如何将 OpenMP 指令 (#pragmas) 包装为函数或类似函数的宏？

Question

我正在尝试将 OpenMP 与另一种发出 C 代码的语言（C 代码生成器）连接起来。从我的角度来看（我不是其他语言的设计者），通过调用 C 函数或类似函数的宏而不是直接使用 #pragma 或 _Pragma 来做到这一点是最简单的。我对 C 预处理器没有太多经验，但我在 wikipedia 上找到了一个简单的例子，它以一种不令人满意的方式工作。这是原始的 C 示例：

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

int main (int argc, char *argv[]) {
  int th_id, nthreads;
  #pragma omp parallel private(th_id)
  {
    th_id = omp_get_thread_num();
    printf("Hello World from thread %d\n", th_id);
    #pragma omp barrier
    if ( th_id == 0 ) {
      nthreads = omp_get_num_threads();
      printf("There are %d threads\n",nthreads);
    }
  }
  return EXIT_SUCCESS;
}

现在我在这里创建一个宏 (pragma_omp_parallel_private) 来做我想做的事：

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

#define STR(x) #x
#define STRINGIFY(x) STR(x) 
#define omp_par_pri STRINGIFY(omp parallel private)

char omp_test[] = omp_par_pri;
#define pragma_omp_parallel_private(thread_id)        \
  _Pragma(STRINGIFY(omp parallel private ## (thread_id)))

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

  // #pragma omp parallel private(th_id)
  pragma_omp_parallel_private(th_id)
  {
    th_id = omp_get_thread_num();
    printf("Hello World from thread %d\n", th_id);
    #pragma omp barrier
    if ( th_id == 0 ) {
      nthreads = omp_get_num_threads();
      printf("There are %d threads\n",nthreads);
    }
  }
  return EXIT_SUCCESS;
}

预处理器会发出错误，但如果我真的编译预处理代码，它会按预期工作。有没有更合法的方法来实现这一点（更不用说在大多数构建管道中不会导致构建错误的方法）？

$ gcc -E -fopenmp wikiHello.c > wikiHello_pp.c
wikiHello.c:11:34: error: pasting "private" and "(" does not give a valid preprocessing token
   _Pragma(STRINGIFY(omp parallel private ## (thread_id)))
                                  ^
wikiHello.c:17:3: note: in expansion of macro ‘pragma_omp_parallel_private’
   pragma_omp_parallel_private(th_id)
   ^
$ gcc -fopenmp wikiHello_pp.c

$ ./a.exe
Hello World from thread 3
Hello World from thread 6
Hello World from thread 4
Hello World from thread 7
Hello World from thread 1
Hello World from thread 5
Hello World from thread 2
Hello World from thread 0
There are 8 threads

Answer 1

我会在将pragma 转换为字符串文字之前将其连接起来。像这样的：

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

#define STR(x) #x
#define STRINGIFY(x) STR(x) 
#define CONCATENATE(X,Y) X ( Y )

#define pragma_omp_parallel_private(x) \
  _Pragma( STRINGIFY( CONCATENATE(omp parallel private,x) ) )

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

  // #pragma omp parallel private(th_id)
  pragma_omp_parallel_private(th_id)
  {
    th_id = omp_get_thread_num();
    printf("Hello World from thread %d\n", th_id);
    #pragma omp barrier
    if ( th_id == 0 ) {
      nthreads = omp_get_num_threads();
      printf("There are %d threads\n",nthreads);
    }
  }
  return EXIT_SUCCESS;
}