header files in C

Should you have to include a header file into its own library? I can't understand why I sometimes, but not always, get compile errors because it's not getting the header file prototypes before it compiles the libraries.

This compiles with no warnings (-Wall -W etc.)

#include <stdio.h>

void x();
extern void x();

int main()
{
        x();
        return 0;
}

void x() {
        puts("Tada!");
}

You want "void somefunc();" in your *.h files, and "extern void somefunc();" in your *.c files, and in the latter case, you usually won't even need it.