable file after the process finishes. Or if you start process recording (*note Reverse Execution::) and 'reverse-step' there, you are presented a virtual layer of the recording target, while the process target remains stopped at the chronologically last point of the process execution. Use the 'core-file' and 'exec-file' commands to select a new core file or executable target (*note Commands to Specify Files: Files.). To specify as a target a process that is already running, use the 'attach' command (*note Debugging an Already-running Process: Attach.).  File: gdb.info, Node: Target Commands, Next: Byte Order, Prev: Active Targets, Up: Targets 19.2 Commands for Managing Targets ================================== 'target TYPE PARAMETERS' Connects the GDB host environment to a target machine or process. A target is typically a protocol for talking to debugging facilities. You use the argument TYPE to specify the type or protocol of the target machine. Further PARAMETERS are interpreted by the target protocol, but typically include things like device names or host names to connect with, process numbers, and baud rates. The 'target' command does not repeat if you press again after executing the command. 'help target' Displays the names of all targets available. To display targets currently selected, use either 'info target' or 'info files' (*note Commands to Specify Files: Files.). 'help target NAME' Describe a particular target, including any parameters necessary to select it. 'set gnutarget ARGS' GDB uses its own library BFD to read your files. GDB knows whether it is reading an "executable", a "core", or a ".o" file; however, you can specify the file format with the 'set gnutarget' command. Unlike most 'target' commands, with 'gnutarget' the 'target' refers to a program, not a machine. _Warning:_ To specify a file format with 'set gnutarget', you must know the actual BFD name. *Note Commands to Specify Files: Files. 'show gnutarget' Use the 'show gnutarget' command to display what file format 'gnutarget' is set to read. If you have not set 'gnutarget', GDB will determine the file format for each file automatically, and 'show gnutarget' displays 'The current BFD target is "auto"'. Here are some common targets (available, or not, depending on the GDB configuration): 'target exec PROGRAM' An executable file. 'target exec PROGRAM' is the same as 'exec-file PROGRAM'. 'target core FILENAME' A core dump file. 'target core FILENAME' is the same as 'core-file FILENAME'. 'target remote MEDIUM' A remote system connected to GDB via a serial line or network connection. This command tells GDB to use its own remote protocol over MEDIUM for debugging. *Note Remote Debugging::. For example, if you have a board connected to '/dev/ttya' on the machine running GDB, you could say: target remote /dev/ttya 'target remote' supports the 'load' command. This is only useful if you have some other way of getting the stub to the target system, and you can put it somewhere in memory where it won't get clobbered by the download. 'target sim [SIMARGS] ...' Builtin CPU simulator. GDB includes simulators for most architectures. In general, target sim load run works; however, you cannot assume that a specific memory map, device drivers, or even basic I/O is available, although some simulators do provide these. For info about any processor-specific simulator details, see the appropriate section in *note Embedded Processors: Embedded Processors. 'target native' Setup for local/native process debugging. Useful to make the 'run' command spawn native processes (likewise 'attach', etc.) even when 'set auto-connect-native-target' is 'off' (*note set auto-connect-native-target::). Different targets are available on different configurations of GDB; your configuration may have more or fewer targets. Many remote targets require you to download the executable's code once you've successfully established a connection. You may wish to control various aspects of this process. 'set hash' This command controls whether a hash mark '#' is displayed while downloading a file to the remote monitor. If on, a hash mark is displayed after each S-record is successfully downloaded to the monitor. 'show hash' Show the current status of displaying the hash mark. 'set debug monitor' Enable or disable display of communications messages between GDB and the remote monitor. 'show debug monitor' Show the current status of displaying communications between GDB and the remote monitor. 'load FILENAME OFFSET' Depending on what remote debugging facilities are configured into GDB, the 'load' command may be available. Where it exists, it is meant to make FILENAME (an executable) available for debugging on the remote system--by downloading, or dynamic linking, for example. 'load' also records the FILENAME symbol table in GDB, like the 'add-symbol-file' command. If your GDB does not have a 'load' command, attempting to execute it gets the error message "'You can't do that when your target is ...'" The file is loaded at whatever address is specified in the executable. For some object file formats, you can specify the load address when you link the program; for other formats, like a.out, the object file format specifies a fixed address. It is also possible to tell GDB to load the executable file at a specific offset described by the optional argument OFFSET. When OFFSET is provided, FILENAME must also be provided. Depending on the remote side capabilities, GDB may be able to load programs into flash memory. 'load' does not repeat if you press again after using it. 'flash-erase' Erases all known flash memory regions on the target.  File: gdb.info, Node: Byte Order, Prev: Target Commands, Up: Targets 19.3 Choosing Target Byte Order =============================== Some types of processors, such as the MIPS, PowerPC, and Renesas SH, offer the ability to run either big-endian or little-endian byte orders. Usually the executable or symbol will include a bit to designate the endian-ness, and you will not need to worry about which to use. However, you may still find it useful to adjust GDB's idea of processor endian-ness manually. 'set endian big' Instruct GDB to assume the target is big-endian. 'set endian little' Instruct GDB to assume the target is little-endian. 'set endian auto' Instruct GDB to use the byte order associated with the executable. 'show endian' Display GDB's current idea of the target byte order. Note that these commands merely adjust interpretation of symbolic data on the host, and that they have absolutely no effect on the target system.  File: gdb.info, Node: Remote Debugging, Next: Configurations, Prev: Targets, Up: Top 20 Debugging Remote Programs **************************** If you are trying to debug a program running on a machine that cannot run GDB in the usual way, it is often useful to use remote debugging. For example, you might use remote debugging on an operating system kernel, or on a small system which does not have a general purpose operating system powerful enough to run a full-featured debugger. Some configurations of GDB have special serial or TCP/IP interfaces to make this work with particular debugging targets. In addition, GDB comes with a generic serial protocol (specific to GDB, but not specific to any particular target system) which you can use if you write the remote stubs--the code that runs on the remote system to communicate with GDB. Other remote targets may be available in your configuration of GDB; use 'help target' to list them. * Menu: * Connecting:: Connecting to a remote target * File Transfer:: Sending files to a remote system * Server:: Using the gdbserver program * Remote Configuration:: Remote configuration * Remote Stub:: Implementing a remote stub  File: gdb.info, Node: Connecting, Next: File Transfer, Up: Remote Debugging 20.1 Connecting to a Remote Target ================================== This section describes how to connect to a remote target, including the types of connections and their differences, how to set up executable and symbol files on the host and target, and the commands used for connecting to and disconnecting from the remote target. 20.1.1 Types of Remote Connections ---------------------------------- GDB supports two types of remote connections, 'target remote' mode and 'target extended-remote' mode. Note that many remote targets support only 'target remote' mode. There are several major differences between the two types of connections, enumerated here: Result of detach or program exit *With target remote mode:* When the debugged program exits or you detach from it, GDB disconnects from the target. When using 'gdbserver', 'gdbserver' will exit. *With target extended-remote mode:* When the debugged program exits or you detach from it, GDB remains connected to the target, even though no program is running. You can rerun the program, attach to a running program, or use 'monitor' commands specific to the target. When using 'gdbserver' in this case, it does not exit unless it was invoked using the '--once' option. If the '--once' option was not used, you can ask 'gdbserver' to exit using the 'monitor exit' command (*note Monitor Commands for gdbserver::). Specifying the program to debug For both connection types you use the 'file' command to specify the program on the host system. If you are using 'gdbserver' there are some differences in how to specify the location of the program on the target. *With target remote mode:* You must either specify the program to debug on the 'gdbserver' command line or use the '--attach' option (*note Attaching to a Running Program: Attaching to a program.). *With target extended-remote mode:* You may specify the program to debug on the 'gdbserver' command line, or you can load the program or attach to it using GDB commands after connecting to 'gdbserver'. You can start 'gdbserver' without supplying an initial command to run or process ID to attach. To do this, use the '--multi' command line option. Then you can connect using 'target extended-remote' and start the program you want to debug (see below for details on using the 'run' command in this scenario). Note that the conditions under which 'gdbserver' terminates depend on how GDB connects to it ('target remote' or 'target extended-remote'). The '--multi' option to 'gdbserver' has no influence on that. The 'run' command *With target remote mode:* The 'run' command is not supported. Once a connection has been established, you can use all the usual GDB commands to examine and change data. The remote program is already running, so you can use commands like 'step' and 'continue'. *With target extended-remote mode:* The 'run' command is supported. The 'run' command uses the value set by 'set remote exec-file' (*note set remote exec-file::) to select the program to run. Command line arguments are supported, except for wildcard expansion and I/O redirection (*note Arguments::). If you specify the program to debug on the command line, then the 'run' command is not required to start execution, and you can resume using commands like 'step' and 'continue' as with 'target remote' mode. Attaching *With target remote mode:* The GDB command 'attach' is not supported. To attach to a running program using 'gdbserver', you must use the '--attach' option (*note Running gdbserver::). *With target extended-remote mode:* To attach to a running program, you may use the 'attach' command after the connection has been established. If you are using 'gdbserver', you may also invoke 'gdbserver' using the '--attach' option (*note Running gdbserver::). 20.1.2 Host and Target Files ---------------------------- GDB, running on the host, needs access to symbol and debugging information for your program running on the target. This requires access to an unstripped copy of your program, and possibly any associated symbol files. Note that this section applies equally to both 'target remote' mode and 'target extended-remote' mode. Some remote targets (*note qXfer executable filename read::, and *note Host I/O Packets::) allow GDB to access program files over the same connection used to communicate with GDB. With such a target, if the remote program is unstripped, the only command you need is 'target remote' (or 'target extended-remote'). If the remote program is stripped, or the target does not support remote program file access, start up GDB using the name of the local unstripped copy of your program as the first argument, or use the 'file' command. Use 'set sysroot' to specify the location (on the host) of target libraries (unless your GDB was compiled with the correct sysroot using '--with-sysroot'). Alternatively, you may use 'set solib-search-path' to specify how GDB locates target libraries. The symbol file and target libraries must exactly match the executable and libraries on the target, with one exception: the files on the host system should not be stripped, even if the files on the target system are. Mismatched or missing files will lead to confusing results during debugging. On GNU/Linux targets, mismatched or missing files may also prevent 'gdbserver' from debugging multi-threaded programs. 20.1.3 Remote Connection Commands --------------------------------- GDB can communicate with the target over a serial line, or over an IP network using TCP or UDP. In each case, GDB uses the same protocol for debugging your program; only the medium carrying the debugging packets varies. The 'target remote' and 'target extended-remote' commands establish a connection to the target. Both commands accept the same arguments, which indicate the medium to use: 'target remote SERIAL-DEVICE' 'target extended-remote SERIAL-DEVICE' Use SERIAL-DEVICE to communicate with the target. For example, to use a serial line connected to the device named '/dev/ttyb': target remote /dev/ttyb If you're using a serial line, you may want to give GDB the '--baud' option, or use the 'set serial baud' command (*note set serial baud: Remote Configuration.) before the 'target' command. 'target remote HOST:PORT' 'target remote tcp:HOST:PORT' 'target extended-remote HOST:PORT' 'target extended-remote tcp:HOST:PORT' Debug using a TCP connection to PORT on HOST. The HOST may be either a host name or a numeric IP address; PORT must be a decimal number. The HOST could be the target machine itself, if it is directly connected to the net, or it might be a terminal server which in turn has a serial line to the target. For example, to connect to port 2828 on a terminal server named 'manyfarms': target remote manyfarms:2828 If your remote target is actually running on the same machine as your debugger session (e.g. a simulator for your target running on the same host), you can omit the hostname. For example, to connect to port 1234 on your local machine: target remote :1234 Note that the colon is still required here. 'target remote udp:HOST:PORT' 'target extended-remote udp:HOST:PORT' Debug using UDP packets to PORT on HOST. For example, to connect to UDP port 2828 on a terminal server named 'manyfarms': target remote udp:manyfarms:2828 When using a UDP connection for remote debugging, you should keep in mind that the 'U' stands for "Unreliable". UDP can silently drop packets on busy or unreliable networks, which will cause havoc with your debugging session. 'target remote | COMMAND' 'target extended-remote | COMMAND' Run COMMAND in the background and communicate with it using a pipe. The COMMAND is a shell command, to be parsed and expanded by the system's command shell, '/bin/sh'; it should expect remote protocol packets on its standard input, and send replies on its standard output. You could use this to run a stand-alone simulator that speaks the remote debugging protocol, to make net connections using programs like 'ssh', or for other similar tricks. If COMMAND closes its standard output (perhaps by exiting), GDB will try to send it a 'SIGTERM' signal. (If the program has already exited, this will have no effect.) Whenever GDB is waiting for the remote program, if you type the interrupt character (often 'Ctrl-c'), GDB attempts to stop the program. This may or may not succeed, depending in part on the hardware and the serial drivers the remote system uses. If you type the interrupt character once again, GDB displays this prompt: Interrupted while waiting for the program. Give up (and stop debugging it)? (y or n) In 'target remote' mode, if you type 'y', GDB abandons the remote debugging session. (If you decide you want to try again later, you can use 'target remote' again to connect once more.) If you type 'n', GDB goes back to waiting. In 'target extended-remote' mode, typing 'n' will leave GDB connected to the target. 'detach' When you have finished debugging the remote program, you can use the 'detach' command to release it from GDB control. Detaching from the target normally resumes its execution, but the results will depend on your particular remote stub. After the 'detach' command in 'target remote' mode, GDB is free to connect to another target. In 'target extended-remote' mode, GDB is still connected to the target. 'disconnect' The 'disconnect' command closes the connection to the target, and the target is generally not resumed. It will wait for GDB (this instance or another one) to connect and continue debugging. After the 'disconnect' command, GDB is again free to connect to another target. 'monitor CMD' This command allows you to send arbitrary commands directly to the remote monitor. Since GDB doesn't care about the commands it sends like this, this command is the way to extend GDB--you can add new commands that only the external monitor will understand and implement.  File: gdb.info, Node: File Transfer, Next: Server, Prev: Connecting, Up: Remote Debugging 20.2 Sending files to a remote system ===================================== Some remote targets offer the ability to transfer files over the same connection used to communicate with GDB. This is convenient for targets accessible through other means, e.g. GNU/Linux systems running 'gdbserver' over a network interface. For other targets, e.g. embedded devices with only a single serial port, this may be the only way to upload or download files. Not all remote targets support these commands. 'remote put HOSTFILE TARGETFILE' Copy file HOSTFILE from the host system (the machine running GDB) to TARGETFILE on the target system. 'remote get TARGETFILE HOSTFILE' Copy file TARGETFILE from the target system to HOSTFILE on the host system. 'remote delete TARGETFILE' Delete TARGETFILE from the target system.  File: gdb.info, Node: Server, Next: Remote Configuration, Prev: File Transfer, Up: Remote Debugging 20.3 Using the 'gdbserver' Program ================================== 'gdbserver' is a control program for Unix-like systems, which allows you to connect your program with a remote GDB via 'target remote' or 'target extended-remote'--but without linking in the usual debugging stub. 'gdbserver' is not a complete replacement for the debugging stubs, because it requires essentially the same operating-system facilities that GDB itself does. In fact, a system that can run 'gdbserver' to connect to a remote GDB could also run GDB locally! 'gdbserver' is sometimes useful nevertheless, because it is a much smaller program than GDB itself. It is also easier to port than all of GDB, so you may be able to get started more quickly on a new system by using 'gdbserver'. Finally, if you develop code for real-time systems, you may find that the tradeoffs involved in real-time operation make it more convenient to do as much development work as possible on another system, for example by cross-compiling. You can use 'gdbserver' to make a similar choice for debugging. GDB and 'gdbserver' communicate via either a serial line or a TCP connection, using the standard GDB remote serial protocol. _Warning:_ 'gdbserver' does not have any built-in security. Do not run 'gdbserver' connected to any public network; a GDB connection to 'gdbserver' provides access to the target system with the same privileges as the user running 'gdbserver'. 20.3.1 Running 'gdbserver' -------------------------- Run 'gdbserver' on the target system. You need a copy of the program you want to debug, including any libraries it requires. 'gdbserver' does not need your program's symbol table, so you can strip the program if necessary to save space. GDB on the host system does all the symbol handling. To use the server, you must tell it how to communicate with GDB; the name of your program; and the arguments for your program. The usual syntax is: target> gdbserver COMM PROGRAM [ ARGS ... ] COMM is either a device name (to use a serial line), or a TCP hostname and portnumber, or '-' or 'stdio' to use stdin/stdout of 'gdbserver'. For example, to debug Emacs with the argument 'foo.txt' and communicate with GDB over the serial port '/dev/com1': target> gdbserver /dev/com1 emacs foo.txt 'gdbserver' waits passively for the host GDB to communicate with it. To use a TCP connection instead of a serial line: target> gdbserver host:2345 emacs foo.txt The only difference from the previous example is the first argument, specifying that you are communicating with the host GDB via TCP. The 'host:2345' argument means that 'gdbserver' is to expect a TCP connection from machine 'host' to local TCP port 2345. (Currently, the 'host' part is ignored.) You can choose any number you want for the port number as long as it does not conflict with any TCP ports already in use on the target system (for example, '23' is reserved for 'telnet').(1) You must use the same port number with the host GDB 'target remote' command. The 'stdio' connection is useful when starting 'gdbserver' with ssh: (gdb) target remote | ssh -T hostname gdbserver - hello The '-T' option to ssh is provided because we don't need a remote pty, and we don't want escape-character handling. Ssh does this by default when a command is provided, the flag is provided to make it explicit. You could elide it if you want to. Programs started with stdio-connected gdbserver have '/dev/null' for 'stdin', and 'stdout','stderr' are sent back to gdb for display through a pipe connected to gdbserver. Both 'stdout' and 'stderr' use the same pipe. 20.3.1.1 Attaching to a Running Program ....................................... On some targets, 'gdbserver' can also attach to running programs. This is accomplished via the '--attach' argument. The syntax is: target> gdbserver --attach COMM PID PID is the process ID of a currently running process. It isn't necessary to point 'gdbserver' at a binary for the running process. In 'target extended-remote' mode, you can also attach using the GDB attach command (*note Attaching in Types of Remote Connections::). You can debug processes by name instead of process ID if your target has the 'pidof' utility: target> gdbserver --attach COMM `pidof PROGRAM` In case more than one copy of PROGRAM is running, or PROGRAM has multiple threads, most versions of 'pidof' support the '-s' option to only return the first process ID. 20.3.1.2 TCP port allocation lifecycle of 'gdbserver' ..................................................... This section applies only when 'gdbserver' is run to listen on a TCP port. 