Compiling multiple C files in a program

You don't need an extern, but file1.c must see a declaration that foo() exists. Usually this declaration is in a header file.

To add a forward declaration without using a header file, simply modify file1.c to:

int foo();  // add this declaration

int main(){
  foo();
  return 0;
}

The correct way is as follows:

file1.c

#include <stdio.h>
#include "file2.h"

int main(void){
    printf("%s:%s:%d \n", __FILE__, __FUNCTION__, __LINE__);
    foo();
    return 0;
}

file2.h

void foo(void);

file2.c

#include <stdio.h>
#include "file2.h"

void foo(void) {
    printf("%s:%s:%d \n", __FILE__, __func__, __LINE__);
    return;
}

output

$
$ gcc file1.c file2.c -o file -Wall
$
$ ./file 
file1.c:main:6 
file2.c:foo:6 
$

Related