C preprocessor #if expression

Question

I am a bit confused on the type of expression we can use with the #IF preprocessor in the C language. I tried the following code, and it isn't working. Please explain and provide examples for expressions that can be used with the preprocessor.

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

int c=1;

#if c==1
    #define check(a) (a==1)?a:5
    #define TABLE_SIZE 100
#endif

int main()
{
    int a = 0, b;
    printf("a = %d\n", a);
    b = check(a);
    printf("a = %d %d\n", a, TABLE_SIZE);
    system("PAUSE");
    return 0;
}

Answer 1

The preprocessor cannot use variables from the C program in expressions - it can only act on preprocessor macros. So when you try to use c in the preprocessor you don't get what you might expect.

However, you also don't get an error because when the preprocessor tries to evaluate an identifier that isn't defined as a macro, it treats the identifier as having a value of zero.

So when you hit this snippet:

#if c==1
#define check(a) (a==1)?a:5
#define TABLE_SIZE 100
#endif

The c used by the preprocessor has nothing to do with the variable c from the C program. The preprocessor looks to see if there's a macro defined for c. Since there isn't, it evaluates the following expression:

#if 0==1

which is false of course.

Since you don't appear to use the variable c in your program, you can do the following to get behavior in line with what you're trying:

#define C 1

#if C==1
#define check(a) (a==1)?a:5
#define TABLE_SIZE 100
#endif

(Note that I also made the macro name uppercase in keeping with convention for macro names.)

Answer 2

The preprocessor is run on the text, before any compilation is done. It doesn't know how to parse C. What you probably wanted instead of int c=1; was

#define C 1

and the test works the way you had it:

#if C == 1

The key here is that this is all defined before compile time. The preprocessor doesn't care about C variables, and certainly doesn't care what their values are.

Note that the convention is to have preprocessor macro names defined in ALL_CAPS.