1<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> 2<html> 3<!-- Copyright (C) 1988-2021 Free Software Foundation, Inc. 4 5Permission is granted to copy, distribute and/or modify this document 6under the terms of the GNU Free Documentation License, Version 1.3 or 7any later version published by the Free Software Foundation; with the 8Invariant Sections being "Free Software" and "Free Software Needs 9Free Documentation", with the Front-Cover Texts being "A GNU Manual," 10and with the Back-Cover Texts as in (a) below. 11 12(a) The FSF's Back-Cover Text is: "You are free to copy and modify 13this GNU Manual. 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The precise semantics 78of threads differ from one operating system to another, but in general 79the threads of a single program are akin to multiple processes—except 80that they share one address space (that is, they can all examine and 81modify the same variables). On the other hand, each thread has its own 82registers and execution stack, and perhaps private memory. 83</p> 84<p><small>GDB</small> provides these facilities for debugging multi-thread 85programs: 86</p> 87<ul> 88<li> automatic notification of new threads 89</li><li> ‘<samp>thread <var>thread-id</var></samp>’, a command to switch among threads 90</li><li> ‘<samp>info threads</samp>’, a command to inquire about existing threads 91</li><li> ‘<samp>thread apply [<var>thread-id-list</var> | all] <var>args</var></samp>’, 92a command to apply a command to a list of threads 93</li><li> thread-specific breakpoints 94</li><li> ‘<samp>set print thread-events</samp>’, which controls printing of 95messages on thread start and exit. 96</li><li> ‘<samp>set libthread-db-search-path <var>path</var></samp>’, which lets 97the user specify which <code>libthread_db</code> to use if the default choice 98isn’t compatible with the program. 99</li></ul> 100 101<a name="index-focus-of-debugging"></a> 102<a name="index-current-thread"></a> 103<p>The <small>GDB</small> thread debugging facility allows you to observe all 104threads while your program runs—but whenever <small>GDB</small> takes 105control, one thread in particular is always the focus of debugging. 106This thread is called the <em>current thread</em>. Debugging commands show 107program information from the perspective of the current thread. 108</p> 109<a name="index-New-systag-message"></a> 110<a name="index-thread-identifier-_0028system_0029"></a> 111<p>Whenever <small>GDB</small> detects a new thread in your program, it displays 112the target system’s identification for the thread with a message in the 113form ‘<samp>[New <var>systag</var>]</samp>’, where <var>systag</var> is a thread identifier 114whose form varies depending on the particular system. For example, on 115<small>GNU</small>/Linux, you might see 116</p> 117<div class="smallexample"> 118<pre class="smallexample">[New Thread 0x41e02940 (LWP 25582)] 119</pre></div> 120 121<p>when <small>GDB</small> notices a new thread. In contrast, on other systems, 122the <var>systag</var> is simply something like ‘<samp>process 368</samp>’, with no 123further qualifier. 124</p> 125 126<a name="thread-numbers"></a><a name="index-thread-number_002c-per-inferior"></a> 127<a name="index-thread-identifier-_0028GDB_0029"></a> 128<p>For debugging purposes, <small>GDB</small> associates its own thread number 129—always a single integer—with each thread of an inferior. This 130number is unique between all threads of an inferior, but not unique 131between threads of different inferiors. 132</p> 133<a name="index-qualified-thread-ID"></a> 134<p>You can refer to a given thread in an inferior using the qualified 135<var>inferior-num</var>.<var>thread-num</var> syntax, also known as 136<em>qualified thread ID</em>, with <var>inferior-num</var> being the inferior 137number and <var>thread-num</var> being the thread number of the given 138inferior. For example, thread <code>2.3</code> refers to thread number 3 of 139inferior 2. If you omit <var>inferior-num</var> (e.g., <code>thread 3</code>), 140then <small>GDB</small> infers you’re referring to a thread of the current 141inferior. 142</p> 143<p>Until you create a second inferior, <small>GDB</small> does not show the 144<var>inferior-num</var> part of thread IDs, even though you can always use 145the full <var>inferior-num</var>.