Thread: Problem using structures (they crash my program)

I'm just learning structures, and I'm having some trouble. The following is the structure I delcared globally:

Code:

struct creatures
{
	int *position;
};

and the next is when I declare members of the structure in my main function:

Code:

char command;
   struct creatures prey1;
   struct creatures pred;

Now, if you need to know more I can post more code, but I don't think it's necessary. Basically, my program runs perfectly fine. Only, if I so much as declare the second member (struct creatures pred), the program crashes on startup. If I comment out that line, everything works as intended again. Wtf? I'm not even doing anything with it yet! Any help would be greatly appreciated.

As far as I'm aware, there is no more allocation code. I don't even know enough to know what that really means. To show more code, here is how I use the first structure element that works:

Code:

int preyh(char command, struct creatures *prey)
{

   move(command, prey);
   grid[prey->position[0]][prey->position[1]]=1;

   return 0;
}

Where it just sends the position array into the "move" function and changes it based on what command I gave (here I'm just moving a character around a 2D array of chars, and displaying - a 1 running around the screen, for example).

My main function is this:

Code:

int main(void)

{
	char command;
   struct creatures prey1;
   struct creatures pred;

   prey1.position[0]= 6;
   prey1.position[1]= 6;

  // pred.position[0]=8;
  // pred.position[1]=14;

    zerobox();

    printf("Please enter a command ('q' to quit).\n");

    do{

    command=input();

    preyh(command, &prey1);
    //predator(&pred, &prey1);
    printbox();
    zerobox();
    }
    while(command != 'q');

    gameover();

    return 0;

}

I don't do anything with the structure anywhere else, so this should cover it. I just want to be able to feed the "pred" element into the move function as well, so that it can chase the "prey" around. I just don't understand why declaring it is crashing my program.

Since I know nothing about memory allocation, maybe the problem is that I haven't been doing it? Can you suggest any resources online for learning more? (I am not taking a class nor do I have a book, I'm just learning because it is a useful skill to help me with my research, so I'm limited to free learning).

Originally Posted by shadow1515

Since I know nothing about memory allocation, maybe the problem is that I haven't been doing it?

Exactly. You define your structs as holding a pointer, but that pointer doesn't point to any allocated memory. Yet you poke data into it in your program. The fact that it doesn't crash when commenting out certain pieces of code is just a matter of luck. Something like if you shoot someone and don't happen to hit any vital organs, they may live.

--
Computer Programming: An Introduction for the Scientifically Inclined

Thanks for the information. I think I'm starting to understand a little better now. I ended up simply declaring the position array as int position[2] because I already knew what size I wanted it to be.

One question though: had I allocated memory using the malloc function, would I do it inside this

Code:

struct creatures
{
	int *position;
};

or in here somewhere

Code:

struct creatures prey1;

   prey1.position[0]= 6;
   prey1.position[1]= 6;

?

If I allocated memory in the second piece of code, where I declare the member of the structure, then would that allow me to make the position pointer a different size (maybe different size array, or whatever) for each member if I wanted to?

It does, thank you, you did a lot to clear up my understanding of structures and pointers.