[C/Cpp] How to include an external function in a C code

If you'd like to use an external function of add.c in main.c, you need to import the function in main.c.
Then, compile main.c with having add.c in same folder.


main.c

#include <stdio.h>
#include "add.c"
 
extern int add(int x, int y);
extern int sum;
 
 
void main(void)
{
 int a=2, b=5;
 
 sum = add(a,b);
 printf("%d\n", sum);
}

add.c

#include <stdio.h>
 
int add(int x, int y);
int sum=0;
 
int add(int x, int y)
{
 sum = x+y;
 return sum;
}