<var>thread-num</var> form to refer to threads 146of inferior 1, the initial inferior. 147</p> 148<a name="thread-ID-lists"></a><a name="index-thread-ID-lists"></a> 149<p>Some commands accept a space-separated <em>thread ID list</em> as 150argument. A list element can be: 151</p> 152<ol> 153<li> A thread ID as shown in the first field of the ‘<samp>info threads</samp>’ 154display, with or without an inferior qualifier. E.g., ‘<samp>2.1</samp>’ or 155‘<samp>1</samp>’. 156 157</li><li> A range of thread numbers, again with or without an inferior 158qualifier, as in <var>inf</var>.<var>thr1</var>-<var>thr2</var> or 159<var>thr1</var>-<var>thr2</var>. E.g., ‘<samp>1.2-4</samp>’ or ‘<samp>2-4</samp>’. 160 161</li><li> All threads of an inferior, specified with a star wildcard, with or 162without an inferior qualifier, as in <var>inf</var>.<code>*</code> (e.g., 163‘<samp>1.*</samp>’) or <code>*</code>. The former refers to all threads of the 164given inferior, and the latter form without an inferior qualifier 165refers to all threads of the current inferior. 166 167</li></ol> 168 169<p>For example, if the current inferior is 1, and inferior 7 has one 170thread with ID 7.1, the thread list ‘<samp>1 2-3 4.5 6.7-9 7.*</samp>’ 171includes threads 1 to 3 of inferior 1, thread 5 of inferior 4, threads 1727 to 9 of inferior 6 and all threads of inferior 7. That is, in 173expanded qualified form, the same as ‘<samp>1.1 1.2 1.3 4.5 6.7 6.8 6.9 1747.1</samp>’. 175</p> 176 177<a name="global-thread-numbers"></a><a name="index-global-thread-number"></a> 178<a name="index-global-thread-identifier-_0028GDB_0029"></a> 179<p>In addition to a <em>per-inferior</em> number, each thread is also 180assigned a unique <em>global</em> number, also known as <em>global 181thread ID</em>, a single integer. Unlike the thread number component of 182the thread ID, no two threads have the same global ID, even when 183you’re debugging multiple inferiors. 184</p> 185<p>From <small>GDB</small>’s perspective, a process always has at least one 186thread. In other words, <small>GDB</small> assigns a thread number to the 187program’s “main thread” even if the program is not multi-threaded. 188</p> 189<a name="index-_0024_005fthread_002c-convenience-variable"></a> 190<a name="index-_0024_005fgthread_002c-convenience-variable"></a> 191<p>The debugger convenience variables ‘<samp>$_thread</samp>’ and 192‘<samp>$_gthread</samp>’ contain, respectively, the per-inferior thread number 193and the global thread number of the current thread. You may find this 194useful in writing breakpoint conditional expressions, command scripts, 195and so forth. See <a href="Convenience-Vars.html#Convenience-Vars">Convenience Variables</a>, for 196general information on convenience variables. 197</p> 198<p>If <small>GDB</small> detects the program is multi-threaded, it augments the 199usual message about stopping at a breakpoint with the ID and name of 200the thread that hit the breakpoint. 201</p> 202<div class="smallexample"> 203<pre class="smallexample">Thread 2 "client" hit Breakpoint 1, send_message () at client.c:68 204</pre></div> 205 206<p>Likewise when the program receives a signal: 207</p> 208<div class="smallexample"> 209<pre class="smallexample">Thread 1 "main" received signal SIGINT, Interrupt. 210</pre></div> 211 212<dl compact="compact"> 213<dd><a name="index-info-threads"></a> 214</dd> 215<dt><code>info threads <span class="roman">[</span><var>thread-id-list</var><span class="roman">]</span></code></dt> 216<dd> 217<p>Display information about one or more threads. With no arguments 218displays information about all threads. You can specify the list of 219threads that you want to display using the thread ID list syntax 220(see <a href="#thread-ID-lists">thread ID lists</a>). 221</p> 222<p><small>GDB</small> displays for each thread (in this order): 223</p> 224<ol> 225<li> the per-inferior thread number assigned by <small>GDB</small> 226 227</li><li> the global thread number assigned by <small>GDB</small>, if the ‘<samp>-gid</samp>’ 228option was specified 229 230</li><li> the target system’s thread identifier (<var>systag</var>) 231 232</li><li> the thread’s name, if one is known. A thread can either be named by 233the user (see <code>thread name</code>, below), or, in some cases, by the 234program itself. 