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<a name="Threads"></a>
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<div class="header">
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<p>
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Next: <a href="Forks.html#Forks" accesskey="n" rel="next">Forks</a>, Previous: <a href="Inferiors-Connections-and-Programs.html#Inferiors-Connections-and-Programs" accesskey="p" rel="previous">Inferiors Connections and Programs</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>
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</div>
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<hr>
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<a name="Debugging-Programs-with-Multiple-Threads"></a>
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<h3 class="section">4.10 Debugging Programs with Multiple Threads</h3>
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<a name="index-threads-of-execution"></a>
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<a name="index-multiple-threads"></a>
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<a name="index-switching-threads"></a>
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<p>In some operating systems, such as GNU/Linux and Solaris, a single program
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may have more than one <em>thread</em> of execution. The precise semantics
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of threads differ from one operating system to another, but in general
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the threads of a single program are akin to multiple processes—except
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that they share one address space (that is, they can all examine and
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modify the same variables). On the other hand, each thread has its own
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registers and execution stack, and perhaps private memory.
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</p>
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<p><small>GDB</small> provides these facilities for debugging multi-thread
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programs:
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</p>
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<ul>
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<li> automatic notification of new threads
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</li><li> ‘<samp>thread <var>thread-id</var></samp>’, a command to switch among threads
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</li><li> ‘<samp>info threads</samp>’, a command to inquire about existing threads
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</li><li> ‘<samp>thread apply [<var>thread-id-list</var> | all] <var>args</var></samp>’,
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a command to apply a command to a list of threads
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</li><li> thread-specific breakpoints
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</li><li> ‘<samp>set print thread-events</samp>’, which controls printing of
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messages on thread start and exit.
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</li><li> ‘<samp>set libthread-db-search-path <var>path</var></samp>’, which lets
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the user specify which <code>libthread_db</code> to use if the default choice
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isn’t compatible with the program.
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</li></ul>
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<a name="index-focus-of-debugging"></a>
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<a name="index-current-thread"></a>
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<p>The <small>GDB</small> thread debugging facility allows you to observe all
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threads while your program runs—but whenever <small>GDB</small> takes
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control, one thread in particular is always the focus of debugging.
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This thread is called the <em>current thread</em>. Debugging commands show
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program information from the perspective of the current thread.
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</p>
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<a name="index-New-systag-message"></a>
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<a name="index-thread-identifier-_0028system_0029"></a>
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<p>Whenever <small>GDB</small> detects a new thread in your program, it displays
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the target system’s identification for the thread with a message in the
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form ‘<samp>[New <var>systag</var>]</samp>’, where <var>systag</var> is a thread identifier
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whose form varies depending on the particular system. For example, on
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<small>GNU</small>/Linux, you might see
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</p>
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<div class="smallexample">
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<pre class="smallexample">[New Thread 0x41e02940 (LWP 25582)]
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</pre></div>
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<p>when <small>GDB</small> notices a new thread. In contrast, on other systems,
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the <var>systag</var> is simply something like ‘<samp>process 368</samp>’, with no
|
further qualifier.
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</p>
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<a name="thread-numbers"></a><a name="index-thread-number_002c-per-inferior"></a>
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<a name="index-thread-identifier-_0028GDB_0029"></a>
|
<p>For debugging purposes, <small>GDB</small> associates its own thread number
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—always a single integer—with each thread of an inferior. This
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number is unique between all threads of an inferior, but not unique
|
between threads of different inferiors.
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</p>
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<a name="index-qualified-thread-ID"></a>
|
<p>You can refer to a given thread in an inferior using the qualified
|
<var>inferior-num</var>.<var>thread-num</var> syntax, also known as
|
<em>qualified thread ID</em>, with <var>inferior-num</var> being the inferior
|
number and <var>thread-num</var> being the thread number of the given
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inferior. For example, thread <code>2.3</code> refers to thread number 3 of
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inferior 2. If you omit <var>inferior-num</var> (e.g., <code>thread 3</code>),
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then <small>GDB</small> infers you’re referring to a thread of the current
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inferior.
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</p>
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<p>Until you create a second inferior, <small>GDB</small> does not show the
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<var>inferior-num</var> part of thread IDs, even though you can always use
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the full <var>inferior-num</var>.<var>thread-num</var> form to refer to threads
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of inferior 1, the initial inferior.
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</p>
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<a name="thread-ID-lists"></a><a name="index-thread-ID-lists"></a>
|
<p>Some commands accept a space-separated <em>thread ID list</em> as
|
argument. A list element can be:
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</p>
|
<ol>
|
<li> A thread ID as shown in the first field of the ‘<samp>info threads</samp>’
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display, with or without an inferior qualifier. E.g., ‘<samp>2.1</samp>’ or
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‘<samp>1</samp>’.
