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203 lines
11 KiB
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<title>Backtrace - Debugging with GDB</title>
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation; with the
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Invariant Sections being ``Free Software'' and ``Free Software Needs
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Free Documentation'', with the Front-Cover Texts being ``A GNU Manual,''
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<body>
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<div class="node">
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<a name="Backtrace"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Selection.html#Selection">Selection</a>,
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Previous: <a rel="previous" accesskey="p" href="Frames.html#Frames">Frames</a>,
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Up: <a rel="up" accesskey="u" href="Stack.html#Stack">Stack</a>
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<hr>
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</div>
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<h3 class="section">8.2 Backtraces</h3>
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<p><a name="index-traceback-513"></a><a name="index-call-stack-traces-514"></a>A backtrace is a summary of how your program got where it is. It shows one
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line per frame, for many frames, starting with the currently executing
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frame (frame zero), followed by its caller (frame one), and on up the
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stack.
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<p><a name="backtrace_002dcommand"></a><a name="index-backtrace-515"></a><a name="index-bt-_0040r_007b_0028_0040code_007bbacktrace_007d_0029_007d-516"></a>To print a backtrace of the entire stack, use the <code>backtrace</code>
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command, or its alias <code>bt</code>. This command will print one line per
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frame for frames in the stack. By default, all stack frames are
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printed. You can stop the backtrace at any time by typing the system
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interrupt character, normally <kbd>Ctrl-c</kbd>.
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<dl>
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<dt><code>backtrace [</code><var>args</var><code>...]</code><dt><code>bt [</code><var>args</var><code>...]</code><dd>Print the backtrace of the entire stack. The optional <var>args</var> can
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be one of the following:
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<dl>
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<dt><var>n</var><dt><var>n</var><dd>Print only the innermost <var>n</var> frames, where <var>n</var> is a positive
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number.
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<br><dt><code>-</code><var>n</var><dt><code>-</code><var>n</var><dd>Print only the outermost <var>n</var> frames, where <var>n</var> is a positive
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number.
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<br><dt><code>full</code><dd>Print the values of the local variables also. This can be combined
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with a number to limit the number of frames shown.
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<br><dt><code>no-filters</code><dd>Do not run Python frame filters on this backtrace. See <a href="Frame-Filter-API.html#Frame-Filter-API">Frame Filter API</a>, for more information. Additionally use <a href="disable-frame_002dfilter-all.html#disable-frame_002dfilter-all">disable frame-filter all</a> to turn off all frame filters. This is only
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relevant when <span class="sc">gdb</span> has been configured with <code>Python</code>
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support.
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<br><dt><code>hide</code><dd>A Python frame filter might decide to “elide” some frames. Normally
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such elided frames are still printed, but they are indented relative
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to the filtered frames that cause them to be elided. The <code>hide</code>
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option causes elided frames to not be printed at all.
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</dl>
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</dl>
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<p><a name="index-where-517"></a><a name="index-info-stack-518"></a>The names <code>where</code> and <code>info stack</code> (abbreviated <code>info s</code>)
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are additional aliases for <code>backtrace</code>.
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<p><a name="index-multiple-threads_002c-backtrace-519"></a>In a multi-threaded program, <span class="sc">gdb</span> by default shows the
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backtrace only for the current thread. To display the backtrace for
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several or all of the threads, use the command <code>thread apply</code>
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(see <a href="Threads.html#Threads">thread apply</a>). For example, if you type <kbd>thread
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apply all backtrace</kbd>, <span class="sc">gdb</span> will display the backtrace for all
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the threads; this is handy when you debug a core dump of a
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multi-threaded program.
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<p>Each line in the backtrace shows the frame number and the function name.
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The program counter value is also shown—unless you use <code>set
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print address off</code>. The backtrace also shows the source file name and
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line number, as well as the arguments to the function. The program
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counter value is omitted if it is at the beginning of the code for that
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line number.
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<p>Here is an example of a backtrace. It was made with the command
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‘<samp><span class="samp">bt 3</span></samp>’, so it shows the innermost three frames.
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<pre class="smallexample"> #0 m4_traceon (obs=0x24eb0, argc=1, argv=0x2b8c8)
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at builtin.c:993
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#1 0x6e38 in expand_macro (sym=0x2b600, data=...) at macro.c:242
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#2 0x6840 in expand_token (obs=0x0, t=177664, td=0xf7fffb08)
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at macro.c:71
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(More stack frames follow...)
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</pre>
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<p class="noindent">The display for frame zero does not begin with a program counter
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value, indicating that your program has stopped at the beginning of the
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code for line <code>993</code> of <code>builtin.c</code>.
