初始化结构段错误

Question

I'm initializing a struct which represents a hash table. I'm attempting to make the frequencies = 0 and the keys = null, but I'm getting a seg fault and not too sure why. Here is my code:

htable htable_new(int capacity) {
  int i;

  htable result = emalloc(sizeof *result);
  result->capacity = capacity;
  result->frequencies = emalloc(capacity * sizeof result->frequencies[0]);
  result->keys = emalloc(capacity * sizeof result->keys[0]);
  for (i = 0; i < capacity; i++) {
    result->frequencies[i] = 0;
  }
  for (i = 0; i < capacity; i++) {
    result->keys[i] = NULL;
  }

  return result;
}

and here is the struct which I'm initializing:

struct htablerec {

  int capacity;
  int num_keys;
  int *frequencies;
  char **keys;

};

I don't see what's wrong as I'm trying to initialize all the int's i've allocated to 0, and all the char pointers to NULL. Is this wrong? Should I be doing this some other way? Thanks for your help!

Answer 1

Try the following (notice pointers near result and return type):

htable *htable_new(int capacity) {
  int i;

  htable *result = emalloc(sizeof *result);
  result->capacity = capacity;
  result->frequencies = emalloc(capacity * sizeof result->frequencies[0]);
  result->keys = emalloc(capacity * sizeof result->keys[0]);
  for (i = 0; i < capacity; i++) {
    result->frequencies[i] = 0;
  }
  for (i = 0; i < capacity; i++) {
    result->keys[i] = NULL;
  }

  return *result;
}

Or, as far as your structure is small, you may want to return it by value:

htable htable_new(int capacity) {
  int i;
  htable result = { 0 };

  result.capacity = capacity;
  result.frequencies = emalloc(capacity * sizeof result->frequencies[0]);
  result.keys = emalloc(capacity * sizeof result->keys[0]);
  for (i = 0; i < capacity; i++) {
    result.frequencies[i] = 0;
  }
  for (i = 0; i < capacity; i++) {
    result.keys[i] = NULL;
  }

  return result;
}

I changed result to be an actual struct, rather than pointer to it. Because of that, it's allocated on stack, fields are initialized as usual and then I return it as is. That means, all struct will be copied to the new place.

Remember that only field of htable are copied; capacity and keys are copied as pointers. That means that if you do htable a = new_htable(10); htable b = a; htable a = new_htable(10); htable b = a; then a.keys == b.keys (they point to the same array).

I don't know, how your code compiled at all. By the way, you forgot to initialize num_keys field and the name of struct you defined doesn't match the name of return type.