As mentioned earlier, there are two ways to use a debugger, either by starting the application from within the debugger (or attaching to the running process), or by loading a core dump file. Within the debugging session, one can either interrupt the running process (with Ctrl+C, which sends the SIGINT signal), or load the debug symbols for the loaded core dump. After this, we can examine the active threads in this frame:

Thread 1 received signal SIGINT, Interrupt.
0x00007fff8a3fff72 in mach_msg_trap () from /usr/lib/system/libsystem_kernel.dylib
(gdb) info threads
Id   Target Id         Frame 
* 1    Thread 0x1703 of process 72492 0x00007fff8a3fff72 in mach_msg_trap () from /usr/lib/system/libsystem_kernel.dylib
3    Thread 0x1a03 of process 72492 0x00007fff8a406efa in kevent_qos () from /usr/lib/system/libsystem_kernel.dylib
10   Thread 0x2063 of process 72492 0x00007fff8a3fff72 in mach_msg_trap () from /usr/lib/system/libsystem_kernel.dylibs
14   Thread 0x1e0f of process 72492 0x00007fff8a405d3e in __pselect () from /usr/lib/system/libsystem_kernel.dylib
(gdb) c
Continuing.
  

In the preceding code, we can see how after sending the SIGINT signal to the application (a Qt-based application running on OS X), we request the list of all threads which exist at this point in time along with their thread number, ID, and the function which they are currently executing. This also shows clearly which threads are likely waiting based on the latter information, as is often the case of a graphical user interface application like this one. Here we also see that the thread which is currently active in the application as marked by the asterisk in front of its number (thread 1).

We can also switch between threads at will by using the thread <ID> command, and move up and down between a thread's stack frames. This allows us to examine every aspect of individual threads.

When full debug information is available, one would generally also see the exact line of code that a thread is executing. This means that during the development stage of an application, it makes sense to have as much debug information available as possible to make debugging much easier.