Debugging with GDB Manual The GNU Source-Level Debugger (769148-001, March 2014)

You can use the thread qualifier on conditional breakpoints
as well; in this case, place 'thread threadno' before
the breakpoint condition, like this:
((gdb)) break frik.c:13 thread 28 if bartab >
lim
Whenever your program stops under GDB for any reason, all threads of execution stop, not just
the current thread. This allows you to examine the overall state of the program, including switching
between threads, without worrying that things may change underfoot.
Conversely, whenever you restart the program, all threads start executing. This is true even when
single-stepping with commands like step or next.
In particular, GDB cannot single-step all threads in lockstep. Since thread scheduling is up to your
debugging target's operating system (not controlled by GDB), other threads may execute more
than one statement while the current thread completes a single step. Moreover, in general other
threads stop in the middle of a statement, rather than at a clean statement boundary, when the
program stops.
You might even find your program stopped in another thread after continuing or even
single-stepping. This happens whenever some other thread runs into a breakpoint, a signal, or an
exception before the first thread completes whatever you requested.
On some OSes, you can lock the OS scheduler and thus allow only a single thread to run.
set scheduler-locking mode Set the scheduler locking mode. If it is off, then there is no
locking and any thread may run at any time. If on, then
only the current thread may run when the inferior is resumed.
The step mode optimizes for single-stepping. It stops other
threads from “seizing the prompt” by preempting the current
thread while you are stepping. Other threads will only rarely
(or never) get a chance to run when you step. They are more
likely to run when you 'next' over a function call, and they
are completely free to run when you use commands like
'continue', 'until', or 'finish'. However, unless
another thread hits a breakpoint during its timeslice, they
will never steal the GDB prompt away from the thread that
you are debugging.
show scheduler-locking Display the current scheduler locking mode.
Stopping and starting multi-thread programs 49