Why is #define throwing "error: expected declaration specifiers"?-CodePudding

I'm new so apologies if my formatting is a little off. I'm writing a simple OpenMP program in order to get the hang of it, and I've been stopped completely dead in my tracks by a strange compilation error. My serial implementation compiles just fine (with gnu11), but my parallel compilation seems to fail for some reason I can't locate.

The entire code up to the point of failure is as follows (and the error I receive from make follows thereafter)

#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#include <string.h>

#define N_THR 1
#define MIN_SIZE 3 //NOTE: min_size is inclusive here
#ifdef _OPENMP
    #include <omp.h>
    #undef N_THR
    #define N_THR 4
    #undef MIN_SIZE
    #define MIN_SIZE N_THR
    //omp_set_dynamic(false); //we need to explicitly disable dynamic teams to force 4 threads
    omp_set_num_threads(N_THR);
#endif

gcc maze.c -o maze
gcc maze.c -fopenmp -o mazep
maze.c:11:16: error: expected declaration specifiers or ‘...’ before numeric constant
  #define N_THR 4
                ^
maze.c:15:22: note: in expansion of macro ‘N_THR’
  omp_set_num_threads(N_THR);
                      ^~~~~
make: *** [Makefile:5: parallel] Error 1

Is there some deep C language syntax hint I'm missing or is it something a little more obvious?

CodePudding user response：

It's because you are calling omp_set_num_threads() outside of a function.

CodePudding user response：

omp_set_num_threads(N_THR);

I don't think you can call functions inside preprocessor blocks