Segmentation fault on function

Question

I'm learning how to use SDL library to graphic programming and I am having a segmentation fault on the next code:

int situador(SDL_Surface * dib,struct sit_per per)
{

    SDL_Rect pos;
    struct sit_per * est;
    est=&per;


    while(est)
    {
        pos.x = est->ac->x;
        pos.y = est->ac->y;
        pos.w = est->ac->img[est->ac->dir]->w;
        pos.h = est->ac->img[est->ac->dir]->h;
        SDL_BlitSurface(est->ac->img[est->ac->dir], NULL, dib, &pos);
        est=est->si;
    }


    return 0;
}

dib variable comes from this function:

SDL_Surface * draw = SDL_CreateRGBSurface(SDL_SWSURFACE, ANCHO, ALTO, 24,rmask, gmask,bmask, amask);

The struct sit_per is the following:

struct sit_per
{
    struct personaje * ac;
    struct sit_per * si;
};

struct personaje
{
    char des[50];
    SDL_Surface * img[8];
    int dir;
    int x;
    int y;
};

And the img array was filled with the next function:

struct personaje * creador(char * cad)
{
    int x;
    struct personaje * per;

    per=(struct personaje *)malloc(sizeof(struct personaje));
    strcpy(per->des,cad);

    per->img[0] = SDL_LoadBMP("/home/andoni/SDL/img/flecha/0.bmp");
    per->img[1] = SDL_LoadBMP("/home/andoni/SDL/img/flecha/1.bmp");
    per->img[2] = SDL_LoadBMP("/home/andoni/SDL/img/flecha/2.bmp");
    per->img[3] = SDL_LoadBMP("/home/andoni/SDL/img/flecha/3.bmp");
    per->img[4] = SDL_LoadBMP("/home/andoni/SDL/img/flecha/4.bmp");
    per->img[5] = SDL_LoadBMP("/home/andoni/SDL/img/flecha/5.bmp");
    per->img[6] = SDL_LoadBMP("/home/andoni/SDL/img/flecha/6.bmp");
    per->img[7] = SDL_LoadBMP("/home/andoni/SDL/img/flecha/7.bmp");
    per->img[8] = SDL_LoadBMP("/home/andoni/SDL/img/flecha/8.bmp");

    for(x=0;x<9;x++)
        if(!per->img[x])
            x=10;
    if(x==10)
        return NULL;
    else
        return per;
}

I'm sure that the route to the images are well writed, and I can say that the x and y coordinates are well too.

I'll apreciate any suggestion, thanks in advance.

Answer 1

There are several oddness in your code.

First,

int situador(SDL_Surface * dib,struct sit_per per)
{
    SDL_Rect pos;
    struct sit_per * est;
    est=&per;

let it be

int situador(SDL_Surface * dib,struct sit_per *per)
{
    SDL_Rect pos;
    struct sit_per * est = per; // assignment can be made later

just style and avoiding too much data and copying on the stack.

The use of the for loop could be also an option;

  for(est = per; est != NULL; est = est->si)

If a function returns always a fixed value, you can make it return nothing...; so, void situador(...) instead of int situador(...).

The function creador load 9 images, when only 8 are needed (one per direction, I suppose), this is the source for the segfault; do not hard encode (absolute) paths to files, as said before.

To exit a for loop, use the break keyword, avoid in general the modification of the loop variable (unless you have a good reason, and here I can't see one). Again, the for loop loops for 9 images, where you have room just for 8 in your array. If you need the 9th image, you have to write SDL_Surface * img[9];. It could be confusing, but when you declare an array, the number between brackets is the number of items, while when accessing, the index is 0-based so that the last element has index N-1.

So the whole final piece can become

for(x = 0; x < NUM_OF_ELEMENT; x++)
    if(!per->img[x])
        break;
if (x < NUM_OF_ELEMENT)
{
    int y;
    for(y = 0; y < x; y++)
       SDL_func_to_free_loaded_images(per->img[y]);
    free(per);
    return NULL;
}
else
    return per;

usually it is better to use named constants, instead of number; e.g.

#define  NUM_OF_ELEMENT 8
#define  NUM_OF_DES  50

and so

   struct personaje
   {
      char des[NUM_OF_DES];
      SDL_Surface * img[NUM_OF_ELEMENT];
      int dir;
      int x;
      int y;
   };

and so on... (of course you will pick another name, like NUM_OF_DIRS or NUM_OF_IMAGES or whatever is your meaningful prefered form in your language)

Answer 2

Your code loads 9 image pointers into an array of 8 elements.

This is not a recipe for happiness.