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carraysdata-structures
antonpug
antonpug

Reputation: 14296

Declaring a dynamic array in C?

I have a struct PACKET which contains two int fields. How would I declare a dynamic array of PACKETs? I will be adding to the array an unknown number of times in the main...that is why I need it to be dynamically sized.

Upvotes: 1

Views: 1761

Answers (4)

Niklas Hansson
Niklas Hansson

Reputation: 473

Encapsulate your dynamic array implementation with malloc/realloc in a .h/.c file pair for easier use.

static int length = 0;

PACKET *dynamic_array_put(PACKET *list, const PACKET new) {
  PACKET *ret_val = NULL;

  ret_val = realloc(list, sizeof(PACKET) * (length + 1));

  if (ret_val == NULL) {
    /* Not enough memory */
    return NULL;
  }

  memmove(list + length, &new, sizeof(PACKET));
  length++;

  return ret_val;
}

PACKET *dynamic_array_get(PACKET *list, int index) {
  PACKET *p = NULL;

  p = malloc(sizeof(PACKET));
  if (p == NULL) {
    /* Not enough memory */
    return NULL;
  }

  memmove(p, list + index, sizeof(PACKET));

  return p;
}

PACKET *dynamic_array_remove(PACKET *list, int index) {
  PACKET *ret_val = NULL;
  PACKET tmp;

  memmove(&tmp, list + index, sizeof(PACKET));
  memmove(list + index, list + index + 1, 
          sizeof(PACKET) * (list - index - 1));

  ret_val = realloc(list, sizeof(PACKET) * (list - 1));
  if (ret_val == NULL) {
    /* Not enough memory, restore! */
    memmove(list + index + 1, list + index, 
            sizeof(PACKET) * (list - index - 1));
    memmove(list + index, &tmp, sizeof(PACKET));
    return NULL;
  }

  length--;

  return ret_val;
}

If memory space is no issue, you can do the memory allocation in blocks, instead of at every addition of PACKETs and speed up the average addition.

Upvotes: 1

Roland Illig
Roland Illig

Reputation: 41627

Depending on the environment you are using, you have these options:

  1. Implement your resizable array using malloc and realloc
  2. Use the GArray or GList data type from GNOME
  3. Switch to C++ and use std::vector<PACKET>
  4. Switch to Java and use ArrayList
  5. Find another library that provides the feature you need

The standard C is very limited in its runtime library, but many people have already extended that standard library to be useful. You should not need to write that code yourself, unless you are in an exotic environment.

Upvotes: 2

Doc Brown
Doc Brown

Reputation: 20044

Make yourself familiar with malloc and realloc - here is some kind of example / small tutorial:

http://fydo.net/gamedev/dynamic-arrays

Here is a more elaborate explanation:

http://www.cse.unt.edu/~donr/courses/4410/dynamicArrays/dynamicArrays.html

When dealing with realloc, you will most likely have to manage the actual size of the array and the reserved capacity of the array as two different values. That is for performance reasons, otherwise, you would have to call realloc every time you add one element to it, which can get extremely slow even for arrays containing just a few hundred elements.

Upvotes: 1

rerun
rerun

Reputation: 25495

There is no dynamic array built into C you have a couple of options. One Build some type of linked storage, the other is to use a static array and realloc as needed but you have to manage the storage your self. Take a looke realloc

Upvotes: 0

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