difference between omp critical and omp single

single and critical are two very different things. As you mentioned:

• single specifies that a section of code should be executed by single thread (not necessarily the master thread)
• critical specifies that code is executed by one thread at a time

So the former will be executed only once while the later will be executed as many times as there are of threads.

For example the following code

int a=0, b=0;
#pragma omp parallel num_threads(4)
{
    #pragma omp single
    a++;
    #pragma omp critical
    b++;
}
printf("single: %d -- critical: %d\n", a, b);

will print

single: 1 -- critical: 4

I hope you see the difference better now.

For the sake of completeness, I can add that:

• master is very similar to single with two differences:
  1. master will be executed by the master only while single can be executed by whichever thread reaching the region first; and
  2. single has an implicit barrier upon completion of the region, where all threads wait for synchronization, while master doesn’t have any.
• atomic is very similar to critical, but is restricted for a selection of simple operations.

I added these precisions since these two pairs of instructions are often the ones people tend to mix-up…