ffmpeg源码分析之avformat_alloc_context

***FormatContext是在整个媒体流的处理流程中都会用到的对象。

***FormatContext必须初始化为NULL或者用avformat_alloc_context()进行初始化。原因在于：

av_regeister_all()中：

if (!s && !(s = avformat_alloc_context()))
        return ***ERROR(ENOMEM);

下面看一下是怎么初始化该对象的。

***FormatContext *avformat_alloc_context(void)
{
    ***FormatContext *ic;
    ic = av_malloc(sizeof(***FormatContext));
    if (!ic) 

        return ic;
    avformat_get_context_defaults(ic);

    ic->internal = av_mallocz(sizeof(*ic->internal));
    if (!ic->internal) {
        avformat_free_context(ic);
        return NULL;
    }

    return ic;
}

static size_t max_alloc_size= INT_MAX;

//INT_MAX定义在limits.h头文件中，具体的数没必要记住，只要知道是很大的就行了！

//好像c中32位的int的范围是：-2147483648->2147483647.

void *av_malloc(size_t size)
{
    void *ptr = NULL;
#if CONFIG_MEMALIGN_HACK//这都是定义在config.h中的。初始定义为0
    long diff;
#endif

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

#if CONFIG_MEMALIGN_HACK
    ptr = malloc(size + ALIGN);//ALIGN=32
    if (!ptr)
        return ptr;//一般会走到这里就结束了
    diff              = ((~(long)ptr)&(ALIGN - 1)) + 1;
    ptr               = (char *)ptr + diff;
    ((char *)ptr)[-1] = diff;
#elif H***E_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 H***E_ALIGNED_MALLOC
    ptr = _aligned_malloc(size, ALIGN);
#elif H***E_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 ***X 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;
}