critical

Specifies that code is only be executed on one thread at a time.

#pragma omp critical [(name)]  
{  
   code_block  
}  

where,

(name) (optional)
A name to identify the critical code. Note that name must be enclosed in parentheses.

The critical directive supports no OpenMP clauses.

For more information, see 2.6.2 critical Construct.

// omp_critical.cpp  
// compile with: /openmp   
#include <omp.h>  
#include <stdio.h>  
#include <stdlib.h>  
  
#define SIZE 10  
  
int main()   
{  
    int i;  
    int max;  
    int a[SIZE];  
  
    for (i = 0; i < SIZE; i++)   
    {  
        a[i] = rand();  
        printf_s("%d\n", a[i]);  
    }  
  
    max = a[0];  
    #pragma omp parallel for num_threads(4)  
        for (i = 1; i < SIZE; i++)   
        {  
            if (a[i] > max)  
            {  
                #pragma omp critical  
                {  
                    // compare a[i] and max again because max   
                    // could have been changed by another thread after   
                    // the comparison outside the critical section  
                    if (a[i] > max)  
                        max = a[i];  
                }  
            }  
        }  
  
    printf_s("max = %d\n", max);  
}  

41  
18467  
6334  
26500  
19169  
15724  
11478  
29358  
26962  
24464  
max = 29358  

Directives