Why multiple definitions? Why are other references not defined? This is really basic, what am I missing?

Question

I have a small project that I need to compile. I have one header and one source that I have created and a nearly empty driver.c that includes my header.
Observe:

// iol.h
#ifndef __IOL_HEADER
#define __IOL_HEADER
/*  program: iol.h
    date:    5 October 2010
*/

#define UNIX 1
#define WINDOWS 2
#define OS UNIX  
#if OS == UNIX
    #include <ncurses.h>
#elif OS == WINDOWS
    #include <conio.h> 
    #include <windows.h>
 // Function declarations!
#endif
void iol_init(void);
#endif

Now my implementation file:

// iol.c
#include <string.h>  
#include <stdlib.h>
#include "iol.h"
void iol_init(void) {
    #if OS == WINDOWS
        /* no startup required for windows */
    #elif OS == UNIX  
        initscr();  
        noecho();  
        cbreak();
        keypad(stdscr, 1);
 // Implmntn continues....  

Now the driver that includes my header and provides the main ():

//main.c
#include "iol.h"

My bash command:

gcc iol.c driver.c -l"ncurses"

I get back:

/tmp/ccmmW6hQ.o:iol.c:(.text+0x83f): first defined here
/tmp/ccwIKUaT.o: In function 'isEscaping':
driver.c:(.text+0xbab): multiple definition of 'isEscaping'
/tmp/ccmmW6hQ.o:iol.c:(.text+0xbab): first defined here
/tmp/ccwIKUaT.o: In function 'initSeq':
..
driver.c:(.text+0x149): undefined reference to 'iol_prnstr'
driver.c:(.text+0x178): undefined reference to 'iol_putch'
..
driver.c:(.text+0x726): undefined reference to 'iol_display'
collect2: ld returned 1 exit status

I just want to get to point where I can compile this, and start ripping my hair out 'cuz of all my seg-faults. What's the problem in my setup? I RTFM on the Gnu C Compiler apparently I'm doing what I'm supposed to, which is declare stuff in iol.h, define in iol.c, and use it in driver.c this is pretty trivial stuff maybe I just need a second set of eyes :S
I'm actually getting a long list of errors, if anyone thinks that's relevant, I'm happy to post the whole source.

Answer 1

Try compiling them separately:

$ gcc -Wall -c ioi.c
$ gcc -Wall -c driver.c
$ gcc ioi.o driver.o -o program -lncurses

To isolate and fix compilation errors...

Answer 2

You didn't mention if you are compiling on Windows or Unix. If on Windows I suspect that there are order dependencies in the .h files. Usually you want windows.h first so that it defines constants that the other .h files will use.

Answer 3

this is the linker complaining. This is what you would get if you had a function defined in the header file that was not declared 'inline'

the missing ones are because you have not added the correct libraries