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|
</li><li> A range of thread numbers, again with or without an inferior
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qualifier, as in <var>inf</var>.<var>thr1</var>-<var>thr2</var> or
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<var>thr1</var>-<var>thr2</var>. E.g., ‘<samp>1.2-4</samp>’ or ‘<samp>2-4</samp>’.
|
|
</li><li> All threads of an inferior, specified with a star wildcard, with or
|
without an inferior qualifier, as in <var>inf</var>.<code>*</code> (e.g.,
|
‘<samp>1.*</samp>’) or <code>*</code>. The former refers to all threads of the
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given inferior, and the latter form without an inferior qualifier
|
refers to all threads of the current inferior.
|
|
</li></ol>
|
|
<p>For example, if the current inferior is 1, and inferior 7 has one
|
thread with ID 7.1, the thread list ‘<samp>1 2-3 4.5 6.7-9 7.*</samp>’
|
includes threads 1 to 3 of inferior 1, thread 5 of inferior 4, threads
|
7 to 9 of inferior 6 and all threads of inferior 7. That is, in
|
expanded qualified form, the same as ‘<samp>1.1 1.2 1.3 4.5 6.7 6.8 6.9
|
7.1</samp>’.
|
</p>
|
|
<a name="global-thread-numbers"></a><a name="index-global-thread-number"></a>
|
<a name="index-global-thread-identifier-_0028GDB_0029"></a>
|
<p>In addition to a <em>per-inferior</em> number, each thread is also
|
assigned a unique <em>global</em> number, also known as <em>global
|
thread ID</em>, a single integer. Unlike the thread number component of
|
the thread ID, no two threads have the same global ID, even when
|
you’re debugging multiple inferiors.
|
</p>
|
<p>From <small>GDB</small>’s perspective, a process always has at least one
|
thread. In other words, <small>GDB</small> assigns a thread number to the
|
program’s “main thread” even if the program is not multi-threaded.
|
</p>
|
<a name="index-_0024_005fthread_002c-convenience-variable"></a>
|
<a name="index-_0024_005fgthread_002c-convenience-variable"></a>
|
<p>The debugger convenience variables ‘<samp>$_thread</samp>’ and
|
‘<samp>$_gthread</samp>’ contain, respectively, the per-inferior thread number
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and the global thread number of the current thread. You may find this
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useful in writing breakpoint conditional expressions, command scripts,
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and so forth. See <a href="Convenience-Vars.html#Convenience-Vars">Convenience Variables</a>, for
|
general information on convenience variables.
|
</p>
|
<p>If <small>GDB</small> detects the program is multi-threaded, it augments the
|
usual message about stopping at a breakpoint with the ID and name of
|
the thread that hit the breakpoint.
|
</p>
|
<div class="smallexample">
|
<pre class="smallexample">Thread 2 "client" hit Breakpoint 1, send_message () at client.c:68
|
</pre></div>
|
|
<p>Likewise when the program receives a signal:
|
</p>
|
<div class="smallexample">
|
<pre class="smallexample">Thread 1 "main" received signal SIGINT, Interrupt.
|
</pre></div>
|
|
<dl compact="compact">
|
<dd><a name="index-info-threads"></a>
|
</dd>
|
<dt><code>info threads <span class="roman">[</span><var>thread-id-list</var><span class="roman">]</span></code></dt>
|
<dd>
|
<p>Display information about one or more threads. With no arguments
|
displays information about all threads. You can specify the list of
|
threads that you want to display using the thread ID list syntax
|
(see <a href="#thread-ID-lists">thread ID lists</a>).
|
</p>
|
<p><small>GDB</small> displays for each thread (in this order):
|
</p>
|
<ol>
|
<li> the per-inferior thread number assigned by <small>GDB</small>
|
|
</li><li> the global thread number assigned by <small>GDB</small>, if the ‘<samp>-gid</samp>’
|
option was specified
|
|
</li><li> the target system’s thread identifier (<var>systag</var>)
|
|
</li><li> the thread’s name, if one is known. A thread can either be named by
|
the user (see <code>thread name</code>, below), or, in some cases, by the
|
program itself.
|
|
</li><li> the current stack frame summary for that thread
|
</li></ol>
|
|
<p>An asterisk ‘<samp>*</samp>’ to the left of the <small>GDB</small> thread number
|
indicates the current thread.
