int sum(int,int);
int mul(int,int);

#include<stdlib.h>
#include<stdio.h>
#include "header.h"
void main(){
    printf("sum of 4 & 5 is %d ", sum(5,4));
    printf("mul of 4 & 5 is %d ", mul(5,4));
}

gcc -c -g file1.c
gcc -c -g file2.c
ar -r libmylib.a file1.o file2.o

gcc -c -g mymain.c
gcc -L. -o mymain mymain.o -lmylib