GNU Info
(gdb.info)Threads
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, for
more information about how GDB behaves when you stop and start programs
with multiple threads.
Note: Setting watchpoints, for information about
watchpoints in programs with multiple threads.
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