|
</p>
|
<p>For example,
|
</p></dd>
|
</dl>
|
|
<div class="smallexample">
|
<pre class="smallexample">(gdb) info threads
|
Id Target Id Frame
|
* 1 process 35 thread 13 main (argc=1, argv=0x7ffffff8)
|
2 process 35 thread 23 0x34e5 in sigpause ()
|
3 process 35 thread 27 0x34e5 in sigpause ()
|
at threadtest.c:68
|
</pre></div>
|
|
<p>If you’re debugging multiple inferiors, <small>GDB</small> displays thread
|
IDs using the qualified <var>inferior-num</var>.<var>thread-num</var> format.
|
Otherwise, only <var>thread-num</var> is shown.
|
</p>
|
<p>If you specify the ‘<samp>-gid</samp>’ option, <small>GDB</small> displays a column
|
indicating each thread’s global thread ID:
|
</p>
|
<div class="smallexample">
|
<pre class="smallexample">(gdb) info threads
|
Id GId Target Id Frame
|
1.1 1 process 35 thread 13 main (argc=1, argv=0x7ffffff8)
|
1.2 3 process 35 thread 23 0x34e5 in sigpause ()
|
1.3 4 process 35 thread 27 0x34e5 in sigpause ()
|
* 2.1 2 process 65 thread 1 main (argc=1, argv=0x7ffffff8)
|
</pre></div>
|
|
<p>On Solaris, you can display more information about user threads with a
|
Solaris-specific command:
|
</p>
|
<dl compact="compact">
|
<dt><code>maint info sol-threads</code></dt>
|
<dd><a name="index-maint-info-sol_002dthreads"></a>
|
<a name="index-thread-info-_0028Solaris_0029"></a>
|
<p>Display info on Solaris user threads.
|
</p></dd>
|
</dl>
|
|
<dl compact="compact">
|
<dd><a name="index-thread-thread_002did"></a>
|
</dd>
|
<dt><code>thread <var>thread-id</var></code></dt>
|
<dd><p>Make thread ID <var>thread-id</var> the current thread. The command
|
argument <var>thread-id</var> is the <small>GDB</small> thread ID, as shown in
|
the first field of the ‘<samp>info threads</samp>’ display, with or without an
|
inferior qualifier (e.g., ‘<samp>2.1</samp>’ or ‘<samp>1</samp>’).
|
</p>
|
<p><small>GDB</small> responds by displaying the system identifier of the
|
thread you selected, and its current stack frame summary:
|
</p>
|
<div class="smallexample">
|
<pre class="smallexample">(gdb) thread 2
|
[Switching to thread 2 (Thread 0xb7fdab70 (LWP 12747))]
|
#0 some_function (ignore=0x0) at example.c:8
|
8 printf ("hello\n");
|
</pre></div>
|
|
<p>As with the ‘<samp>[New …]</samp>’ message, the form of the text after
|
‘<samp>Switching to</samp>’ depends on your system’s conventions for identifying
|
threads.
|
</p>
|
<a name="thread-apply-all"></a><a name="index-thread-apply"></a>
|
<a name="index-apply-command-to-several-threads"></a>
|
</dd>
|
<dt><code>thread apply [<var>thread-id-list</var> | all [-ascending]] [<var>flag</var>]… <var>command</var></code></dt>
|
<dd><p>The <code>thread apply</code> command allows you to apply the named
|
<var>command</var> to one or more threads. Specify the threads that you
|
want affected using the thread ID list syntax (see <a href="#thread-ID-lists">thread ID lists</a>), or specify <code>all</code> to apply to all threads. To apply a
|
command to all threads in descending order, type <kbd>thread apply all
|
<var>command</var></kbd>. To apply a command to all threads in ascending order,
|
type <kbd>thread apply all -ascending <var>command</var></kbd>.
|
</p>
|
<p>The <var>flag</var> arguments control what output to produce and how to handle
|
errors raised when applying <var>command</var> to a thread. <var>flag</var>
|
must start with a <code>-</code> directly followed by one letter in
|
<code>qcs</code>. If several flags are provided, they must be given
|
individually, such as <code>-c -q</code>.
|
</p>
|
<p>By default, <small>GDB</small> displays some thread information before the
|
output produced by <var>command</var>, and an error raised during the
|
execution of a <var>command</var> will abort <code>thread apply</code>. The
|
following flags can be used to fine-tune this behavior:
|
</p>
|
<dl compact="compact">
|
<dt><code>-c</code></dt>
|
<dd><p>The flag <code>-c</code>, which stands for ‘<samp>continue</samp>’, causes any
|
errors in <var>command</var> to be displayed, and the execution of
|
<code>thread apply</code> then continues.