'gdbserver' normally terminates after all of its debugged processes have terminated in 'target remote' mode. On the other hand, for 'target extended-remote', 'gdbserver' stays running even with no processes left. GDB normally terminates the spawned debugged process on its exit, which normally also terminates 'gdbserver' in the 'target remote' mode. Therefore, when the connection drops unexpectedly, and GDB cannot ask 'gdbserver' to kill its debugged processes, 'gdbserver' stays running even in the 'target remote' mode. When 'gdbserver' stays running, GDB can connect to it again later. Such reconnecting is useful for features like *note disconnected tracing::. For completeness, at most one GDB can be connected at a time. By default, 'gdbserver' keeps the listening TCP port open, so that subsequent connections are possible. However, if you start 'gdbserver' with the '--once' option, it will stop listening for any further connection attempts after connecting to the first GDB session. This means no further connections to 'gdbserver' will be possible after the first one. It also means 'gdbserver' will terminate after the first connection with remote GDB has closed, even for unexpectedly closed connections and even in the 'target extended-remote' mode. The '--once' option allows reusing the same port number for connecting to multiple instances of 'gdbserver' running on the same host, since each instance closes its port after the first connection. 20.3.1.3 Other Command-Line Arguments for 'gdbserver' ..................................................... You can use the '--multi' option to start 'gdbserver' without specifying a program to debug or a process to attach to. Then you can attach in 'target extended-remote' mode and run or attach to a program. For more information, *note --multi Option in Types of Remote Connnections::. The '--debug' option tells 'gdbserver' to display extra status information about the debugging process. The '--remote-debug' option tells 'gdbserver' to display remote protocol debug output. These options are intended for 'gdbserver' development and for bug reports to the developers. The '--debug-format=option1[,option2,...]' option tells 'gdbserver' to include additional information in each output. Possible options are: 'none' Turn off all extra information in debugging output. 'all' Turn on all extra information in debugging output. 'timestamps' Include a timestamp in each line of debugging output. Options are processed in order. Thus, for example, if 'none' appears last then no additional information is added to debugging output. The '--wrapper' option specifies a wrapper to launch programs for debugging. The option should be followed by the name of the wrapper, then any command-line arguments to pass to the wrapper, then '--' indicating the end of the wrapper arguments. 'gdbserver' runs the specified wrapper program with a combined command line including the wrapper arguments, then the name of the program to debug, then any arguments to the program. The wrapper runs until it executes your program, and then GDB gains control. You can use any program that eventually calls 'execve' with its arguments as a wrapper. Several standard Unix utilities do this, e.g. 'env' and 'nohup'. Any Unix shell script ending with 'exec "$@"' will also work. For example, you can use 'env' to pass an environment variable to the debugged program, without setting the variable in 'gdbserver''s environment: $ gdbserver --wrapper env LD_PRELOAD=libtest.so -- :2222 ./testprog 20.3.2 Connecting to 'gdbserver' -------------------------------- The basic procedure for connecting to the remote target is: * Run GDB on the host system. * Make sure you have the necessary symbol files (*note Host and target files::). Load symbols for your application using the 'file' command before you connect. Use 'set sysroot' to locate target libraries (unless your GDB was compiled with the correct sysroot using '--with-sysroot'). * Connect to your target (*note Connecting to a Remote Target: Connecting.). For TCP connections, you must start up 'gdbserver' prior to using the 'target' command. Otherwise you may get an error whose text depends on the host system, but which usually looks something like 'Connection refused'. Don't use the 'load' command in GDB when using 'target remote' mode, since the program is already on the target. 20.3.3 Monitor Commands for 'gdbserver' --------------------------------------- During a GDB session using 'gdbserver', you can use the 'monitor' command to send special requests to 'gdbserver'. Here are the available commands. 'monitor help' List the available monitor commands. 'monitor set debug 0' 'monitor set debug 1' Disable or enable general debugging messages. 'monitor set remote-debug 0' 'monitor set remote-debug 1' Disable or enable specific debugging messages associated with the remote protocol (*note Remote Protocol::). 'monitor set debug-format option1[,option2,...]' Specify additional text to add to debugging messages. Possible options are: 'none' Turn off all extra information in debugging output. 'all' Turn on all extra information in debugging output. 'timestamps' Include a timestamp in each line of debugging output. Options are processed in order. Thus, for example, if 'none' appears last then no additional information is added to debugging output. 'monitor set libthread-db-search-path [PATH]' When this command is issued, PATH is a colon-separated list of directories to search for 'libthread_db' (*note set libthread-db-search-path: Threads.). If you omit PATH, 'libthread-db-search-path' will be reset to its default value. The special entry '$pdir' for 'libthread-db-search-path' is not supported in 'gdbserver'. 'monitor exit' Tell gdbserver to exit immediately. This command should be followed by 'disconnect' to close the debugging session. 'gdbserver' will detach from any attached processes and kill any processes it created. Use 'monitor exit' to terminate 'gdbserver' at the end of a multi-process mode debug session. 20.3.4 Tracepoints support in 'gdbserver' ----------------------------------------- On some targets, 'gdbserver' supports tracepoints, fast tracepoints and static tracepoints. For fast or static tracepoints to work, a special library called the "in-process agent" (IPA), must be loaded in the inferior process. This library is built and distributed as an integral part of 'gdbserver'. In addition, support for static tracepoints requires building the in-process agent library with static tracepoints support. At present, the UST (LTTng Userspace Tracer, ) tracing engine is supported. This support is automatically available if UST development headers are found in the standard include path when 'gdbserver' is built, or if 'gdbserver' was explicitly configured using '--with-ust' to point at such headers. You can explicitly disable the support using '--with-ust=no'. There are several ways to load the in-process agent in your program: 'Specifying it as dependency at link time' You can link your program dynamically with the in-process agent library. On most systems, this is accomplished by adding '-linproctrace' to the link command. 'Using the system's preloading mechanisms' You can force loading the in-process agent at startup time by using your system's support for preloading shared libraries. Many Unixes support the concept of preloading user defined libraries. In most cases, you do that by specifying 'LD_PRELOAD=libinproctrace.so' in the environment. See also the description of 'gdbserver''s '--wrapper' command line option. 'Using GDB to force loading the agent at run time' On some systems, you can force the inferior to load a shared library, by calling a dynamic loader function in the inferior that takes care of dynamically looking up and loading a shared library. On most Unix systems, the function is 'dlopen'. You'll use the 'call' command for that. For example: (gdb) call dlopen ("libinproctrace.so", ...) Note that on most Unix systems, for the 'dlopen' function to be available, the program needs to be linked with '-ldl'. On systems that have a userspace dynamic loader, like most Unix systems, when you connect to 'gdbserver' using 'target remote', you'll find that the program is stopped at the dynamic loader's entry point, and no shared library has been loaded in the program's address space yet, including the in-process agent. In that case, before being able to use any of the fast or static tracepoints features, you need to let the loader run and load the shared libraries. The simplest way to do that is to run the program to the main procedure. E.g., if debugging a C or C++ program, start 'gdbserver' like so: $ gdbserver :9999 myprogram Start GDB and connect to 'gdbserver' like so, and run to main: $ gdb myprogram (gdb) target remote myhost:9999 0x00007f215893ba60 in ?? () from /lib64/ld-linux-x86-64.so.2 (gdb) b main (gdb) continue The in-process tracing agent library should now be loaded into the process; you can confirm it with the 'info sharedlibrary' command, which will list 'libinproctrace.so' as loaded in the process. You are now ready to install fast tracepoints, list static tracepoint markers, probe static tracepoints markers, and start tracing. ---------- Footnotes ---------- (1) If you choose a port number that conflicts with another service, 'gdbserver' prints an error message and exits.  File: gdb.info, Node: Remote Configuration, Next: Remote Stub, Prev: Server, Up: Remote Debugging 20.4 Remote Configuration ========================= This section documents the configuration options available when debugging remote programs. For the options related to the File I/O extensions of the remote protocol, see *note system-call-allowed: system. 'set remoteaddresssize BITS' Set the maximum size of address in a memory packet to the specified number of bits. GDB will mask off the address bits above that number, when it passes addresses to the remote target. The default value is the number of bits in the target's address. 'show remoteaddresssize' Show the current value of remote address size in bits. 'set serial baud N' Set the baud rate for the remote serial I/O to N baud. The value is used to set the speed of the serial port used for debugging remote targets. 'show serial baud' Show the current speed of the remote connection. 'set serial parity PARITY' Set the parity for the remote serial I/O. Supported values of PARITY are: 'even', 'none', and 'odd'. The default is 'none'. 'show serial parity' Show the current parity of the serial port. 'set remotebreak' If set to on, GDB sends a 'BREAK' signal to the remote when you type 'Ctrl-c' to interrupt the program running on the remote. If set to off, GDB sends the 'Ctrl-C' character instead. The default is off, since most remote systems expect to see 'Ctrl-C' as the interrupt signal. 'show remotebreak' Show whether GDB sends 'BREAK' or 'Ctrl-C' to interrupt the remote program. 'set remoteflow on' 'set remoteflow off' Enable or disable hardware flow control ('RTS'/'CTS') on the serial port used to communicate to the remote target. 'show remoteflow' Show the current setting of hardware flow control. 'set remotelogbase BASE' Set the base (a.k.a. radix) of logging serial protocol communications to BASE. Supported values of BASE are: 'ascii', 'octal', and 'hex'. The default is 'ascii'. 'show remotelogbase' Show the current setting of the radix for logging remote serial protocol. 'set remotelogfile FILE' Record remote serial communications on the named FILE. The default is not to record at all. 'show remotelogfile.' Show the current setting of the file name on which to record the serial communications. 'set remotetimeout NUM' Set the timeout limit to wait for the remote target to respond to NUM seconds. The default is 2 seconds. 'show remotetimeout' Show the current number of seconds to wait for the remote target responses. 'set remote hardware-watchpoint-limit LIMIT' 'set remote hardware-breakpoint-limit LIMIT' Restrict GDB to using LIMIT remote hardware breakpoint or watchpoints. A limit of -1, the default, is treated as unlimited. 'set remote hardware-watchpoint-length-limit LIMIT' Restrict GDB to using LIMIT bytes for the maximum length of a remote hardware watchpoint. A limit of -1, the default, is treated as unlimited. 'show remote hardware-watchpoint-length-limit' Show the current limit (in bytes) of the maximum length of a remote hardware watchpoint. 'set remote exec-file FILENAME' 'show remote exec-file' Select the file used for 'run' with 'target extended-remote'. This should be set to a filename valid on the target system. If it is not set, the target will use a default filename (e.g. the last program run). 'set remote interrupt-sequence' Allow the user to select one of 'Ctrl-C', a 'BREAK' or 'BREAK-g' as the sequence to the remote target in order to interrupt the execution. 'Ctrl-C' is a default. Some system prefers 'BREAK' which is high level of serial line for some certain time. Linux kernel prefers 'BREAK-g', a.k.a Magic SysRq g. It is 'BREAK' signal followed by character 'g'. 'show interrupt-sequence' Show which of 'Ctrl-C', 'BREAK' or 'BREAK-g' is sent by GDB to interrupt the remote program. 'BREAK-g' is BREAK signal followed by 'g' and also known as Magic SysRq g. 'set remote interrupt-on-connect' Specify whether interrupt-sequence is sent to remote target when GDB connects to it. This is mostly needed when you debug Linux kernel. Linux kernel expects 'BREAK' followed by 'g' which is known as Magic SysRq g in order to connect GDB. 'show interrupt-on-connect' Show whether interrupt-sequence is sent to remote target when GDB connects to it. 'set tcp auto-retry on' Enable auto-retry for remote TCP connections. This is useful if the remote debugging agent is launched in parallel with GDB; there is a race condition because the agent may not become ready to accept the connection before GDB attempts to connect. When auto-retry is enabled, if the initial attempt to connect fails, GDB reattempts to establish the connection using the timeout specified by 'set tcp connect-timeout'. 'set tcp auto-retry off' Do not auto-retry failed TCP connections. 'show tcp auto-retry' Show the current auto-retry setting. 'set tcp connect-timeout SECONDS' 'set tcp connect-timeout unlimited' Set the timeout for establishing a TCP connection to the remote target to SECONDS. The timeout affects both polling to retry failed connections (enabled by 'set tcp auto-retry on') and waiting for connections that are merely slow to complete, and represents an approximate cumulative value. If SECONDS is 'unlimited', there is no timeout and GDB will keep attempting to establish a connection forever, unless interrupted with 'Ctrl-c'. The default is 15 seconds. 'show tcp connect-timeout' Show the current connection timeout setting. The GDB remote protocol autodetects the packets supported by your debugging stub. If you need to override the autodetection, you can use these commands to enable or disable individual packets. Each packet can be set to 'on' (the remote target supports this packet), 'off' (the remote target does not support this packet), or 'auto' (detect remote target support for this packet). They all default to 'auto'. For more information about each packet, see *note Remote Protocol::. During normal use, you should not have to use any of these commands. If you do, that may be a bug in your remote debugging stub, or a bug in GDB. You may want to report the problem to the GDB developers. For each packet NAME, the command to enable or disable the packet is 'set remote NAME-packet'. The available settings are: Command Name Remote Packet Related Features 'fetch-register' 'p' 'info registers' 'set-register' 'P' 'set' 'binary-download' 'X' 'load', 'set' 'read-aux-vector' 'qXfer:auxv:read' 'info auxv' 'symbol-lookup' 'qSymbol' Detecting multiple threads 'attach' 'vAttach' 'attach' 'verbose-resume' 'vCont' Stepping or resuming multiple threads 'run' 'vRun' 'run' 'software-breakpoint''Z0' 'break' 'hardware-breakpoint''Z1' 'hbreak' 'write-watchpoint' 'Z2' 'watch' 'read-watchpoint' 'Z3' 'rwatch' 'access-watchpoint' 'Z4' 'awatch' 'pid-to-exec-file' 'qXfer:exec-file:read' 'attach', 'run' 'target-features' 'qXfer:features:read' 'set architecture' 'library-info' 'qXfer:libraries:read' 'info sharedlibrary' 'memory-map' 'qXfer:memory-map:read' 'info mem' 'read-sdata-object' 'qXfer:sdata:read' 'print $_sdata' 'read-spu-object' 'qXfer:spu:read' 'info spu' 'write-spu-object' 'qXfer:spu:write' 'info spu' 'read-siginfo-object''qXfer:siginfo:read' 'print $_siginfo' 'write-siginfo-object''qXfer:siginfo:write' 'set $_siginfo' 'threads' 'qXfer:threads:read' 'info threads' 'get-thread-local- 'qGetTLSAddr' Displaying storage-address' '__thread' variables 'get-thread-information-block-address''qGetTIBAddr'Display MS-Windows Thread Information Block. 'search-memory' 'qSearch:memory' 'find' 'supported-packets' 'qSupported' Remote communications parameters 'catch-syscalls' 'QCatchSyscalls' 'catch syscall' 'pass-signals' 'QPassSignals' 'handle SIGNAL' 'program-signals' 'QProgramSignals' 'handle SIGNAL' 'hostio-close-packet''vFile:close' 'remote get', 'remote put' 'hostio-open-packet' 'vFile:open' 'remote get', 'remote put' 'hostio-pread-packet''vFile:pread' 'remote get', 'remote put' 'hostio-pwrite-packet''vFile:pwrite' 'remote get', 'remote put' 'hostio-unlink-packet''vFile:unlink' 'remote delete' 'hostio-readlink-packet''vFile:readlink' Host I/O 'hostio-fstat-packet''vFile:fstat' Host I/O 'hostio-setfs-packet''vFile:setfs' Host I/O 'noack-packet' 'QStartNoAckMode' Packet acknowledgment 'osdata' 'qXfer:osdata:read' 'info os' 'query-attached' 'qAttached' Querying remote process attach state. 'trace-buffer-size' 'QTBuffer:size' 'set trace-buffer-size' 'trace-status' 'qTStatus' 'tstatus' 'traceframe-info' 'qXfer:traceframe-info:read'Traceframe info 'install-in-trace' 'InstallInTrace' Install tracepoint in tracing 'disable-randomization''QDisableRandomization''set disable-randomization' 'conditional-breakpoints-packet''Z0 and Z1' 'Support for target-side breakpoint condition evaluation' 'multiprocess-extensions''multiprocess Debug multiple extensions' processes and remote process PID awareness 'swbreak-feature' 'swbreak stop reason' 'break' 'hwbreak-feature' 'hwbreak stop reason' 'hbreak' 'fork-event-feature' 'fork stop reason' 'fork' 'vfork-event-feature''vfork stop reason' 'vfork' 'exec-event-feature' 'exec stop reason' 'exec' 'thread-events' 'QThreadEvents' Tracking thread lifetime. 'no-resumed-stop-reply''no resumed thread Tracking thread left stop reply' lifetime.  File: gdb.info, Node: Remote Stub, Prev: Remote Configuration, Up: Remote Debugging 20.5 Implementing a Remote Stub =============================== The stub files provided with GDB implement the target side of the communication protocol, and the GDB side is implemented in the GDB source file 'remote.c'. Normally, you can simply allow these subroutines to communicate, and ignore the details. (If you're implementing your own stub file, you can still ignore the details: start with one of the existing stub files. 'sparc-stub.c' is the best organized, and therefore the easiest to read.) To debug a program running on another machine (the debugging "target" machine), you must first arrange for all the usual prerequisites for the program to run by itself. For example, for a C program, you need: 1. A startup routine to set up the C runtime environment; these usually have a name like 'crt0'. The startup routine may be supplied by your hardware supplier, or you may have to write your own. 2. A C subroutine library to support your program's subroutine calls, notably managing input and output. 3. A way of getting your program to the other machine--for example, a download program. These are often supplied by the hardware manufacturer, but you may have to write your own from hardware documentation. The next step is to arrange for your program to use a serial port to communicate with the machine where GDB is running (the "host" machine). In general terms, the scheme looks like this: _On the host,_ GDB already understands how to use this protocol; when everything else is set up, you can simply use the 'target remote' command (*note Specifying a Debugging Target: Targets.). _On the target,_ you must link with your program a few special-purpose subroutines that implement the GDB remote serial protocol. The file containing these subroutines is called a "debugging stub". On certain remote targets, you can use an auxiliary program 'gdbserver' instead of linking a stub into your program. *Note Using the 'gdbserver' Program: Server, for details. The debugging stub is specific to the architecture of the remote machine; for example, use 'sparc-stub.c' to debug programs on SPARC boards. These working remote stubs are distributed with GDB: 'i386-stub.c' For Intel 386 and compatible architectures. 'm68k-stub.c' For Motorola 680x0 architectures. 'sh-stub.c' For Renesas SH architectures. 'sparc-stub.c' For SPARC architectures. 'sparcl-stub.c' For Fujitsu SPARCLITE architectures. The 'README' file in the GDB distribution may list other recently added stubs. * Menu: * Stub Contents:: What the stub can do for you * Bootstrapping:: What you must do for the stub * Debug Session:: Putting it all together  File: gdb.info, Node: Stub Contents, Next: Bootstrapping, Up: Remote Stub 20.5.1 What the Stub Can Do for You ----------------------------------- The debugging stub for your architecture supplies these three subroutines: 'set_debug_traps' This routine arranges for 'handle_exception' to run when your program stops. You must call this subroutine explicitly in your program's startup code. 'handle_exception' This is the central workhorse, but your program never calls it explicitly--the setup code arranges for 'handle_exception' to run when a trap is triggered. 'handle_exception' takes control when your program stops during execution (for example, on a breakpoint), and mediates communications with GDB on the host machine. This is where the communications protocol is implemented; 'handle_exception' acts as the GDB representative on the target machine. It begins by sending summary information on the state of your program, then continues to execute, retrieving and transmitting any information GDB needs, until you execute a GDB command that makes your program resume; at that point, 'handle_exception' returns control to your own code on the target machine. 'breakpoint' Use this auxiliary subroutine to make your program contain a breakpoint. Depending on the particular situation, this may be the only way for GDB to get control. For instance, if your target machine has some sort of interrupt button, you won't need to call this; pressing the interrupt button transfers control to 'handle_exception'--in effect, to GDB. On some machines, simply receiving characters on the serial port may also trigger a trap; again, in that situation, you don't need to call 'breakpoint' from your own program--simply running 'target remote' from the host GDB session gets control. Call 'breakpoint' if none of these is true, or if you simply want to make certain your program stops at a predetermined point for the start of your debugging session.  File: gdb.info, Node: Bootstrapping, Next: Debug Session, Prev: Stub Contents, Up: Remote Stub 20.5.2 What You Must Do for the Stub ------------------------------------ The debugging stubs that come with GDB are set up for a particular chip architecture, but they have no information about the rest of your debugging target machine. First of all you need to tell the stub how to communicate with the serial port. 