Debugging programs with multiple threads

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Debugging programs with multiple threads

In some operating systems, 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 the following 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 instance, a system without thread support shows no output from ‘info threads’ and always rejects the thread command, like the following example shows.

(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 the following order):

a. The thread number assigned by GDB.

b. The target system’s thread identifier (systag).

c. The current stack frame summary for that thread.

An asterisk ‘*’ to the left of the GDB thread number indicates the current thread. Use the following example for clarity.

(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

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.
See Stopping and starting multi-thread programs for more information about how GDB behaves when you stop and start programs with multiple threads.
See Setting watchpoints for information about watchpoints in programs with multiple threads.

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