compiler warning while using modules

Question

I made functions in different files(modules) when i compile them it says implicit declaration of function b and same warning for C . i used gcc -c *.c and linked object files using gcc *.o -o prog.

in file m1.c---> #include<stdio.h>

int main() {
    b();
    int num;
    printf("%s\n", "enter number:");
    scanf("%d", &num);
    for (int i = 0; i < c(num); i++) {
        printf("%d\n", i);
    } 
    return 0;
}

in file m2.c--->

#include<stdio.h>

void b() {
    printf("%s\n", "Hello i am a module...\n");
}

in file m3.c--->

#include<stdio.h>

int c(int num) {
    return num;
}