.hgignore

syntax:glob
main
*.o

image.h

typedef void Image;

Image *make_image(int foo);
char access_pixel(Image *img, int r, int c);

image_cv.c

#include <stdlib.h>
#include <image.h>

typedef struct {
    int foo, bar;
} IplImage;

Image *make_image(int foo) {
    IplImage *img = (Image *) malloc(sizeof(IplImage));
    if (!img)
        return NULL;
    img->foo = foo;
    img->bar = foo + 14;
    return img;
}

char access_pixel(Image *img, int r, int c) {
    IplImage *iplimg = (IplImage *) img;
    return iplimg->foo + r + c;
}

main.c

#include <stdio.h>
#include <image.h>

int main() {
    Image *img = make_image(42);
    if (!img) {
        fprintf(stderr, "error: querying an image for 42 failed\n");
        return 1;
    }
    printf("access_pixel(img, 0, 0) = %d\n", access_pixel(img, 0, 0));
    return 0;
}

Makefile

SOURCES = image_cv.c main.c
OBJECTS = $(patsubst %.c,%.o,$(SOURCES))
CC = gcc
CFLAGS = -std=c99 -Wall -Wextra -pedantic
INCLUDE = -I.

default: all

all: $(OBJECTS) main

main: main.o image_cv.o
	gcc -o main main.o image_cv.o

image_cv.c: image.h

%.o: %.c
	$(CC) -c -o $@ $(CFLAGS) $(INCLUDE) $<

clean:
	rm -f $(OBJECTS)