235 236</li><li> the current stack frame summary for that thread 237</li></ol> 238 239<p>An asterisk ‘<samp>*</samp>’ to the left of the <small>GDB</small> thread number 240indicates the current thread. 241</p> 242<p>For example, 243</p></dd> 244</dl> 245 246<div class="smallexample"> 247<pre class="smallexample">(gdb) info threads 248 Id Target Id Frame 249* 1 process 35 thread 13 main (argc=1, argv=0x7ffffff8) 250 2 process 35 thread 23 0x34e5 in sigpause () 251 3 process 35 thread 27 0x34e5 in sigpause () 252 at threadtest.c:68 253</pre></div> 254 255<p>If you’re debugging multiple inferiors, <small>GDB</small> displays thread 256IDs using the qualified <var>inferior-num</var>.<var>thread-num</var> format. 257Otherwise, only <var>thread-num</var> is shown. 258</p> 259<p>If you specify the ‘<samp>-gid</samp>’ option, <small>GDB</small> displays a column 260indicating each thread’s global thread ID: 261</p> 262<div class="smallexample"> 263<pre class="smallexample">(gdb) info threads 264 Id GId Target Id Frame 265 1.1 1 process 35 thread 13 main (argc=1, argv=0x7ffffff8) 266 1.2 3 process 35 thread 23 0x34e5 in sigpause () 267 1.3 4 process 35 thread 27 0x34e5 in sigpause () 268* 2.1 2 process 65 thread 1 main (argc=1, argv=0x7ffffff8) 269</pre></div> 270 271<p>On Solaris, you can display more information about user threads with a 272Solaris-specific command: 273</p> 274<dl compact="compact"> 275<dt><code>maint info sol-threads</code></dt> 276<dd><a name="index-maint-info-sol_002dthreads"></a> 277<a name="index-thread-info-_0028Solaris_0029"></a> 278<p>Display info on Solaris user threads. 279</p></dd> 280</dl> 281 282<dl compact="compact"> 283<dd><a name="index-thread-thread_002did"></a> 284</dd> 285<dt><code>thread <var>thread-id</var></code></dt> 286<dd><p>Make thread ID <var>thread-id</var> the current thread. The command 287argument <var>thread-id</var> is the <small>GDB</small> thread ID, as shown in 288the first field of the ‘<samp>info threads</samp>’ display, with or without an 289inferior qualifier (e.g., ‘<samp>2.1</samp>’ or ‘<samp>1</samp>’). 290</p> 291<p><small>GDB</small> responds by displaying the system identifier of the 292thread you selected, and its current stack frame summary: 293</p> 294<div class="smallexample"> 295<pre class="smallexample">(gdb) thread 2 296[Switching to thread 2 (Thread 0xb7fdab70 (LWP 12747))] 297#0 some_function (ignore=0x0) at example.c:8 2988 printf ("hello\n"); 299</pre></div> 300 301<p>As with the ‘<samp>[New …]</samp>’ message, the form of the text after 302‘<samp>Switching to</samp>’ depends on your system’s conventions for identifying 303threads. 304</p> 305<a name="thread-apply-all"></a><a name="index-thread-apply"></a> 306<a name="index-apply-command-to-several-threads"></a> 307</dd> 308<dt><code>thread apply [<var>thread-id-list</var> | all [-ascending]] [<var>flag</var>]… <var>command</var></code></dt> 309<dd><p>The <code>thread apply</code> command allows you to apply the named 310<var>command</var> to one or more threads. Specify the threads that you 311want 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 312command to all threads in descending order, type <kbd>thread apply all 313<var>command</var></kbd>. To apply a command to all threads in ascending order, 314type <kbd>thread apply all -ascending <var>command</var></kbd>. 315</p> 316<p>The <var>flag</var> arguments control what output to produce and how to handle 317errors raised when applying <var>command</var> to a thread. <var>flag</var> 318must start with a <code>-</code> directly followed by one letter in 319<code>qcs</code>. If several flags are provided, they must be given 320individually, such as <code>-c -q</code>. 321</p> 322<p>By default, <small>GDB</small> displays some thread information before the 323output produced by <var>command</var>, and an error raised during the 324execution of a <var>command</var> will abort <code>thread apply</code>. The 325following flags can be used to fine-tune this behavior: 326</p> 327<dl compact="compact"> 328<dt><code>-c</code></dt> 329<dd><p>The flag <code>-c</code>, which stands for ‘<samp>continue</samp>’, causes any 330errors in <var>command</var> to be displayed, and the execution of 331<code>thread apply</code> then continues. 