Preprocessor error when defining =

Question

I was trying some awkward preprocessing and came up with something like this:

#include <stdio.h>

#define SIX =6

int main(void)
{
  int x=6;
  int y=2;

  if(x=SIX)
    printf("X == 6\n");
  if(y=SIX)
    printf("Y==6\n");

  return 0;
}

gcc gives me the errors:

test.c: In function ‘main’:
test.c:10:8: error: expected expression before ‘=’ token
test.c:12:8: error: expected expression before ‘=’ token

Why is that?

Answer 1

The == is a single token, it cannot be split in half. You should run gcc -E on your code

From GCC manual pages:

-E Stop after the preprocessing stage; do not run the compiler proper. The output is in the form of preprocessed source code, which is sent to the standard output.

Input files that don't require preprocessing are ignored.

For your code gcc -E gives the following output

  if(x= =6)
    printf("X == 6\n");

  if(y= =6)
    printf("Y==6\n");

The second = is what causes the error message expected expression before ‘=’ token