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8.2 Backtraces

A backtrace is a summary of how your program got where it is. It shows one line per frame, for many frames, starting with the currently executing frame (frame zero), followed by its caller (frame one), and on up the stack.

backtrace
bt

Print a backtrace of the entire stack: one line per frame for all frames in the stack.

You can stop the backtrace at any time by typing the system interrupt character, normally Ctrl-c.

backtrace n
bt n

Similar, but print only the innermost n frames.

backtrace -n
bt -n

Similar, but print only the outermost n frames.

backtrace full
bt full
bt full n
bt full -n

Print the values of the local variables also. n specifies the number of frames to print, as described above.

backtrace no-filters
bt no-filters
bt no-filters n
bt no-filters -n
bt no-filters full
bt no-filters full n
bt no-filters full -n

Do not run Python frame filters on this backtrace. See Frame Filter API, for more information. Additionally use disable frame-filter all to turn off all frame filters. This is only relevant when GDB has been configured with Python support.

The names where and info stack (abbreviated info s) are additional aliases for backtrace.

In a multi-threaded program, GDB by default shows the backtrace only for the current thread. To display the backtrace for several or all of the threads, use the command thread apply (see thread apply). For example, if you type thread apply all backtrace, GDB will display the backtrace for all the threads; this is handy when you debug a core dump of a multi-threaded program.

Each line in the backtrace shows the frame number and the function name. The program counter value is also shown—unless you use set print address off. The backtrace also shows the source file name and line number, as well as the arguments to the function. The program counter value is omitted if it is at the beginning of the code for that line number.

Here is an example of a backtrace. It was made with the command ‘bt 3’, so it shows the innermost three frames.

#0  m4_traceon (obs=0x24eb0, argc=1, argv=0x2b8c8)
    at builtin.c:993
#1  0x6e38 in expand_macro (sym=0x2b600, data=...) at macro.c:242
#2  0x6840 in expand_token (obs=0x0, t=177664, td=0xf7fffb08)
    at macro.c:71
(More stack frames follow...)

The display for frame zero does not begin with a program counter value, indicating that your program has stopped at the beginning of the code for line 993 of builtin.c.

The value of parameter data in frame 1 has been replaced by . By default, GDB prints the value of a parameter only if it is a scalar (integer, pointer, enumeration, etc). See command set print frame-arguments in Print Settings for more details on how to configure the way function parameter values are printed.

If your program was compiled with optimizations, some compilers will optimize away arguments passed to functions if those arguments are never used after the call. Such optimizations generate code that passes arguments through registers, but doesn’t store those arguments in the stack frame. GDB has no way of displaying such arguments in stack frames other than the innermost one. Here’s what such a backtrace might look like:

#0  m4_traceon (obs=0x24eb0, argc=1, argv=0x2b8c8)
    at builtin.c:993
#1  0x6e38 in expand_macro (sym=<optimized out>) at macro.c:242
#2  0x6840 in expand_token (obs=0x0, t=<optimized out>, td=0xf7fffb08)
    at macro.c:71
(More stack frames follow...)

The values of arguments that were not saved in their stack frames are shown as ‘<optimized out>’.

If you need to display the values of such optimized-out arguments, either deduce that from other variables whose values depend on the one you are interested in, or recompile without optimizations.

Most programs have a standard user entry point—a place where system libraries and startup code transition into user code. For C this is main6. When GDB finds the entry function in a backtrace it will terminate the backtrace, to avoid tracing into highly system-specific (and generally uninteresting) code.

If you need to examine the startup code, or limit the number of levels in a backtrace, you can change this behavior:

set backtrace past-main
set backtrace past-main on

Backtraces will continue past the user entry point.

set backtrace past-main off

Backtraces will stop when they encounter the user entry point. This is the default.

show backtrace past-main

Display the current user entry point backtrace policy.

set backtrace past-entry
set backtrace past-entry on

Backtraces will continue past the internal entry point of an application. This entry point is encoded by the linker when the application is built, and is likely before the user entry point main (or equivalent) is called.

set backtrace past-entry off

Backtraces will stop when they encounter the internal entry point of an application. This is the default.

show backtrace past-entry

Display the current internal entry point backtrace policy.

set backtrace limit n
set backtrace limit 0
set backtrace limit unlimited

Limit the backtrace to n levels. A value of unlimited or zero means unlimited levels.

show backtrace limit

Display the current limit on backtrace levels.

You can control how file names are displayed.

set filename-display
set filename-display relative

Display file names relative to the compilation directory. This is the default.

set filename-display basename

Display only basename of a filename.

set filename-display absolute

Display an absolute filename.

show filename-display

Show the current way to display filenames.


Footnotes

(6)

Note that embedded programs (the so-called “free-standing” environment) are not required to have a main function as the entry point. They could even have multiple entry points.


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