English ▾ Topics ▾ Latest version ▾ api-allocation-growing last updated in 2.2.3

Dynamically growing an array using realloc() is error prone and boring.

Define your array with:

  • a pointer (item) that points at the array, initialized to NULL (although please name the variable based on its contents, not on its type);

  • an integer variable (alloc) that keeps track of how big the current allocation is, initialized to 0;

  • another integer variable (nr) to keep track of how many elements the array currently has, initialized to 0.

Then before adding nth element to the item, call ALLOC_GROW(item, n, alloc). This ensures that the array can hold at least n elements by calling realloc(3) and adjusting alloc variable.

sometype *item;
size_t nr;
size_t alloc

for (i = 0; i < nr; i++)
	if (we like item[i] already)
		return;

/* we did not like any existing one, so add one */
ALLOC_GROW(item, nr + 1, alloc);
item[nr++] = value you like;

You are responsible for updating the nr variable.

If you need to specify the number of elements to allocate explicitly then use the macro REALLOC_ARRAY(item, alloc) instead of ALLOC_GROW.