How to manage memory alignments and generic pointer arithmetics in a portable way in C?

Question

I have to implement an optimized version of malloc/realloc/free (tailored for my particular application). At the moment the code runs on a particular platform, but I would l

Answer 1

The main problem is that you only provide the total size of the memory block to malloc() and friends, without any information about the object granularity. If you view an allocation as an array of objects, then you have a size that is the sizeof of the basic object, and a number n that is the number of objects in the array, e.g.:

p = malloc(sizeof(*p) * n);

If you have only the total size, then you don't know if s=4 and n=10, or if s=2 and n=20, or s=1 and n=40, because all multiply to the total size of 40 bytes.

So the basic question is, do you want a direct substitute for the original functions, e.g. when you have thrown native calls all over your code base, or do you have a centralized and DRY modularity with wrapper functions. There you could use functions that provide s and n.

void *my_malloc (size_t s, size_t n)

Most of the time it should be a safe bet when the returned absolute memory address is a multiple of s to guarantee correct alignment.

Alternatively, when porting your implementation, you simply look at the alignment that the native malloc() uses for the target platform (e.g. multiples of 16), and use this for your own implementation.