C macros beyond the usual examples

preprocessor.c

/* 
A single '#' will create a string from the given argument, 
regardless of what that argument contains, 
while the double '##' will create a new token by concatenating the arguments.
*/

/* Run with compiler flag `-E` using gcc to see the pre-processor output. */

#ifdef ERR
#error "Hey! You've got a compile time error."
#endif

#define f(a,b) a##b
#define s(a) #a

#define PRINT_TOKEN(token) printf(#token " is %d\n", token)
#define BUILD_FIELD(name) my_struct.inner_struct.field_##name

#include <stdio.h>

typedef struct {
    int field_age;
} inner_struct_t;

typedef struct {
    inner_struct_t inner_struct;
} my_struct_t;

int main() {
    int GET_A = 9;

    printf("%d\n", f(GET_, A)); // 9
    printf("%s\n", s(GET_A)); // GET_A
    // printf("%s\n", #GET_A); // Error
    // printf("%d\n", GET_##A) // Error

    PRINT_TOKEN(GET_A); // GET_A is 9

    my_struct_t my_struct = {
        .inner_struct = {
            .field_age = 45
        }
    };

    PRINT_TOKEN(BUILD_FIELD(age)); // BUILD_FIELD(age) is 45

    return 0;
}

/*
Expected console output
-----------------------

9
GET_A
GET_A is 9
BUILD_FIELD(age) is 45
*/