Is it possible to use OpenMP directives in multiple files in a program?

I've got a C program which consists of multiple .c files and multiple .h files. I'd like to have one #pragma omp parallel directive (so that all of the threads are on开发者_StackOverflow社区ly created once) in the main function, and then do other OpenMP things like #pragma omp for in other files.

However, I can't seem to do this. When compiling the main file it complains that some of the variables mentioned in the private() and shared() bits of the #pragma omp parallel directive don't exist (which they don't, in that file - because they're in the other file), and when compiling the other file it complains that I have an #pragma omp for without an enclosing #pragma omp parallel.

The code is nicely split between the files, and I'd hate to have to put it all back into one file. Is there any way around this?

---

You definitely can:

outer.c:

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

void inner(int n);

int main(int argc, char **argv) {
    int n=15;

    #pragma omp parallel shared(n) num_threads(4)
    {
        inner(n);
    }

    return 0;
}

inner.c:

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

void inner(int n) {
    int thread = omp_get_thread_num();
    printf("%3d: got %d\n", thread, n);

    #pragma omp for
    for  (int i=0;i<n;i++) {
        int newthread=omp_get_thread_num();
        printf("%3d: doing iter %d.\n",newthread,i);
    }
}

and running:

$ make
gcc -fopenmp -std=c99   -c -o outer.o outer.c
gcc -fopenmp -std=c99   -c -o inner.o inner.c
gcc -o nested outer.o inner.o -fopenmp -std=c99 -lgomp    
$ ./nested 
  3: got 15
  3: doing iter 12.
  3: doing iter 13.
  3: doing iter 14.
  0: got 15
  0: doing iter 0.
  0: doing iter 1.
  0: doing iter 2.
  0: doing iter 3.
  1: got 15
  1: doing iter 4.
  1: doing iter 5.
  1: doing iter 6.
  1: doing iter 7.
  2: got 15
  2: doing iter 8.
  2: doing iter 9.
  2: doing iter 10.
  2: doing iter 11.

But no, you can't set the sharing attributes of variables in one routine from another -- they're just not in scope. You can't set their sharing any more than you can set their value.

Once you've launched (say) inner, everything there is private; you'd have to pass any shared things in as shared.

Just to clairify that bit about "everything there is private": the above isn't any different than

    int n=15;

    #pragma omp parallel shared(n) num_threads(4)
    {
        int thread = omp_get_thread_num();
        printf("%3d: got %d\n", thread, n);

        #pragma omp for
        for  (int i=0;i<n;i++) {
            int newthread=omp_get_thread_num();
            printf("%3d: doing iter %d.\n",newthread,i);
        }
    }

since thread, i, and newthread are defined inside the parallel block -- whether inside a function or not -- they're all necessarily private.

---

Did you try "extern XY" to make the variables known in the other file?

Can't you use a common header for all your files that declares the variables in question?

Are they local variables (then you cannot do this anyway)?