How does the C compiler find function definitions?

[Javasnob]

I thought I'd increase my knowledge of compilation and linking technologies, so I thought I'd ask you all this question. When I declare a function in C, how does the compiler/linker know where to find the definition. For example, if I declare a function with the same parameters and return value as free(), and I don't implement it, will the compiler use free declared in stdlib, or will it complain because I didn't include it?

Thanks!

[dafl00]

c programming 101:

if you declare it as

void free(void *);

somewhere and decide to

free(ptr);

elsewhere without implementation then you'll have an error

however if you try to

extern void free(void *);

and link with c standard libraries then it will use stdlib free

if you

#include "stdlib.h"
void free(void *);

then you'll have an error