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| <a name="Backtraces"></a> |
| <h3 class="section">8.2 Backtraces</h3> |
| |
| <a name="index-traceback"></a> |
| <a name="index-call-stack-traces"></a> |
| <p>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. |
| </p> |
| <a name="backtrace_002dcommand"></a><dl compact="compact"> |
| <dd><a name="index-backtrace"></a> |
| <a name="index-bt-_0028backtrace_0029"></a> |
| </dd> |
| <dt><code>backtrace</code></dt> |
| <dt><code>bt</code></dt> |
| <dd><p>Print a backtrace of the entire stack: one line per frame for all |
| frames in the stack. |
| </p> |
| <p>You can stop the backtrace at any time by typing the system interrupt |
| character, normally <kbd>Ctrl-c</kbd>. |
| </p> |
| </dd> |
| <dt><code>backtrace <var>n</var></code></dt> |
| <dt><code>bt <var>n</var></code></dt> |
| <dd><p>Similar, but print only the innermost <var>n</var> frames. |
| </p> |
| </dd> |
| <dt><code>backtrace -<var>n</var></code></dt> |
| <dt><code>bt -<var>n</var></code></dt> |
| <dd><p>Similar, but print only the outermost <var>n</var> frames. |
| </p> |
| </dd> |
| <dt><code>backtrace full</code></dt> |
| <dt><code>bt full</code></dt> |
| <dt><code>bt full <var>n</var></code></dt> |
| <dt><code>bt full -<var>n</var></code></dt> |
| <dd><p>Print the values of the local variables also. As described above, |
| <var>n</var> specifies the number of frames to print. |
| </p> |
| </dd> |
| <dt><code>backtrace no-filters</code></dt> |
| <dt><code>bt no-filters</code></dt> |
| <dt><code>bt no-filters <var>n</var></code></dt> |
| <dt><code>bt no-filters -<var>n</var></code></dt> |
| <dt><code>bt no-filters full</code></dt> |
| <dt><code>bt no-filters full <var>n</var></code></dt> |
| <dt><code>bt no-filters full -<var>n</var></code></dt> |
| <dd><p>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="Frame-Filter-Management.html#disable-frame_002dfilter-all">disable frame-filter all</a> to turn off all frame filters. This is only |
| relevant when <small>GDB</small> has been configured with <code>Python</code> |
| support. |
| </p></dd> |
| </dl> |
| |
| <a name="index-where"></a> |
| <a name="index-info-stack"></a> |
| <p>The names <code>where</code> and <code>info stack</code> (abbreviated <code>info s</code>) |
| are additional aliases for <code>backtrace</code>. |
| </p> |
| <a name="index-multiple-threads_002c-backtrace"></a> |
| <p>In a multi-threaded program, <small>GDB</small> by default shows the |
| backtrace only for the current thread. To display the backtrace for |
| several or all of the threads, use the command <code>thread apply</code> |
| (see <a href="Threads.html#Threads">thread apply</a>). For example, if you type <kbd>thread |
| apply all backtrace</kbd>, <small>GDB</small> will display the backtrace for all |
| the threads; this is handy when you debug a core dump of a |
| multi-threaded program. |
| </p> |
| <p>Each line in the backtrace shows the frame number and the function name. |
| The program counter value is also shown—unless you use <code>set |
| print address off</code>. 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. |
| </p> |
| <p>Here is an example of a backtrace. It was made with the command |
| ‘<samp>bt 3</samp>’, so it shows the innermost three frames. |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">#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...) |
| </pre></div> |
| |
| <p>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 <code>993</code> of <code>builtin.c</code>. |
| </p> |
| <p>The value of parameter <code>data</code> in frame 1 has been replaced by |
| <code>…</code>. By default, <small>GDB</small> prints the value of a parameter |
| only if it is a scalar (integer, pointer, enumeration, etc). See command |
| <kbd>set print frame-arguments</kbd> in <a href="Print-Settings.html#Print-Settings">Print Settings</a> for more details |
| on how to configure the way function parameter values are printed. |
| </p> |
| <a name="index-optimized-out_002c-in-backtrace"></a> |
| <a name="index-function-call-arguments_002c-optimized-out"></a> |