'int getDebugChar()' Write this subroutine to read a single character from the serial port. It may be identical to 'getchar' for your target system; a different name is used to allow you to distinguish the two if you wish. 'void putDebugChar(int)' Write this subroutine to write a single character to the serial port. It may be identical to 'putchar' for your target system; a different name is used to allow you to distinguish the two if you wish. If you want GDB to be able to stop your program while it is running, you need to use an interrupt-driven serial driver, and arrange for it to stop when it receives a '^C' ('\003', the control-C character). That is the character which GDB uses to tell the remote system to stop. Getting the debugging target to return the proper status to GDB probably requires changes to the standard stub; one quick and dirty way is to just execute a breakpoint instruction (the "dirty" part is that GDB reports a 'SIGTRAP' instead of a 'SIGINT'). Other routines you need to supply are: 'void exceptionHandler (int EXCEPTION_NUMBER, void *EXCEPTION_ADDRESS)' Write this function to install EXCEPTION_ADDRESS in the exception handling tables. You need to do this because the stub does not have any way of knowing what the exception handling tables on your target system are like (for example, the processor's table might be in ROM, containing entries which point to a table in RAM). The EXCEPTION_NUMBER specifies the exception which should be changed; its meaning is architecture-dependent (for example, different numbers might represent divide by zero, misaligned access, etc). When this exception occurs, control should be transferred directly to EXCEPTION_ADDRESS, and the processor state (stack, registers, and so on) should be just as it is when a processor exception occurs. So if you want to use a jump instruction to reach EXCEPTION_ADDRESS, it should be a simple jump, not a jump to subroutine. For the 386, EXCEPTION_ADDRESS should be installed as an interrupt gate so that interrupts are masked while the handler runs. The gate should be at privilege level 0 (the most privileged level). The SPARC and 68k stubs are able to mask interrupts themselves without help from 'exceptionHandler'. 'void flush_i_cache()' On SPARC and SPARCLITE only, write this subroutine to flush the instruction cache, if any, on your target machine. If there is no instruction cache, this subroutine may be a no-op. On target machines that have instruction caches, GDB requires this function to make certain that the state of your program is stable. You must also make sure this library routine is available: 'void *memset(void *, int, int)' This is the standard library function 'memset' that sets an area of memory to a known value. If you have one of the free versions of 'libc.a', 'memset' can be found there; otherwise, you must either obtain it from your hardware manufacturer, or write your own. If you do not use the GNU C compiler, you may need other standard library subroutines as well; this varies from one stub to another, but in general the stubs are likely to use any of the common library subroutines which 'GCC' generates as inline code.  File: gdb.info, Node: Debug Session, Prev: Bootstrapping, Up: Remote Stub 20.5.3 Putting it All Together ------------------------------ In summary, when your program is ready to debug, you must follow these steps. 1. Make sure you have defined the supporting low-level routines (*note What You Must Do for the Stub: Bootstrapping.): 'getDebugChar', 'putDebugChar', 'flush_i_cache', 'memset', 'exceptionHandler'. 2. Insert these lines in your program's startup code, before the main procedure is called: set_debug_traps(); breakpoint(); On some machines, when a breakpoint trap is raised, the hardware automatically makes the PC point to the instruction after the breakpoint. If your machine doesn't do that, you may need to adjust 'handle_exception' to arrange for it to return to the instruction after the breakpoint on this first invocation, so that your program doesn't keep hitting the initial breakpoint instead of making progress. 3. For the 680x0 stub only, you need to provide a variable called 'exceptionHook'. Normally you just use: void (*exceptionHook)() = 0; but if before calling 'set_debug_traps', you set it to point to a function in your program, that function is called when 'GDB' continues after stopping on a trap (for example, bus error). The function indicated by 'exceptionHook' is called with one parameter: an 'int' which is the exception number. 4. Compile and link together: your program, the GDB debugging stub for your target architecture, and the supporting subroutines. 5. Make sure you have a serial connection between your target machine and the GDB host, and identify the serial port on the host. 6. Download your program to your target machine (or get it there by whatever means the manufacturer provides), and start it. 7. Start GDB on the host, and connect to the target (*note Connecting to a Remote Target: Connecting.).  File: gdb.info, Node: Configurations, Next: Controlling GDB, Prev: Remote Debugging, Up: Top 21 Configuration-Specific Information ************************************* While nearly all GDB commands are available for all native and cross versions of the debugger, there are some exceptions. This chapter describes things that are only available in certain configurations. There are three major categories of configurations: native configurations, where the host and target are the same, embedded operating system configurations, which are usually the same for several different processor architectures, and bare embedded processors, which are quite different from each other. * Menu: * Native:: * Embedded OS:: * Embedded Processors:: * Architectures::  File: gdb.info, Node: Native, Next: Embedded OS, Up: Configurations 21.1 Native =========== This section describes details specific to particular native configurations. * Menu: * BSD libkvm Interface:: Debugging BSD kernel memory images * SVR4 Process Information:: SVR4 process information * DJGPP Native:: Features specific to the DJGPP port * Cygwin Native:: Features specific to the Cygwin port * Hurd Native:: Features specific to GNU Hurd * Darwin:: Features specific to Darwin  File: gdb.info, Node: BSD libkvm Interface, Next: SVR4 Process Information, Up: Native 21.1.1 BSD libkvm Interface --------------------------- BSD-derived systems (FreeBSD/NetBSD/OpenBSD) have a kernel memory interface that provides a uniform interface for accessing kernel virtual memory images, including live systems and crash dumps. GDB uses this interface to allow you to debug live kernels and kernel crash dumps on many native BSD configurations. This is implemented as a special 'kvm' debugging target. For debugging a live system, load the currently running kernel into GDB and connect to the 'kvm' target: (gdb) target kvm For debugging crash dumps, provide the file name of the crash dump as an argument: (gdb) target kvm /var/crash/bsd.0 Once connected to the 'kvm' target, the following commands are available: 'kvm pcb' Set current context from the "Process Control Block" (PCB) address. 'kvm proc' Set current context from proc address. This command isn't available on modern FreeBSD systems.  File: gdb.info, Node: SVR4 Process Information, Next: DJGPP Native, Prev: BSD libkvm Interface, Up: Native 21.1.2 SVR4 Process Information ------------------------------- Many versions of SVR4 and compatible systems provide a facility called '/proc' that can be used to examine the image of a running process using file-system subroutines. If GDB is configured for an operating system with this facility, the command 'info proc' is available to report information about the process running your program, or about any process running on your system. This includes, as of this writing, GNU/Linux and Solaris, for example. This command may also work on core files that were created on a system that has the '/proc' facility. 'info proc' 'info proc PROCESS-ID' Summarize available information about any running process. If a process ID is specified by PROCESS-ID, display information about that process; otherwise display information about the program being debugged. The summary includes the debugged process ID, the command line used to invoke it, its current working directory, and its executable file's absolute file name. On some systems, PROCESS-ID can be of the form '[PID]/TID' which specifies a certain thread ID within a process. If the optional PID part is missing, it means a thread from the process being debugged (the leading '/' still needs to be present, or else GDB will interpret the number as a process ID rather than a thread ID). 'info proc cmdline' Show the original command line of the process. This command is specific to GNU/Linux. 'info proc cwd' Show the current working directory of the process. This command is specific to GNU/Linux. 'info proc exe' Show the name of executable of the process. This command is specific to GNU/Linux. 'info proc mappings' Report the memory address space ranges accessible in the program, with information on whether the process has read, write, or execute access rights to each range. On GNU/Linux systems, each memory range includes the object file which is mapped to that range, instead of the memory access rights to that range. 'info proc stat' 'info proc status' These subcommands are specific to GNU/Linux systems. They show the process-related information, including the user ID and group ID; how many threads are there in the process; its virtual memory usage; the signals that are pending, blocked, and ignored; its TTY; its consumption of system and user time; its stack size; its 'nice' value; etc. For more information, see the 'proc' man page (type 'man 5 proc' from your shell prompt). 'info proc all' Show all the information about the process described under all of the above 'info proc' subcommands. 'set procfs-trace' This command enables and disables tracing of 'procfs' API calls. 'show procfs-trace' Show the current state of 'procfs' API call tracing. 'set procfs-file FILE' Tell GDB to write 'procfs' API trace to the named FILE. GDB appends the trace info to the previous contents of the file. The default is to display the trace on the standard output. 'show procfs-file' Show the file to which 'procfs' API trace is written. 'proc-trace-entry' 'proc-trace-exit' 'proc-untrace-entry' 'proc-untrace-exit' These commands enable and disable tracing of entries into and exits from the 'syscall' interface. 'info pidlist' For QNX Neutrino only, this command displays the list of all the processes and all the threads within each process. 'info meminfo' For QNX Neutrino only, this command displays the list of all mapinfos.  File: gdb.info, Node: DJGPP Native, Next: Cygwin Native, Prev: SVR4 Process Information, Up: Native 21.1.3 Features for Debugging DJGPP Programs -------------------------------------------- DJGPP is a port of the GNU development tools to MS-DOS and MS-Windows. DJGPP programs are 32-bit protected-mode programs that use the "DPMI" (DOS Protected-Mode Interface) API to run on top of real-mode DOS systems and their emulations. GDB supports native debugging of DJGPP programs, and defines a few commands specific to the DJGPP port. This subsection describes those commands. 'info dos' This is a prefix of DJGPP-specific commands which print information about the target system and important OS structures. 'info dos sysinfo' This command displays assorted information about the underlying platform: the CPU type and features, the OS version and flavor, the DPMI version, and the available conventional and DPMI memory. 'info dos gdt' 'info dos ldt' 'info dos idt' These 3 commands display entries from, respectively, Global, Local, and Interrupt Descriptor Tables (GDT, LDT, and IDT). The descriptor tables are data structures which store a descriptor for each segment that is currently in use. The segment's selector is an index into a descriptor table; the table entry for that index holds the descriptor's base address and limit, and its attributes and access rights. A typical DJGPP program uses 3 segments: a code segment, a data segment (used for both data and the stack), and a DOS segment (which allows access to DOS/BIOS data structures and absolute addresses in conventional memory). However, the DPMI host will usually define additional segments in order to support the DPMI environment. These commands allow to display entries from the descriptor tables. Without an argument, all entries from the specified table are displayed. An argument, which should be an integer expression, means display a single entry whose index is given by the argument. For example, here's a convenient way to display information about the debugged program's data segment: (gdb) info dos ldt $ds 0x13f: base=0x11970000 limit=0x0009ffff 32-Bit Data (Read/Write, Exp-up) This comes in handy when you want to see whether a pointer is outside the data segment's limit (i.e. "garbled"). 'info dos pde' 'info dos pte' These two commands display entries from, respectively, the Page Directory and the Page Tables. Page Directories and Page Tables are data structures which control how virtual memory addresses are mapped into physical addresses. A Page Table includes an entry for every page of memory that is mapped into the program's address space; there may be several Page Tables, each one holding up to 4096 entries. A Page Directory has up to 4096 entries, one each for every Page Table that is currently in use. Without an argument, 'info dos pde' displays the entire Page Directory, and 'info dos pte' displays all the entries in all of the Page Tables. An argument, an integer expression, given to the 'info dos pde' command means display only that entry from the Page Directory table. An argument given to the 'info dos pte' command means display entries from a single Page Table, the one pointed to by the specified entry in the Page Directory. These commands are useful when your program uses "DMA" (Direct Memory Access), which needs physical addresses to program the DMA controller. These commands are supported only with some DPMI servers. 'info dos address-pte ADDR' This command displays the Page Table entry for a specified linear address. The argument ADDR is a linear address which should already have the appropriate segment's base address added to it, because this command accepts addresses which may belong to _any_ segment. For example, here's how to display the Page Table entry for the page where a variable 'i' is stored: (gdb) info dos address-pte __djgpp_base_address + (char *)&i Page Table entry for address 0x11a00d30: Base=0x02698000 Dirty Acc. Not-Cached Write-Back Usr Read-Write +0xd30 This says that 'i' is stored at offset '0xd30' from the page whose physical base address is '0x02698000', and shows all the attributes of that page. Note that you must cast the addresses of variables to a 'char *', since otherwise the value of '__djgpp_base_address', the base address of all variables and functions in a DJGPP program, will be added using the rules of C pointer arithmetics: if 'i' is declared an 'int', GDB will add 4 times the value of '__djgpp_base_address' to the address of 'i'. Here's another example, it displays the Page Table entry for the transfer buffer: (gdb) info dos address-pte *((unsigned *)&_go32_info_block + 3) Page Table entry for address 0x29110: Base=0x00029000 Dirty Acc. Not-Cached Write-Back Usr Read-Write +0x110 (The '+ 3' offset is because the transfer buffer's address is the 3rd member of the '_go32_info_block' structure.) The output clearly shows that this DPMI server maps the addresses in conventional memory 1:1, i.e. the physical ('0x00029000' + '0x110') and linear ('0x29110') addresses are identical. This command is supported only with some DPMI servers. In addition to native debugging, the DJGPP port supports remote debugging via a serial data link. The following commands are specific to remote serial debugging in the DJGPP port of GDB. 'set com1base ADDR' This command sets the base I/O port address of the 'COM1' serial port. 'set com1irq IRQ' This command sets the "Interrupt Request" ('IRQ') line to use for the 'COM1' serial port. There are similar commands 'set com2base', 'set com3irq', etc. for setting the port address and the 'IRQ' lines for the other 3 COM ports. The related commands 'show com1base', 'show com1irq' etc. display the current settings of the base address and the 'IRQ' lines used by the COM ports. 'info serial' This command prints the status of the 4 DOS serial ports. For each port, it prints whether it's active or not, its I/O base address and IRQ number, whether it uses a 16550-style FIFO, its baudrate, and the counts of various errors encountered so far.  File: gdb.info, Node: Cygwin Native, Next: Hurd Native, Prev: DJGPP Native, Up: Native 21.1.4 Features for Debugging MS Windows PE Executables ------------------------------------------------------- GDB supports native debugging of MS Windows programs, including DLLs with and without symbolic debugging information. MS-Windows programs that call 'SetConsoleMode' to switch off the special meaning of the 'Ctrl-C' keystroke cannot be interrupted by typing 'C-c'. For this reason, GDB on MS-Windows supports 'C-' as an alternative interrupt key sequence, which can be used to interrupt the debuggee even if it ignores 'C-c'. There are various additional Cygwin-specific commands, described in this section. Working with DLLs that have no debugging symbols is described in *note Non-debug DLL Symbols::. 'info w32' This is a prefix of MS Windows-specific commands which print information about the target system and important OS structures. 'info w32 selector' This command displays information returned by the Win32 API 'GetThreadSelectorEntry' function. It takes an optional argument that is evaluated to a long value to give the information about this given selector. Without argument, this command displays information about the six segment registers. 'info w32 thread-information-block' This command displays thread specific information stored in the Thread Information Block (readable on the X86 CPU family using '$fs' selector for 32-bit programs and '$gs' for 64-bit programs). 'signal-event ID' This command signals an event with user-provided ID. Used to resume crashing process when attached to it using MS-Windows JIT debugging (AeDebug). To use it, create or edit the following keys in 'HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\AeDebug' and/or 'HKLM\SOFTWARE\Wow6432Node\Microsoft\Windows NT\CurrentVersion\AeDebug' (for x86_64 versions): - 'Debugger' (REG_SZ) -- a command to launch the debugger. Suggested command is: 'FULLY-QUALIFIED-PATH-TO-GDB.EXE -ex "attach %ld" -ex "signal-event %ld" -ex "continue"'. The first '%ld' will be replaced by the process ID of the crashing process, the second '%ld' will be replaced by the ID of the event that blocks the crashing process, waiting for GDB to attach. - 'Auto' (REG_SZ) -- either '1' or '0'. '1' will make the system run debugger specified by the Debugger key automatically, '0' will cause a dialog box with "OK" and "Cancel" buttons to appear, which allows the user to either terminate the crashing process (OK) or debug it (Cancel). 'set cygwin-exceptions MODE' If MODE is 'on', GDB will break on exceptions that happen inside the Cygwin DLL. If MODE is 'off', GDB will delay recognition of exceptions, and may ignore some exceptions which seem to be caused by internal Cygwin DLL "bookkeeping". This option is meant primarily for debugging the Cygwin DLL itself; the default value is 'off' to avoid annoying GDB users with false 'SIGSEGV' signals. 'show cygwin-exceptions' Displays whether GDB will break on exceptions that happen inside the Cygwin DLL itself. 'set new-console MODE' If MODE is 'on' the debuggee will be started in a new console on next start. If MODE is 'off', the debuggee will be started in the same console as the debugger. 'show new-console' Displays whether a new console is used when the debuggee is started. 'set new-group MODE' This boolean value controls whether the debuggee should start a new group or stay in the same group as the debugger. This affects the way the Windows OS handles 'Ctrl-C'. 'show new-group' Displays current value of new-group boolean. 'set debugevents' This boolean value adds debug output concerning kernel events related to the debuggee seen by the debugger. This includes events that signal thread and process creation and exit, DLL loading and unloading, console interrupts, and debugging messages produced by the Windows 'OutputDebugString' API call. 'set debugexec' This boolean value adds debug output concerning execute events (such as resume thread) seen by the debugger. 'set debugexceptions' This boolean value adds debug output concerning exceptions in the debuggee seen by the debugger. 'set debugmemory' This boolean value adds debug output concerning debuggee memory reads and writes by the debugger. 'set shell' This boolean values specifies whether the debuggee is called via a shell or directly (default value is on). 'show shell' Displays if the debuggee will be started with a shell. * Menu: * Non-debug DLL Symbols:: Support for DLLs without debugging symbols  File: gdb.info, Node: Non-debug DLL Symbols, Up: Cygwin Native 21.1.4.1 Support for DLLs without Debugging Symbols ................................................... Very often on windows, some of the DLLs that your program relies on do not include symbolic debugging information (for example, 'kernel32.dll'). When GDB doesn't recognize any debugging symbols in a DLL, it relies on the minimal amount of symbolic information contained in the DLL's export table. This section describes working with such symbols, known internally to GDB as "minimal symbols". Note that before the debugged program has started execution, no DLLs will have been loaded. The easiest way around this problem is simply to start the program -- either by setting a breakpoint or letting the program run once to completion. 21.1.4.2 DLL Name Prefixes .......................... In keeping with the naming conventions used by the Microsoft debugging tools, DLL export symbols are made available with a prefix based on the DLL name, for instance 'KERNEL32!CreateFileA'. The plain name is also entered into the symbol table, so 'CreateFileA' is often sufficient. In some cases there will be name clashes within a program (particularly if the executable itself includes full debugging symbols) necessitating the use of the fully qualified name when referring to the contents of the DLL. Use single-quotes around the name to avoid the exclamation mark ("!") being interpreted as a language operator. Note that the internal name of the DLL may be all upper-case, even though the file name of the DLL is lower-case, or vice-versa. Since symbols within GDB are _case-sensitive_ this may cause some confusion. If in doubt, try the 'info functions' and 'info variables' commands or even 'maint print msymbols' (*note Symbols::). Here's an example: (gdb) info function CreateFileA All functions matching regular expression "CreateFileA": Non-debugging symbols: 0x77e885f4 CreateFileA 0x77e885f4 KERNEL32!CreateFileA (gdb) info function ! All functions matching regular expression "!": Non-debugging symbols: 0x6100114c cygwin1!__assert 0x61004034 cygwin1!