Debugging with GDB: Backtrace

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.

To print a backtrace of the entire stack, use the backtrace command, or its alias bt. This command will print one line per frame for frames in the stack. By default, all stack frames are printed. You can stop the backtrace at any time by typing the system interrupt character, normally Ctrl-c.

backtrace [option]… [qualifier]… [count]

bt [option]… [qualifier]… [count]

Print the backtrace of the entire stack.

The optional count can be one of the following:

n
n: Print only the innermost n frames, where n is a positive number.
-n
-n: Print only the outermost n frames, where n is a positive number.

Options:

-full: Print the values of the local variables also. This can be combined with the optional count to limit the number of frames shown.
-no-filters: 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.
-hide: A Python frame filter might decide to “elide” some frames. Normally such elided frames are still printed, but they are indented relative to the filtered frames that cause them to be elided. The -hide option causes elided frames to not be printed at all.

The backtrace command also supports a number of options that allow overriding relevant global print settings as set by set backtrace and set print subcommands:

-past-main [on|off]: Set whether backtraces should continue past main. Related setting: set backtrace past-main.
-past-entry [on|off]: Set whether backtraces should continue past the entry point of a program. Related setting: set backtrace past-entry.
-entry-values no|only|preferred|if-needed|both|compact|default: Set printing of function arguments at function entry. Related setting: set print entry-values.
-frame-arguments all|scalars|none: Set printing of non-scalar frame arguments. Related setting: set print frame-arguments.
-raw-frame-arguments [on|off]: Set whether to print frame arguments in raw form. Related setting: set print raw-frame-arguments.
-frame-info auto|source-line|location|source-and-location|location-and-address|short-location: Set printing of frame information. Related setting: set print frame-info.

The optional qualifier is maintained for backward compatibility. It can be one of the following:

full: Equivalent to the -full option.
no-filters: Equivalent to the -no-filters option.
hide: Equivalent to the -hide option.

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. The command set print frame-info (see Print Settings) controls what frame information is 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 main⁹. 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

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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.