You cannot select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
296 lines
12 KiB
HTML
296 lines
12 KiB
HTML
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
|
|
<html>
|
|
<!-- Copyright (C) 1988-2019 Free Software Foundation, Inc.
|
|
|
|
Permission is granted to copy, distribute and/or modify this document
|
|
under the terms of the GNU Free Documentation License, Version 1.3 or
|
|
any later version published by the Free Software Foundation; with the
|
|
Invariant Sections being "Free Software" and "Free Software Needs
|
|
Free Documentation", with the Front-Cover Texts being "A GNU Manual,"
|
|
and with the Back-Cover Texts as in (a) below.
|
|
|
|
(a) The FSF's Back-Cover Text is: "You are free to copy and modify
|
|
this GNU Manual. Buying copies from GNU Press supports the FSF in
|
|
developing GNU and promoting software freedom." -->
|
|
<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
|
|
<head>
|
|
<title>Backtrace (Debugging with GDB)</title>
|
|
|
|
<meta name="description" content="Backtrace (Debugging with GDB)">
|
|
<meta name="keywords" content="Backtrace (Debugging with GDB)">
|
|
<meta name="resource-type" content="document">
|
|
<meta name="distribution" content="global">
|
|
<meta name="Generator" content="makeinfo">
|
|
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
|
|
<link href="index.html#Top" rel="start" title="Top">
|
|
<link href="Concept-Index.html#Concept-Index" rel="index" title="Concept Index">
|
|
<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
|
|
<link href="Stack.html#Stack" rel="up" title="Stack">
|
|
<link href="Selection.html#Selection" rel="next" title="Selection">
|
|
<link href="Frames.html#Frames" rel="prev" title="Frames">
|
|
<style type="text/css">
|
|
<!--
|
|
a.summary-letter {text-decoration: none}
|
|
blockquote.indentedblock {margin-right: 0em}
|
|
blockquote.smallindentedblock {margin-right: 0em; font-size: smaller}
|
|
blockquote.smallquotation {font-size: smaller}
|
|
div.display {margin-left: 3.2em}
|
|
div.example {margin-left: 3.2em}
|
|
div.lisp {margin-left: 3.2em}
|
|
div.smalldisplay {margin-left: 3.2em}
|
|
div.smallexample {margin-left: 3.2em}
|
|
div.smalllisp {margin-left: 3.2em}
|
|
kbd {font-style: oblique}
|
|
pre.display {font-family: inherit}
|
|
pre.format {font-family: inherit}
|
|
pre.menu-comment {font-family: serif}
|
|
pre.menu-preformatted {font-family: serif}
|
|
pre.smalldisplay {font-family: inherit; font-size: smaller}
|
|
pre.smallexample {font-size: smaller}
|
|
pre.smallformat {font-family: inherit; font-size: smaller}
|
|
pre.smalllisp {font-size: smaller}
|
|
span.nolinebreak {white-space: nowrap}
|
|
span.roman {font-family: initial; font-weight: normal}
|
|
span.sansserif {font-family: sans-serif; font-weight: normal}
|
|
ul.no-bullet {list-style: none}
|
|
-->
|
|
</style>
|
|
|
|
|
|
</head>
|
|
|
|
<body lang="en">
|
|
<a name="Backtrace"></a>
|
|
<div class="header">
|
|
<p>
|
|
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>
|
|
</div>
|
|
<hr>
|
|
<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><a name="index-backtrace"></a>
|
|
<a name="index-bt-_0028backtrace_0029"></a>
|
|
<p>To print a backtrace of the entire stack, use the <code>backtrace</code>
|
|
command, or its alias <code>bt</code>. 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 <kbd>Ctrl-c</kbd>.
|
|
</p>
|
|
<dl compact="compact">
|
|
<dt><code>backtrace [<var>args</var>…]</code></dt>
|
|
<dt><code>bt [<var>args</var>…]</code></dt>
|
|
<dd><p>Print the backtrace of the entire stack. The optional <var>args</var> can
|
|
be one of the following:
|
|
</p>
|
|
<dl compact="compact">
|
|
<dt><code><var>n</var></code></dt>
|
|
<dt><code><var>n</var></code></dt>
|
|
<dd><p>Print only the innermost <var>n</var> frames, where <var>n</var> is a positive
|
|
number.
|
|
</p>
|
|
</dd>
|
|
<dt><code>-<var>n</var></code></dt>
|
|
<dt><code>-<var>n</var></code></dt>
|
|
<dd><p>Print only the outermost <var>n</var> frames, where <var>n</var> is a positive
|
|
number.
|
|
</p>
|
|
</dd>
|
|
<dt><code>full</code></dt>
|
|
<dd><p>Print the values of the local variables also. This can be combined
|
|
with a number to limit the number of frames shown.
|
|
</p>
|
|
</dd>
|
|
<dt><code>no-filters</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>
|
|
<dt><code>hide</code></dt>
|
|
<dd><p>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 <code>hide</code>
|
|
option causes elided frames to not be printed at all.
|
|
</p></dd>
|
|
</dl>
|
|
</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>
|
|
<hr>
|
|
<div class="header">
|
|
<p>
|
|
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>
|
|
</div>
|
|
|
|
|
|
|
|
</body>
|
|
</html>
|