Thread: main.c: In function 'main': main.c:12:28: warning: implicit declaration of function '

First, I have created two header files: add.h and subtract.h
Next, created two source files: add.c and subtract.c.

Finally, created main program in main.c:

gcc compiler

Code:

//add.h

#ifndef ADD_H
#define ADD_H

int add(int x, int y);

#endif

//subtract.h

#ifndef SUBTRACT_H
#define SUBTRACT_H

int subtract(int x, int y);

#endif


//add.c
#include "add.h"

int add(int x, int y) {
    return x + y;
}

//subtract.c
#include "subtract.h"

int subtract(int x, int y) {
    return x - y;
}

//main.c 

#include <stdio.h>
#include "add.h"
#include "subtract.h"

int main() {
    int x = 10, y = 5;
    
    printf("x + y = %d\n", add(x, y));
    printf("x - y = %d\n", subtract(x, y));
    
    return 0;
}

Here's the compile command:
gcc -o main add.c subtract.c main.c

Code:

main.c: In function 'main':
main.c:12:28: warning: implicit declaration of function 'subtract' [-Wimplicit-function-declaration]
     printf("x - y = %d\n", subtract(x, y));
                            ^~~~~~~~
C:\Users\33\AppData\Local\Temp\ccfJeU2u.o:subtract.c:(.text+0x0): multiple definition of `add'
C:\Users\33\AppData\Local\Temp\ccGHzJAD.o:add.c:(.text+0x0): first defined here
C:\Users\33\AppData\Local\Temp\ccqKrrPu.o:main.c:(.text+0x0): multiple definition of `add'
C:\Users\33\AppData\Local\Temp\ccGHzJAD.o:add.c:(.text+0x0): first defined here
collect2.exe: error: ld returned 1 exit status