gdb.info: Threads

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Debugging programs with multiple threads

In some operating systems, such as HP-UX and Solaris, a single program
may have more than one "thread" 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.
   GDB provides these facilities for debugging multi-thread programs:
   * automatic notification of new threads
   * `thread THREADNO', a command to switch among threads
   * `info threads', a command to inquire about existing threads
   * `thread apply [THREADNO] [ALL] ARGS', a command to apply a command
     to a list of threads
   * thread-specific breakpoints
     _Warning:_ These facilities are not yet available on every GDB
     configuration where the operating system supports threads.  If
     your GDB does not support threads, these commands have no effect.
     For example, a system without thread support shows no output from
     `info threads', and always rejects the `thread' command, like this:
          (gdb) info threads
          (gdb) thread 1
          Thread ID 1 not known.  Use the "info threads" command to
          see the IDs of currently known threads.
   The GDB thread debugging facility allows you to observe all threads
while your program runs--but whenever GDB takes control, one thread in
particular is always the focus of debugging.  This thread is called the
"current thread".  Debugging commands show program information from the
perspective of the current thread.
   Whenever GDB detects a new thread in your program, it displays the
target system's identification for the thread with a message in the
form `[New SYSTAG]'.  SYSTAG is a thread identifier whose form varies
depending on the particular system.  For example, on LynxOS, you might
see
     [New process 35 thread 27]
when GDB notices a new thread.  In contrast, on an SGI system, the
SYSTAG is simply something like `process 368', with no further
qualifier.
   For debugging purposes, GDB associates its own thread number--always
a single integer--with each thread in your program.
`info threads'
     Display a summary of all threads currently in your program.  GDB
     displays for each thread (in this order):
       1. the thread number assigned by GDB
       2. the target system's thread identifier (SYSTAG)
       3. the current stack frame summary for that thread
     An asterisk `*' to the left of the GDB thread number indicates the
     current thread.
     For example,
     (gdb) info threads
       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
   On HP-UX systems:
   For debugging purposes, GDB associates its own thread number--a
small integer assigned in thread-creation order--with each thread in
your program.
   Whenever GDB detects a new thread in your program, it displays both
GDB's thread number and the target system's identification for the
thread with a message in the form `[New SYSTAG]'.  SYSTAG is a thread
identifier whose form varies depending on the particular system.  For
example, on HP-UX, you see
     [New thread 2 (system thread 26594)]
when GDB notices a new thread.
`info threads'
     Display a summary of all threads currently in your program.  GDB
     displays for each thread (in this order):
       1. the thread number assigned by GDB
       2. the target system's thread identifier (SYSTAG)
       3. the current stack frame summary for that thread
     An asterisk `*' to the left of the GDB thread number indicates the
     current thread.
     For example,
     (gdb) info threads
         * 3 system thread 26607  worker (wptr=0x7b09c318 "@") \
     at quicksort.c:137
           2 system thread 26606  0x7b0030d8 in __ksleep () \
     from /usr/lib/libc.2
           1 system thread 27905  0x7b003498 in _brk () \
     from /usr/lib/libc.2
`thread THREADNO'
     Make thread number THREADNO the current thread.  The command
     argument THREADNO is the internal GDB thread number, as shown in
     the first field of the `info threads' display.  GDB responds by
     displaying the system identifier of the thread you selected, and
     its current stack frame summary:
          (gdb) thread 2
          [Switching to process 35 thread 23]
          0x34e5 in sigpause ()
     As with the `[New ...]' message, the form of the text after
     `Switching to' depends on your system's conventions for identifying
     threads.
`thread apply [THREADNO] [ALL]  ARGS'
     The `thread apply' command allows you to apply a command to one or
     more threads.  Specify the numbers of the threads that you want
     affected with the command argument THREADNO.  THREADNO is the
     internal GDB thread number, as shown in the first field of the
     `info threads' display.  To apply a command to all threads, use
     `thread apply all' ARGS.
   Whenever GDB stops your program, due to a breakpoint or a signal, it
automatically selects the thread where that breakpoint or signal
happened.  GDB alerts you to the context switch with a message of the
form `[Switching to SYSTAG]' to identify the thread.
   *Note Stopping and starting multi-thread programs: Thread Stops, for
more information about how GDB behaves when you stop and start programs
with multiple threads.
   *Note Setting watchpoints: Set Watchpoints, for information about
watchpoints in programs with multiple threads.