_dll_crt0@0 0x61004240 cygwin1!dll_crt0(per_process *) [etc...] 21.1.4.3 Working with Minimal Symbols ..................................... Symbols extracted from a DLL's export table do not contain very much type information. All that GDB can do is guess whether a symbol refers to a function or variable depending on the linker section that contains the symbol. Also note that the actual contents of the memory contained in a DLL are not available unless the program is running. This means that you cannot examine the contents of a variable or disassemble a function within a DLL without a running program. Variables are generally treated as pointers and dereferenced automatically. For this reason, it is often necessary to prefix a variable name with the address-of operator ("&") and provide explicit type information in the command. Here's an example of the type of problem: (gdb) print 'cygwin1!__argv' $1 = 268572168 (gdb) x 'cygwin1!__argv' 0x10021610: "\230y\"" And two possible solutions: (gdb) print ((char **)'cygwin1!__argv')[0] $2 = 0x22fd98 "/cygdrive/c/mydirectory/myprogram" (gdb) x/2x &'cygwin1!__argv' 0x610c0aa8 : 0x10021608 0x00000000 (gdb) x/x 0x10021608 0x10021608: 0x0022fd98 (gdb) x/s 0x0022fd98 0x22fd98: "/cygdrive/c/mydirectory/myprogram" Setting a break point within a DLL is possible even before the program starts execution. However, under these circumstances, GDB can't examine the initial instructions of the function in order to skip the function's frame set-up code. You can work around this by using "*&" to set the breakpoint at a raw memory address: (gdb) break *&'python22!PyOS_Readline' Breakpoint 1 at 0x1e04eff0 The author of these extensions is not entirely convinced that setting a break point within a shared DLL like 'kernel32.dll' is completely safe.  File: gdb.info, Node: Hurd Native, Next: Darwin, Prev: Cygwin Native, Up: Native 21.1.5 Commands Specific to GNU Hurd Systems -------------------------------------------- This subsection describes GDB commands specific to the GNU Hurd native debugging. 'set signals' 'set sigs' This command toggles the state of inferior signal interception by GDB. Mach exceptions, such as breakpoint traps, are not affected by this command. 'sigs' is a shorthand alias for 'signals'. 'show signals' 'show sigs' Show the current state of intercepting inferior's signals. 'set signal-thread' 'set sigthread' This command tells GDB which thread is the 'libc' signal thread. That thread is run when a signal is delivered to a running process. 'set sigthread' is the shorthand alias of 'set signal-thread'. 'show signal-thread' 'show sigthread' These two commands show which thread will run when the inferior is delivered a signal. 'set stopped' This commands tells GDB that the inferior process is stopped, as with the 'SIGSTOP' signal. The stopped process can be continued by delivering a signal to it. 'show stopped' This command shows whether GDB thinks the debuggee is stopped. 'set exceptions' Use this command to turn off trapping of exceptions in the inferior. When exception trapping is off, neither breakpoints nor single-stepping will work. To restore the default, set exception trapping on. 'show exceptions' Show the current state of trapping exceptions in the inferior. 'set task pause' This command toggles task suspension when GDB has control. Setting it to on takes effect immediately, and the task is suspended whenever GDB gets control. Setting it to off will take effect the next time the inferior is continued. If this option is set to off, you can use 'set thread default pause on' or 'set thread pause on' (see below) to pause individual threads. 'show task pause' Show the current state of task suspension. 'set task detach-suspend-count' This command sets the suspend count the task will be left with when GDB detaches from it. 'show task detach-suspend-count' Show the suspend count the task will be left with when detaching. 'set task exception-port' 'set task excp' This command sets the task exception port to which GDB will forward exceptions. The argument should be the value of the "send rights" of the task. 'set task excp' is a shorthand alias. 'set noninvasive' This command switches GDB to a mode that is the least invasive as far as interfering with the inferior is concerned. This is the same as using 'set task pause', 'set exceptions', and 'set signals' to values opposite to the defaults. 'info send-rights' 'info receive-rights' 'info port-rights' 'info port-sets' 'info dead-names' 'info ports' 'info psets' These commands display information about, respectively, send rights, receive rights, port rights, port sets, and dead names of a task. There are also shorthand aliases: 'info ports' for 'info port-rights' and 'info psets' for 'info port-sets'. 'set thread pause' This command toggles current thread suspension when GDB has control. Setting it to on takes effect immediately, and the current thread is suspended whenever GDB gets control. Setting it to off will take effect the next time the inferior is continued. Normally, this command has no effect, since when GDB has control, the whole task is suspended. However, if you used 'set task pause off' (see above), this command comes in handy to suspend only the current thread. 'show thread pause' This command shows the state of current thread suspension. 'set thread run' This command sets whether the current thread is allowed to run. 'show thread run' Show whether the current thread is allowed to run. 'set thread detach-suspend-count' This command sets the suspend count GDB will leave on a thread when detaching. This number is relative to the suspend count found by GDB when it notices the thread; use 'set thread takeover-suspend-count' to force it to an absolute value. 'show thread detach-suspend-count' Show the suspend count GDB will leave on the thread when detaching. 'set thread exception-port' 'set thread excp' Set the thread exception port to which to forward exceptions. This overrides the port set by 'set task exception-port' (see above). 'set thread excp' is the shorthand alias. 'set thread takeover-suspend-count' Normally, GDB's thread suspend counts are relative to the value GDB finds when it notices each thread. This command changes the suspend counts to be absolute instead. 'set thread default' 'show thread default' Each of the above 'set thread' commands has a 'set thread default' counterpart (e.g., 'set thread default pause', 'set thread default exception-port', etc.). The 'thread default' variety of commands sets the default thread properties for all threads; you can then change the properties of individual threads with the non-default commands.  File: gdb.info, Node: Darwin, Prev: Hurd Native, Up: Native 21.1.6 Darwin ------------- GDB provides the following commands specific to the Darwin target: 'set debug darwin NUM' When set to a non zero value, enables debugging messages specific to the Darwin support. Higher values produce more verbose output. 'show debug darwin' Show the current state of Darwin messages. 'set debug mach-o NUM' When set to a non zero value, enables debugging messages while GDB is reading Darwin object files. ("Mach-O" is the file format used on Darwin for object and executable files.) Higher values produce more verbose output. This is a command to diagnose problems internal to GDB and should not be needed in normal usage. 'show debug mach-o' Show the current state of Mach-O file messages. 'set mach-exceptions on' 'set mach-exceptions off' On Darwin, faults are first reported as a Mach exception and are then mapped to a Posix signal. Use this command to turn on trapping of Mach exceptions in the inferior. This might be sometimes useful to better understand the cause of a fault. The default is off. 'show mach-exceptions' Show the current state of exceptions trapping.  File: gdb.info, Node: Embedded OS, Next: Embedded Processors, Prev: Native, Up: Configurations 21.2 Embedded Operating Systems =============================== This section describes configurations involving the debugging of embedded operating systems that are available for several different architectures. GDB includes the ability to debug programs running on various real-time operating systems.  File: gdb.info, Node: Embedded Processors, Next: Architectures, Prev: Embedded OS, Up: Configurations 21.3 Embedded Processors ======================== This section goes into details specific to particular embedded configurations. Whenever a specific embedded processor has a simulator, GDB allows to send an arbitrary command to the simulator. 'sim COMMAND' Send an arbitrary COMMAND string to the simulator. Consult the documentation for the specific simulator in use for information about acceptable commands. * Menu: * ARC:: Synopsys ARC * ARM:: ARM * M68K:: Motorola M68K * MicroBlaze:: Xilinx MicroBlaze * MIPS Embedded:: MIPS Embedded * PowerPC Embedded:: PowerPC Embedded * AVR:: Atmel AVR * CRIS:: CRIS * Super-H:: Renesas Super-H  File: gdb.info, Node: ARC, Next: ARM, Up: Embedded Processors 21.3.1 Synopsys ARC ------------------- GDB provides the following ARC-specific commands: 'set debug arc' Control the level of ARC specific debug messages. Use 0 for no messages (the default), 1 for debug messages, and 2 for even more debug messages. 'show debug arc' Show the level of ARC specific debugging in operation. 'maint print arc arc-instruction ADDRESS' Print internal disassembler information about instruction at a given address.  File: gdb.info, Node: ARM, Next: M68K, Prev: ARC, Up: Embedded Processors 21.3.2 ARM ---------- GDB provides the following ARM-specific commands: 'set arm disassembler' This commands selects from a list of disassembly styles. The '"std"' style is the standard style. 'show arm disassembler' Show the current disassembly style. 'set arm apcs32' This command toggles ARM operation mode between 32-bit and 26-bit. 'show arm apcs32' Display the current usage of the ARM 32-bit mode. 'set arm fpu FPUTYPE' This command sets the ARM floating-point unit (FPU) type. The argument FPUTYPE can be one of these: 'auto' Determine the FPU type by querying the OS ABI. 'softfpa' Software FPU, with mixed-endian doubles on little-endian ARM processors. 'fpa' GCC-compiled FPA co-processor. 'softvfp' Software FPU with pure-endian doubles. 'vfp' VFP co-processor. 'show arm fpu' Show the current type of the FPU. 'set arm abi' This command forces GDB to use the specified ABI. 'show arm abi' Show the currently used ABI. 'set arm fallback-mode (arm|thumb|auto)' GDB uses the symbol table, when available, to determine whether instructions are ARM or Thumb. This command controls GDB's default behavior when the symbol table is not available. The default is 'auto', which causes GDB to use the current execution mode (from the 'T' bit in the 'CPSR' register). 'show arm fallback-mode' Show the current fallback instruction mode. 'set arm force-mode (arm|thumb|auto)' This command overrides use of the symbol table to determine whether instructions are ARM or Thumb. The default is 'auto', which causes GDB to use the symbol table and then the setting of 'set arm fallback-mode'. 'show arm force-mode' Show the current forced instruction mode. 'set debug arm' Toggle whether to display ARM-specific debugging messages from the ARM target support subsystem. 'show debug arm' Show whether ARM-specific debugging messages are enabled. 'target sim [SIMARGS] ...' The GDB ARM simulator accepts the following optional arguments. '--swi-support=TYPE' Tell the simulator which SWI interfaces to support. The argument TYPE may be a comma separated list of the following values. The default value is 'all'. 'none' 'demon' 'angel' 'redboot' 'all'  File: gdb.info, Node: M68K, Next: MicroBlaze, Prev: ARM, Up: Embedded Processors 21.3.3 M68k ----------- The Motorola m68k configuration includes ColdFire support.  File: gdb.info, Node: MicroBlaze, Next: MIPS Embedded, Prev: M68K, Up: Embedded Processors 21.3.4 MicroBlaze ----------------- The MicroBlaze is a soft-core processor supported on various Xilinx FPGAs, such as Spartan or Virtex series. Boards with these processors usually have JTAG ports which connect to a host system running the Xilinx Embedded Development Kit (EDK) or Software Development Kit (SDK). This host system is used to download the configuration bitstream to the target FPGA. The Xilinx Microprocessor Debugger (XMD) program communicates with the target board using the JTAG interface and presents a 'gdbserver' interface to the board. By default 'xmd' uses port '1234'. (While it is possible to change this default port, it requires the use of undocumented 'xmd' commands. Contact Xilinx support if you need to do this.) Use these GDB commands to connect to the MicroBlaze target processor. 'target remote :1234' Use this command to connect to the target if you are running GDB on the same system as 'xmd'. 'target remote XMD-HOST:1234' Use this command to connect to the target if it is connected to 'xmd' running on a different system named XMD-HOST. 'load' Use this command to download a program to the MicroBlaze target. 'set debug microblaze N' Enable MicroBlaze-specific debugging messages if non-zero. 'show debug microblaze N' Show MicroBlaze-specific debugging level.  File: gdb.info, Node: MIPS Embedded, Next: PowerPC Embedded, Prev: MicroBlaze, Up: Embedded Processors 21.3.5 MIPS Embedded -------------------- GDB supports these special commands for MIPS targets: 'set mipsfpu double' 'set mipsfpu single' 'set mipsfpu none' 'set mipsfpu auto' 'show mipsfpu' If your target board does not support the MIPS floating point coprocessor, you should use the command 'set mipsfpu none' (if you need this, you may wish to put the command in your GDB init file). This tells GDB how to find the return value of functions which return floating point values. It also allows GDB to avoid saving the floating point registers when calling functions on the board. If you are using a floating point coprocessor with only single precision floating point support, as on the R4650 processor, use the command 'set mipsfpu single'. The default double precision floating point coprocessor may be selected using 'set mipsfpu double'. In previous versions the only choices were double precision or no floating point, so 'set mipsfpu on' will select double precision and 'set mipsfpu off' will select no floating point. As usual, you can inquire about the 'mipsfpu' variable with 'show mipsfpu'.  File: gdb.info, Node: PowerPC Embedded, Next: AVR, Prev: MIPS Embedded, Up: Embedded Processors 21.3.6 PowerPC Embedded ----------------------- GDB supports using the DVC (Data Value Compare) register to implement in hardware simple hardware watchpoint conditions of the form: (gdb) watch ADDRESS|VARIABLE \ if ADDRESS|VARIABLE == CONSTANT EXPRESSION The DVC register will be automatically used when GDB detects such pattern in a condition expression, and the created watchpoint uses one debug register (either the 'exact-watchpoints' option is on and the variable is scalar, or the variable has a length of one byte). This feature is available in native GDB running on a Linux kernel version 2.6.34 or newer. When running on PowerPC embedded processors, GDB automatically uses ranged hardware watchpoints, unless the 'exact-watchpoints' option is on, in which case watchpoints using only one debug register are created when watching variables of scalar types. You can create an artificial array to watch an arbitrary memory region using one of the following commands (*note Expressions::): (gdb) watch *((char *) ADDRESS)@LENGTH (gdb) watch {char[LENGTH]} ADDRESS PowerPC embedded processors support masked watchpoints. See the discussion about the 'mask' argument in *note Set Watchpoints::. PowerPC embedded processors support hardware accelerated "ranged breakpoints". A ranged breakpoint stops execution of the inferior whenever it executes an instruction at any address within the range it specifies. To set a ranged breakpoint in GDB, use the 'break-range' command. GDB provides the following PowerPC-specific commands: 'break-range START-LOCATION, END-LOCATION' Set a breakpoint for an address range given by START-LOCATION and END-LOCATION, which can specify a function name, a line number, an offset of lines from the current line or from the start location, or an address of an instruction (see *note Specify Location::, for a list of all the possible ways to specify a LOCATION.) The breakpoint will stop execution of the inferior whenever it executes an instruction at any address within the specified range, (including START-LOCATION and END-LOCATION.) 'set powerpc soft-float' 'show powerpc soft-float' Force GDB to use (or not use) a software floating point calling convention. By default, GDB selects the calling convention based on the selected architecture and the provided executable file. 'set powerpc vector-abi' 'show powerpc vector-abi' Force GDB to use the specified calling convention for vector arguments and return values. The valid options are 'auto'; 'generic', to avoid vector registers even if they are present; 'altivec', to use AltiVec registers; and 'spe' to use SPE registers. By default, GDB selects the calling convention based on the selected architecture and the provided executable file. 'set powerpc exact-watchpoints' 'show powerpc exact-watchpoints' Allow GDB to use only one debug register when watching a variable of scalar type, thus assuming that the variable is accessed through the address of its first byte.  File: gdb.info, Node: AVR, Next: CRIS, Prev: PowerPC Embedded, Up: Embedded Processors 21.3.7 Atmel AVR ---------------- When configured for debugging the Atmel AVR, GDB supports the following AVR-specific commands: 'info io_registers' This command displays information about the AVR I/O registers. For each register, GDB prints its number and value.  File: gdb.info, Node: CRIS, Next: Super-H, Prev: AVR, Up: Embedded Processors 21.3.8 CRIS ----------- When configured for debugging CRIS, GDB provides the following CRIS-specific commands: 'set cris-version VER' Set the current CRIS version to VER, either '10' or '32'. The CRIS version affects register names and sizes. This command is useful in case autodetection of the CRIS version fails. 'show cris-version' Show the current CRIS version. 'set cris-dwarf2-cfi' Set the usage of DWARF-2 CFI for CRIS debugging. The default is 'on'. Change to 'off' when using 'gcc-cris' whose version is below 'R59'. 'show cris-dwarf2-cfi' Show the current state of using DWARF-2 CFI. 'set cris-mode MODE' Set the current CRIS mode to MODE. It should only be changed when debugging in guru mode, in which case it should be set to 'guru' (the default is 'normal'). 'show cris-mode' Show the current CRIS mode.  File: gdb.info, Node: Super-H, Prev: CRIS, Up: Embedded Processors 21.3.9 Renesas Super-H ---------------------- For the Renesas Super-H processor, GDB provides these commands: 'set sh calling-convention CONVENTION' Set the calling-convention used when calling functions from GDB. Allowed values are 'gcc', which is the default setting, and 'renesas'. With the 'gcc' setting, functions are called using the GCC calling convention. If the DWARF-2 information of the called function specifies that the function follows the Renesas calling convention, the function is called using the Renesas calling convention. If the calling convention is set to 'renesas', the Renesas calling convention is always used, regardless of the DWARF-2 information. This can be used to override the default of 'gcc' if debug information is missing, or the compiler does not emit the DWARF-2 calling convention entry for a function. 'show sh calling-convention' Show the current calling convention setting.  File: gdb.info, Node: Architectures, Prev: Embedded Processors, Up: Configurations 21.4 Architectures ================== This section describes characteristics of architectures that affect all uses of GDB with the architecture, both native and cross. * Menu: * AArch64:: * i386:: * Alpha:: * MIPS:: * HPPA:: HP PA architecture * SPU:: Cell Broadband Engine SPU architecture * PowerPC:: * Nios II::  File: gdb.info, Node: AArch64, Next: i386, Up: Architectures 21.4.1 AArch64 -------------- When GDB is debugging the AArch64 architecture, it provides the following special commands: 'set debug aarch64' This command determines whether AArch64 architecture-specific debugging messages are to be displayed. 'show debug aarch64' Show whether AArch64 debugging messages are displayed.  File: gdb.info, Node: i386, Next: Alpha, Prev: AArch64, Up: Architectures 21.4.2 x86 Architecture-specific Issues --------------------------------------- 'set struct-convention MODE' Set the convention used by the inferior to return 'struct's and 'union's from functions to MODE. Possible values of MODE are '"pcc"', '"reg"', and '"default"' (the default). '"default"' or '"pcc"' means that 'struct's are returned on the stack, while '"reg"' means that a 'struct' or a 'union' whose size is 1, 2, 4, or 8 bytes will be returned in a register. 'show struct-convention' Show the current setting of the convention to return 'struct's from functions. 21.4.2.1 Intel "Memory Protection Extensions" (MPX). .................................................... Memory Protection Extension (MPX) adds the bound registers 'BND0' (1) through 'BND3'. Bound registers store a pair of 64-bit values which are the lower bound and upper bound. Bounds are effective addresses or memory locations. The upper bounds are architecturally represented in 1's complement form. A bound having lower bound = 0, and upper bound = 0 (1's complement of all bits set) will allow access to the entire address space. 'BND0' through 'BND3' are represented in GDB as 'bnd0raw' through 'bnd3raw'. Pseudo registers 'bnd0' through 'bnd3' display the upper bound performing the complement of one operation on the upper bound value, i.e. when upper bound in 'bnd0raw' is 0 in the GDB 'bnd0' it will be '0xfff...'. In this sense it can also be noted that the upper bounds are inclusive. As an example, assume that the register BND0 holds bounds for a pointer having access allowed for the range between 0x32 and 0x71. The values present on bnd0raw and bnd registers are presented as follows: bnd0raw = {0x32, 0xffffffff8e} bnd0 = {lbound = 0x32, ubound = 0x71} : size 64 This way the raw value can be accessed via bnd0raw...bnd3raw. Any change on bnd0...bnd3 or bnd0raw...bnd3raw is reflect on its counterpart. When the bnd0...bnd3 registers are displayed via Python, the display includes the memory size, in bits, accessible to the pointer. Bounds can also be stored in bounds tables, which are stored in application memory. These tables store bounds for pointers by specifying the bounds pointer's value along with its bounds. Evaluating and changing bounds located in bound tables is therefore interesting while investigating bugs on MPX context. GDB provides commands for this purpose: 'show mpx bound POINTER' Display bounds of the given POINTER. 