c - Why does #define INVALID_VALUE -999; give a syntax error when used?

Question

I am trying to compare to a defined constants in C, and I have simplified my program to the following:

#include "stdio.h"
#include "stdlib.h"
#define INVALID_VALUE -999;

int main(void)
{
    int test=0;
    if(test==INVALID_VALUE) //The error line..
        return INVALID_VALUE;
    return 0;
}

And when I use gcc to compile, it gives out error "error: expected ‘)’ before ‘;’ token".

Is there any reason that this cannot be done?

Answer 1

Remove the semicolon from your INVALID_VALUE definition.

Macros are replaced lexically (character-by-character) with no understanding of the syntax around them. Your macro INVALID_VALUE is set to -999;, so your if line expands the macro to:

if (test==-999;)

which is invalid C syntax.