Thread: strange number problem

Hi I just started C and since I started with a scripting language, pointers are hard to grasp.
I'm hoping someone can help me with this baffling problem.
I wrote the code below hoping to make a image manipulating program. I create a white image and output it as ppm type 3 format but I get a strange result.

Code:

#include <stdio.h>
#include <stdarg.h>
#include <stdlib.h>
#include <math.h>
#include <string.h>


typedef struct pixel {
	float r, g, b;
} pixel;

typedef struct image {
	int		width, height;
	pixel	*(data[]);
} image;

void createWhiteImage(image **img, int width, int height) {
	int i;
	pixel pixels[width*height];
	for(i = 0; i < width*height; ++i)
	{
		pixels[i].r = 255;
		pixels[i].g = 255;
		pixels[i].b = 255;
	}
	*img = (image *)malloc(sizeof(image));
	(**img).width = width;
	(**img).height = height;
	(*((**img).data)) = pixels;
}

void imageToP3(char *str, image *img) {
	int i;
	char tmp[70];
	strcpy(str, "P3\n");
	sprintf(tmp, "%d %d\n", img->width, img->height);
	strcat(str, tmp);
	strcat(str, "255\n");
	for(i = 0; i < img->width*img->height; ++i)
	{
		sprintf(tmp, "% 3.0f % 3.0f % 3.0f  ", (*(img->data))[i].r, (*(img->data))[i].g, (*(img->data))[i].b);
		strcat(str, tmp);
		if ( i%10 ==9 ) {
			strcat(str, "\n");
		}
	}
}

pixel getPixel(image *img, int x, int y) {
	return (*((*img).data))[img->width * y + x - 1];
}

int main (int argc, char const *argv[])
{
	image *b;
	char str[10000];
	createWhiteImage(&b, 10, 10);
	imageToP3(str, b);
	printf("%s\n", str);
	return 0;
}

Result:

Code:

P3
10 10
255
 255  255  255   255  255  255   255  255  255   255  255  255   255  255  255   255  255  255   255  255  255   255  255  255   255  255  255   255  255  255  
 255  255  255   255  255  255   255  255  255   255  255  255   255  255  -2   255  255  255   255  255  255   255  255  255   255  255  255   255  255  255  
 255  255  255   255  255  255   255  255  255   255  255  255   255  255  255   255  255  255   255  255  -2   255  255  255   255  255  255   255  255  255  
 255  255   0   -2   0   0   -2   0  -0   -0  -2   0   -0  255  255   255  255  255   255  255  255   255  -2  255   255  255  255   255  255   0  
  0   0   0    0  -2   0    0   0  -2   -2  -0   0    0   0   0   -0   0  2422584382005669456248832   -0  255  255   -0  -0   0    0   0   0    0   0   0  
  0   0   0    0   0   0    0   0  -2    0  -2  -2    0  -0  -2    0  -0   0   -2  -0  -0   -0   0   0   -0   0   0    0  -2   0  
  0   0   0    0   0   0    0   0   0    0  -2  -0  nan   0  -2  nan  -0   0    0   0   0   -0  255  255   -2  255   0   -0   0   0  
  0  -0   0    0   0   0    0   0   0    0   0   0    0   0   0    0   0   0    0   0   0    0   0   0    0   0   0    0   0   0  
  0   0   0    0   0   0    0   0   0    0   0   0    0   0   0    0   0   0    0  255  255   255  255  255   255  255  255    0   0  -2  
 -2   0  -2   255   0  255   255  255  -2    0   4   0    2   0   0    0   0   0    0   0   0    0   0   0    0   0   0   255  255  255

Why do I get these strange numbers? What am I doing wrong?
Environment: Mac OS X, PPC

You might want to try using a library that allows you to display some proper visual output. SDL might be a good one to start with. Lodes computer graphics tutorials explains a lot of the stuff you might want to do as well.

For storing colours you would probably be better using an integer RGB value than 3 floats. It would save a lot of space.

Thanks swoopy! So my guess is that I could use malloc to allocate sizeof(pixel)*width*height bytes of memory, but then I would have to manually move the pointer sizeof(pixel) at a time. Is this ok or is there a better proper way?

Thanks mike_g! Right now I'm only writing this to learn more about C but SDL will be great to use in the future.

Thanks for your advice swoopy! I've managed to write a working script that reads and writes p3 files. thanks alot!

Actually I have one more question...
I don't know how to free memory.
This is my code

Code:

typedef struct pixel {
	int r, g, b;
} pixel;

typedef struct image {
	int		width, height;
	pixel	*data;
} image;

void createWhiteImage(image **img, int width, int height)
{
	int i;
	*img = (image *)malloc(sizeof(image));
	(*img)->data = malloc(sizeof(pixel)*width*height);
	for(i = 0; i < width*height; ++i)
	{
		((*img)->data)[i].r = 255;
		((*img)->data)[i].g = 255;
		((*img)->data)[i].b = 255;
	}
	(*img)->width = width;
	(*img)->height = height;
}

and I wrote a function that will free the memory

Code:

void freeImage(image **img)
{
	free((*img)->data);
	free((*img));
}

Am I doing enough freeing? I always get confused about what to free.

thanks vart and Salem. I've followed both advices and successfully implemented in my code. I was casting malloc without thinking so it was very interesting read.