gdb : multithreading

Question

Hi,

I have a program which uses two threads. I have put the break point in both the threads. While running the program under gdb I want to switch between the threads and make them run. (thread t1 is active and running and thread t2; when paused on the breakpoint. I want to stop T1 running and run the T2).

Is there any way that I can schedule the threads in gdb?

Thanks Arpit

Answer 1

use break conditions

(gdb) break frik.c:13 thread 28 if bartab > lim

see Debugging with GDB

Edit:

(gdb) break <thread_function_entry_point> thread 2
(gdb) break <thread_function_entry_point> thread 1
(gdb) thread 1
(gdb) continue
(gdb) ... thread 1 finishes
(gdb) thread 2
(gdb) continue

You can put these commands inside a .gdbrc file.

Answer 2

See the GDB manual page on Debugging Programs with Multiple Threads, specifically you want to use the thread command to switch the active thread.

Answer 3

By default, GDB stops all threads when any breakpoint is hit, and resumes all threads when you issue any command (such as continue, next, step, finish, etc.) which requires that the inferior process (the one you are debugging) start to execute.

However, you can tell GDB not to do that:

(gdb) help set scheduler-locking 
Set mode for locking scheduler during execution.
off  == no locking (threads may preempt at any time)
on   == full locking (no thread except the current thread may run)
step == scheduler locked during every single-step operation.
    In this mode, no other thread may run during a step command.
    Other threads may run while stepping over a function call ('next').

So you'll want to set breakpoints, then set scheduler-locking on, then continue or finish in thread 1 (thread 2 is still stopped), then Ctrl-C to regain control of GDB, switch to thread 2, continue (thread 1 is still stopped), etc.

Beware: by setting scheduler-locking on it is very easy to cause the inferior process to self-deadlock.