'set mpx bound POINTER, LBOUND, UBOUND' Set the bounds of a pointer in the bound table. This command takes three parameters: POINTER is the pointers whose bounds are to be changed, LBOUND and UBOUND are new values for lower and upper bounds respectively. When you call an inferior function on an Intel MPX enabled program, GDB sets the inferior's bound registers to the init (disabled) state before calling the function. As a consequence, bounds checks for the pointer arguments passed to the function will always pass. This is necessary because when you call an inferior function, the program is usually in the middle of the execution of other function. Since at that point bound registers are in an arbitrary state, not clearing them would lead to random bound violations in the called function. You can still examine the influence of the bound registers on the execution of the called function by stopping the execution of the called function at its prologue, setting bound registers, and continuing the execution. For example: $ break *upper Breakpoint 2 at 0x4009de: file i386-mpx-call.c, line 47. $ print upper (a, b, c, d, 1) Breakpoint 2, upper (a=0x0, b=0x6e0000005b, c=0x0, d=0x0, len=48).... $ print $bnd0 {lbound = 0x0, ubound = ffffffff} : size -1 At this last step the value of bnd0 can be changed for investigation of bound violations caused along the execution of the call. In order to know how to set the bound registers or bound table for the call consult the ABI. ---------- Footnotes ---------- (1) The register named with capital letters represent the architecture registers.  File: gdb.info, Node: Alpha, Next: MIPS, Prev: i386, Up: Architectures 21.4.3 Alpha ------------ See the following section.  File: gdb.info, Node: MIPS, Next: HPPA, Prev: Alpha, Up: Architectures 21.4.4 MIPS ----------- Alpha- and MIPS-based computers use an unusual stack frame, which sometimes requires GDB to search backward in the object code to find the beginning of a function. To improve response time (especially for embedded applications, where GDB may be restricted to a slow serial line for this search) you may want to limit the size of this search, using one of these commands: 'set heuristic-fence-post LIMIT' Restrict GDB to examining at most LIMIT bytes in its search for the beginning of a function. A value of 0 (the default) means there is no limit. However, except for 0, the larger the limit the more bytes 'heuristic-fence-post' must search and therefore the longer it takes to run. You should only need to use this command when debugging a stripped executable. 'show heuristic-fence-post' Display the current limit. These commands are available _only_ when GDB is configured for debugging programs on Alpha or MIPS processors. Several MIPS-specific commands are available when debugging MIPS programs: 'set mips abi ARG' Tell GDB which MIPS ABI is used by the inferior. Possible values of ARG are: 'auto' The default ABI associated with the current binary (this is the default). 'o32' 'o64' 'n32' 'n64' 'eabi32' 'eabi64' 'show mips abi' Show the MIPS ABI used by GDB to debug the inferior. 'set mips compression ARG' Tell GDB which MIPS compressed ISA (Instruction Set Architecture) encoding is used by the inferior. GDB uses this for code disassembly and other internal interpretation purposes. This setting is only referred to when no executable has been associated with the debugging session or the executable does not provide information about the encoding it uses. Otherwise this setting is automatically updated from information provided by the executable. Possible values of ARG are 'mips16' and 'micromips'. The default compressed ISA encoding is 'mips16', as executables containing MIPS16 code frequently are not identified as such. This setting is "sticky"; that is, it retains its value across debugging sessions until reset either explicitly with this command or implicitly from an executable. The compiler and/or assembler typically add symbol table annotations to identify functions compiled for the MIPS16 or microMIPS ISAs. If these function-scope annotations are present, GDB uses them in preference to the global compressed ISA encoding setting. 'show mips compression' Show the MIPS compressed ISA encoding used by GDB to debug the inferior. 'set mipsfpu' 'show mipsfpu' *Note set mipsfpu: MIPS Embedded. 'set mips mask-address ARG' This command determines whether the most-significant 32 bits of 64-bit MIPS addresses are masked off. The argument ARG can be 'on', 'off', or 'auto'. The latter is the default setting, which lets GDB determine the correct value. 'show mips mask-address' Show whether the upper 32 bits of MIPS addresses are masked off or not. 'set remote-mips64-transfers-32bit-regs' This command controls compatibility with 64-bit MIPS targets that transfer data in 32-bit quantities. If you have an old MIPS 64 target that transfers 32 bits for some registers, like SR and FSR, and 64 bits for other registers, set this option to 'on'. 'show remote-mips64-transfers-32bit-regs' Show the current setting of compatibility with older MIPS 64 targets. 'set debug mips' This command turns on and off debugging messages for the MIPS-specific target code in GDB. 'show debug mips' Show the current setting of MIPS debugging messages.  File: gdb.info, Node: HPPA, Next: SPU, Prev: MIPS, Up: Architectures 21.4.5 HPPA ----------- When GDB is debugging the HP PA architecture, it provides the following special commands: 'set debug hppa' This command determines whether HPPA architecture-specific debugging messages are to be displayed. 'show debug hppa' Show whether HPPA debugging messages are displayed. 'maint print unwind ADDRESS' This command displays the contents of the unwind table entry at the given ADDRESS.  File: gdb.info, Node: SPU, Next: PowerPC, Prev: HPPA, Up: Architectures 21.4.6 Cell Broadband Engine SPU architecture --------------------------------------------- When GDB is debugging the Cell Broadband Engine SPU architecture, it provides the following special commands: 'info spu event' Display SPU event facility status. Shows current event mask and pending event status. 'info spu signal' Display SPU signal notification facility status. Shows pending signal-control word and signal notification mode of both signal notification channels. 'info spu mailbox' Display SPU mailbox facility status. Shows all pending entries, in order of processing, in each of the SPU Write Outbound, SPU Write Outbound Interrupt, and SPU Read Inbound mailboxes. 'info spu dma' Display MFC DMA status. Shows all pending commands in the MFC DMA queue. For each entry, opcode, tag, class IDs, effective and local store addresses and transfer size are shown. 'info spu proxydma' Display MFC Proxy-DMA status. Shows all pending commands in the MFC Proxy-DMA queue. For each entry, opcode, tag, class IDs, effective and local store addresses and transfer size are shown. When GDB is debugging a combined PowerPC/SPU application on the Cell Broadband Engine, it provides in addition the following special commands: 'set spu stop-on-load ARG' Set whether to stop for new SPE threads. When set to 'on', GDB will give control to the user when a new SPE thread enters its 'main' function. The default is 'off'. 'show spu stop-on-load' Show whether to stop for new SPE threads. 'set spu auto-flush-cache ARG' Set whether to automatically flush the software-managed cache. When set to 'on', GDB will automatically cause the SPE software-managed cache to be flushed whenever SPE execution stops. This provides a consistent view of PowerPC memory that is accessed via the cache. If an application does not use the software-managed cache, this option has no effect. 'show spu auto-flush-cache' Show whether to automatically flush the software-managed cache.  File: gdb.info, Node: PowerPC, Next: Nios II, Prev: SPU, Up: Architectures 21.4.7 PowerPC -------------- When GDB is debugging the PowerPC architecture, it provides a set of pseudo-registers to enable inspection of 128-bit wide Decimal Floating Point numbers stored in the floating point registers. These values must be stored in two consecutive registers, always starting at an even register like 'f0' or 'f2'. The pseudo-registers go from '$dl0' through '$dl15', and are formed by joining the even/odd register pairs 'f0' and 'f1' for '$dl0', 'f2' and 'f3' for '$dl1' and so on. For POWER7 processors, GDB provides a set of pseudo-registers, the 64-bit wide Extended Floating Point Registers ('f32' through 'f63').  File: gdb.info, Node: Nios II, Prev: PowerPC, Up: Architectures 21.4.8 Nios II -------------- When GDB is debugging the Nios II architecture, it provides the following special commands: 'set debug nios2' This command turns on and off debugging messages for the Nios II target code in GDB. 'show debug nios2' Show the current setting of Nios II debugging messages.  File: gdb.info, Node: Controlling GDB, Next: Extending GDB, Prev: Configurations, Up: Top 22 Controlling GDB ****************** You can alter the way GDB interacts with you by using the 'set' command. For commands controlling how GDB displays data, see *note Print Settings: Print Settings. Other settings are described here. * Menu: * Prompt:: Prompt * Editing:: Command editing * Command History:: Command history * Screen Size:: Screen size * Numbers:: Numbers * ABI:: Configuring the current ABI * Auto-loading:: Automatically loading associated files * Messages/Warnings:: Optional warnings and messages * Debugging Output:: Optional messages about internal happenings * Other Misc Settings:: Other Miscellaneous Settings  File: gdb.info, Node: Prompt, Next: Editing, Up: Controlling GDB 22.1 Prompt =========== GDB indicates its readiness to read a command by printing a string called the "prompt". This string is normally '(gdb)'. You can change the prompt string with the 'set prompt' command. For instance, when debugging GDB with GDB, it is useful to change the prompt in one of the GDB sessions so that you can always tell which one you are talking to. _Note:_ 'set prompt' does not add a space for you after the prompt you set. This allows you to set a prompt which ends in a space or a prompt that does not. 'set prompt NEWPROMPT' Directs GDB to use NEWPROMPT as its prompt string henceforth. 'show prompt' Prints a line of the form: 'Gdb's prompt is: YOUR-PROMPT' Versions of GDB that ship with Python scripting enabled have prompt extensions. The commands for interacting with these extensions are: 'set extended-prompt PROMPT' Set an extended prompt that allows for substitutions. *Note gdb.prompt::, for a list of escape sequences that can be used for substitution. Any escape sequences specified as part of the prompt string are replaced with the corresponding strings each time the prompt is displayed. For example: set extended-prompt Current working directory: \w (gdb) Note that when an extended-prompt is set, it takes control of the PROMPT_HOOK hook. *Note prompt_hook::, for further information. 'show extended-prompt' Prints the extended prompt. Any escape sequences specified as part of the prompt string with 'set extended-prompt', are replaced with the corresponding strings each time the prompt is displayed.  File: gdb.info, Node: Editing, Next: Command History, Prev: Prompt, Up: Controlling GDB 22.2 Command Editing ==================== GDB reads its input commands via the "Readline" interface. This GNU library provides consistent behavior for programs which provide a command line interface to the user. Advantages are GNU Emacs-style or "vi"-style inline editing of commands, 'csh'-like history substitution, and a storage and recall of command history across debugging sessions. You may control the behavior of command line editing in GDB with the command 'set'. 'set editing' 'set editing on' Enable command line editing (enabled by default). 'set editing off' Disable command line editing. 'show editing' Show whether command line editing is enabled. *Note Command Line Editing::, for more details about the Readline interface. Users unfamiliar with GNU Emacs or 'vi' are encouraged to read that chapter.  File: gdb.info, Node: Command History, Next: Screen Size, Prev: Editing, Up: Controlling GDB 22.3 Command History ==================== GDB can keep track of the commands you type during your debugging sessions, so that you can be certain of precisely what happened. Use these commands to manage the GDB command history facility. GDB uses the GNU History library, a part of the Readline package, to provide the history facility. *Note Using History Interactively::, for the detailed description of the History library. To issue a command to GDB without affecting certain aspects of the state which is seen by users, prefix it with 'server ' (*note Server Prefix::). This means that this command will not affect the command history, nor will it affect GDB's notion of which command to repeat if is pressed on a line by itself. The server prefix does not affect the recording of values into the value history; to print a value without recording it into the value history, use the 'output' command instead of the 'print' command. Here is the description of GDB commands related to command history. 'set history filename FNAME' Set the name of the GDB command history file to FNAME. This is the file where GDB reads an initial command history list, and where it writes the command history from this session when it exits. You can access this list through history expansion or through the history command editing characters listed below. This file defaults to the value of the environment variable 'GDBHISTFILE', or to './.gdb_history' ('./_gdb_history' on MS-DOS) if this variable is not set. 'set history save' 'set history save on' Record command history in a file, whose name may be specified with the 'set history filename' command. By default, this option is disabled. 'set history save off' Stop recording command history in a file. 'set history size SIZE' 'set history size unlimited' Set the number of commands which GDB keeps in its history list. This defaults to the value of the environment variable 'GDBHISTSIZE', or to 256 if this variable is not set. Non-numeric values of 'GDBHISTSIZE' are ignored. If SIZE is 'unlimited' or if 'GDBHISTSIZE' is either a negative number or the empty string, then the number of commands GDB keeps in the history list is unlimited. 'set history remove-duplicates COUNT' 'set history remove-duplicates unlimited' Control the removal of duplicate history entries in the command history list. If COUNT is non-zero, GDB will look back at the last COUNT history entries and remove the first entry that is a duplicate of the current entry being added to the command history list. If COUNT is 'unlimited' then this lookbehind is unbounded. If COUNT is 0, then removal of duplicate history entries is disabled. Only history entries added during the current session are considered for removal. This option is set to 0 by default. History expansion assigns special meaning to the character '!'. *Note Event Designators::, for more details. Since '!' is also the logical not operator in C, history expansion is off by default. If you decide to enable history expansion with the 'set history expansion on' command, you may sometimes need to follow '!' (when it is used as logical not, in an expression) with a space or a tab to prevent it from being expanded. The readline history facilities do not attempt substitution on the strings '!=' and '!(', even when history expansion is enabled. The commands to control history expansion are: 'set history expansion on' 'set history expansion' Enable history expansion. History expansion is off by default. 'set history expansion off' Disable history expansion. 'show history' 'show history filename' 'show history save' 'show history size' 'show history expansion' These commands display the state of the GDB history parameters. 'show history' by itself displays all four states. 'show commands' Display the last ten commands in the command history. 'show commands N' Print ten commands centered on command number N. 'show commands +' Print ten commands just after the commands last printed.  File: gdb.info, Node: Screen Size, Next: Numbers, Prev: Command History, Up: Controlling GDB 22.4 Screen Size ================ Certain commands to GDB may produce large amounts of information output to the screen. To help you read all of it, GDB pauses and asks you for input at the end of each page of output. Type when you want to continue the output, or 'q' to discard the remaining output. Also, the screen width setting determines when to wrap lines of output. Depending on what is being printed, GDB tries to break the line at a readable place, rather than simply letting it overflow onto the following line. Normally GDB knows the size of the screen from the terminal driver software. For example, on Unix GDB uses the termcap data base together with the value of the 'TERM' environment variable and the 'stty rows' and 'stty cols' settings. If this is not correct, you can override it with the 'set height' and 'set width' commands: 'set height LPP' 'set height unlimited' 'show height' 'set width CPL' 'set width unlimited' 'show width' These 'set' commands specify a screen height of LPP lines and a screen width of CPL characters. The associated 'show' commands display the current settings. If you specify a height of either 'unlimited' or zero lines, GDB does not pause during output no matter how long the output is. This is useful if output is to a file or to an editor buffer. Likewise, you can specify 'set width unlimited' or 'set width 0' to prevent GDB from wrapping its output. 'set pagination on' 'set pagination off' Turn the output pagination on or off; the default is on. Turning pagination off is the alternative to 'set height unlimited'. Note that running GDB with the '--batch' option (*note -batch: Mode Options.) also automatically disables pagination. 'show pagination' Show the current pagination mode.  File: gdb.info, Node: Numbers, Next: ABI, Prev: Screen Size, Up: Controlling GDB 22.5 Numbers ============ You can always enter numbers in octal, decimal, or hexadecimal in GDB by the usual conventions: octal numbers begin with '0', decimal numbers end with '.', and hexadecimal numbers begin with '0x'. Numbers that neither begin with '0' or '0x', nor end with a '.' are, by default, entered in base 10; likewise, the default display for numbers--when no particular format is specified--is base 10. You can change the default base for both input and output with the commands described below. 'set input-radix BASE' Set the default base for numeric input. Supported choices for BASE are decimal 8, 10, or 16. The base must itself be specified either unambiguously or using the current input radix; for example, any of set input-radix 012 set input-radix 10. set input-radix 0xa sets the input base to decimal. On the other hand, 'set input-radix 10' leaves the input radix unchanged, no matter what it was, since '10', being without any leading or trailing signs of its base, is interpreted in the current radix. Thus, if the current radix is 16, '10' is interpreted in hex, i.e. as 16 decimal, which doesn't change the radix. 'set output-radix BASE' Set the default base for numeric display. Supported choices for BASE are decimal 8, 10, or 16. The base must itself be specified either unambiguously or using the current input radix. 'show input-radix' Display the current default base for numeric input. 'show output-radix' Display the current default base for numeric display. 'set radix [BASE]' 'show radix' These commands set and show the default base for both input and output of numbers. 'set radix' sets the radix of input and output to the same base; without an argument, it resets the radix back to its default value of 10.  File: gdb.info, Node: ABI, Next: Auto-loading, Prev: Numbers, Up: Controlling GDB 22.6 Configuring the Current ABI ================================ GDB can determine the "ABI" (Application Binary Interface) of your application automatically. However, sometimes you need to override its conclusions. Use these commands to manage GDB's view of the current ABI. One GDB configuration can debug binaries for multiple operating system targets, either via remote debugging or native emulation. GDB will autodetect the "OS ABI" (Operating System ABI) in use, but you can override its conclusion using the 'set osabi' command. One example where this is useful is in debugging of binaries which use an alternate C library (e.g. UCLIBC for GNU/Linux) which does not have the same identifying marks that the standard C library for your platform provides. When GDB is debugging the AArch64 architecture, it provides a "Newlib" OS ABI. This is useful for handling 'setjmp' and 'longjmp' when debugging binaries that use the NEWLIB C library. The "Newlib" OS ABI can be selected by 'set osabi Newlib'. 'show osabi' Show the OS ABI currently in use. 'set osabi' With no argument, show the list of registered available OS ABI's. 'set osabi ABI' Set the current OS ABI to ABI. Generally, the way that an argument of type 'float' is passed to a function depends on whether the function is prototyped. For a prototyped (i.e. ANSI/ISO style) function, 'float' arguments are passed unchanged, according to the architecture's convention for 'float'. For unprototyped (i.e. K&R style) functions, 'float' arguments are first promoted to type 'double' and then passed. Unfortunately, some forms of debug information do not reliably indicate whether a function is prototyped. If GDB calls a function that is not marked as prototyped, it consults 'set coerce-float-to-double'. 'set coerce-float-to-double' 'set coerce-float-to-double on' Arguments of type 'float' will be promoted to 'double' when passed to an unprototyped function. This is the default setting. 'set coerce-float-to-double off' Arguments of type 'float' will be passed directly to unprototyped functions. 'show coerce-float-to-double' Show the current setting of promoting 'float' to 'double'. GDB needs to know the ABI used for your program's C++ objects. The correct C++ ABI depends on which C++ compiler was used to build your application. GDB only fully supports programs with a single C++ ABI; if your program contains code using multiple C++ ABI's or if GDB can not identify your program's ABI correctly, you can tell GDB which ABI to use. Currently supported ABI's include "gnu-v2", for 'g++' versions before 3.0, "gnu-v3", for 'g++' versions 3.0 and later, and "hpaCC" for the HP ANSI C++ compiler. Other C++ compilers may use the "gnu-v2" or "gnu-v3" ABI's as well. The default setting is "auto". 'show cp-abi' Show the C++ ABI currently in use. 'set cp-abi' With no argument, show the list of supported C++ ABI's. 'set cp-abi ABI' 'set cp-abi auto' Set the current C++ ABI to ABI, or return to automatic detection.  File: gdb.info, Node: Auto-loading, Next: Messages/Warnings, Prev: ABI, Up: Controlling GDB 22.7 Automatically loading associated files =========================================== GDB sometimes reads files with commands and settings automatically, without being explicitly told so by the user. We call this feature "auto-loading". While auto-loading is useful for automatically adapting GDB to the needs of your project, it can sometimes produce unexpected results or introduce security risks (e.g., if the file comes from untrusted sources). * Menu: * Init File in the Current Directory:: 'set/show/info auto-load local-gdbinit' * libthread_db.so.1 file:: 'set/show/info auto-load libthread-db' * Auto-loading safe path:: 'set/show/info auto-load safe-path' * Auto-loading verbose mode:: 'set/show debug auto-load' There are various kinds of files GDB can automatically load. In addition to these files, GDB supports auto-loading code written in various extension languages. *Note Auto-loading extensions::. Note that loading of these associated files (including the local '.gdbinit' file) requires accordingly configured 'auto-load safe-path' (*note Auto-loading safe path::). For these reasons, GDB includes commands and options to let you control when to auto-load files and which files should be auto-loaded. 