ffmpeg源码分析之avformat_alloc_context

来源：互联网发布：c语言课程设计编辑：程序博客网时间：2024/06/06 01:48

AVFormatContext是在整个媒体流的处理流程中都会用到的对象。

AVFormatContext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 AVERROR(ENOMEM);

下面看一下是怎么初始化该对象的。

AVFormatContext *avformat_alloc_context(void){    AVFormatContext *ic;    ic = av_malloc(sizeof(AVFormatContext));    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 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;}

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