332</p></dd> 333<dt><code>-s</code></dt> 334<dd><p>The flag <code>-s</code>, which stands for ‘<samp>silent</samp>’, causes any errors 335or empty output produced by a <var>command</var> to be silently ignored. 336That is, the execution continues, but the thread information and errors 337are not printed. 338</p></dd> 339<dt><code>-q</code></dt> 340<dd><p>The flag <code>-q</code> (‘<samp>quiet</samp>’) disables printing the thread 341information. 342</p></dd> 343</dl> 344 345<p>Flags <code>-c</code> and <code>-s</code> cannot be used together. 346</p> 347<a name="index-taas"></a> 348<a name="index-apply-command-to-all-threads-_0028ignoring-errors-and-empty-output_0029"></a> 349</dd> 350<dt><code>taas [<var>option</var>]… <var>command</var></code></dt> 351<dd><p>Shortcut for <code>thread apply all -s [<var>option</var>]… <var>command</var></code>. 352Applies <var>command</var> on all threads, ignoring errors and empty output. 353</p> 354<p>The <code>taas</code> command accepts the same options as the <code>thread 355apply all</code> command. See <a href="#thread-apply-all">thread apply all</a>. 356</p> 357<a name="index-tfaas"></a> 358<a name="index-apply-a-command-to-all-frames-of-all-threads-_0028ignoring-errors-and-empty-output_0029"></a> 359</dd> 360<dt><code>tfaas [<var>option</var>]… <var>command</var></code></dt> 361<dd><p>Shortcut for <code>thread apply all -s -- frame apply all -s [<var>option</var>]… <var>command</var></code>. 362Applies <var>command</var> on all frames of all threads, ignoring errors 363and empty output. Note that the flag <code>-s</code> is specified twice: 364The first <code>-s</code> ensures that <code>thread apply</code> only shows the thread 365information of the threads for which <code>frame apply</code> produces 366some output. The second <code>-s</code> is needed to ensure that <code>frame 367apply</code> shows the frame information of a frame only if the 368<var>command</var> successfully produced some output. 369</p> 370<p>It can for example be used to print a local variable or a function 371argument without knowing the thread or frame where this variable or argument 372is, using: 373</p><div class="smallexample"> 374<pre class="smallexample">(gdb) tfaas p some_local_var_i_do_not_remember_where_it_is 375</pre></div> 376 377<p>The <code>tfaas</code> command accepts the same options as the <code>frame 378apply</code> command. See <a href="Frame-Apply.html#Frame-Apply">frame apply</a>. 379</p> 380<a name="index-thread-name"></a> 381<a name="index-name-a-thread"></a> 382</dd> 383<dt><code>thread name [<var>name</var>]</code></dt> 384<dd><p>This command assigns a name to the current thread. If no argument is 385given, any existing user-specified name is removed. The thread name 386appears in the ‘<samp>info threads</samp>’ display. 387</p> 388<p>On some systems, such as <small>GNU</small>/Linux, <small>GDB</small> is able to 389determine the name of the thread as given by the OS. On these 390systems, a name specified with ‘<samp>thread name</samp>’ will override the 391system-give name, and removing the user-specified name will cause 392<small>GDB</small> to once again display the system-specified name. 393</p> 394<a name="index-thread-find"></a> 395<a name="index-search-for-a-thread"></a> 396</dd> 397<dt><code>thread find [<var>regexp</var>]</code></dt> 398<dd><p>Search for and display thread ids whose name or <var>systag</var> 399matches the supplied regular expression. 400</p> 401<p>As well as being the complement to the ‘<samp>thread name</samp>’ command, 402this command also allows you to identify a thread by its target 403<var>systag</var>. For instance, on <small>GNU</small>/Linux, the target <var>systag</var> 404is the LWP id. 405</p> 406<div class="smallexample"> 407<pre class="smallexample">(GDB) thread find 26688 408Thread 4 has target id 'Thread 0x41e02940 (LWP 26688)' 409(GDB) info thread 4 410 Id Target Id Frame 411 4 Thread 0x41e02940 (LWP 26688) 0x00000031ca6cd372 in select () 412</pre></div> 413 414<a name="index-set-print-thread_002devents"></a> 415<a name="index-print-messages-on-thread-start-and-exit"></a> 416</dd> 417<dt><code>set print thread-events</code></dt> 418<dt><code>set print thread-events on</code></dt> 419<dt><code>set print thread-events off</code></dt> 420<dd><p>The <code>set print thread-events</code> command allows you to enable or 421disable printing of messages when <small>GDB</small> notices that new threads have 422started or that threads have exited. By default, these messages will 423be printed if detection of these events is supported by the target. 