'set auto-load off' Globally disable loading of all auto-loaded files. You may want to use this command with the '-iex' option (*note Option -init-eval-command::) such as: $ gdb -iex "set auto-load off" untrusted-executable corefile Be aware that system init file (*note System-wide configuration::) and init files from your home directory (*note Home Directory Init File::) still get read (as they come from generally trusted directories). To prevent GDB from auto-loading even those init files, use the '-nx' option (*note Mode Options::), in addition to 'set auto-load no'. 'show auto-load' Show whether auto-loading of each specific 'auto-load' file(s) is enabled or disabled. (gdb) show auto-load gdb-scripts: Auto-loading of canned sequences of commands scripts is on. libthread-db: Auto-loading of inferior specific libthread_db is on. local-gdbinit: Auto-loading of .gdbinit script from current directory is on. python-scripts: Auto-loading of Python scripts is on. safe-path: List of directories from which it is safe to auto-load files is $debugdir:$datadir/auto-load. scripts-directory: List of directories from which to load auto-loaded scripts is $debugdir:$datadir/auto-load. 'info auto-load' Print whether each specific 'auto-load' file(s) have been auto-loaded or not. (gdb) info auto-load gdb-scripts: Loaded Script Yes /home/user/gdb/gdb-gdb.gdb libthread-db: No auto-loaded libthread-db. local-gdbinit: Local .gdbinit file "/home/user/gdb/.gdbinit" has been loaded. python-scripts: Loaded Script Yes /home/user/gdb/gdb-gdb.py These are GDB control commands for the auto-loading: *Note set auto-load off::. Disable auto-loading globally. *Note show auto-load::. Show setting of all kinds of files. *Note info auto-load::. Show state of all kinds of files. *Note set auto-load gdb-scripts::. Control for GDB command scripts. *Note show auto-load gdb-scripts::. Show setting of GDB command scripts. *Note info auto-load gdb-scripts::. Show state of GDB command scripts. *Note set auto-load python-scripts::.Control for GDB Python scripts. *Note show auto-load python-scripts::.Show setting of GDB Python scripts. *Note info auto-load python-scripts::.Show state of GDB Python scripts. *Note set auto-load guile-scripts::. Control for GDB Guile scripts. *Note show auto-load guile-scripts::.Show setting of GDB Guile scripts. *Note info auto-load guile-scripts::.Show state of GDB Guile scripts. *Note set auto-load scripts-directory::.Control for GDB auto-loaded scripts location. *Note show auto-load scripts-directory::.Show GDB auto-loaded scripts location. *Note add-auto-load-scripts-directory::.Add directory for auto-loaded scripts location list. *Note set auto-load local-gdbinit::. Control for init file in the current directory. *Note show auto-load local-gdbinit::.Show setting of init file in the current directory. *Note info auto-load local-gdbinit::.Show state of init file in the current directory. *Note set auto-load libthread-db::. Control for thread debugging library. *Note show auto-load libthread-db::. Show setting of thread debugging library. *Note info auto-load libthread-db::. Show state of thread debugging library. *Note set auto-load safe-path::. Control directories trusted for automatic loading. *Note show auto-load safe-path::. Show directories trusted for automatic loading. *Note add-auto-load-safe-path::. Add directory trusted for automatic loading.  File: gdb.info, Node: Init File in the Current Directory, Next: libthread_db.so.1 file, Up: Auto-loading 22.7.1 Automatically loading init file in the current directory --------------------------------------------------------------- By default, GDB reads and executes the canned sequences of commands from init file (if any) in the current working directory, see *note Init File in the Current Directory during Startup::. Note that loading of this local '.gdbinit' file also requires accordingly configured 'auto-load safe-path' (*note Auto-loading safe path::). 'set auto-load local-gdbinit [on|off]' Enable or disable the auto-loading of canned sequences of commands (*note Sequences::) found in init file in the current directory. 'show auto-load local-gdbinit' Show whether auto-loading of canned sequences of commands from init file in the current directory is enabled or disabled. 'info auto-load local-gdbinit' Print whether canned sequences of commands from init file in the current directory have been auto-loaded.  File: gdb.info, Node: libthread_db.so.1 file, Next: Auto-loading safe path, Prev: Init File in the Current Directory, Up: Auto-loading 22.7.2 Automatically loading thread debugging library ----------------------------------------------------- This feature is currently present only on GNU/Linux native hosts. GDB reads in some cases thread debugging library from places specific to the inferior (*note set libthread-db-search-path::). The special 'libthread-db-search-path' entry '$sdir' is processed without checking this 'set auto-load libthread-db' switch as system libraries have to be trusted in general. In all other cases of 'libthread-db-search-path' entries GDB checks first if 'set auto-load libthread-db' is enabled before trying to open such thread debugging library. Note that loading of this debugging library also requires accordingly configured 'auto-load safe-path' (*note Auto-loading safe path::). 'set auto-load libthread-db [on|off]' Enable or disable the auto-loading of inferior specific thread debugging library. 'show auto-load libthread-db' Show whether auto-loading of inferior specific thread debugging library is enabled or disabled. 'info auto-load libthread-db' Print the list of all loaded inferior specific thread debugging libraries and for each such library print list of inferior PIDs using it.  File: gdb.info, Node: Auto-loading safe path, Next: Auto-loading verbose mode, Prev: libthread_db.so.1 file, Up: Auto-loading 22.7.3 Security restriction for auto-loading -------------------------------------------- As the files of inferior can come from untrusted source (such as submitted by an application user) GDB does not always load any files automatically. GDB provides the 'set auto-load safe-path' setting to list directories trusted for loading files not explicitly requested by user. Each directory can also be a shell wildcard pattern. If the path is not set properly you will see a warning and the file will not get loaded: $ ./gdb -q ./gdb Reading symbols from /home/user/gdb/gdb...done. warning: File "/home/user/gdb/gdb-gdb.gdb" auto-loading has been declined by your `auto-load safe-path' set to "$debugdir:$datadir/auto-load". warning: File "/home/user/gdb/gdb-gdb.py" auto-loading has been declined by your `auto-load safe-path' set to "$debugdir:$datadir/auto-load". To instruct GDB to go ahead and use the init files anyway, invoke GDB like this: $ gdb -q -iex "set auto-load safe-path /home/user/gdb" ./gdb The list of trusted directories is controlled by the following commands: 'set auto-load safe-path [DIRECTORIES]' Set the list of directories (and their subdirectories) trusted for automatic loading and execution of scripts. You can also enter a specific trusted file. Each directory can also be a shell wildcard pattern; wildcards do not match directory separator - see 'FNM_PATHNAME' for system function 'fnmatch' (*note fnmatch: (libc)Wildcard Matching.). If you omit DIRECTORIES, 'auto-load safe-path' will be reset to its default value as specified during GDB compilation. The list of directories uses path separator (':' on GNU and Unix systems, ';' on MS-Windows and MS-DOS) to separate directories, similarly to the 'PATH' environment variable. 'show auto-load safe-path' Show the list of directories trusted for automatic loading and execution of scripts. 'add-auto-load-safe-path' Add an entry (or list of entries) to the list of directories trusted for automatic loading and execution of scripts. Multiple entries may be delimited by the host platform path separator in use. This variable defaults to what '--with-auto-load-dir' has been configured to (*note with-auto-load-dir::). '$debugdir' and '$datadir' substitution applies the same as for *note set auto-load scripts-directory::. The default 'set auto-load safe-path' value can be also overriden by GDB configuration option '--with-auto-load-safe-path'. Setting this variable to '/' disables this security protection, corresponding GDB configuration option is '--without-auto-load-safe-path'. This variable is supposed to be set to the system directories writable by the system superuser only. Users can add their source directories in init files in their home directories (*note Home Directory Init File::). See also deprecated init file in the current directory (*note Init File in the Current Directory during Startup::). To force GDB to load the files it declined to load in the previous example, you could use one of the following ways: '~/.gdbinit': 'add-auto-load-safe-path ~/src/gdb' Specify this trusted directory (or a file) as additional component of the list. You have to specify also any existing directories displayed by by 'show auto-load safe-path' (such as '/usr:/bin' in this example). 'gdb -iex "set auto-load safe-path /usr:/bin:~/src/gdb" ...' Specify this directory as in the previous case but just for a single GDB session. 'gdb -iex "set auto-load safe-path /" ...' Disable auto-loading safety for a single GDB session. This assumes all the files you debug during this GDB session will come from trusted sources. './configure --without-auto-load-safe-path' During compilation of GDB you may disable any auto-loading safety. This assumes all the files you will ever debug with this GDB come from trusted sources. On the other hand you can also explicitly forbid automatic files loading which also suppresses any such warning messages: 'gdb -iex "set auto-load no" ...' You can use GDB command-line option for a single GDB session. '~/.gdbinit': 'set auto-load no' Disable auto-loading globally for the user (*note Home Directory Init File::). While it is improbable, you could also use system init file instead (*note System-wide configuration::). This setting applies to the file names as entered by user. If no entry matches GDB tries as a last resort to also resolve all the file names into their canonical form (typically resolving symbolic links) and compare the entries again. GDB already canonicalizes most of the filenames on its own before starting the comparison so a canonical form of directories is recommended to be entered.  File: gdb.info, Node: Auto-loading verbose mode, Prev: Auto-loading safe path, Up: Auto-loading 22.7.4 Displaying files tried for auto-load ------------------------------------------- For better visibility of all the file locations where you can place scripts to be auto-loaded with inferior -- or to protect yourself against accidental execution of untrusted scripts -- GDB provides a feature for printing all the files attempted to be loaded. Both existing and non-existing files may be printed. For example the list of directories from which it is safe to auto-load files (*note Auto-loading safe path::) applies also to canonicalized filenames which may not be too obvious while setting it up. (gdb) set debug auto-load on (gdb) file ~/src/t/true auto-load: Loading canned sequences of commands script "/tmp/true-gdb.gdb" for objfile "/tmp/true". auto-load: Updating directories of "/usr:/opt". auto-load: Using directory "/usr". auto-load: Using directory "/opt". warning: File "/tmp/true-gdb.gdb" auto-loading has been declined by your `auto-load safe-path' set to "/usr:/opt". 'set debug auto-load [on|off]' Set whether to print the filenames attempted to be auto-loaded. 'show debug auto-load' Show whether printing of the filenames attempted to be auto-loaded is turned on or off.  File: gdb.info, Node: Messages/Warnings, Next: Debugging Output, Prev: Auto-loading, Up: Controlling GDB 22.8 Optional Warnings and Messages =================================== By default, GDB is silent about its inner workings. If you are running on a slow machine, you may want to use the 'set verbose' command. This makes GDB tell you when it does a lengthy internal operation, so you will not think it has crashed. Currently, the messages controlled by 'set verbose' are those which announce that the symbol table for a source file is being read; see 'symbol-file' in *note Commands to Specify Files: Files. 'set verbose on' Enables GDB output of certain informational messages. 'set verbose off' Disables GDB output of certain informational messages. 'show verbose' Displays whether 'set verbose' is on or off. By default, if GDB encounters bugs in the symbol table of an object file, it is silent; but if you are debugging a compiler, you may find this information useful (*note Errors Reading Symbol Files: Symbol Errors.). 'set complaints LIMIT' Permits GDB to output LIMIT complaints about each type of unusual symbols before becoming silent about the problem. Set LIMIT to zero to suppress all complaints; set it to a large number to prevent complaints from being suppressed. 'show complaints' Displays how many symbol complaints GDB is permitted to produce. By default, GDB is cautious, and asks what sometimes seems to be a lot of stupid questions to confirm certain commands. For example, if you try to run a program which is already running: (gdb) run The program being debugged has been started already. Start it from the beginning? (y or n) If you are willing to unflinchingly face the consequences of your own commands, you can disable this "feature": 'set confirm off' Disables confirmation requests. Note that running GDB with the '--batch' option (*note -batch: Mode Options.) also automatically disables confirmation requests. 'set confirm on' Enables confirmation requests (the default). 'show confirm' Displays state of confirmation requests. If you need to debug user-defined commands or sourced files you may find it useful to enable "command tracing". In this mode each command will be printed as it is executed, prefixed with one or more '+' symbols, the quantity denoting the call depth of each command. 'set trace-commands on' Enable command tracing. 'set trace-commands off' Disable command tracing. 'show trace-commands' Display the current state of command tracing.  File: gdb.info, Node: Debugging Output, Next: Other Misc Settings, Prev: Messages/Warnings, Up: Controlling GDB 22.9 Optional Messages about Internal Happenings ================================================ GDB has commands that enable optional debugging messages from various GDB subsystems; normally these commands are of interest to GDB maintainers, or when reporting a bug. This section documents those commands. 'set exec-done-display' Turns on or off the notification of asynchronous commands' completion. When on, GDB will print a message when an asynchronous command finishes its execution. The default is off. 'show exec-done-display' Displays the current setting of asynchronous command completion notification. 'set debug aarch64' Turns on or off display of debugging messages related to ARM AArch64. The default is off. 'show debug aarch64' Displays the current state of displaying debugging messages related to ARM AArch64. 'set debug arch' Turns on or off display of gdbarch debugging info. The default is off 'show debug arch' Displays the current state of displaying gdbarch debugging info. 'set debug aix-solib' Control display of debugging messages from the AIX shared library support module. The default is off. 'show debug aix-thread' Show the current state of displaying AIX shared library debugging messages. 'set debug aix-thread' Display debugging messages about inner workings of the AIX thread module. 'show debug aix-thread' Show the current state of AIX thread debugging info display. 'set debug check-physname' Check the results of the "physname" computation. When reading DWARF debugging information for C++, GDB attempts to compute each entity's name. GDB can do this computation in two different ways, depending on exactly what information is present. When enabled, this setting causes GDB to compute the names both ways and display any discrepancies. 'show debug check-physname' Show the current state of "physname" checking. 'set debug coff-pe-read' Control display of debugging messages related to reading of COFF/PE exported symbols. The default is off. 'show debug coff-pe-read' Displays the current state of displaying debugging messages related to reading of COFF/PE exported symbols. 'set debug dwarf-die' Dump DWARF DIEs after they are read in. The value is the number of nesting levels to print. A value of zero turns off the display. 'show debug dwarf-die' Show the current state of DWARF DIE debugging. 'set debug dwarf-line' Turns on or off display of debugging messages related to reading DWARF line tables. The default is 0 (off). A value of 1 provides basic information. A value greater than 1 provides more verbose information. 'show debug dwarf-line' Show the current state of DWARF line table debugging. 'set debug dwarf-read' Turns on or off display of debugging messages related to reading DWARF debug info. The default is 0 (off). A value of 1 provides basic information. A value greater than 1 provides more verbose information. 'show debug dwarf-read' Show the current state of DWARF reader debugging. 'set debug displaced' Turns on or off display of GDB debugging info for the displaced stepping support. The default is off. 'show debug displaced' Displays the current state of displaying GDB debugging info related to displaced stepping. 'set debug event' Turns on or off display of GDB event debugging info. The default is off. 'show debug event' Displays the current state of displaying GDB event debugging info. 'set debug expression' Turns on or off display of debugging info about GDB expression parsing. The default is off. 'show debug expression' Displays the current state of displaying debugging info about GDB expression parsing. 'set debug fbsd-lwp' Turns on or off debugging messages from the FreeBSD LWP debug support. 'show debug fbsd-lwp' Show the current state of FreeBSD LWP debugging messages. 'set debug frame' Turns on or off display of GDB frame debugging info. The default is off. 'show debug frame' Displays the current state of displaying GDB frame debugging info. 'set debug gnu-nat' Turn on or off debugging messages from the GNU/Hurd debug support. 'show debug gnu-nat' Show the current state of GNU/Hurd debugging messages. 'set debug infrun' Turns on or off display of GDB debugging info for running the inferior. The default is off. 'infrun.c' contains GDB's runtime state machine used for implementing operations such as single-stepping the inferior. 'show debug infrun' Displays the current state of GDB inferior debugging. 'set debug jit' Turn on or off debugging messages from JIT debug support. 'show debug jit' Displays the current state of GDB JIT debugging. 'set debug lin-lwp' Turn on or off debugging messages from the Linux LWP debug support. 'show debug lin-lwp' Show the current state of Linux LWP debugging messages. 'set debug linux-namespaces' Turn on or off debugging messages from the Linux namespaces debug support. 'show debug linux-namespaces' Show the current state of Linux namespaces debugging messages. 'set debug mach-o' Control display of debugging messages related to Mach-O symbols processing. The default is off. 'show debug mach-o' Displays the current state of displaying debugging messages related to reading of COFF/PE exported symbols. 'set debug notification' Turn on or off debugging messages about remote async notification. The default is off. 'show debug notification' Displays the current state of remote async notification debugging messages. 'set debug observer' Turns on or off display of GDB observer debugging. This includes info such as the notification of observable events. 'show debug observer' Displays the current state of observer debugging. 'set debug overload' Turns on or off display of GDB C++ overload debugging info. This includes info such as ranking of functions, etc. The default is off. 'show debug overload' Displays the current state of displaying GDB C++ overload debugging info. 'set debug parser' Turns on or off the display of expression parser debugging output. Internally, this sets the 'yydebug' variable in the expression parser. *Note Tracing Your Parser: (bison)Tracing, for details. The default is off. 'show debug parser' Show the current state of expression parser debugging. 'set debug remote' Turns on or off display of reports on all packets sent back and forth across the serial line to the remote machine. The info is printed on the GDB standard output stream. The default is off. 'show debug remote' Displays the state of display of remote packets. 'set debug serial' Turns on or off display of GDB serial debugging info. The default is off. 'show debug serial' Displays the current state of displaying GDB serial debugging info. 'set debug solib-frv' Turn on or off debugging messages for FR-V shared-library code. 'show debug solib-frv' Display the current state of FR-V shared-library code debugging messages. 'set debug symbol-lookup' Turns on or off display of debugging messages related to symbol lookup. The default is 0 (off). A value of 1 provides basic information. A value greater than 1 provides more verbose information. 'show debug symbol-lookup' Show the current state of symbol lookup debugging messages. 'set debug symfile' Turns on or off display of debugging messages related to symbol file functions. The default is off. *Note Files::. 'show debug symfile' Show the current state of symbol file debugging messages. 'set debug symtab-create' Turns on or off display of debugging messages related to symbol table creation. The default is 0 (off). A value of 1 provides basic information. A value greater than 1 provides more verbose information. 'show debug symtab-create' Show the current state of symbol table creation debugging. 'set debug target' Turns on or off display of GDB target debugging info. This info includes what is going on at the target level of GDB, as it happens. The default is 0. Set it to 1 to track events, and to 2 to also track the value of large memory transfers. 'show debug target' Displays the current state of displaying GDB target debugging info. 'set debug timestamp' Turns on or off display of timestamps with GDB debugging info. When enabled, seconds and microseconds are displayed before each debugging message. 'show debug timestamp' Displays the current state of displaying timestamps with GDB debugging info. 'set debug varobj' Turns on or off display of GDB variable object debugging info. The default is off. 'show debug varobj' Displays the current state of displaying GDB variable object debugging info. 'set debug xml' Turn on or off debugging messages for built-in XML parsers. 'show debug xml' Displays the current state of XML debugging messages.  File: gdb.info, Node: Other Misc Settings, Prev: Debugging Output, Up: Controlling GDB 22.10 Other Miscellaneous Settings ================================== 'set interactive-mode' If 'on', forces GDB to assume that GDB was started in a terminal. In practice, this means that GDB should wait for the user to answer queries generated by commands entered at the command prompt. If 'off', forces GDB to operate in the opposite mode, and it uses the default answers to all queries. If 'auto' (the default), GDB tries to determine whether its standard input is a terminal, and works in interactive-mode if it is, non-interactively otherwise. In the vast majority of cases, the debugger should be able to guess correctly which mode should be used. But this setting can be useful in certain specific cases, such as running a MinGW GDB inside a cygwin window. 