malloc - Does C allocate memory automatically for me?

Question

I have been writing C for only a scant few weeks and have not taken the time to worry myself too much about malloc(). Recently, though, a program of mine returned a string of happy faces instead of the true/false values I had expected to it.

If I create a struct like this:

typedef struct Cell {
  struct Cell* subcells;
} 

and then later initialize it like this

Cell makeCell(int dim) {
  Cell newCell;

  for(int i = 0; i < dim; i++) {
    newCell.subcells[i] = makeCell(dim -1);
  }

  return newCell; //ha ha ha, this is here in my program don't worry!
}

Am I going to end up accessing happy faces stored in memory somewhere, or perhaps writing over previously existing cells, or what? My question is, how does C allocate memory when I haven't actually malloc()ed the appropriate amount of memory? What's the default?

Answer 1

Short answer: It isn't allocated for you.

Slightly longer answer: The subcells pointer is uninitialized and may point anywhere. This is a bug, and you should never allow it to happen.

Longer answer still: Automatic variables are allocated on the stack, global variables are allocated by the compiler and often occupy a special segment or may be in the heap. Global variables are initialized to zero by default. Automatic variables do not have a default value (they simply get the value found in memory) and the programmer is responsible for making sure they have good starting values (though many compilers will try to clue you in when you forget).

The newCell variable in you function is automatic, and is not initialized. You should fix that pronto. Either give newCell.subcells a meaningful value promptly, or point it at NULL until you allocate some space for it. That way you'll throw a segmentation violation if you try to dereference it before allocating some memory for it.

Worse still, you are returning a Cell by value, but assigning it to a Cell * when you try to fill the subcells array. Either return a pointer to a heap allocated object, or assign the value to a locally allocated object.

A usual idiom for this would have the form something like

Cell* makeCell(dim){
  Cell *newCell = malloc(sizeof(Cell));
  // error checking here
  newCell->subcells = malloc(sizeof(Cell*)*dim); // what if dim=0?
  // more error checking
  for (int i=0; i<dim; ++i){
    newCell->subCells[i] = makeCell(dim-1);
    // what error checking do you need here? 
    // depends on your other error checking...
  }
  return newCell;
}

though I've left you a few problems to hammer out..

And note that you have to keep track of all the bits of memory that will eventually need to be deallocated...