Are #define-s in a source file local to that object?-CodePudding

I'm unsure of the terminology, but I would like to know if in the following scenario, main.c has access to x.cs #defines:

/*x.h*/
#define ONE 1

/*x.c*/
#include "x.h"
#define TWO 2

/*main.c*/
#include "x.h"
int a = ONE;
int b = TWO;

You could just tell me to run gcc -o out main.c x.c if I want to know. But it might work, yet not be correct. This is why I am asking you. My goal is to have the #defines in x.c be local to that file, so they don't clash with any others in main.c. On the other hand, I do want #defines from x.h to be available to main.c because they need to be used when implementing the functions from x.c

Please advise

CodePudding user response：

Your two .c files are equivalent to the following:
x.c :

#define ONE 1
#define TWO 2

main.c :

#define ONE 1
int a = ONE;
int b = TWO;

Any #define in a .c file will be local to that file. This is a common practice when optimizing code, leave a #define that is exclusively used in a .c file declared in that same c file.

Edit : BUT as #defines and #includes (and many other preprocessor directives) are expanded, this is done from top to bottom. meaning that one directive can then recieve #defines from some #include that was expanded earlier.
Here's an example of what I'm saying:
a.h :

#define A 1

b.h :

#define NEW_A A 1

main.c :

#include "a.h"
#include "b.h"
#include <stdio.h>

int main() {
    printf("%i, %i\n", A, NEW_A);
    return 0;
}

Will output:

1, 2

I'd suggest reading about compilation units and preprocessor directives in C.