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.
276 lines
12 KiB
HTML
276 lines
12 KiB
HTML
4 years ago
|
<!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>Forks (Debugging with GDB)</title>
|
||
|
|
|
||
|
|
<meta name="description" content="Forks (Debugging with GDB)">
|
||
|
|
<meta name="keywords" content="Forks (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="Running.html#Running" rel="up" title="Running">
|
||
|
|
<link href="Checkpoint_002fRestart.html#Checkpoint_002fRestart" rel="next" title="Checkpoint/Restart">
|
||
|
|
<link href="Threads.html#Threads" rel="prev" title="Threads">
|
||
|
|
<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="Forks"></a>
|
||
|
|
<div class="header">
|
||
|
|
<p>
|
||
|
|
Next: <a href="Checkpoint_002fRestart.html#Checkpoint_002fRestart" accesskey="n" rel="next">Checkpoint/Restart</a>, Previous: <a href="Threads.html#Threads" accesskey="p" rel="prev">Threads</a>, Up: <a href="Running.html#Running" accesskey="u" rel="up">Running</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="Debugging-Forks"></a>
|
||
|
|
<h3 class="section">4.11 Debugging Forks</h3>
|
||
|
|
|
||
|
|
<a name="index-fork_002c-debugging-programs-which-call"></a>
|
||
|
|
<a name="index-multiple-processes"></a>
|
||
|
|
<a name="index-processes_002c-multiple"></a>
|
||
|
|
<p>On most systems, <small>GDB</small> has no special support for debugging
|
||
|
|
programs which create additional processes using the <code>fork</code>
|
||
|
|
function. When a program forks, <small>GDB</small> will continue to debug the
|
||
|
|
parent process and the child process will run unimpeded. If you have
|
||
|
|
set a breakpoint in any code which the child then executes, the child
|
||
|
|
will get a <code>SIGTRAP</code> signal which (unless it catches the signal)
|
||
|
|
will cause it to terminate.
|
||
|
|
</p>
|
||
|
|
<p>However, if you want to debug the child process there is a workaround
|
||
|
|
which isn’t too painful. Put a call to <code>sleep</code> in the code which
|
||
|
|
the child process executes after the fork. It may be useful to sleep
|
||
|
|
only if a certain environment variable is set, or a certain file exists,
|
||
|
|
so that the delay need not occur when you don’t want to run <small>GDB</small>
|
||
|
|
on the child. While the child is sleeping, use the <code>ps</code> program to
|
||
|
|
get its process ID. Then tell <small>GDB</small> (a new invocation of
|
||
|
|
<small>GDB</small> if you are also debugging the parent process) to attach to
|
||
|
|
the child process (see <a href="Attach.html#Attach">Attach</a>). From that point on you can debug
|
||
|
|
the child process just like any other process which you attached to.
|
||
|
|
</p>
|
||
|
|
<p>On some systems, <small>GDB</small> provides support for debugging programs
|
||
|
|
that create additional processes using the <code>fork</code> or <code>vfork</code>
|
||
|
|
functions. On <small>GNU</small>/Linux platforms, this feature is supported
|
||
|
|
with kernel version 2.5.46 and later.
|
||
|
|
</p>
|
||
|
|
<p>The fork debugging commands are supported in native mode and when
|
||
|
|
connected to <code>gdbserver</code> in either <code>target remote</code> mode or
|
||
|
|
<code>target extended-remote</code> mode.
|
||
|
|
</p>
|
||
|
|
<p>By default, when a program forks, <small>GDB</small> will continue to debug
|
||
|
|
the parent process and the child process will run unimpeded.
|
||
|
|
</p>
|
||
|
|
<p>If you want to follow the child process instead of the parent process,
|
||
|
|
use the command <code>set <span class="nolinebreak">follow-fork-mode</span></code><!-- /@w -->.
|
||
|
|
</p>
|
||
|
|
<dl compact="compact">
|
||
|
|
<dd><a name="index-set-follow_002dfork_002dmode"></a>
|
||
|
|
</dd>
|
||
|
|
<dt><code>set follow-fork-mode <var>mode</var></code></dt>
|
||
|
|
<dd><p>Set the debugger response to a program call of <code>fork</code> or
|
||
|
|
<code>vfork</code>. A call to <code>fork</code> or <code>vfork</code> creates a new
|
||
|
|
process. The <var>mode</var> argument can be:
|
||
|
|
</p>
|
||
|
|
<dl compact="compact">
|
||
|
|
<dt><code>parent</code></dt>
|
||
|
|
<dd><p>The original process is debugged after a fork. The child process runs
|
||
|
|
unimpeded. This is the default.
