External functions

I'm having trouble understanding how to use external functions. Does an external function have to be prototyped in a header file or can it just be prototyped in a C source file and defined in another C source file? Can you provide an example of using an external function involving 2 or more files.

file1:

void myfunction( void ) {...do stuff...}

file2:

void myfunction( void ); //prototype it before you use it
...stuff goes here...
...now call the function...
myfunction( );

Additionally, you could do:

file1.h
void myfunction( void );

file2:
#include "file1.h"

...now call the function...
myfunction( );


Quzah.

file1.c

int function (void)
{
return 1;
}


file2.c

extern int function (void);

int other_function (void)
{
int retval;

retval = function (void);
return retval;
}