|
</p></dd>
|
<dt><code>-s</code></dt>
|
<dd><p>The flag <code>-s</code>, which stands for ‘<samp>silent</samp>’, causes any errors
|
or empty output produced by a <var>command</var> to be silently ignored.
|
That is, the execution continues, but the thread information and errors
|
are not printed.
|
</p></dd>
|
<dt><code>-q</code></dt>
|
<dd><p>The flag <code>-q</code> (‘<samp>quiet</samp>’) disables printing the thread
|
information.
|
</p></dd>
|
</dl>
|
|
<p>Flags <code>-c</code> and <code>-s</code> cannot be used together.
|
</p>
|
<a name="index-taas"></a>
|
<a name="index-apply-command-to-all-threads-_0028ignoring-errors-and-empty-output_0029"></a>
|
</dd>
|
<dt><code>taas [<var>option</var>]… <var>command</var></code></dt>
|
<dd><p>Shortcut for <code>thread apply all -s [<var>option</var>]… <var>command</var></code>.
|
Applies <var>command</var> on all threads, ignoring errors and empty output.
|
</p>
|
<p>The <code>taas</code> command accepts the same options as the <code>thread
|
apply all</code> command. See <a href="#thread-apply-all">thread apply all</a>.
|
</p>
|
<a name="index-tfaas"></a>
|
<a name="index-apply-a-command-to-all-frames-of-all-threads-_0028ignoring-errors-and-empty-output_0029"></a>
|
</dd>
|
<dt><code>tfaas [<var>option</var>]… <var>command</var></code></dt>
|
<dd><p>Shortcut for <code>thread apply all -s -- frame apply all -s [<var>option</var>]… <var>command</var></code>.
|
Applies <var>command</var> on all frames of all threads, ignoring errors
|
and empty output. Note that the flag <code>-s</code> is specified twice:
|
The first <code>-s</code> ensures that <code>thread apply</code> only shows the thread
|
information of the threads for which <code>frame apply</code> produces
|
some output. The second <code>-s</code> is needed to ensure that <code>frame
|
apply</code> shows the frame information of a frame only if the
|
<var>command</var> successfully produced some output.
|
</p>
|
<p>It can for example be used to print a local variable or a function
|
argument without knowing the thread or frame where this variable or argument
|
is, using:
|
</p><div class="smallexample">
|
<pre class="smallexample">(gdb) tfaas p some_local_var_i_do_not_remember_where_it_is
|
</pre></div>
|
|
<p>The <code>tfaas</code> command accepts the same options as the <code>frame
|
apply</code> command. See <a href="Frame-Apply.html#Frame-Apply">frame apply</a>.
|
</p>
|
<a name="index-thread-name"></a>
|
<a name="index-name-a-thread"></a>
|
</dd>
|
<dt><code>thread name [<var>name</var>]</code></dt>
|
<dd><p>This command assigns a name to the current thread. If no argument is
|
given, any existing user-specified name is removed. The thread name
|
appears in the ‘<samp>info threads</samp>’ display.
|
</p>
|
<p>On some systems, such as <small>GNU</small>/Linux, <small>GDB</small> is able to
|
determine the name of the thread as given by the OS. On these
|
systems, a name specified with ‘<samp>thread name</samp>’ will override the
|
system-give name, and removing the user-specified name will cause
|
<small>GDB</small> to once again display the system-specified name.
|
</p>
|
<a name="index-thread-find"></a>
|
<a name="index-search-for-a-thread"></a>
|
</dd>
|
<dt><code>thread find [<var>regexp</var>]</code></dt>
|
<dd><p>Search for and display thread ids whose name or <var>systag</var>
|
matches the supplied regular expression.
|
</p>
|
<p>As well as being the complement to the ‘<samp>thread name</samp>’ command,
|
this command also allows you to identify a thread by its target
|
<var>systag</var>. For instance, on <small>GNU</small>/Linux, the target <var>systag</var>
|
is the LWP id.
|
</p>
|
<div class="smallexample">
|
<pre class="smallexample">(GDB) thread find 26688
|
Thread 4 has target id 'Thread 0x41e02940 (LWP 26688)'
|
(GDB) info thread 4
|
Id Target Id Frame
|
4 Thread 0x41e02940 (LWP 26688) 0x00000031ca6cd372 in select ()
|
</pre></div>
|
|
<a name="index-set-print-thread_002devents"></a>
|
<a name="index-print-messages-on-thread-start-and-exit"></a>
|
</dd>
|
<dt><code>set print thread-events</code></dt>
|
<dt><code>set print thread-events on</code></dt>
|
<dt><code>set print thread-events off</code></dt>
|
<dd><p>The <code>set print thread-events</code> command allows you to enable or
|
disable printing of messages when <small>GDB</small> notices that new threads have
|
started or that threads have exited. By default, these messages will
|
be printed if detection of these events is supported by the target.
