openmp for block error

Question

Why does openmp give me this error :-

error: for statement expected before ‘{’ token

#include <stdio.h>
#include <stdlib.h>
#include <omp.h>
#include <unistd.h>

int main (int argc, char *argv[]) 
{

#pragma omp parallel 
{

int a[100],b[100],c[100];
int i =0;

    for(; i < 100; i++){
    a[i] = i;
    b[i] = i;
    }

    #pragma omp parallel for schedule(static,5)
    {
    int i = 0;
        for( ; i < 100 ; i++){ // this is the for loop that is referred in the error message
    c[i] = a[i] + b[i];
    }

    }

}

printf("Outside parallel block \n");

}

Jonathan Dursi · Accepted Answer · 2012-02-13 17:15:37Z

6

First, the second OpenMP pragma shouldn't have a "parallel" in it; you've already opened a parallel block, you just now need to share the work of the for loop.

Second, you can't have the parallel for enclose a general block; it has to be a for loop. If you really want a different i than is used above, do:

#pragma omp for schedule(static,5)
for (int i=0; i < 100; i++)
{
    c[i] = a[i] + b[i];
}

answered Feb 13, 2012 at 17:15

Jonathan Dursi

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Community · Accepted Answer · 2017-05-23 12:00:53Z

This is an answer I posted to this question before I realised it is a duplicate of this one. As it somehow expands on the answers already given here, I'm reposing it.

parallel for is a combined directive. It combines parallel and for in a single directive, which allows for some space saving.

#pragma omp parallel for shared(sum) private(i,j,k)
...

is basically a shorthand notation for

#pragma omp parallel shared(sum) private(i,j,k)
{
   #pragma omp for
   ...
}

(the private clause could also be attributed to the for directive)

Since the syntax of the for directive requires that it is immediately followed by a for-loop, the same syntax requirement applies to the parallel for directive. In other words, the ... above can only be a for-loop with its associated (block) body:

#pragma omp parallel for ...
for (...)
{
   ...
}

Having the for-loop inside a block is erroneous. Having the entire parallel region inside a block is not though. The following is a perfectly valid syntax:

{
   #pragma omp parallel for ...
   for (...)
   {
      ...
   }
}

mikithskegg · Accepted Answer · 2012-02-13 17:12:09Z

1

Do so

int i;
#pragma omp parallel for schedule(static,5)
for (i=0;...

answered Feb 13, 2012 at 17:12

mikithskegg

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2 Comments

user9869932 Over a year ago

I have been thinking about this. My concern is, if 'i' becomes a shared variable for different threads (instead of individual to each thread). Will the result be erratic due to different threads writing on the same variable?

NoseKnowsAll Over a year ago

@julianromera Yes. This answer needs to also state private(i) in order to be correct.

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