D.1.17 _Pragma

6.10.9 Pragma operator

A unary operator expression of the form: _Pragma ( string-literal ) is processed as follows:

• The L prefix of the string literal is deleted, if it is present.

• The leading and trailing double-quotes are deleted.

• Each escape sequence ’ is replaced by a double-quote.

• Each escape sequence \\ is replaced by a single backslash.

The resulting sequence of preprocessing tokens are processed as if they were the preprocessor tokens in a pragma directive.

The original four preprocessing tokens in the unary operator expression are removed.

_Pragma offers an advantage over #pragma in that _Pragma can be used in a macro definition.

_Pragma("string") behaves exactly the same as #pragma string. Consider the following example. First, the example’s source code is listed and then the example’s source is listed after the preprocessor has made it’s pass.

example% cat test.c

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

#define Pragma(x) _Pragma(#x)
#define OMP(directive) Pragma(omp directive)

void main()
{
  omp_set_dynamic(0);
  omp_set_num_threads(2);
  OMP(parallel)
  {
  printf("Hello!\n");
  }
}

example% cc test.c -P -xopenmp -x03
example% cat test.i

Here’s the source after the preprocessor has finished.

void main()
{
  omp_set_dynamic(0);
  omp_set_num_threads(2);
  # pragma omp parallel
  {
    printf("Hellow!\n");
  }
}

example% cc test.c -xopenmp -->
example% ./a.out
Hello!
Hello!
example%