Pre-processor

At this stage, the macro such as #define, #include and etc are expanded to C code.

For example,

/* hello.c */
#define TXT "hello"

int main(void) {
        printf("%s\n", TXT);
        return 0;
}

Try to compile it with gcc -E:

$ gcc -E hello.c

The entry TXT is replaced the content defined by #define.

Also the marco is being processed by the pre-processor.

/* swap.c */
#include <stdio.h>
#define SWAP(a,b)  { int c; c = a; a = b; b = c; }

int main(void)
{
        int i = 10, j = 20;
        printf("before swap: i = %d, j = %d\n", i, j);
        SWAP(i, j);
        printf("after swap: i = %d, j = %d\n", i, j);
        return 0;
}

By using gcc -E:

$ gcc -E swap.c

For #include, the pre-processor will also expand the expression and put it in code before compilation.

For example,

// header.h
int add_fun(int a, int b) {
        return (a + b);
}

// include.c
#include "header.h"

int main(void) {
        add_fun(1,2);
        return 0;
}

By using gcc -E :

gcc –E program/include.c

*Output*