A.19 Examples Showing Incorrect Nesting of Work-sharing Directives

 

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The examples in this section illustrate the directive nesting rules. For more information on directive nesting, see Section 2.9 on page 33.

The following example is noncompliant because the inner and outer for directives are nested and bind to the same parallel directive:

void wrong1(int n)  
{  
  #pragma omp parallel default(shared)  
  {  
      int i, j;  
      #pragma omp for  
      for (i=0; i<n; i++) {  
          #pragma omp for  
              for (j=0; j<n; j++)  
                 work(i, j);  
     }  
   }  
}  

The following dynamically nested version of the preceding example is also noncompliant:

void wrong2(int n)  
{  
  #pragma omp parallel default(shared)  
  {  
    int i;  
    #pragma omp for  
      for (i=0; i<n; i++)  
        work1(i, n);  
  }  
}  
  
void work1(int i, int n)  
{  
  int j;  
  #pragma omp for  
    for (j=0; j<n; j++)  
      work2(i, j);  
}  

The following example is noncompliant because the for and single directives are nested, and they bind to the same parallel region:

void wrong3(int n)  
{  
  #pragma omp parallel default(shared)  
  {  
    int i;  
    #pragma omp for  
      for (i=0; i<n; i++) {  
        #pragma omp single  
          work(i);  
      }  
  }  
}  

The following example is noncompliant because a barrier directive inside a for can result in deadlock:

void wrong4(int n)  
{  
  #pragma omp parallel default(shared)  
  {  
    int i;  
    #pragma omp for  
      for (i=0; i<n; i++) {  
        work1(i);  
        #pragma omp barrier  
        work2(i);  
      }  
  }  
}  

The following example is noncompliant because the barrier results in deadlock due to the fact that only one thread at a time can enter the critical section:

void wrong5()  
{  
  #pragma omp parallel  
  {  
    #pragma omp critical  
    {  
       work1();  
       #pragma omp barrier  
       work2();  
    }  
  }  
}  

The following example is noncompliant because the barrier results in deadlock due to the fact that only one thread executes the single section:

void wrong6()  
{  
  #pragma omp parallel  
  {  
    setup();  
    #pragma omp single  
    {  
      work1();  
      #pragma omp barrier  
      work2();  
    }  
    finish();  
  }  
}