| <p>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. <small>GDB</small> has no way of displaying such |
| arguments in stack frames other than the innermost one. Here’s what |
| such a backtrace might look like: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">#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...) |
| </pre></div> |
| |
| <p>The values of arguments that were not saved in their stack frames are |
| shown as ‘<samp><optimized out></samp>’. |
| </p> |
| <p>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. |
| </p> |
| <a name="index-backtrace-beyond-main-function"></a> |
| <a name="index-program-entry-point"></a> |
| <a name="index-startup-code_002c-and-backtrace"></a> |
| <p>Most programs have a standard user entry point—a place where system |
| libraries and startup code transition into user code. For C this is |
| <code>main</code><a name="DOCF8" href="#FOOT8"><sup>8</sup></a>. |
| When <small>GDB</small> finds the entry function in a backtrace |
| it will terminate the backtrace, to avoid tracing into highly |
| system-specific (and generally uninteresting) code. |
| </p> |
| <p>If you need to examine the startup code, or limit the number of levels |
| in a backtrace, you can change this behavior: |
| </p> |
| <dl compact="compact"> |
| <dt><code>set backtrace past-main</code></dt> |
| <dt><code>set backtrace past-main on</code></dt> |
| <dd><a name="index-set-backtrace"></a> |
| <p>Backtraces will continue past the user entry point. |
| </p> |
| </dd> |
| <dt><code>set backtrace past-main off</code></dt> |
| <dd><p>Backtraces will stop when they encounter the user entry point. This is the |
| default. |
| </p> |
| </dd> |
| <dt><code>show backtrace past-main</code></dt> |
| <dd><a name="index-show-backtrace"></a> |
| <p>Display the current user entry point backtrace policy. |
| </p> |
| </dd> |
| <dt><code>set backtrace past-entry</code></dt> |
| <dt><code>set backtrace past-entry on</code></dt> |
| <dd><p>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 <code>main</code> (or equivalent) is called. |
| </p> |
| </dd> |
| <dt><code>set backtrace past-entry off</code></dt> |
| <dd><p>Backtraces will stop when they encounter the internal entry point of an |
| application. This is the default. |
| </p> |
| </dd> |
| <dt><code>show backtrace past-entry</code></dt> |
| <dd><p>Display the current internal entry point backtrace policy. |
| </p> |
| </dd> |
| <dt><code>set backtrace limit <var>n</var></code></dt> |
| <dt><code>set backtrace limit 0</code></dt> |
| <dt><code>set backtrace limit unlimited</code></dt> |
| <dd><a name="index-backtrace-limit"></a> |
| <p>Limit the backtrace to <var>n</var> levels. A value of <code>unlimited</code> |
| or zero means unlimited levels. |
| </p> |
| </dd> |
| <dt><code>show backtrace limit</code></dt> |
| <dd><p>Display the current limit on backtrace levels. |
| </p></dd> |
| </dl> |
| |
| <p>You can control how file names are displayed. |
| </p> |
| <dl compact="compact"> |
| <dt><code>set filename-display</code></dt> |
| <dt><code>set filename-display relative</code></dt> |
| <dd><a name="index-filename_002ddisplay"></a> |
| <p>Display file names relative to the compilation directory. This is the default. |
| </p> |
| </dd> |
| <dt><code>set filename-display basename</code></dt> |
| <dd><p>Display only basename of a filename. |
| </p> |
| </dd> |
| <dt><code>set filename-display absolute</code></dt> |
| <dd><p>Display an absolute filename. |
| </p> |
| </dd> |
| <dt><code>show filename-display</code></dt> |
| <dd><p>Show the current way to display filenames. |
| </p></dd> |
| </dl> |
| |
| <div class="footnote"> |
| <hr> |
| <h4 class="footnotes-heading">Footnotes</h4> |
| |
| <h3><a name="FOOT8" href="#DOCF8">(8)</a></h3> |
| <p>Note that embedded programs (the so-called “free-standing” |
| environment) are not required to have a <code>main</code> function as the |
| entry point. They could even have multiple entry points.</p> |
| </div> |
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