aligned_malloc.c

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/*
 * Copyright © 2017 Embedded Artistry LLC.
 * License: MIT. See LICENSE file for details.
 */

#include <assert.h>
#include <stdbool.h>
#include <stdint.h>
#include <stdlib.h>

#include "aligned_malloc.h"
#include "malloc.h"

#pragma mark - Definitions -

#ifndef align_up
#define align_up(num, align) (((num) + ((align)-1)) & ~((align)-1))
#endif

typedef uint16_t offset_t;

#define PTR_OFFSET_SZ sizeof(offset_t)

#pragma mark - APIs -

void* aligned_malloc(size_t align, size_t size)
{
    void* ptr = NULL;

    // We want it to be a power of two since align_up operates on powers of two
    assert((align & (align - 1)) == 0);

    if(align && size)
    {
        /*
         * We know we have to fit an offset value
         * We also allocate extra bytes to ensure we can meet the alignment
         */
        size_t hdr_size = PTR_OFFSET_SZ + (align - 1);
        void* p = malloc(size + hdr_size);

        if(p)
        {
            /*
             * Add the offset size to malloc's pointer (we will always store that)
             * Then align the resulting value to the arget alignment
             */
            ptr = (void*)align_up(((uintptr_t)p + PTR_OFFSET_SZ), align);

            // Calculate the offset and store it behind our aligned pointer
            *((offset_t*)ptr - 1) = (offset_t)((uintptr_t)ptr - (uintptr_t)p);

        } // else NULL, could not malloc
    } // else NULL, invalid arguments

    return ptr;
}

void aligned_free(void* ptr)
{
    assert(ptr);

    /*
     * Walk backwards from the passed-in pointer to get the pointer offset
     * We convert to an offset_t pointer and rely on pointer math to get the data
     */
    offset_t offset = *((offset_t*)ptr - 1);

    /*
     * Once we have the offset, we can get our original pointer and call free
     */
    void* p = (void*)((uint8_t*)ptr - offset);
    free(p);
}