FFmpeg 內(nèi)存管理函數(shù)說明

    FFmpeg內(nèi)存操作的常見函數(shù)位于libavutil\mem.c中，有如下幾個常見的內(nèi)存管理函數(shù)：av_malloc()，av_realloc()，av_mallocz()，av_calloc()，av_free()，av_freep()。

1)av_mallocz分配內(nèi)存，然后清零內(nèi)存中的數(shù)據(jù)
內(nèi)部的實現(xiàn)就是av_malloc+memset
void *av_mallocz(size_t size)
{
    void *ptr = av_malloc(size);
    if (ptr)
        memset(ptr, 0, size);
    return ptr;
}

2)av_freep釋放內(nèi)存，然后將指針置空
內(nèi)存實現(xiàn)就是av_free+NULL
void av_freep(void *arg)
{
    void *val;

    memcpy(&val, arg, sizeof(val));
    memcpy(arg, &(void *){ NULL }, sizeof(val));
    av_free(val);
}

3)av_malloc復(fù)雜的內(nèi)存分配方式主要是為了字節(jié)對齊
void *av_malloc(size_t size)
{
    void *ptr = NULL;

    /* let's disallow possibly ambiguous cases */
    if (size > (max_alloc_size - 32))
        return NULL;

#if HAVE_POSIX_MEMALIGN
    if (size) //OS X on SDK 10.6 has a broken posix_memalign implementation
    if (posix_memalign(&ptr, ALIGN, size))
        ptr = NULL;
#elif HAVE_ALIGNED_MALLOC
    ptr = _aligned_malloc(size, ALIGN);
#elif HAVE_MEMALIGN
#ifndef __DJGPP__
    ptr = memalign(ALIGN, size);
#else
    ptr = memalign(size, ALIGN);
#endif
    /* Why 64?
     * Indeed, we should align it:
     *   on  4 for 386
     *   on 16 for 486
     *   on 32 for 586, PPro - K6-III
     *   on 64 for K7 (maybe for P3 too).
     * Because L1 and L2 caches are aligned on those values.
     * But I don't want to code such logic here!
     */
    /* Why 32?
     * For AVX ASM. SSE / NEON needs only 16.
     * Why not larger? Because I did not see a difference in benchmarks ...
     */
    /* benchmarks with P3
     * memalign(64) + 1          3071, 3051, 3032
     * memalign(64) + 2          3051, 3032, 3041
     * memalign(64) + 4          2911, 2896, 2915
     * memalign(64) + 8          2545, 2554, 2550
     * memalign(64) + 16         2543, 2572, 2563
     * memalign(64) + 32         2546, 2545, 2571
     * memalign(64) + 64         2570, 2533, 2558
     *
     * BTW, malloc seems to do 8-byte alignment by default here.
     */
#else
    ptr = malloc(size);
#endif
    if(!ptr && !size) {
        size = 1;
        ptr= av_malloc(1);
    }
#if CONFIG_MEMORY_POISONING
    if (ptr)
        memset(ptr, FF_MEMORY_POISON, size);
#endif
    return ptr;
}