424Note that these messages cannot be disabled on all targets. 425</p> 426<a name="index-show-print-thread_002devents"></a> 427</dd> 428<dt><code>show print thread-events</code></dt> 429<dd><p>Show whether messages will be printed when <small>GDB</small> detects that threads 430have started and exited. 431</p></dd> 432</dl> 433 434<p>See <a href="Thread-Stops.html#Thread-Stops">Stopping and Starting Multi-thread Programs</a>, for 435more information about how <small>GDB</small> behaves when you stop and start 436programs with multiple threads. 437</p> 438<p>See <a href="Set-Watchpoints.html#Set-Watchpoints">Setting Watchpoints</a>, for information about 439watchpoints in programs with multiple threads. 440</p> 441<a name="set-libthread_002ddb_002dsearch_002dpath"></a><dl compact="compact"> 442<dd><a name="index-set-libthread_002ddb_002dsearch_002dpath"></a> 443<a name="index-search-path-for-libthread_005fdb"></a> 444</dd> 445<dt><code>set libthread-db-search-path <span class="roman">[</span><var>path</var><span class="roman">]</span></code></dt> 446<dd><p>If this variable is set, <var>path</var> is a colon-separated list of 447directories <small>GDB</small> will use to search for <code>libthread_db</code>. 448If you omit <var>path</var>, ‘<samp>libthread-db-search-path</samp>’ will be reset to 449its default value (<code>$sdir:$pdir</code> on <small>GNU</small>/Linux and Solaris systems). 450Internally, the default value comes from the <code>LIBTHREAD_DB_SEARCH_PATH</code> 451macro. 452</p> 453<p>On <small>GNU</small>/Linux and Solaris systems, <small>GDB</small> uses a “helper” 454<code>libthread_db</code> library to obtain information about threads in the 455inferior process. <small>GDB</small> will use ‘<samp>libthread-db-search-path</samp>’ 456to find <code>libthread_db</code>. <small>GDB</small> also consults first if inferior 457specific thread debugging library loading is enabled 458by ‘<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>). 459</p> 460<p>A special entry ‘<samp>$sdir</samp>’ for ‘<samp>libthread-db-search-path</samp>’ 461refers to the default system directories that are 462normally searched for loading shared libraries. The ‘<samp>$sdir</samp>’ entry 463is the only kind not needing to be enabled by ‘<samp>set auto-load libthread-db</samp>’ 464(see <a href="libthread_005fdb_002eso_002e1-file.html#libthread_005fdb_002eso_002e1-file">libthread_db.so.1 file</a>). 465</p> 466<p>A special entry ‘<samp>$pdir</samp>’ for ‘<samp>libthread-db-search-path</samp>’ 467refers to the directory from which <code>libpthread</code> 468was loaded in the inferior process. 469</p> 470<p>For any <code>libthread_db</code> library <small>GDB</small> finds in above directories, 471<small>GDB</small> attempts to initialize it with the current inferior process. 472If this initialization fails (which could happen because of a version 473mismatch between <code>libthread_db</code> and <code>libpthread</code>), <small>GDB</small> 474will unload <code>libthread_db</code>, and continue with the next directory. 475If none of <code>libthread_db</code> libraries initialize successfully, 476<small>GDB</small> will issue a warning and thread debugging will be disabled. 477</p> 478<p>Setting <code>libthread-db-search-path</code> is currently implemented 479only on some platforms. 480</p> 481<a name="index-show-libthread_002ddb_002dsearch_002dpath"></a> 482</dd> 483<dt><code>show libthread-db-search-path</code></dt> 484<dd><p>Display current libthread_db search path. 485</p> 486<a name="index-set-debug-libthread_002ddb"></a> 487<a name="index-show-debug-libthread_002ddb"></a> 488<a name="index-debugging-libthread_005fdb"></a> 489</dd> 490<dt><code>set debug libthread-db</code></dt> 491<dt><code>show debug libthread-db</code></dt> 492<dd><p>Turns on or off display of <code>libthread_db</code>-related events. 493Use <code>1</code> to enable, <code>0</code> to disable. 494</p></dd> 495</dl> 496 497<hr> 498<div class="header"> 499<p> 500Next: <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> 501</div> 502 503 504 505</body> 506</html> 507