Interprocess semaphores sometimes not working as expected

Question

I have the following C code, where variables prefixed by sm are shared by two processes proc1 and proc2. Therefore the sem

Answer 1

Maybe this is a copy/paste error in the question (you are using copy/paste from the real code, right?), but it looks like you have a bug in proc2's handling:

// ....

else // is proc2
{
    sem_wait( sm_l2f );
    if ( sm_condition )
    {
        sm_value_proc1 = calc_value();  // <---  this should be assigning to
                                        //       sm_value_proc2
        printf( ">>> proc2 value = %u!\n", sm_value_proc2 );
    }
    sem_post( sm_f2l );
} 

Then again, maybe it's a copy/paste error in the actual code?

Also - don't forget that sem_wait() can unblock due to a signal.