Debugging with GDB Manual (5900-1473; WDB 6.2; January 2011)

((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.
5.4 Stopping and starting multi-thread programs 65