How to extern SDL_Surface array in C?

Question

I want to extern a SDL_Surface array from one function (load_level) to another (initialize_balls). I have to say that the SDL_Surface (brickim) is a dynamic pointer-struct which is defined inside of load_level function via malloc function and declared as SDL_Surface **brickim as well. I got a segmentation fault when I try to access brickim[0]->w in the initialize_balls function but not in the load_level function. This is the piece of code I hope you could help me to fix.

file1.c

#include <stdio.h>
#include <SDL.h>
SDL_Surface **brickim; /*as global*/
SDL_Surface* load_map(char *map_name , int tran );

void load_level(int stage){
int n;
SDL_Surface **brickim=( SDL_Surface **)malloc(sizeof(SDL_Surface)*input.N);
    for (n=0;n < input.N;n++){
        **brickim[n]=load_map("brick1.bmp",1); /*load SDL_Surfaces*/**
        printf("%d\n",brickim[n]->w);  /***access succesfully (returns 100 N times)***/
        }
...
}}
SDL_Surface* load_map(char *map_name , int tran ){
SDL_Surface *temp,*map;
Uint32 colorkey;
        printf("Loading bit map %s %d...\n",map_name,tran);
        temp = SDL_LoadBMP(map_name);
        if (temp == NULL){
                printf("Unable to load bitmap: %s\n", SDL_GetError());
                SDL_FreeSurface(temp);
                exit(0);
        }else{
                map = SDL_DisplayFormat(temp);
                colorkey = SDL_MapRGB( map -> format, 132, 0, 12);
                if ( tran==1 ) SDL_SetColorKey( map, SDL_SRCCOLORKEY, colorkey );
                SDL_FreeSurface(temp);
        }
                return map;
 }

file2.c

#include <SDL.h>
#include <stdlib.h>
#include <stdio.h>

extern SDL_Surface **brickim;/*brings brickim to this function*/

void initialize_balls()
{
      printf("Into initialization :)\n ");
    printf("%d \n",brickim[0]->w); /***returns segmentation fault***/
}

$./kuranes
Loading bit map brick1.bmp 1...
level image:) 100 (x10 times)
Into initialization :)
Violación de segmento (`core' generado)

Everything was running ok when I used a single SDL_Surface compiled with gcc but I need to do it dynamically. I think something very basic is missing.

Answer 1

Your problem seems to be here. You declare a global brickim, this will be what the extern declaration finds. You then, in load_level, declare a local brickim variable. You can declare a variable with the same name in different scopes, but the global brickim will not be used within load_level, the local one will. Therefore, while you do malloc into the local brickim, you don't ever assign into the global brickim - and therefore if you access it via extern, it will be NULL and you'll segfault.

SDL_Surface **brickim; /*as global*/
SDL_Surface* load_map(char *map_name , int tran );

void load_level(int stage){
int n;
SDL_Surface **brickim=( SDL_Surface **)malloc(sizeof(SDL_Surface)*input.N);
    for (n=0;n < input.N;n++){
        **brickim[n]=load_map("brick1.bmp",1); /*load SDL_Surfaces*/**
        printf("%d\n",brickim[n]->w);  /***access succesfully (returns 100 N times)***/
        }
...
}}

edit: you might want to check that whole allocation in general. I'm not familiar with the SDL_Surface struct, but i assume it's not a pointer type (it's a generic struct). Your allocation is actually allocating N structures, but you're casting as if you're allocating N pointers to SDL_Surface structures. Don't cast malloc.

If you want N SDL_Surface structures, you only need to have:

SDL_Surface * brickim = malloc(sizeof(SDL_Surface) * N)

If you want N pointers to SDL_Surface structures, you would need:

SDL_Surface ** brickim = malloc(sizeof(SDL_Surface *) * N)
for( i = 0; i < N; i++){
   // you've only allocated an array of pointers so far, you now need
   // allocate N structures  
   brickim[0] = malloc(sizeof(SDL_Surface));
}