explanation to aligned malloc implementation

Question

This is not homework, this is purely for my own personal education.

I couldn\'t figure out how to implement an aligned malloc so looked online and found this website. Fo

Answer 1

implementation does work

Perhaps, but I wouldn't be too sure. IMO you'd be better off working from first principles. Right off the bat,

p1 = (void*)malloc

is a red flag. malloc returns void. In C, any pointer can be assigned from void *. Casting from malloc is usually considered bad form because any effect it has can only be bad.

Why we need an offset

The offset provides room to stash the pointer returned by malloc, used later by free.

p1 is retrieved from malloc. Later, it has to be provide to free to be released. aligned_malloc reserves sizeof(void*) bytes at p1, stashes p1 there, and returns p2 (the first "aligned" address in the block that p1 points to). Later, when the caller passes p2 to aligned_free, it converts p2 in effect to void *p2[], and fetches the original p1 using -1 as an index.

What does anding with ~(alignment - 1) accomplish

It's what puts p2 on the boundary. Say alignment is 16; alignment -1 is 15, 0xF. ~OxF is all bits on except the last 4. For any pointer P, P & ~0xF will be a multiple of 16.

p2 is a double pointer.

pointer schmointer. malloc returns void*. It's a block of memory; you address it as you will. You wouldn't blink at

char **args = calloc(7, sizeof(char*));

to allocate an array of 7 char * pointers, would you? The code picks some "aligned" location at least sizeof(void*) bytes from p1 and, for the purposes of free, treats it as void **.

What is the general approach

There is no one answer. Best is probably to use a standard (or popular) library. If you build atop malloc, allocating enough to keep the "real" pointer and returning an aligned one is pretty standard, although I would code it differently. The syscall mmap returns a page-aligned pointer, which will satisfy most criteria for "aligned". Depending on need, that might be better or worse than piggybacking on malloc.