|
||
|
|
</p>
|
||
|
|
</dd>
|
||
|
|
<dt><code>child</code></dt>
|
||
|
|
<dd><p>The new process is debugged after a fork. The parent process runs
|
||
|
|
unimpeded.
|
||
|
|
</p>
|
||
|
|
</dd>
|
||
|
|
</dl>
|
||
|
|
|
||
|
|
<a name="index-show-follow_002dfork_002dmode"></a>
|
||
|
|
</dd>
|
||
|
|
<dt><code>show follow-fork-mode</code></dt>
|
||
|
|
<dd><p>Display the current debugger response to a <code>fork</code> or <code>vfork</code> call.
|
||
|
|
</p></dd>
|
||
|
|
</dl>
|
||
|
|
|
||
|
|
<a name="index-debugging-multiple-processes"></a>
|
||
|
|
<p>On Linux, if you want to debug both the parent and child processes, use the
|
||
|
|
command <code>set <span class="nolinebreak">detach-on-fork</span></code><!-- /@w -->.
|
||
|
|
</p>
|
||
|
|
<dl compact="compact">
|
||
|
|
<dd><a name="index-set-detach_002don_002dfork"></a>
|
||
|
|
</dd>
|
||
|
|
<dt><code>set detach-on-fork <var>mode</var></code></dt>
|
||
|
|
<dd><p>Tells gdb whether to detach one of the processes after a fork, or
|
||
|
|
retain debugger control over them both.
|
||
|
|
</p>
|
||
|
|
<dl compact="compact">
|
||
|
|
<dt><code>on</code></dt>
|
||
|
|
<dd><p>The child process (or parent process, depending on the value of
|
||
|
|
<code>follow-fork-mode</code>) will be detached and allowed to run
|
||
|
|
independently. This is the default.
|
||
|
|
</p>
|
||
|
|
</dd>
|
||
|
|
<dt><code>off</code></dt>
|
||
|
|
<dd><p>Both processes will be held under the control of <small>GDB</small>.
|
||
|
|
One process (child or parent, depending on the value of
|
||
|
|
<code>follow-fork-mode</code>) is debugged as usual, while the other
|
||
|
|
is held suspended.
|
||
|
|
</p>
|
||
|
|
</dd>
|
||
|
|
</dl>
|
||
|
|
|
||
|
|
<a name="index-show-detach_002don_002dfork"></a>
|
||
|
|
</dd>
|
||
|
|
<dt><code>show detach-on-fork</code></dt>
|
||
|
|
<dd><p>Show whether detach-on-fork mode is on/off.
|
||
|
|
</p></dd>
|
||
|
|
</dl>
|
||
|
|
|
||
|
|
<p>If you choose to set ‘<samp>detach-on-fork</samp>’ mode off, then <small>GDB</small>
|
||
|
|
will retain control of all forked processes (including nested forks).
|
||
|
|
You can list the forked processes under the control of <small>GDB</small> by
|
||
|
|
using the <code>info inferiors</code><!-- /@w --> command, and switch from one fork
|
||
|
|
to another by using the <code>inferior</code> command (see <a href="Inferiors-and-Programs.html#Inferiors-and-Programs">Debugging Multiple Inferiors and Programs</a>).
|
||
|
|
</p>
|
||
|
|
<p>To quit debugging one of the forked processes, you can either detach
|
||
|
|
from it by using the <code>detach inferiors</code><!-- /@w --> command (allowing it
|
||
|
|
to run independently), or kill it using the <code>kill inferiors</code><!-- /@w -->
|
||
|
|
command. See <a href="Inferiors-and-Programs.html#Inferiors-and-Programs">Debugging Multiple Inferiors
|
||
|
|
and Programs</a>.
|
||
|
|
</p>
|
||
|
|
<p>If you ask to debug a child process and a <code>vfork</code> is followed by an
|
||
|
|
<code>exec</code>, <small>GDB</small> executes the new target up to the first
|
||
|
|
breakpoint in the new target. If you have a breakpoint set on
|
||
|
|
<code>main</code> in your original program, the breakpoint will also be set on
|
||
|
|
the child process’s <code>main</code>.
|
||
|
|
</p>
|
||
|
|
<p>On some systems, when a child process is spawned by <code>vfork</code>, you
|
||
|
|
cannot debug the child or parent until an <code>exec</code> call completes.