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<p class="noindent">The value of parameter <code>data</code> in frame 1 has been replaced by
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<code>...</code>. By default, <span class="sc">gdb</span> prints the value of a parameter
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only if it is a scalar (integer, pointer, enumeration, etc). See command
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<kbd>set print frame-arguments</kbd> in <a href="Print-Settings.html#Print-Settings">Print Settings</a> for more details
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on how to configure the way function parameter values are printed.
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<p><a name="index-optimized-out_002c-in-backtrace-520"></a><a name="index-function-call-arguments_002c-optimized-out-521"></a>If your program was compiled with optimizations, some compilers will
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optimize away arguments passed to functions if those arguments are
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never used after the call. Such optimizations generate code that
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passes arguments through registers, but doesn't store those arguments
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in the stack frame. <span class="sc">gdb</span> has no way of displaying such
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arguments in stack frames other than the innermost one. Here's what
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such a backtrace might look like:
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<pre class="smallexample"> #0 m4_traceon (obs=0x24eb0, argc=1, argv=0x2b8c8)
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at builtin.c:993
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#1 0x6e38 in expand_macro (sym=<optimized out>) at macro.c:242
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#2 0x6840 in expand_token (obs=0x0, t=<optimized out>, td=0xf7fffb08)
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at macro.c:71
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(More stack frames follow...)
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</pre>
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<p class="noindent">The values of arguments that were not saved in their stack frames are
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shown as ‘<samp><span class="samp"><optimized out></span></samp>’.
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<p>If you need to display the values of such optimized-out arguments,
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either deduce that from other variables whose values depend on the one
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you are interested in, or recompile without optimizations.
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<p><a name="index-backtrace-beyond-_0040code_007bmain_007d-function-522"></a><a name="index-program-entry-point-523"></a><a name="index-startup-code_002c-and-backtrace-524"></a>Most programs have a standard user entry point—a place where system
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libraries and startup code transition into user code. For C this is
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<code>main</code><a rel="footnote" href="#fn-1" name="fnd-1"><sup>1</sup></a>.
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When <span class="sc">gdb</span> finds the entry function in a backtrace
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it will terminate the backtrace, to avoid tracing into highly
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system-specific (and generally uninteresting) code.
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<p>If you need to examine the startup code, or limit the number of levels
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in a backtrace, you can change this behavior:
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<dl>
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<dt><code>set backtrace past-main</code><dt><code>set backtrace past-main on</code><dd><a name="index-set-backtrace-525"></a>Backtraces will continue past the user entry point.
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<br><dt><code>set backtrace past-main off</code><dd>Backtraces will stop when they encounter the user entry point. This is the
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default.
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<br><dt><code>show backtrace past-main</code><dd><a name="index-show-backtrace-526"></a>Display the current user entry point backtrace policy.
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<br><dt><code>set backtrace past-entry</code><dt><code>set backtrace past-entry on</code><dd>Backtraces will continue past the internal entry point of an application.
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This entry point is encoded by the linker when the application is built,
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and is likely before the user entry point <code>main</code> (or equivalent) is called.
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<br><dt><code>set backtrace past-entry off</code><dd>Backtraces will stop when they encounter the internal entry point of an
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application. This is the default.
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<br><dt><code>show backtrace past-entry</code><dd>Display the current internal entry point backtrace policy.
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<br><dt><code>set backtrace limit </code><var>n</var><dt><code>set backtrace limit 0</code><dt><code>set backtrace limit unlimited</code><dd><a name="index-backtrace-limit-527"></a>Limit the backtrace to <var>n</var> levels. A value of <code>unlimited</code>
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or zero means unlimited levels.
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<br><dt><code>show backtrace limit</code><dd>Display the current limit on backtrace levels.
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</dl>
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<p>You can control how file names are displayed.
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<dl>
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<dt><code>set filename-display</code><dt><code>set filename-display relative</code><dd><a name="index-filename_002ddisplay-528"></a>Display file names relative to the compilation directory. This is the default.
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<br><dt><code>set filename-display basename</code><dd>Display only basename of a filename.
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<br><dt><code>set filename-display absolute</code><dd>Display an absolute filename.
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<br><dt><code>show filename-display</code><dd>Show the current way to display filenames.
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</dl>
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<div class="footnote">
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<hr>
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<h4>Footnotes</h4><p class="footnote"><small>[<a name="fn-1" href="#fnd-1">1</a>]</small>
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Note that embedded programs (the so-called “free-standing”
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environment) are not required to have a <code>main</code> function as the
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entry point. They could even have multiple entry points.</p>
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<hr></div>
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</body></html>
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