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| <a name="Threads"></a> |
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| <hr> |
| <a name="Debugging-Programs-with-Multiple-Threads"></a> |
| <h3 class="section">4.10 Debugging Programs with Multiple Threads</h3> |
| |
| <a name="index-threads-of-execution"></a> |
| <a name="index-multiple-threads"></a> |
| <a name="index-switching-threads"></a> |
| <p>In some operating systems, such as HP-UX and Solaris, a single program |
| may have more than one <em>thread</em> of execution. The precise semantics |
| of threads differ from one operating system to another, but in general |
| the threads of a single program are akin to multiple processes—except |
| that they share one address space (that is, they can all examine and |
| modify the same variables). On the other hand, each thread has its own |
| registers and execution stack, and perhaps private memory. |
| </p> |
| <p><small>GDB</small> provides these facilities for debugging multi-thread |
| programs: |
| </p> |
| <ul> |
| <li> automatic notification of new threads |
| </li><li> ‘<samp>thread <var>threadno</var></samp>’, a command to switch among threads |
| </li><li> ‘<samp>info threads</samp>’, a command to inquire about existing threads |
| </li><li> ‘<samp>thread apply [<var>threadno</var>] [<var>all</var>] <var>args</var></samp>’, |
| a command to apply a command to a list of threads |
| </li><li> thread-specific breakpoints |
| </li><li> ‘<samp>set print thread-events</samp>’, which controls printing of |
| messages on thread start and exit. |
| </li><li> ‘<samp>set libthread-db-search-path <var>path</var></samp>’, which lets |
| the user specify which <code>libthread_db</code> to use if the default choice |
| isn’t compatible with the program. |
| </li></ul> |
| |
| <blockquote> |
| <p><em>Warning:</em> These facilities are not yet available on every |
| <small>GDB</small> configuration where the operating system supports threads. |
| If your <small>GDB</small> does not support threads, these commands have no |
| effect. For example, a system without thread support shows no output |
| from ‘<samp>info threads</samp>’, and always rejects the <code>thread</code> command, |
| like this: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">(gdb) info threads |
| (gdb) thread 1 |
| Thread ID 1 not known. Use the "info threads" command to |
| see the IDs of currently known threads. |
| </pre></div> |
| </blockquote> |
| |
| <a name="index-focus-of-debugging"></a> |
| <a name="index-current-thread"></a> |
| <p>The <small>GDB</small> thread debugging facility allows you to observe all |
| threads while your program runs—but whenever <small>GDB</small> takes |
| control, one thread in particular is always the focus of debugging. |
| This thread is called the <em>current thread</em>. Debugging commands show |
| program information from the perspective of the current thread. |
| </p> |
| <a name="index-New-systag-message"></a> |
| <a name="index-thread-identifier-_0028system_0029"></a> |
| <p>Whenever <small>GDB</small> detects a new thread in your program, it displays |
| the target system’s identification for the thread with a message in the |
| form ‘<samp>[New <var>systag</var>]</samp>’, where <var>systag</var> is a thread identifier |
| whose form varies depending on the particular system. For example, on |
| <small>GNU</small>/Linux, you might see |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">[New Thread 0x41e02940 (LWP 25582)] |
| </pre></div> |
| |
| <p>when <small>GDB</small> notices a new thread. In contrast, on an SGI system, |
| the <var>systag</var> is simply something like ‘<samp>process 368</samp>’, with no |
| further qualifier. |
| </p> |
| |
| <a name="index-thread-number"></a> |
| <a name="index-thread-identifier-_0028GDB_0029"></a> |
| <p>For debugging purposes, <small>GDB</small> associates its own thread |
| number—always a single integer—with each thread in your program. |
| </p> |
| <dl compact="compact"> |
| <dd><a name="index-info-threads"></a> |
| </dd> |
| <dt><code>info threads <span class="roman">[</span><var>id</var>…<span class="roman">]</span></code></dt> |
| <dd><p>Display a summary of all threads currently in your program. Optional |
| argument <var>id</var>… is one or more thread ids separated by spaces, and |
| means to print information only about the specified thread or threads. |
| <small>GDB</small> displays for each thread (in this order): |
| </p> |
| <ol> |
| <li> the thread number assigned by <small>GDB</small> |
| |
| </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 |
| 3 process 35 thread 27 0x34e5 in sigpause () |
| 2 process 35 thread 23 0x34e5 in sigpause () |
| * 1 process 35 thread 13 main (argc=1, argv=0x7ffffff8) |
| at threadtest.c:68 |
| </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-threadno"></a> |
| </dd> |
| <dt><code>thread <var>threadno</var></code></dt> |
| <dd><p>Make thread number <var>threadno</var> the current thread. The command |
| argument <var>threadno</var> is the internal <small>GDB</small> thread number, as |
| shown in the first field of the ‘<samp>info threads</samp>’ display. |
| <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="index-_0024_005fthread_002c-convenience-variable"></a> |
| <p>The debugger convenience variable ‘<samp>$_thread</samp>’ contains the number |
| of the current thread. You may find this useful in writing breakpoint |
| conditional expressions, command scripts, and so forth. See |
| See <a href="Convenience-Vars.html#Convenience-Vars">Convenience Variables</a>, for general |
| information on convenience variables. |
| </p> |
| <a name="index-thread-apply"></a> |
| <a name="index-apply-command-to-several-threads"></a> |
| </dd> |
| <dt><code>thread apply [<var>threadno</var> | all [-ascending]] <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 numbers of the |
| threads that you want affected with the command argument |
| <var>threadno</var>. It can be a single thread number, one of the numbers |
| shown in the first field of the ‘<samp>info threads</samp>’ display; or it |
| could be a range of thread numbers, as in <code>2-4</code>. 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> |
| |
| <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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