How to process return in OpenMP parallel code?

2017-01-27

An answer to this question on Stack Overflow.

Question

My requirement is like this: every thread allocates memory itself, then processes it:

typedef struct
{
	......
}A;
A *p[N];
#pragma omp parallel
{
	#pragma omp for
	for (int i = 0; i < N; i++) {
		p[i] = (A*)calloc(sizeof(*p[i]), N);
		if (NULL == p[i]) {
			return;
		}
		......			
	}
}

But the compiler will complain:

error: invalid exit from OpenMP structured block
     return;

So except put the allocating memory code out of the #pragma omp parallel:

for (int i = 0; i < N; i++) {
    p[i] = (A*)calloc(sizeof(*p[i]), N);
	if (NULL == p[i]) {
		return;
	}		
}
#pragma omp parallel
{
	#pragma omp for
	......
}

Is there any better method?

Answer

You're looking for this, I think:

#pragma omp parallel
{
    #pragma omp for
    for (int i = 0; i < N; i++) {
        p[i] = (A*)calloc(sizeof(*p[i]), N);
        if (NULL == p[i]) {
            #pragma omp cancel for
        }
        ......          
    }
}

But you'll need to set the environment variable OMP_CANCELLATION to true for this to work.

You should try to avoid doing this, though, because cancellation is expensive.