GCC pragma: error: expected expression before ‘#pragma’

Please consider the following minimal example:

#include <stdio.h>

// Compile with:
// gcc -g -o myprogram main.c

//#define SPECIAL

typedef struct {
  int id;
  char name[50];
  float value;
} MyStruct;

MyStruct example = {
  #ifdef SPECIAL
  .id = 43,
  #else
  #pragma message("Hello from structure")
  .id = 42,
  #endif
  .name = "Example Name",
  .value = 3.14f,
};

int main() {
  printf("ID: %d\n", example.id);

  return 0;
}

When I try to build this, I get:

$  gcc -g -o myprogram main.c && ./myprogram
main.c:18:11: error: expected expression before ‘#pragma’
   18 |   #pragma message("Hello from structure")
      |           ^~~~~~~

Compiler I use:

$  gcc --version | head -1
gcc (Ubuntu 11.4.0-1ubuntu1~22.04.2) 11.4.0

I always thought, the preprocessor is independent of C code, so I can put preprocessor statements wherever I want? So is the problem here that I have the #pragma message in between initializing fields of a structure? But then - there is no problem using #if etc in between fields of a structure, as the above example shows if you comment the #pragma message line ?!

So - is there any way to get this #pragma message to work at the position it is in?

Solution

That is actually a long running half-bug half-design-inconsistency in the gcc itself.

It boils down to a "#pragma is a statement". Pragmas can be used anywhere a statement can, but not be part of another statement. Even if other preprocessor's words like #define and #ifdef can.

Said that, the easiest way to solve your problem is to move the #pragma message out of struct definition and replace it with a special define:

#include <stdio.h>

// Compile with:
// gcc -g -o myprogram main.c

//#define SPECIAL

typedef struct {
  int id;
  char name[50];
  float value;
} MyStruct;

MyStruct example = {
  #ifdef SPECIAL
  .id = 43,
  #else
  #define HIT_STRUCT_WITH_42
  .id = 42,
  #endif
  .name = "Example Name",
  .value = 3.14f,
};

#ifdef HIT_STRUCT_WITH_42
  #pragma message("Hello from structure")
#endif

int main() {
  printf("ID: %d\n", example.id);

  return 0;
}