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allocation growing API
======================
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 `n`th 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`.
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