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296 lines
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HTML
296 lines
12 KiB
HTML
4 years ago
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<!-- Copyright (C) 1988-2019 Free Software Foundation, Inc.
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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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and with the Back-Cover Texts as in (a) below.
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(a) The FSF's Back-Cover Text is: "You are free to copy and modify
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this GNU Manual. Buying copies from GNU Press supports the FSF in
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developing GNU and promoting software freedom." -->
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<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>Backtrace (Debugging with GDB)</title>
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<meta name="description" content="Backtrace (Debugging with GDB)">
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<meta name="keywords" content="Backtrace (Debugging with GDB)">
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<meta name="Generator" content="makeinfo">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="Concept-Index.html#Concept-Index" rel="index" title="Concept Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Stack.html#Stack" rel="up" title="Stack">
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<link href="Selection.html#Selection" rel="next" title="Selection">
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<link href="Frames.html#Frames" rel="prev" title="Frames">
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</head>
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<body lang="en">
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<a name="Backtrace"></a>
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<div class="header">
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<p>
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Next: <a href="Selection.html#Selection" accesskey="n" rel="next">Selection</a>, Previous: <a href="Frames.html#Frames" accesskey="p" rel="prev">Frames</a>, Up: <a href="Stack.html#Stack" accesskey="u" rel="up">Stack</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="Backtraces"></a>
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<h3 class="section">8.2 Backtraces</h3>
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<a name="index-traceback"></a>
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<a name="index-call-stack-traces"></a>
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<p>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>
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<a name="backtrace_002dcommand"></a><a name="index-backtrace"></a>
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<a name="index-bt-_0028backtrace_0029"></a>
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<p>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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</p>
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<dl compact="compact">
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<dt><code>backtrace [<var>args</var>…]</code></dt>
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<dt><code>bt [<var>args</var>…]</code></dt>
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<dd><p>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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</p>
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<dl compact="compact">
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<dt><code><var>n</var></code></dt>
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<dt><code><var>n</var></code></dt>
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<dd><p>Print only the innermost <var>n</var> frames, where <var>n</var> is a positive
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number.
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</p>
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</dd>
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<dt><code>-<var>n</var></code></dt>
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<dt><code>-<var>n</var></code></dt>
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<dd><p>Print only the outermost <var>n</var> frames, where <var>n</var> is a positive
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number.
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</p>
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</dd>
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<dt><code>full</code></dt>
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<dd><p>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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</p>
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</dd>
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<dt><code>no-filters</code></dt>
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<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
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relevant when <small>GDB</small> has been configured with <code>Python</code>
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support.
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</p>
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</dd>
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<dt><code>hide</code></dt>
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<dd><p>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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</p></dd>
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</dl>
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</dd>
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</dl>
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<a name="index-where"></a>
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<a name="index-info-stack"></a>
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<p>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>
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<a name="index-multiple-threads_002c-backtrace"></a>
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<p>In a multi-threaded program, <small>GDB</small> 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>, <small>GDB</small> 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>
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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>
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<p>Here is an example of a backtrace. It was made with the command
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‘<samp>bt 3</samp>’, so it shows the innermost three frames.
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</p>
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<div class="smallexample">
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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></div>
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<p>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>
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<p>The value of parameter <code>data</code> in frame 1 has been replaced by
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<code>…</code>. By default, <small>GDB</small> 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>
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<a name="index-optimized-out_002c-in-backtrace"></a>
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<a name="index-function-call-arguments_002c-optimized-out"></a>
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<p>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. <small>GDB</small> 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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</p>
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<div class="smallexample">
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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></div>
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<p>The values of arguments that were not saved in their stack frames are
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shown as ‘<samp><optimized out></samp>’.
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</p>
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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>
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<a name="index-backtrace-beyond-main-function"></a>
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<a name="index-program-entry-point"></a>
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<a name="index-startup-code_002c-and-backtrace"></a>
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<p>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 name="DOCF8" href="#FOOT8"><sup>8</sup></a>.
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When <small>GDB</small> 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>
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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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</p>
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<dl compact="compact">
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<dt><code>set backtrace past-main</code></dt>
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<dt><code>set backtrace past-main on</code></dt>
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<dd><a name="index-set-backtrace"></a>
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<p>Backtraces will continue past the user entry point.
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</p>
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</dd>
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<dt><code>set backtrace past-main off</code></dt>
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<dd><p>Backtraces will stop when they encounter the user entry point. This is the
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default.
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</p>
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</dd>
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<dt><code>show backtrace past-main</code></dt>
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<dd><a name="index-show-backtrace"></a>
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<p>Display the current user entry point backtrace policy.
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</p>
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</dd>
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<dt><code>set backtrace past-entry</code></dt>
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<dt><code>set backtrace past-entry on</code></dt>
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<dd><p>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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</p>
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</dd>
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<dt><code>set backtrace past-entry off</code></dt>
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<dd><p>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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</p>
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</dd>
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<dt><code>show backtrace past-entry</code></dt>
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<dd><p>Display the current internal entry point backtrace policy.
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</p>
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</dd>
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<dt><code>set backtrace limit <var>n</var></code></dt>
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<dt><code>set backtrace limit 0</code></dt>
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<dt><code>set backtrace limit unlimited</code></dt>
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<dd><a name="index-backtrace-limit"></a>
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<p>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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</p>
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</dd>
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<dt><code>show backtrace limit</code></dt>
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<dd><p>Display the current limit on backtrace levels.
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</p></dd>
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</dl>
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<p>You can control how file names are displayed.
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</p>
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<dl compact="compact">
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<dt><code>set filename-display</code></dt>
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<dt><code>set filename-display relative</code></dt>
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<dd><a name="index-filename_002ddisplay"></a>
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<p>Display file names relative to the compilation directory. This is the default.
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</p>
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</dd>
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<dt><code>set filename-display basename</code></dt>
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<dd><p>Display only basename of a filename.
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</p>
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</dd>
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<dt><code>set filename-display absolute</code></dt>
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<dd><p>Display an absolute filename.
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</p>
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</dd>
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<dt><code>show filename-display</code></dt>
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<dd><p>Show the current way to display filenames.
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</p></dd>
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</dl>
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<div class="footnote">
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<hr>
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<h4 class="footnotes-heading">Footnotes</h4>
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<h3><a name="FOOT8" href="#DOCF8">(8)</a></h3>
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<p>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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</div>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Selection.html#Selection" accesskey="n" rel="next">Selection</a>, Previous: <a href="Frames.html#Frames" accesskey="p" rel="prev">Frames</a>, Up: <a href="Stack.html#Stack" accesskey="u" rel="up">Stack</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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</body>
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</html>
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