Is it necessary to set loop index as a private variable in OpenMP? -


i reading openmp tutorial, , come across following program: 

#include <omp.h> #include <stdio.h> #include <stdlib.h> #define n       100  int main (int argc, char *argv[]) {  int nthreads, tid, i; float a[n], b[n], c[n];  /* initializations */ (i=0; < n; i++)   a[i] = b[i] = i;   #pragma omp parallel shared(a,b,c,nthreads) private(i,tid)   {   tid = omp_get_thread_num();   if (tid == 0)     {     nthreads = omp_get_num_threads();     printf("number of threads = %d\n", nthreads);     }    printf("thread %d starting...\n",tid);    #pragma omp    (i=0; i<n; i++)     {     c[i] = a[i] + b[i];     printf("thread %d: c[%d]= %f\n",tid,i,c[i]);     }    }  /* end of parallel section */  }

i little confused whether loop index i must private variable or not. since same tutorial:

all threads can modify , access variables (except loop index)

so seems threads can't control i, right? btw, try remove i private variable, , result seems ok.

no it's not necessary specify loop index variable private. openmp enforces that, , somewhere in many many pages standard states much.

further requirements of openmp standard forbid adjustments loop index variable inside loop. in effect openmp enforces on c (and c++) programs 1 of constraints built fortran. enables run-time schedule multiple iterations across threads when loop first encountered, without worrying distribution might invalidated during execution.

bear in mind threads allocated, @ loop initialisation, sets of values of i distribute individual loop iterations according schedule specified (either programmer or implementation-defined default). allowing threads update 'local' value i lead mad code


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