Address boundary error on dynamic array

Question

I'm just starting with C from a interpreted language background, and I'm having problems understanding why this is not working.

I have a Dynamic_Array struct (http://pastebin.com/h4k7Sk3U), and a Task struct (http://pastebin.com/evs4TEQA).

When creating a Dynamic_Array instance with an initial size I get:

“./ctask ” terminated by signal SIGSEGV (Address boundary error)

I tracked the error with gdb, who shows that its originated on dynamic_array.h:16:

Dynamic_Array *dynamic_array_create(size_t initialSize) {
  Dynamic_Array *a;
  a->array = (Task *)malloc(initialSize * sizeof(Task));   // line 16
  a->used = 0;
  a->size = initialSize;
  return a;
}

What am I doing wrong there? I'm multiplying the initial size with the size of the Task struct, which is supposed to be enough memory for them.

Answer 1

You never allocated a itself, before you started creating its members.

Until you initialize it, a is pointing to some random memory location that you shouldn't be touching.

Dynamic_Array *dynamic_array_create(size_t initialSize) {
  Dynamic_Array *a = malloc(sizeof(Dynamic_Array));  // create 'a' first

  a->array = (Task *)malloc(initialSize * sizeof(Task));
  a->used = 0;
  a->size = initialSize;
  return a;
}