lib.c

static int addone(int a) {
    return a + 1;
}

int addtwo(int a) {
    return addone(addone(a));
}

main.c

#include <stdio.h>

static int addone(int);

int main(int argc, char** argv) {
    int a = 4;
    printf("a is %d\n", a);
    int b = addone(a);
    printf("b is %d\n", b);

    return 0;
}