|
Note that these messages cannot be disabled on all targets.
|
</p>
|
<a name="index-show-print-thread_002devents"></a>
|
</dd>
|
<dt><code>show print thread-events</code></dt>
|
<dd><p>Show whether messages will be printed when <small>GDB</small> detects that threads
|
have started and exited.
|
</p></dd>
|
</dl>
|
|
<p>See <a href="Thread-Stops.html#Thread-Stops">Stopping and Starting Multi-thread Programs</a>, for
|
more information about how <small>GDB</small> behaves when you stop and start
|
programs with multiple threads.
|
</p>
|
<p>See <a href="Set-Watchpoints.html#Set-Watchpoints">Setting Watchpoints</a>, for information about
|
watchpoints in programs with multiple threads.
|
</p>
|
<a name="set-libthread_002ddb_002dsearch_002dpath"></a><dl compact="compact">
|
<dd><a name="index-set-libthread_002ddb_002dsearch_002dpath"></a>
|
<a name="index-search-path-for-libthread_005fdb"></a>
|
</dd>
|
<dt><code>set libthread-db-search-path <span class="roman">[</span><var>path</var><span class="roman">]</span></code></dt>
|
<dd><p>If this variable is set, <var>path</var> is a colon-separated list of
|
directories <small>GDB</small> will use to search for <code>libthread_db</code>.
|
If you omit <var>path</var>, ‘<samp>libthread-db-search-path</samp>’ will be reset to
|
its default value (<code>$sdir:$pdir</code> on <small>GNU</small>/Linux and Solaris systems).
|
Internally, the default value comes from the <code>LIBTHREAD_DB_SEARCH_PATH</code>
|
macro.
|
</p>
|
<p>On <small>GNU</small>/Linux and Solaris systems, <small>GDB</small> uses a “helper”
|
<code>libthread_db</code> library to obtain information about threads in the
|
inferior process. <small>GDB</small> will use ‘<samp>libthread-db-search-path</samp>’
|
to find <code>libthread_db</code>. <small>GDB</small> also consults first if inferior
|
specific thread debugging library loading is enabled
|
by ‘<samp>set auto-load libthread-db</samp>’ (see <a href="libthread_005fdb_002eso_002e1-file.html#libthread_005fdb_002eso_002e1-file">libthread_db.so.1 file</a>).
|
</p>
|
<p>A special entry ‘<samp>$sdir</samp>’ for ‘<samp>libthread-db-search-path</samp>’
|
refers to the default system directories that are
|
normally searched for loading shared libraries. The ‘<samp>$sdir</samp>’ entry
|
is the only kind not needing to be enabled by ‘<samp>set auto-load libthread-db</samp>’
|
(see <a href="libthread_005fdb_002eso_002e1-file.html#libthread_005fdb_002eso_002e1-file">libthread_db.so.1 file</a>).
|
</p>
|
<p>A special entry ‘<samp>$pdir</samp>’ for ‘<samp>libthread-db-search-path</samp>’
|
refers to the directory from which <code>libpthread</code>
|
was loaded in the inferior process.
|
</p>
|
<p>For any <code>libthread_db</code> library <small>GDB</small> finds in above directories,
|
<small>GDB</small> attempts to initialize it with the current inferior process.
|
If this initialization fails (which could happen because of a version
|
mismatch between <code>libthread_db</code> and <code>libpthread</code>), <small>GDB</small>
|
will unload <code>libthread_db</code>, and continue with the next directory.
|
If none of <code>libthread_db</code> libraries initialize successfully,
|
<small>GDB</small> will issue a warning and thread debugging will be disabled.
|
</p>
|
<p>Setting <code>libthread-db-search-path</code> is currently implemented
|
only on some platforms.
|
</p>
|
<a name="index-show-libthread_002ddb_002dsearch_002dpath"></a>
|
</dd>
|
<dt><code>show libthread-db-search-path</code></dt>
|
<dd><p>Display current libthread_db search path.
|
</p>
|
<a name="index-set-debug-libthread_002ddb"></a>
|
<a name="index-show-debug-libthread_002ddb"></a>
|
<a name="index-debugging-libthread_005fdb"></a>
|
</dd>
|
<dt><code>set debug libthread-db</code></dt>
|
<dt><code>show debug libthread-db</code></dt>
|
<dd><p>Turns on or off display of <code>libthread_db</code>-related events.
|
Use <code>1</code> to enable, <code>0</code> to disable.
|
</p></dd>
|
</dl>
|
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|
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