mingw/info/gdb/Thread-Stops.html

   1 <html lang="en">
   2 <head>
   3 <title>Debugging with GDB</title>
   4 <meta http-equiv="Content-Type" content="text/html">
   5 <meta name="description" content="Debugging with GDB">
   6 <meta name="generator" content="makeinfo 4.3">
   7 <link href="http://www.gnu.org/software/texinfo/" rel="generator-home">
   8 </head>
   9 <body>
  10 <div class="node">
  11 <p>
  12 Node:<a name="Thread%20Stops">Thread Stops</a>,
  13 Previous:<a rel="previous" accesskey="p" href="Signals.html#Signals">Signals</a>,
  14 Up:<a rel="up" accesskey="u" href="Stopping.html#Stopping">Stopping</a>
  15 <hr><br>
  16 </div>
  17
  18 <h3 class="section">Stopping and starting multi-thread programs</h3>
  19
  20    <p>When your program has multiple threads (see <a href="Threads.html#Threads">Debugging programs with multiple threads</a>), you can choose whether to set
  21 breakpoints on all threads, or on a particular thread.
  22
  23      <dl>
  24 <dt><code>break </code><var>linespec</var><code> thread </code><var>threadno</var><code></code>
  25      <dd><dt><code>break </code><var>linespec</var><code> thread </code><var>threadno</var><code> if ...</code>
  26      <dd><var>linespec</var> specifies source lines; there are several ways of
  27 writing them, but the effect is always to specify some source line.
  28
  29      <p>Use the qualifier <code>thread </code><var>threadno</var><code></code> with a breakpoint command
  30 to specify that you only want GDB to stop the program when a
  31 particular thread reaches this breakpoint.  <var>threadno</var> is one of the
  32 numeric thread identifiers assigned by GDB, shown in the first
  33 column of the <code>info threads</code> display.
  34
  35      <p>If you do not specify <code>thread </code><var>threadno</var><code></code> when you set a
  36 breakpoint, the breakpoint applies to <em>all</em> threads of your
  37 program.
  38
  39      <p>You can use the <code>thread</code> qualifier on conditional breakpoints as
  40 well; in this case, place <code>thread </code><var>threadno</var><code></code> before the
  41 breakpoint condition, like this:
  42
  43      <pre class="smallexample">          (gdb) break frik.c:13 thread 28 if bartab &gt; lim
  44           </pre>
  45
  46    </dl>
  47
  48    <p>Whenever your program stops under GDB for any reason,
  49 <em>all</em> threads of execution stop, not just the current thread.  This
  50 allows you to examine the overall state of the program, including
  51 switching between threads, without worrying that things may change
  52 underfoot.
  53
  54    <p>Conversely, whenever you restart the program, <em>all</em> threads start
  55 executing.  <em>This is true even when single-stepping</em> with commands
  56 like <code>step</code> or <code>next</code>.
  57
  58    <p>In particular, GDB cannot single-step all threads in lockstep.
  59 Since thread scheduling is up to your debugging target's operating
  60 system (not controlled by GDB), other threads may
  61 execute more than one statement while the current thread completes a
  62 single step.  Moreover, in general other threads stop in the middle of a
  63 statement, rather than at a clean statement boundary, when the program
  64 stops.
  65
  66    <p>You might even find your program stopped in another thread after
  67 continuing or even single-stepping.  This happens whenever some other
  68 thread runs into a breakpoint, a signal, or an exception before the
  69 first thread completes whatever you requested.
  70
  71    <p>On some OSes, you can lock the OS scheduler and thus allow only a single
  72 thread to run.
  73
  74      <dl>
  75 <dt><code>set scheduler-locking </code><var>mode</var><code></code>
  76      <dd>Set the scheduler locking mode.  If it is <code>off</code>, then there is no
  77 locking and any thread may run at any time.  If <code>on</code>, then only the
  78 current thread may run when the inferior is resumed.  The <code>step</code>
  79 mode optimizes for single-stepping.  It stops other threads from
  80 "seizing the prompt" by preempting the current thread while you are
  81 stepping.  Other threads will only rarely (or never) get a chance to run
  82 when you step.  They are more likely to run when you <code>next</code> over a
  83 function call, and they are completely free to run when you use commands
  84 like <code>continue</code>, <code>until</code>, or <code>finish</code>.  However, unless another
  85 thread hits a breakpoint during its timeslice, they will never steal the
  86 GDB prompt away from the thread that you are debugging.
  87
  88      <br><dt><code>show scheduler-locking</code>
  89      <dd>Display the current scheduler locking mode.
  90 </dl>
  91
  92    </body></html>
  93