'show interactive-mode' Displays whether the debugger is operating in interactive mode or not.  File: gdb.info, Node: Extending GDB, Next: Interpreters, Prev: Controlling GDB, Up: Top 23 Extending GDB **************** GDB provides several mechanisms for extension. GDB also provides the ability to automatically load extensions when it reads a file for debugging. This allows the user to automatically customize GDB for the program being debugged. * Menu: * Sequences:: Canned Sequences of GDB Commands * Python:: Extending GDB using Python * Guile:: Extending GDB using Guile * Auto-loading extensions:: Automatically loading extensions * Multiple Extension Languages:: Working with multiple extension languages * Aliases:: Creating new spellings of existing commands To facilitate the use of extension languages, GDB is capable of evaluating the contents of a file. When doing so, GDB can recognize which extension language is being used by looking at the filename extension. Files with an unrecognized filename extension are always treated as a GDB Command Files. *Note Command files: Command Files. You can control how GDB evaluates these files with the following setting: 'set script-extension off' All scripts are always evaluated as GDB Command Files. 'set script-extension soft' The debugger determines the scripting language based on filename extension. If this scripting language is supported, GDB evaluates the script using that language. Otherwise, it evaluates the file as a GDB Command File. 'set script-extension strict' The debugger determines the scripting language based on filename extension, and evaluates the script using that language. If the language is not supported, then the evaluation fails. 'show script-extension' Display the current value of the 'script-extension' option.  File: gdb.info, Node: Sequences, Next: Python, Up: Extending GDB 23.1 Canned Sequences of Commands ================================= Aside from breakpoint commands (*note Breakpoint Command Lists: Break Commands.), GDB provides two ways to store sequences of commands for execution as a unit: user-defined commands and command files. * Menu: * Define:: How to define your own commands * Hooks:: Hooks for user-defined commands * Command Files:: How to write scripts of commands to be stored in a file * Output:: Commands for controlled output * Auto-loading sequences:: Controlling auto-loaded command files  File: gdb.info, Node: Define, Next: Hooks, Up: Sequences 23.1.1 User-defined Commands ---------------------------- A "user-defined command" is a sequence of GDB commands to which you assign a new name as a command. This is done with the 'define' command. User commands may accept an unlimited number of arguments separated by whitespace. Arguments are accessed within the user command via '$arg0...$argN'. A trivial example: define adder print $arg0 + $arg1 + $arg2 end To execute the command use: adder 1 2 3 This defines the command 'adder', which prints the sum of its three arguments. Note the arguments are text substitutions, so they may reference variables, use complex expressions, or even perform inferior functions calls. In addition, '$argc' may be used to find out how many arguments have been passed. define adder if $argc == 2 print $arg0 + $arg1 end if $argc == 3 print $arg0 + $arg1 + $arg2 end end Combining with the 'eval' command (*note eval::) makes it easier to process a variable number of arguments: define adder set $i = 0 set $sum = 0 while $i < $argc eval "set $sum = $sum + $arg%d", $i set $i = $i + 1 end print $sum end 'define COMMANDNAME' Define a command named COMMANDNAME. If there is already a command by that name, you are asked to confirm that you want to redefine it. The argument COMMANDNAME may be a bare command name consisting of letters, numbers, dashes, and underscores. It may also start with any predefined prefix command. For example, 'define target my-target' creates a user-defined 'target my-target' command. The definition of the command is made up of other GDB command lines, which are given following the 'define' command. The end of these commands is marked by a line containing 'end'. 'document COMMANDNAME' Document the user-defined command COMMANDNAME, so that it can be accessed by 'help'. The command COMMANDNAME must already be defined. This command reads lines of documentation just as 'define' reads the lines of the command definition, ending with 'end'. After the 'document' command is finished, 'help' on command COMMANDNAME displays the documentation you have written. You may use the 'document' command again to change the documentation of a command. Redefining the command with 'define' does not change the documentation. 'dont-repeat' Used inside a user-defined command, this tells GDB that this command should not be repeated when the user hits (*note repeat last command: Command Syntax.). 'help user-defined' List all user-defined commands and all python commands defined in class COMAND_USER. The first line of the documentation or docstring is included (if any). 'show user' 'show user COMMANDNAME' Display the GDB commands used to define COMMANDNAME (but not its documentation). If no COMMANDNAME is given, display the definitions for all user-defined commands. This does not work for user-defined python commands. 'show max-user-call-depth' 'set max-user-call-depth' The value of 'max-user-call-depth' controls how many recursion levels are allowed in user-defined commands before GDB suspects an infinite recursion and aborts the command. This does not apply to user-defined python commands. In addition to the above commands, user-defined commands frequently use control flow commands, described in *note Command Files::. When user-defined commands are executed, the commands of the definition are not printed. An error in any command stops execution of the user-defined command. If used interactively, commands that would ask for confirmation proceed without asking when used inside a user-defined command. Many GDB commands that normally print messages to say what they are doing omit the messages when used in a user-defined command.  File: gdb.info, Node: Hooks, Next: Command Files, Prev: Define, Up: Sequences 23.1.2 User-defined Command Hooks --------------------------------- You may define "hooks", which are a special kind of user-defined command. Whenever you run the command 'foo', if the user-defined command 'hook-foo' exists, it is executed (with no arguments) before that command. A hook may also be defined which is run after the command you executed. Whenever you run the command 'foo', if the user-defined command 'hookpost-foo' exists, it is executed (with no arguments) after that command. Post-execution hooks may exist simultaneously with pre-execution hooks, for the same command. It is valid for a hook to call the command which it hooks. If this occurs, the hook is not re-executed, thereby avoiding infinite recursion. In addition, a pseudo-command, 'stop' exists. Defining ('hook-stop') makes the associated commands execute every time execution stops in your program: before breakpoint commands are run, displays are printed, or the stack frame is printed. For example, to ignore 'SIGALRM' signals while single-stepping, but treat them normally during normal execution, you could define: define hook-stop handle SIGALRM nopass end define hook-run handle SIGALRM pass end define hook-continue handle SIGALRM pass end As a further example, to hook at the beginning and end of the 'echo' command, and to add extra text to the beginning and end of the message, you could define: define hook-echo echo <<<--- end define hookpost-echo echo --->>>\n end (gdb) echo Hello World <<<---Hello World--->>> (gdb) You can define a hook for any single-word command in GDB, but not for command aliases; you should define a hook for the basic command name, e.g. 'backtrace' rather than 'bt'. You can hook a multi-word command by adding 'hook-' or 'hookpost-' to the last word of the command, e.g. 'define target hook-remote' to add a hook to 'target remote'. If an error occurs during the execution of your hook, execution of GDB commands stops and GDB issues a prompt (before the command that you actually typed had a chance to run). If you try to define a hook which does not match any known command, you get a warning from the 'define' command.  File: gdb.info, Node: Command Files, Next: Output, Prev: Hooks, Up: Sequences 23.1.3 Command Files -------------------- A command file for GDB is a text file made of lines that are GDB commands. Comments (lines starting with '#') may also be included. An empty line in a command file does nothing; it does not mean to repeat the last command, as it would from the terminal. You can request the execution of a command file with the 'source' command. Note that the 'source' command is also used to evaluate scripts that are not Command Files. The exact behavior can be configured using the 'script-extension' setting. *Note Extending GDB: Extending GDB. 'source [-s] [-v] FILENAME' Execute the command file FILENAME. The lines in a command file are generally executed sequentially, unless the order of execution is changed by one of the _flow-control commands_ described below. The commands are not printed as they are executed. An error in any command terminates execution of the command file and control is returned to the console. GDB first searches for FILENAME in the current directory. If the file is not found there, and FILENAME does not specify a directory, then GDB also looks for the file on the source search path (specified with the 'directory' command); except that '$cdir' is not searched because the compilation directory is not relevant to scripts. If '-s' is specified, then GDB searches for FILENAME on the search path even if FILENAME specifies a directory. The search is done by appending FILENAME to each element of the search path. So, for example, if FILENAME is 'mylib/myscript' and the search path contains '/home/user' then GDB will look for the script '/home/user/mylib/myscript'. The search is also done if FILENAME is an absolute path. For example, if FILENAME is '/tmp/myscript' and the search path contains '/home/user' then GDB will look for the script '/home/user/tmp/myscript'. For DOS-like systems, if FILENAME contains a drive specification, it is stripped before concatenation. For example, if FILENAME is 'd:myscript' and the search path contains 'c:/tmp' then GDB will look for the script 'c:/tmp/myscript'. If '-v', for verbose mode, is given then GDB displays each command as it is executed. The option must be given before FILENAME, and is interpreted as part of the filename anywhere else. Commands that would ask for confirmation if used interactively proceed without asking when used in a command file. Many GDB commands that normally print messages to say what they are doing omit the messages when called from command files. GDB also accepts command input from standard input. In this mode, normal output goes to standard output and error output goes to standard error. Errors in a command file supplied on standard input do not terminate execution of the command file--execution continues with the next command. gdb < cmds > log 2>&1 (The syntax above will vary depending on the shell used.) This example will execute commands from the file 'cmds'. All output and errors would be directed to 'log'. Since commands stored on command files tend to be more general than commands typed interactively, they frequently need to deal with complicated situations, such as different or unexpected values of variables and symbols, changes in how the program being debugged is built, etc. GDB provides a set of flow-control commands to deal with these complexities. Using these commands, you can write complex scripts that loop over data structures, execute commands conditionally, etc. 'if' 'else' This command allows to include in your script conditionally executed commands. The 'if' command takes a single argument, which is an expression to evaluate. It is followed by a series of commands that are executed only if the expression is true (its value is nonzero). There can then optionally be an 'else' line, followed by a series of commands that are only executed if the expression was false. The end of the list is marked by a line containing 'end'. 'while' This command allows to write loops. Its syntax is similar to 'if': the command takes a single argument, which is an expression to evaluate, and must be followed by the commands to execute, one per line, terminated by an 'end'. These commands are called the "body" of the loop. The commands in the body of 'while' are executed repeatedly as long as the expression evaluates to true. 'loop_break' This command exits the 'while' loop in whose body it is included. Execution of the script continues after that 'while's 'end' line. 'loop_continue' This command skips the execution of the rest of the body of commands in the 'while' loop in whose body it is included. Execution branches to the beginning of the 'while' loop, where it evaluates the controlling expression. 'end' Terminate the block of commands that are the body of 'if', 'else', or 'while' flow-control commands.  File: gdb.info, Node: Output, Next: Auto-loading sequences, Prev: Command Files, Up: Sequences 23.1.4 Commands for Controlled Output ------------------------------------- During the execution of a command file or a user-defined command, normal GDB output is suppressed; the only output that appears is what is explicitly printed by the commands in the definition. This section describes three commands useful for generating exactly the output you want. 'echo TEXT' Print TEXT. Nonprinting characters can be included in TEXT using C escape sequences, such as '\n' to print a newline. *No newline is printed unless you specify one.* In addition to the standard C escape sequences, a backslash followed by a space stands for a space. This is useful for displaying a string with spaces at the beginning or the end, since leading and trailing spaces are otherwise trimmed from all arguments. To print ' and foo = ', use the command 'echo \ and foo = \ '. A backslash at the end of TEXT can be used, as in C, to continue the command onto subsequent lines. For example, echo This is some text\n\ which is continued\n\ onto several lines.\n produces the same output as echo This is some text\n echo which is continued\n echo onto several lines.\n 'output EXPRESSION' Print the value of EXPRESSION and nothing but that value: no newlines, no '$NN = '. The value is not entered in the value history either. *Note Expressions: Expressions, for more information on expressions. 'output/FMT EXPRESSION' Print the value of EXPRESSION in format FMT. You can use the same formats as for 'print'. *Note Output Formats: Output Formats, for more information. 'printf TEMPLATE, EXPRESSIONS...' Print the values of one or more EXPRESSIONS under the control of the string TEMPLATE. To print several values, make EXPRESSIONS be a comma-separated list of individual expressions, which may be either numbers or pointers. Their values are printed as specified by TEMPLATE, exactly as a C program would do by executing the code below: printf (TEMPLATE, EXPRESSIONS...); As in 'C' 'printf', ordinary characters in TEMPLATE are printed verbatim, while "conversion specification" introduced by the '%' character cause subsequent EXPRESSIONS to be evaluated, their values converted and formatted according to type and style information encoded in the conversion specifications, and then printed. For example, you can print two values in hex like this: printf "foo, bar-foo = 0x%x, 0x%x\n", foo, bar-foo 'printf' supports all the standard 'C' conversion specifications, including the flags and modifiers between the '%' character and the conversion letter, with the following exceptions: * The argument-ordering modifiers, such as '2$', are not supported. * The modifier '*' is not supported for specifying precision or width. * The ''' flag (for separation of digits into groups according to 'LC_NUMERIC'') is not supported. * The type modifiers 'hh', 'j', 't', and 'z' are not supported. * The conversion letter 'n' (as in '%n') is not supported. * The conversion letters 'a' and 'A' are not supported. Note that the 'll' type modifier is supported only if the underlying 'C' implementation used to build GDB supports the 'long long int' type, and the 'L' type modifier is supported only if 'long double' type is available. As in 'C', 'printf' supports simple backslash-escape sequences, such as '\n', '\t', '\\', '\"', '\a', and '\f', that consist of backslash followed by a single character. Octal and hexadecimal escape sequences are not supported. Additionally, 'printf' supports conversion specifications for DFP ("Decimal Floating Point") types using the following length modifiers together with a floating point specifier. letters: * 'H' for printing 'Decimal32' types. * 'D' for printing 'Decimal64' types. * 'DD' for printing 'Decimal128' types. If the underlying 'C' implementation used to build GDB has support for the three length modifiers for DFP types, other modifiers such as width and precision will also be available for GDB to use. In case there is no such 'C' support, no additional modifiers will be available and the value will be printed in the standard way. Here's an example of printing DFP types using the above conversion letters: printf "D32: %Hf - D64: %Df - D128: %DDf\n",1.2345df,1.2E10dd,1.2E1dl 'eval TEMPLATE, EXPRESSIONS...' Convert the values of one or more EXPRESSIONS under the control of the string TEMPLATE to a command line, and call it.  File: gdb.info, Node: Auto-loading sequences, Prev: Output, Up: Sequences 23.1.5 Controlling auto-loading native GDB scripts -------------------------------------------------- When a new object file is read (for example, due to the 'file' command, or because the inferior has loaded a shared library), GDB will look for the command file 'OBJFILE-gdb.gdb'. *Note Auto-loading extensions::. Auto-loading can be enabled or disabled, and the list of auto-loaded scripts can be printed. 'set auto-load gdb-scripts [on|off]' Enable or disable the auto-loading of canned sequences of commands scripts. 'show auto-load gdb-scripts' Show whether auto-loading of canned sequences of commands scripts is enabled or disabled. 'info auto-load gdb-scripts [REGEXP]' Print the list of all canned sequences of commands scripts that GDB auto-loaded. If REGEXP is supplied only canned sequences of commands scripts with matching names are printed.  File: gdb.info, Node: Python, Next: Guile, Prev: Sequences, Up: Extending GDB 23.2 Extending GDB using Python =============================== You can extend GDB using the Python programming language (http://www.python.org/). This feature is available only if GDB was configured using '--with-python'. Python scripts used by GDB should be installed in 'DATA-DIRECTORY/python', where DATA-DIRECTORY is the data directory as determined at GDB startup (*note Data Files::). This directory, known as the "python directory", is automatically added to the Python Search Path in order to allow the Python interpreter to locate all scripts installed at this location. Additionally, GDB commands and convenience functions which are written in Python and are located in the 'DATA-DIRECTORY/python/gdb/command' or 'DATA-DIRECTORY/python/gdb/function' directories are automatically imported when GDB starts. * Menu: * Python Commands:: Accessing Python from GDB. * Python API:: Accessing GDB from Python. * Python Auto-loading:: Automatically loading Python code. * Python modules:: Python modules provided by GDB.  File: gdb.info, Node: Python Commands, Next: Python API, Up: Python 23.2.1 Python Commands ---------------------- GDB provides two commands for accessing the Python interpreter, and one related setting: 'python-interactive [COMMAND]' 'pi [COMMAND]' Without an argument, the 'python-interactive' command can be used to start an interactive Python prompt. To return to GDB, type the 'EOF' character (e.g., 'Ctrl-D' on an empty prompt). Alternatively, a single-line Python command can be given as an argument and evaluated. If the command is an expression, the result will be printed; otherwise, nothing will be printed. For example: (gdb) python-interactive 2 + 3 5 'python [COMMAND]' 'py [COMMAND]' The 'python' command can be used to evaluate Python code. If given an argument, the 'python' command will evaluate the argument as a Python command. For example: (gdb) python print 23 23 If you do not provide an argument to 'python', it will act as a multi-line command, like 'define'. In this case, the Python script is made up of subsequent command lines, given after the 'python' command. This command list is terminated using a line containing 'end'. For example: (gdb) python Type python script End with a line saying just "end". >print 23 >end 23 'set python print-stack' By default, GDB will print only the message component of a Python exception when an error occurs in a Python script. This can be controlled using 'set python print-stack': if 'full', then full Python stack printing is enabled; if 'none', then Python stack and message printing is disabled; if 'message', the default, only the message component of the error is printed. It is also possible to execute a Python script from the GDB interpreter: 'source script-name' The script name must end with '.py' and GDB must be configured to recognize the script language based on filename extension using the 'script-extension' setting. *Note Extending GDB: Extending GDB. 'python execfile ("script-name")' This method is based on the 'execfile' Python built-in function, and thus is always available.  File: gdb.info, Node: Python API, Next: Python Auto-loading, Prev: Python Commands, Up: Python 23.2.2 Python API ----------------- You can get quick online help for GDB's Python API by issuing the command 'python help (gdb)'. Functions and methods which have two or more optional arguments allow them to be specified using keyword syntax. This allows passing some optional arguments while skipping others. Example: 'gdb.some_function ('foo', bar = 1, baz = 2)'. * Menu: * Basic Python:: Basic Python Functions. * Exception Handling:: How Python exceptions are translated. * Values From Inferior:: Python representation of values. * Types In Python:: Python representation of types. * Pretty Printing API:: Pretty-printing values. * Selecting Pretty-Printers:: How GDB chooses a pretty-printer. * Writing a Pretty-Printer:: Writing a Pretty-Printer. * Type Printing API:: Pretty-printing types. * Frame Filter API:: Filtering Frames. * Frame Decorator API:: Decorating Frames. * Writing a Frame Filter:: Writing a Frame Filter. * Unwinding Frames in Python:: Writing frame unwinder. * Xmethods In Python:: Adding and replacing methods of C++ classes. * Xmethod API:: Xmethod types. * Writing an Xmethod:: Writing an xmethod. * Inferiors In Python:: Python representation of inferiors (processes) * Events In Python:: Listening for events from GDB. * Threads In Python:: Accessing inferior threads from Python. * Recordings In Python:: Accessing recordings from Python. * Commands In Python:: Implementing new commands in Python. * Parameters In Python:: Adding new GDB parameters. * Functions In Python:: Writing new convenience functions. * Progspaces In Python:: Program spaces. * Objfiles In Python:: Object files. * Frames In Python:: Accessing inferior stack frames from Python. * Blocks In Python:: Accessing blocks from Python. * Symbols In Python:: Python representation of symbols. * Symbol Tables In Python:: Python representation of symbol tables. * Line Tables In Python:: Python representation of line tables. * Breakpoints In Python:: Manipulating breakpoints using Python. * Finish Breakpoints in Python:: Setting Breakpoints on function return using Python. * Lazy Strings In Python:: Python representation of lazy strings. * Architectures In Python:: Python representation of architectures.  