Thread Synchronization

In our study of MPI, we learned that it is sometimes necessary to force all processes to stop so that they can catch up and synchronize. Similar circumstances can arise in threaded programming.

Barrier

When a thread encounters a barrier construct, it will wait till all threads have reached the barrier.

C/C++

#pragma omp barrier

Fortran

$!omp barrier

Python

omp("barrier")

This enables the threads to synchronize but does tend to serialize the code.

Barrier is a single directive and does not introduce a code block. In particular, it is not used with with in omp4py.

Single and Master

Suppose we only want to see the output from within a parallel region once.

The single construct directs the compiler that only a single thread should execute the block of code within the block.

Syntax

C/C++

#pragma omp single 
  {code block}

Fortran

!$omp single
   code
!$omp end single

Python

with omp("single"):

With `single` the first thread that reaches it executes the segment. The `master` directive causes only the master thread (thread ID 0) to execute the section.

C/C++
```c
#pragma omp master {code block}

Fortran

!$omp master
   code
!$omp end master

Python

with omp("master"):

The compiler inserts an implicit barrier synchronization at the end of every parallel or parallel for or for directive (but not, as we have seen, loop), or after a master or single block.

Examples

It’s annoying for every thread to print the exit condition, since it’s the same for all. Let’s have only the master thread print it.

C/C++

   for (int i=0; i<M; i++) {
       low = a[i];
       high = b[i];

       if (low > high) {
          #pragma omp master
          printf ("Exiting (%d)\n", i);
          break;
       }

Fortran

do i=1,M
      low = a(i)
      high = b(i)

      if (low > high) then
         !$omp master
         write(*,*) "Exiting ",i
         !$omp end master
         exit
      endif

Python Omp4py currently has some limitations on the master directive so we will not discuss it for this case.

Last updated on Aug 19, 2026

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