A.17 Using Nestable Locks

 

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The following example (for Section 3.2 on page 41) demonstrates how a nestable lock can be used to synchronize updates both to a whole structure and to one of its members.

#include <omp.h>  
typedef struct {int a,b; omp_nest_lock_t lck;} pair;  
  
void incr_a(pair *p, int a)  
{  
    // Called only from incr_pair, no need to lock.  
    p->a += a;  
}  
  
void incr_b(pair *p, int b)  
{  
    // Called both from incr_pair and elsewhere,  
    // so need a nestable lock.  
  
    omp_set_nest_lock(&p->lck);  
    p->b += b;  
    omp_unset_nest_lock(&p->lck);  
}  
  
void incr_pair(pair *p, int a, int b)  
{  
    omp_set_nest_lock(&p->lck);  
    incr_a(p, a);  
    incr_b(p, b);  
    omp_unset_nest_lock(&p->lck);  
}  
  
void f(pair *p)  
{  
    extern int work1(), work2(), work3();  
    #pragma omp parallel sections  
    {  
        #pragma omp section  
            incr_pair(p, work1(), work2());  
        #pragma omp section  
            incr_b(p, work3());  
    }  
}