c - Segmentation fault when using global struct variable - c

Question

This is in a program meant to work with ppm image files.
I'm getting a segmentation fault when trying to read an image from a file and assigning that image to my global struct image.
These are the relevant parts of my ppmIO.c file:
#include <stdlib.h>
#include <stdio.h>
#include <ctype.h>
#include <ppmIO.h>
struct Image *instance;
void ImageRead(char *filename)
{
printf("hi 0!");
int width, height, num, size;
//unsigned char *p;
//Image *image = (Image *) malloc(sizeof(Image));
FILE *fp = fopen(filename, "r");
//if (!image) die("cannot allocate memory for new image");
if (!fp) die("cannot open file for reading");
readPPMHeader(fp, &width, &height);
size = width * height * 3;
printf("hi!");
//instance->data = (unsigned char *) malloc(size);
printf("hi 2!");
instance->width = width;
printf("hi 3!");
instance->height = height;
printf("hi 4!");
if (!instance->data) die("cannot allocate memory for new image");
num = fread((void *) instance->data, 1, (size_t) size, fp);
if (num != size) die("cannot read image data from file");
fclose(fp);
}
This is my ppmIO.h file:
#ifndef PPMIO_H
#define PPMIO_H
struct Image
{
int width;
int height;
unsigned char *data;
};
extern struct Image *instance;
//extern Image *ImageCreate(int width, int height);
//extern void ImageClear(struct Image *image, unsigned char red, unsigned char green, unsigned char blue);
extern void ImageRead(char *filename);
extern void ImageWrite(char *filename);
extern void ImageSetPixel(int x, int y, int chan, unsigned char val);
extern unsigned char ImageGetPixel(int x, int y, int chan);
#endif /* PPMIO_H */
And this is the segmentation fault that GDB reports:
Program received signal SIGSEGV, Segmentation fault.
0x0000000000400fff in ImageRead (filename=0x7fffffffdc32 "nika.ppm")
at ppmIO.c:126
126 instance->width = width;
I think there is a problem with how I'm trying to use the Image *instance... but I really have no real clue what is causing this mess. :(

Answer 1

You're getting this error as you have not allocated any memory for instance. Before you try and use any member of instance (i.e. width or data) you must allocate memory (from inside a function), i.e:
instance = malloc(sizeof *instance);
You should not cast the return value of instance (See: this), and there is no need to specify the type since the compiler already knows. You can't allocate the memory when you declare the variable as static initializations must be to constant values (See: this) (the return value of a function is not constant).
You will also need to allocate memory for the instance->data part of the structure, based on the size you read from the file.