How to use lock in OpenMP?

Question

You want the OMP_SET_LOCK/OMP_UNSET_LOCK functions: https://hpc.llnl.gov/tuts/openMP/#OMP_SET_LOCK

Basically:

omp_lock_t writelock;

omp_init_lock(&writelock);

#pragma omp parallel for
for ( i = 0; i < x; i++ )
{
    // some stuff
   omp_set_lock(&writelock);
    // one thread at a time stuff
    omp_unset_lock(&writelock);
    // some stuff
}

omp_destroy_lock(&writelock);

Most locking routines such as pthreads semaphores and sysv semaphores work on that sort of logic, although the specific API calls are different.

For the benefit of those coming after, using critical is another option. You can even make named critical sections.

For example:

#include <omp.h>

void myParallelFunction()
{
    #pragma omp parallel for
    for(int i=0;i<1000;++i)
    {

        // some expensive work 

        #pragma omp critical LogUpdate
        {
            // critical section where you update file        
        }

        // other work

        #pragma omp critical LogUpdate
        {
            // critical section where you update file      
        }
    }
} 

Edit: There's a great thread in the comments initiated by Victor Eijkhout. Summarizing and paraphrasing: In short critical locks a code segment. That can be overkill in more complex examples where all you want to do is lock a specific data item. It's important to understand this before you make a choice between the two methods.

#pragma omp critical
{
    // write to file here
}