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
• `set print thread-events', which controls printing of messages on thread start and exit.
• `set libthread-db-search-path path', which lets the user specify which libthread_db to use if the default choice isn't compatible with the program.

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 gnu/Linux, you might see

     [New Thread 46912507313328 (LWP 25582)]

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

On Solaris, you can display more information about user threads with a Solaris-specific command:

maint info sol-threads

Display info on Solaris user threads.

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.

The debugger convenience variable `$_thread' contains the number of the current thread. You may find this useful in writing breakpoint conditional expressions, command scripts, and so forth. See See Convenience Variables, for general information on convenience variables.

thread apply [threadno] [all] command

The thread apply command allows you to apply the named command to one or more threads. Specify the numbers of the threads that you want affected with the command argument threadno. It can be a single thread number, one of the numbers shown in the first field of the `info threads' display; or it could be a range of thread numbers, as in 2-4. To apply a command to all threads, type thread apply all command.

set print thread-events

set print thread-events on

set print thread-events off

The set print thread-events command allows you to enable or disable printing of messages when gdb notices that new threads have started or that threads have exited. By default, these messages will be printed if detection of these events is supported by the target. Note that these messages cannot be disabled on all targets.

show print thread-events

Show whether messages will be printed when gdb detects that threads have started and exited.

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.

set libthread-db-search-path [path]

If this variable is set, path is a colon-separated list of directories gdb will use to search for libthread_db. If you omit path, `libthread-db-search-path' will be reset to an empty list.

On gnu/Linux and Solaris systems, gdb uses a “helper” libthread_db library to obtain information about threads in the inferior process. gdb will use `libthread-db-search-path' to find libthread_db. If that fails, gdb will continue with default system shared library directories, and finally the directory from which libpthread was loaded in the inferior process.

For any libthread_db library gdb finds in above directories, gdb attempts to initialize it with the current inferior process. If this initialization fails (which could happen because of a version mismatch between libthread_db and libpthread), gdb will unload libthread_db, and continue with the next directory. If none of libthread_db libraries initialize successfully, gdb will issue a warning and thread debugging will be disabled.

Setting libthread-db-search-path is currently implemented only on some platforms.

show libthread-db-search-path

Display current libthread_db search path.

set debug libthread-db

show debug libthread-db

Turns on or off display of libthread_db-related events. Use 1 to enable, 0 to disable.

 

gdb supports debugging programs with multiple threads (see Debugging Programs with Multiple Threads). There are two modes of controlling execution of your program within the debugger. In the default mode, referred to as all-stop mode, when any thread in your program stops (for example, at a breakpoint or while being stepped), all other threads in the program are also stopped by gdb. On some targets, gdb also supports non-stop mode, in which other threads can continue to run freely while you examine the stopped thread in the debugger.

• All-Stop Mode: All threads stop when GDB takes control
• Non-Stop Mode: Other threads continue to execute
• Background Execution: Running your program asynchronously
• Thread-Specific Breakpoints: Controlling breakpoints
• Interrupted System Calls: GDB may interfere with system calls
• Observer Mode: GDB does not alter program behavior