How to allocate and free aligned memory in C

Question

How do you allocate memory that's aligned to a specific boundary in C (e.g., cache line boundary)? I'm looking for malloc/free like implementation that ideally would be as portable as possible --- at least between 32 and 64 bit architectures.

Edit to add: In other words, I'm looking for something that would behave like (the now obsolete?) memalign function, which can be freed using free.

Answer 1

Here is a solution, which encapsulates the call to malloc, allocates a bigger buffer for alignment purpose, and stores the original allocated address just before the aligned buffer for a later call to free.

// cache line
#define ALIGN 64

void *aligned_malloc(int size) {
    void *mem = malloc(size+ALIGN+sizeof(void*));
    void **ptr = (void**)((uintptr_t)(mem+ALIGN+sizeof(void*)) & ~(ALIGN-1));
    ptr[-1] = mem;
    return ptr;
}

void aligned_free(void *ptr) {
    free(((void**)ptr)[-1]);
}