Undefined Reference? But I've already implemented the function

display.h

#ifndef PRO_DISPLAY_H
#define PRO_DISPLAY_H

/** Initializes the display **/
int pro_display_init(void);

#endif /* PRO_DISPLAY_H */

display.c

#include "main.h"

static int height_ = 300;
static int width_ = 300;
static int bpp_ = 16;

static SDL_Surface* screen_ = NULL;

int pro_display_init(void)
{
    screen_ = SDL_SetVideoMode(width_, height_, bpp_, SDL_HWSURFACE|SDL_DOUBLEBUF);
    if (!screen_)
    {
        pro_sdl_error("Video initialization failed.");
        return 0;
    }

    return 1;
}

main.h

#ifndef PRO_MAIN_H
#define PRO_MAIN_H

// standard headers
#include <stdlib.h>
#include <stdlib.h>

// conditional headers
#if defined(WIN32) || defined(_WIN32)
#include <windows.h>
#endif

// our own headers
#include "scripter.h"
#include "ttf_util.h"
#include "events.h"
#include "display.h"

// some macros
#define pro_error(...) fprintf(stderr, __VA_ARGS__)
#define pro_sdl_error(x) fprintf(stderr, "%s. \n=> %s\n", x, SDL_GetError())
#define pro_ttf_error(x) fprintf(stderr, "%s. \n=> %s\n", x, TTF_GetError())

#endif /* PRO_MAIN_H */

** main.c**

#include "main.h"

int main(int argc, char* argv[])
{
    pro_display_init();
    return 0;
}

The Error:

main.c|5|undefined reference to `pro_display_init()'|

Checked the build process. Made sure I was adding "display.c" to gcc's input files. I'm at my wit's end. Why the error?

display.c and main.c are compiled into their own "translation unit". What happens is that when trying to resolve symbols name (i.e. looking for pro_display_init), the C compiler thinks it's compiling a standalone .c unit. The proper way to go is to compile them separately and then link them, e.g.

gcc -c display.c # creates display.o
gcc main.c display.o # compiles main.o and then link with display.o

Of course, you'll be creating/reusing a Makefile soon that lets you define rules for all this.