File: gdb.info, Node: Basic Python, Next: Exception Handling, Up: Python API 23.2.2.1 Basic Python ..................... At startup, GDB overrides Python's 'sys.stdout' and 'sys.stderr' to print using GDB's output-paging streams. A Python program which outputs to one of these streams may have its output interrupted by the user (*note Screen Size::). In this situation, a Python 'KeyboardInterrupt' exception is thrown. Some care must be taken when writing Python code to run in GDB. Two things worth noting in particular: * GDB install handlers for 'SIGCHLD' and 'SIGINT'. Python code must not override these, or even change the options using 'sigaction'. If your program changes the handling of these signals, GDB will most likely stop working correctly. Note that it is unfortunately common for GUI toolkits to install a 'SIGCHLD' handler. * GDB takes care to mark its internal file descriptors as close-on-exec. However, this cannot be done in a thread-safe way on all platforms. Your Python programs should be aware of this and should both create new file descriptors with the close-on-exec flag set and arrange to close unneeded file descriptors before starting a child process. GDB introduces a new Python module, named 'gdb'. All methods and classes added by GDB are placed in this module. GDB automatically 'import's the 'gdb' module for use in all scripts evaluated by the 'python' command. -- Variable: gdb.PYTHONDIR A string containing the python directory (*note Python::). -- Function: gdb.execute (command [, from_tty [, to_string]]) Evaluate COMMAND, a string, as a GDB CLI command. If a GDB exception happens while COMMAND runs, it is translated as described in *note Exception Handling: Exception Handling. The FROM_TTY flag specifies whether GDB ought to consider this command as having originated from the user invoking it interactively. It must be a boolean value. If omitted, it defaults to 'False'. By default, any output produced by COMMAND is sent to GDB's standard output (and to the log output if logging is turned on). If the TO_STRING parameter is 'True', then output will be collected by 'gdb.execute' and returned as a string. The default is 'False', in which case the return value is 'None'. If TO_STRING is 'True', the GDB virtual terminal will be temporarily set to unlimited width and height, and its pagination will be disabled; *note Screen Size::. -- Function: gdb.breakpoints () Return a sequence holding all of GDB's breakpoints. *Note Breakpoints In Python::, for more information. In GDB version 7.11 and earlier, this function returned 'None' if there were no breakpoints. This peculiarity was subsequently fixed, and now 'gdb.breakpoints' returns an empty sequence in this case. -- Function: gdb.parameter (parameter) Return the value of a GDB PARAMETER given by its name, a string; the parameter name string may contain spaces if the parameter has a multi-part name. For example, 'print object' is a valid parameter name. If the named parameter does not exist, this function throws a 'gdb.error' (*note Exception Handling::). Otherwise, the parameter's value is converted to a Python value of the appropriate type, and returned. -- Function: gdb.history (number) Return a value from GDB's value history (*note Value History::). The NUMBER argument indicates which history element to return. If NUMBER is negative, then GDB will take its absolute value and count backward from the last element (i.e., the most recent element) to find the value to return. If NUMBER is zero, then GDB will return the most recent element. If the element specified by NUMBER doesn't exist in the value history, a 'gdb.error' exception will be raised. If no exception is raised, the return value is always an instance of 'gdb.Value' (*note Values From Inferior::). -- Function: gdb.parse_and_eval (expression) Parse EXPRESSION, which must be a string, as an expression in the current language, evaluate it, and return the result as a 'gdb.Value'. This function can be useful when implementing a new command (*note Commands In Python::), as it provides a way to parse the command's argument as an expression. It is also useful simply to compute values, for example, it is the only way to get the value of a convenience variable (*note Convenience Vars::) as a 'gdb.Value'. -- Function: gdb.find_pc_line (pc) Return the 'gdb.Symtab_and_line' object corresponding to the PC value. *Note Symbol Tables In Python::. If an invalid value of PC is passed as an argument, then the 'symtab' and 'line' attributes of the returned 'gdb.Symtab_and_line' object will be 'None' and 0 respectively. -- Function: gdb.post_event (event) Put EVENT, a callable object taking no arguments, into GDB's internal event queue. This callable will be invoked at some later point, during GDB's event processing. Events posted using 'post_event' will be run in the order in which they were posted; however, there is no way to know when they will be processed relative to other events inside GDB. GDB is not thread-safe. If your Python program uses multiple threads, you must be careful to only call GDB-specific functions in the GDB thread. 'post_event' ensures this. For example: (gdb) python >import threading > >class Writer(): > def __init__(self, message): > self.message = message; > def __call__(self): > gdb.write(self.message) > >class MyThread1 (threading.Thread): > def run (self): > gdb.post_event(Writer("Hello ")) > >class MyThread2 (threading.Thread): > def run (self): > gdb.post_event(Writer("World\n")) > >MyThread1().start() >MyThread2().start() >end (gdb) Hello World -- Function: gdb.write (string [, stream]) Print a string to GDB's paginated output stream. The optional STREAM determines the stream to print to. The default stream is GDB's standard output stream. Possible stream values are: 'gdb.STDOUT' GDB's standard output stream. 'gdb.STDERR' GDB's standard error stream. 'gdb.STDLOG' GDB's log stream (*note Logging Output::). Writing to 'sys.stdout' or 'sys.stderr' will automatically call this function and will automatically direct the output to the relevant stream. -- Function: gdb.flush () Flush the buffer of a GDB paginated stream so that the contents are displayed immediately. GDB will flush the contents of a stream automatically when it encounters a newline in the buffer. The optional STREAM determines the stream to flush. The default stream is GDB's standard output stream. Possible stream values are: 'gdb.STDOUT' GDB's standard output stream. 'gdb.STDERR' GDB's standard error stream. 'gdb.STDLOG' GDB's log stream (*note Logging Output::). Flushing 'sys.stdout' or 'sys.stderr' will automatically call this function for the relevant stream. -- Function: gdb.target_charset () Return the name of the current target character set (*note Character Sets::). This differs from 'gdb.parameter('target-charset')' in that 'auto' is never returned. -- Function: gdb.target_wide_charset () Return the name of the current target wide character set (*note Character Sets::). This differs from 'gdb.parameter('target-wide-charset')' in that 'auto' is never returned. -- Function: gdb.solib_name (address) Return the name of the shared library holding the given ADDRESS as a string, or 'None'. -- Function: gdb.decode_line [expression] Return locations of the line specified by EXPRESSION, or of the current line if no argument was given. This function returns a Python tuple containing two elements. The first element contains a string holding any unparsed section of EXPRESSION (or 'None' if the expression has been fully parsed). The second element contains either 'None' or another tuple that contains all the locations that match the expression represented as 'gdb.Symtab_and_line' objects (*note Symbol Tables In Python::). If EXPRESSION is provided, it is decoded the way that GDB's inbuilt 'break' or 'edit' commands do (*note Specify Location::). -- Function: gdb.prompt_hook (current_prompt) If PROMPT_HOOK is callable, GDB will call the method assigned to this operation before a prompt is displayed by GDB. The parameter 'current_prompt' contains the current GDB prompt. This method must return a Python string, or 'None'. If a string is returned, the GDB prompt will be set to that string. If 'None' is returned, GDB will continue to use the current prompt. Some prompts cannot be substituted in GDB. Secondary prompts such as those used by readline for command input, and annotation related prompts are prohibited from being changed.  File: gdb.info, Node: Exception Handling, Next: Values From Inferior, Prev: Basic Python, Up: Python API 23.2.2.2 Exception Handling ........................... When executing the 'python' command, Python exceptions uncaught within the Python code are translated to calls to GDB error-reporting mechanism. If the command that called 'python' does not handle the error, GDB will terminate it and print an error message containing the Python exception name, the associated value, and the Python call stack backtrace at the point where the exception was raised. Example: (gdb) python print foo Traceback (most recent call last): File "", line 1, in NameError: name 'foo' is not defined GDB errors that happen in GDB commands invoked by Python code are converted to Python exceptions. The type of the Python exception depends on the error. 'gdb.error' This is the base class for most exceptions generated by GDB. It is derived from 'RuntimeError', for compatibility with earlier versions of GDB. If an error occurring in GDB does not fit into some more specific category, then the generated exception will have this type. 'gdb.MemoryError' This is a subclass of 'gdb.error' which is thrown when an operation tried to access invalid memory in the inferior. 'KeyboardInterrupt' User interrupt (via 'C-c' or by typing 'q' at a pagination prompt) is translated to a Python 'KeyboardInterrupt' exception. In all cases, your exception handler will see the GDB error message as its value and the Python call stack backtrace at the Python statement closest to where the GDB error occured as the traceback. When implementing GDB commands in Python via 'gdb.Command', it is useful to be able to throw an exception that doesn't cause a traceback to be printed. For example, the user may have invoked the command incorrectly. Use the 'gdb.GdbError' exception to handle this case. Example: (gdb) python >class HelloWorld (gdb.Command): > """Greet the whole world.""" > def __init__ (self): > super (HelloWorld, self).__init__ ("hello-world", gdb.COMMAND_USER) > def invoke (self, args, from_tty): > argv = gdb.string_to_argv (args) > if len (argv) != 0: > raise gdb.GdbError ("hello-world takes no arguments") > print "Hello, World!" >HelloWorld () >end (gdb) hello-world 42 hello-world takes no arguments  File: gdb.info, Node: Values From Inferior, Next: Types In Python, Prev: Exception Handling, Up: Python API 23.2.2.3 Values From Inferior ............................. GDB provides values it obtains from the inferior program in an object of type 'gdb.Value'. GDB uses this object for its internal bookkeeping of the inferior's values, and for fetching values when necessary. Inferior values that are simple scalars can be used directly in Python expressions that are valid for the value's data type. Here's an example for an integer or floating-point value 'some_val': bar = some_val + 2 As result of this, 'bar' will also be a 'gdb.Value' object whose values are of the same type as those of 'some_val'. Valid Python operations can also be performed on 'gdb.Value' objects representing a 'struct' or 'class' object. For such cases, the overloaded operator (if present), is used to perform the operation. For example, if 'val1' and 'val2' are 'gdb.Value' objects representing instances of a 'class' which overloads the '+' operator, then one can use the '+' operator in their Python script as follows: val3 = val1 + val2 The result of the operation 'val3' is also a 'gdb.Value' object corresponding to the value returned by the overloaded '+' operator. In general, overloaded operators are invoked for the following operations: '+' (binary addition), '-' (binary subtraction), '*' (multiplication), '/', '%', '<<', '>>', '|', '&', '^'. Inferior values that are structures or instances of some class can be accessed using the Python "dictionary syntax". For example, if 'some_val' is a 'gdb.Value' instance holding a structure, you can access its 'foo' element with: bar = some_val['foo'] Again, 'bar' will also be a 'gdb.Value' object. Structure elements can also be accessed by using 'gdb.Field' objects as subscripts (*note Types In Python::, for more information on 'gdb.Field' objects). For example, if 'foo_field' is a 'gdb.Field' object corresponding to element 'foo' of the above structure, then 'bar' can also be accessed as follows: bar = some_val[foo_field] A 'gdb.Value' that represents a function can be executed via inferior function call. Any arguments provided to the call must match the function's prototype, and must be provided in the order specified by that prototype. For example, 'some_val' is a 'gdb.Value' instance representing a function that takes two integers as arguments. To execute this function, call it like so: result = some_val (10,20) Any values returned from a function call will be stored as a 'gdb.Value'. The following attributes are provided: -- Variable: Value.address If this object is addressable, this read-only attribute holds a 'gdb.Value' object representing the address. Otherwise, this attribute holds 'None'. -- Variable: Value.is_optimized_out This read-only boolean attribute is true if the compiler optimized out this value, thus it is not available for fetching from the inferior. -- Variable: Value.type The type of this 'gdb.Value'. The value of this attribute is a 'gdb.Type' object (*note Types In Python::). -- Variable: Value.dynamic_type The dynamic type of this 'gdb.Value'. This uses C++ run-time type information (RTTI) to determine the dynamic type of the value. If this value is of class type, it will return the class in which the value is embedded, if any. If this value is of pointer or reference to a class type, it will compute the dynamic type of the referenced object, and return a pointer or reference to that type, respectively. In all other cases, it will return the value's static type. Note that this feature will only work when debugging a C++ program that includes RTTI for the object in question. Otherwise, it will just return the static type of the value as in 'ptype foo' (*note ptype: Symbols.). -- Variable: Value.is_lazy The value of this read-only boolean attribute is 'True' if this 'gdb.Value' has not yet been fetched from the inferior. GDB does not fetch values until necessary, for efficiency. For example: myval = gdb.parse_and_eval ('somevar') The value of 'somevar' is not fetched at this time. It will be fetched when the value is needed, or when the 'fetch_lazy' method is invoked. The following methods are provided: -- Function: Value.__init__ (VAL) Many Python values can be converted directly to a 'gdb.Value' via this object initializer. Specifically: Python boolean A Python boolean is converted to the boolean type from the current language. Python integer A Python integer is converted to the C 'long' type for the current architecture. Python long A Python long is converted to the C 'long long' type for the current architecture. Python float A Python float is converted to the C 'double' type for the current architecture. Python string A Python string is converted to a target string in the current target language using the current target encoding. If a character cannot be represented in the current target encoding, then an exception is thrown. 'gdb.Value' If 'val' is a 'gdb.Value', then a copy of the value is made. 'gdb.LazyString' If 'val' is a 'gdb.LazyString' (*note Lazy Strings In Python::), then the lazy string's 'value' method is called, and its result is used. -- Function: Value.cast (type) Return a new instance of 'gdb.Value' that is the result of casting this instance to the type described by TYPE, which must be a 'gdb.Type' object. If the cast cannot be performed for some reason, this method throws an exception. -- Function: Value.dereference () For pointer data types, this method returns a new 'gdb.Value' object whose contents is the object pointed to by the pointer. For example, if 'foo' is a C pointer to an 'int', declared in your C program as int *foo; then you can use the corresponding 'gdb.Value' to access what 'foo' points to like this: bar = foo.dereference () The result 'bar' will be a 'gdb.Value' object holding the value pointed to by 'foo'. A similar function 'Value.referenced_value' exists which also returns 'gdb.Value' objects corresonding to the values pointed to by pointer values (and additionally, values referenced by reference values). However, the behavior of 'Value.dereference' differs from 'Value.referenced_value' by the fact that the behavior of 'Value.dereference' is identical to applying the C unary operator '*' on a given value. For example, consider a reference to a pointer 'ptrref', declared in your C++ program as typedef int *intptr; ... int val = 10; intptr ptr = &val; intptr &ptrref = ptr; Though 'ptrref' is a reference value, one can apply the method 'Value.dereference' to the 'gdb.Value' object corresponding to it and obtain a 'gdb.Value' which is identical to that corresponding to 'val'. However, if you apply the method 'Value.referenced_value', the result would be a 'gdb.Value' object identical to that corresponding to 'ptr'. py_ptrref = gdb.parse_and_eval ("ptrref") py_val = py_ptrref.dereference () py_ptr = py_ptrref.referenced_value () The 'gdb.Value' object 'py_val' is identical to that corresponding to 'val', and 'py_ptr' is identical to that corresponding to 'ptr'. In general, 'Value.dereference' can be applied whenever the C unary operator '*' can be applied to the corresponding C value. For those cases where applying both 'Value.dereference' and 'Value.referenced_value' is allowed, the results obtained need not be identical (as we have seen in the above example). The results are however identical when applied on 'gdb.Value' objects corresponding to pointers ('gdb.Value' objects with type code 'TYPE_CODE_PTR') in a C/C++ program. -- Function: Value.referenced_value () For pointer or reference data types, this method returns a new 'gdb.Value' object corresponding to the value referenced by the pointer/reference value. For pointer data types, 'Value.dereference' and 'Value.referenced_value' produce identical results. The difference between these methods is that 'Value.dereference' cannot get the values referenced by reference values. For example, consider a reference to an 'int', declared in your C++ program as int val = 10; int &ref = val; then applying 'Value.dereference' to the 'gdb.Value' object corresponding to 'ref' will result in an error, while applying 'Value.referenced_value' will result in a 'gdb.Value' object identical to that corresponding to 'val'. py_ref = gdb.parse_and_eval ("ref") er_ref = py_ref.dereference () # Results in error py_val = py_ref.referenced_value () # Returns the referenced value The 'gdb.Value' object 'py_val' is identical to that corresponding to 'val'. -- Function: Value.reference_value () Return a 'gdb.Value' object which is a reference to the value encapsulated by this instance. -- Function: Value.const_value () Return a 'gdb.Value' object which is a 'const' version of the value encapsulated by this instance. -- Function: Value.dynamic_cast (type) Like 'Value.cast', but works as if the C++ 'dynamic_cast' operator were used. Consult a C++ reference for details. -- Function: Value.reinterpret_cast (type) Like 'Value.cast', but works as if the C++ 'reinterpret_cast' operator were used. Consult a C++ reference for details. -- Function: Value.string ([encoding[, errors[, length]]]) If this 'gdb.Value' represents a string, then this method converts the contents to a Python string. Otherwise, this method will throw an exception. Values are interpreted as strings according to the rules of the current language. If the optional length argument is given, the string will be converted to that length, and will include any embedded zeroes that the string may contain. Otherwise, for languages where the string is zero-terminated, the entire string will be converted. For example, in C-like languages, a value is a string if it is a pointer to or an array of characters or ints of type 'wchar_t', 'char16_t', or 'char32_t'. If the optional ENCODING argument is given, it must be a string naming the encoding of the string in the 'gdb.Value', such as '"ascii"', '"iso-8859-6"' or '"utf-8"'. It accepts the same encodings as the corresponding argument to Python's 'string.decode' method, and the Python codec machinery will be used to convert the string. If ENCODING is not given, or if ENCODING is the empty string, then either the 'target-charset' (*note Character Sets::) will be used, or a language-specific encoding will be used, if the current language is able to supply one. The optional ERRORS argument is the same as the corresponding argument to Python's 'string.decode' method. If the optional LENGTH argument is given, the string will be fetched and converted to the given length. -- Function: Value.lazy_string ([encoding [, length]]) If this 'gdb.Value' represents a string, then this method converts the contents to a 'gdb.LazyString' (*note Lazy Strings In Python::). Otherwise, this method will throw an exception. If the optional ENCODING argument is given, it must be a string naming the encoding of the 'gdb.LazyString'. Some examples are: 'ascii', 'iso-8859-6' or 'utf-8'. If the ENCODING argument is an encoding that GDB does recognize, GDB will raise an error. When a lazy string is printed, the GDB encoding machinery is used to convert the string during printing. If the optional ENCODING argument is not provided, or is an empty string, GDB will automatically select the encoding most suitable for the string type. For further information on encoding in GDB please see *note Character Sets::. If the optional LENGTH argument is given, the string will be fetched and encoded to the length of characters specified. If the LENGTH argument is not provided, the string will be fetched and encoded until a null of appropriate width is found. -- Function: Value.fetch_lazy () If the 'gdb.Value' object is currently a lazy value ('gdb.Value.is_lazy' is 'True'), then the value is fetched from the inferior. Any errors that occur in the process will produce a Python exception. If the 'gdb.Value' object is not a lazy value, this method has no effect. This method does not return a value.