Linking two object files and the C library

Suppose there are two files :
1) a.c

Code:

---

#include<stdio.h>
extern int func(int);
int main()
{
    int x;
    x=5;
    x=func(x);
    printf("%d",x);
    return 0;
}

---

2) b.c

Code:

---

int func(int n)
{
    return n+n;
}

---

After that I compile them separately...but not link them..(with the -c flag into a.o and b.o)
Then how do I invoke ld so that they get together into a single executable?
..(whatever I try results in an 'undefined reference to main')..