dynamic array, how many bytes to realloc?

sneakyimp · 11-07-2011, 02:31 PM

I'm writing an addToArray function in C like the one described here.

However, instead of passing struct of type DATA to this function, I want to pass a pointer to a struct of type zval -- the basic idea is that I want to remember the address of the zval object. Here's the actual code of my function:

Code:

static int amf_remember_object(zval *obj) /* {{{ */
{
	if(AMF_G(objects_and_arrays_count) == AMF_G(objects_and_arrays_allocated)) // Are more refs required?
	{
		// Feel free to change the initial number of refs
        // and the rate at which refs are allocated.
            if (AMF_G(objects_and_arrays_allocated) == 0)
            	AMF_G(objects_and_arrays_allocated) = 3; // Start off with 3 refs
            else
            	AMF_G(objects_and_arrays_allocated) *= 2; // Double the number of refs allocated

            // Make the reallocation transactional
            // by using a temporary variable first
            // TODO: use erealloc?
            void *_tmp = realloc(AMF_G(objects_and_arrays), (AMF_G(objects_and_arrays_allocated) * sizeof(obj)));

            // If the reallocation didn't go so well,
            // inform the user and bail out
            if (!_tmp)
            {
    			php_error_docref(NULL TSRMLS_CC, E_ERROR,
    					"ERROR! Couldn't realloc memory");
    			return(-1);
            }

            // Things are looking good so far
            AMF_G(objects_and_arrays) = (zval**)_tmp;
    }

	AMF_G(objects_and_arrays)[AMF_G(objects_and_arrays_count)] = obj;
	AMF_G(objects_and_arrays_count)++;

    return AMF_G(objects_and_arrays_count);
} // amf_remember_object()
/* }}} */

Note that I'm using sizeof(obj) to determine how many bytes to allocate -- i have no idea if that is correct. Since I am storing pointers in my array, what expression should I use for calculating my byte count? will sizeof(obj) return the number of bytes required for a pointer or something else? Will this work on 32-bit and also 64-bit machines?

Any help would be much appreciated.

smallpond · 11-07-2011, 05:12 PM

Since obj is a pointer, sizeof(obj) is the size of a pointer in bytes.
Your realloc looks ok as long as AMF_G(objects_and_arrays) is initially NULL.

The AMF_G() thing is a little weird. Its used to get global values. If you have a global structure declared somewhere, why not just use a pointer to it?

Code:

if (amf_global->objects_and_arrays_allocated == 0)
        amf_global->objects_and_arrays_allocated = 3;

ewqdsacxz · 11-07-2011, 05:30 PM

sizeof returns the size in bytes of the datatype of the variable you pass to it, in this case obj of type (zval*), on the architecture you are compiling for (x86, x86-64, ARM...). You are storing (zval*)s, so this is what you want. At compile time, it just takes the size of a pointer to zvals for that architecture and substitutes in the literal number: 4 in the case of x86, 8 in the case of x86-64.

Also see http://en.wikipedia.org/wiki/Sizeof
Edit: ninja'd

smallpond · 11-07-2011, 06:40 PM

Quote:

Originally Posted by ewqdsacxz

Edit: ninja'd

With cat-like tread,
Upon our prey we steal.
In silence dread,
Our cautious way we feel

sundialsvcs · 11-07-2011, 09:44 PM

If it were me, I'd be looking into setting up an abstract data type (or, more likely, finding one that already exists) which provides "an arbitrary-sized collection of objects indexed by an integer." Reallocating a contiguous block of storage can get messy, but who says it has to actually be one contiguous block?