|
||
|
|
</p>
|
||
|
|
<p>If you issue a <code>run</code> command to <small>GDB</small> after an <code>exec</code>
|
||
|
|
call executes, the new target restarts. To restart the parent
|
||
|
|
process, use the <code>file</code> command with the parent executable name
|
||
|
|
as its argument. By default, after an <code>exec</code> call executes,
|
||
|
|
<small>GDB</small> discards the symbols of the previous executable image.
|
||
|
|
You can change this behaviour with the <code>set <span class="nolinebreak">follow-exec-mode</span></code><!-- /@w -->
|
||
|
|
command.
|
||
|
|
</p>
|
||
|
|
<dl compact="compact">
|
||
|
|
<dd><a name="index-set-follow_002dexec_002dmode"></a>
|
||
|
|
</dd>
|
||
|
|
<dt><code>set follow-exec-mode <var>mode</var></code></dt>
|
||
|
|
<dd>
|
||
|
|
<p>Set debugger response to a program call of <code>exec</code>. An
|
||
|
|
<code>exec</code> call replaces the program image of a process.
|
||
|
|
</p>
|
||
|
|
<p><code>follow-exec-mode</code> can be:
|
||
|
|
</p>
|
||
|
|
<dl compact="compact">
|
||
|
|
<dt><code>new</code></dt>
|
||
|
|
<dd><p><small>GDB</small> creates a new inferior and rebinds the process to this
|
||
|
|
new inferior. The program the process was running before the
|
||
|
|
<code>exec</code> call can be restarted afterwards by restarting the
|
||
|
|
original inferior.
|
||
|
|
</p>
|
||
|
|
<p>For example:
|
||
|
|
</p>
|
||
|
|
<div class="smallexample">
|
||
|
|
<pre class="smallexample">(gdb) info inferiors
|
||
|
|
(gdb) info inferior
|
||
|
|
Id Description Executable
|
||
|
|
* 1 <null> prog1
|
||
|
|
(gdb) run
|
||
|
|
process 12020 is executing new program: prog2
|
||
|
|
Program exited normally.
|
||
|
|
(gdb) info inferiors
|
||
|
|
Id Description Executable
|
||
|
|
1 <null> prog1
|
||
|
|
* 2 <null> prog2
|
||
|
|
</pre></div>
|
||
|
|
|
||
|
|
</dd>
|
||
|
|
<dt><code>same</code></dt>
|
||
|
|
<dd><p><small>GDB</small> keeps the process bound to the same inferior. The new
|
||
|
|
executable image replaces the previous executable loaded in the
|
||
|
|
inferior. Restarting the inferior after the <code>exec</code> call, with
|
||
|
|
e.g., the <code>run</code> command, restarts the executable the process was
|
||
|
|
running after the <code>exec</code> call. This is the default mode.
|
||
|
|
</p>
|
||
|
|
<p>For example:
|
||
|
|
</p>
|
||
|
|
<div class="smallexample">
|
||
|
|
<pre class="smallexample">(gdb) info inferiors
|
||
|
|
Id Description Executable
|
||
|
|
* 1 <null> prog1
|
||
|
|
(gdb) run
|
||
|
|
process 12020 is executing new program: prog2
|
||
|
|
Program exited normally.
|
||
|
|
(gdb) info inferiors
|
||
|
|
Id Description Executable
|
||
|
|
* 1 <null> prog2
|
||
|
|
</pre></div>
|
||
|
|
|
||
|
|
</dd>
|
||
|
|
</dl>
|
||
|
|
</dd>
|
||
|
|
</dl>
|
||
|
|
|
||
|
|
<p><code>follow-exec-mode</code> is supported in native mode and
|
||
|
|
<code>target extended-remote</code> mode.
|
||
|
|
</p>
|
||
|
|
<p>You can use the <code>catch</code> command to make <small>GDB</small> stop whenever
|
||
|
|
a <code>fork</code>, <code>vfork</code>, or <code>exec</code> call is made. See <a href="Set-Catchpoints.html#Set-Catchpoints">Setting Catchpoints</a>.
|
||
|
|
</p>
|
||
|
|
<hr>
|
||
|
|
<div class="header">
|
||
|
|
<p>
|
||
|
|
Next: <a href="Checkpoint_002fRestart.html#Checkpoint_002fRestart" accesskey="n" rel="next">Checkpoint/Restart</a>, Previous: <a href="Threads.html#Threads" accesskey="p" rel="prev">Threads</a>, Up: <a href="Running.html#Running" accesskey="u" rel="up">Running</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>
|