How to continue one thread at a time when debugging a multithreaded program in GDB?

Question

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 th

Answer 1

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.

Answer 2

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 3

If you're using GDB 7 or later, try "non-stop mode".

http://sourceware.org/gdb/current/onlinedocs/gdb/Non_002dStop-Mode.html

The "scheduler-locking on" command previously mentioned allows you step one thread with the others stopped. Non-stop mode allows you to step one thread with the others active.