6.2 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.

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

In a multi-threaded program, No value for GDBN 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 section thread apply). For example, if you type thread apply all backtrace, No value for GDBN 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) 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.

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. No value for GDBN 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=<value optimized out>) at macro.c:242
#2  0x6840 in expand_token (obs=0x0, t=<value 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 ‘<value 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 main(3). When No value for GDBN 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

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

show backtrace limit

Display the current limit on backtrace levels.