Using Stack Traces With OpenMP Code

When execution is stopped in parallel region, a where command shows a stack trace that contains the outlined subroutine as well as several runtime library calls. Using the Fortran example from How Compilers Transform OpenMP Code, and stopping execution at line 15, the where command produces the following stack trace.

[t@4 l@4]: where
current thread: t@4
=>[1] _$d1A12.MAIN_(), line 15 in "example.f90"
[2] __mt_run_my_job_(0x45720, 0xff82ee48, 0x0, 0xff82ee58, 0x0, 0x0), at 0x16860
[3] __mt_SlaveFunction_(0x45720, 0x0, 0xff82ee48, 0x0, 0x455e0, 0x1), at 0x1aaf0

The top frame on the stack is the frame of the outlined function. Even though the code is outlined, the source line number still maps back to 15. The other two frames are for runtime library routines.

When execution is stopped in a parallel region, a where command from a slave thread does not have a stack traceback to its parent thread, as shown in the above example. A where command from the master thread, however, has a full traceback:

[t@4 l@4]: thread t@1
t@1 (l@1) stopped in _$d1A12.MAIN_ at line 15 in file "example.f90"
15           sum = sum + a(i)
[t@1 l@1]: where
current thread: t@1
=>[1] _$d1A12.MAIN_(), line 15 in "example.f90"
[2] __mt_run_my_job_(0x41568, 0xff82ee48, 0x0, 0xff82ee58, 0x0, 0x0), at 0x16860
[3] __mt_MasterFunction_(0x1, 0x0, 0x6, 0x0, 0x0, 0x40d78), at 0x16150
[4] MAIN(), line 12 in "example.f90"

If the number of threads is not large, you might be able to determine how execution reached the breakpoint in a slave thread by using the threads command (see threads Command) to list all the threads, and then switch to each thread to determine which one is the master thread.