FFmpeg源码剖析-通用：av_register_all()

av_register_all()函数的定义在 libavformat/allformats.c，

它的作用的注册ffmpeg的各种mux和demux。

1. 函数主体

void av_register_all(void)

{

    static int initialized;

    if (initialized)

        return;

    avcodec_register_all();

    /* (de)muxers */

    REGISTER_MUXER   (A64,              a64);

    REGISTER_DEMUXER (AA,               aa);

    ...

    /* image demuxers */

    REGISTER_DEMUXER (IMAGE_BMP_PIPE,        image_bmp_pipe);

    REGISTER_DEMUXER (IMAGE_DDS_PIPE,        image_dds_pipe);

    ...

    /* external libraries */

    REGISTER_MUXER   (CHROMAPRINT,      chromaprint);

    REGISTER_DEMUXER (LIBGME,           libgme);

    ...

    initialized = 1;

}

2. demux的注册

#define REGISTER_DEMUXER(X, x)                                          \

    {                                                                   \

        extern AVInputFormat ff_##x##_demuxer;                          \

        if (CONFIG_##X##_DEMUXER)                                       \

            av_register_input_format(&ff_##x##_demuxer);                \

    }

宏展开后:

{

  extern AVInputFormat ff_aa_muxer;

  if (CONFIG_A64_MUXER)

    av_register_input_format(&ff_aa_muxer);

}

libavformat/format.c

/** head of registered input format linked list */

static AVInputFormat *first_iformat = NULL;

/** head of registered output format linked list */

static AVOutputFormat *first_oformat = NULL;

static AVInputFormat **last_iformat = &first_iformat;

static AVOutputFormat **last_oformat = &first_oformat;

​

void av_register_input_format(AVInputFormat *format)

{

    AVInputFormat **p = last_iformat;

    // Note, format could be added after the first 2 checks but that implies that *p is no longer NULL

    while(p != &format->next && !format->next && avpriv_atomic_ptr_cas((void * volatile *)p, NULL, format))

        p = &(*p)->next;

    if (!format->next)

        last_iformat = &format->next;

}

将新的用于处理输入format的demux添加到链表的尾部；

3. mux的注册

#define REGISTER_MUXER(X, x)                                            \

    {                                                                   \

        extern AVOutputFormat ff_##x##_muxer;                           \

        if (CONFIG_##X##_MUXER)                                         \

            av_register_output_format(&ff_##x##_muxer);                 \

    }

宏展开后:

{

  extern AVOutputFormat ff_a64_muxer;

  if (CONFIG_A64_MUXER)

    av_register_output_format(&ff_a64_muxer);

}

liavformat/format.c

/** head of registered input format linked list */

static AVInputFormat *first_iformat = NULL;

/** head of registered output format linked list */

static AVOutputFormat *first_oformat = NULL;

static AVInputFormat **last_iformat = &first_iformat;

static AVOutputFormat **last_oformat = &first_oformat;

void av_register_output_format(AVOutputFormat *format)

{

    AVOutputFormat **p = last_oformat;

    // Note, format could be added after the first 2 checks but that implies that *p is no longer NULL

    while(p != &format->next && !format->next && avpriv_atomic_ptr_cas((void * volatile *)p, NULL, format))

        p = &(*p)->next;

    if (!format->next)

        last_oformat = &format->next;

}

将新的用于处理输入format的mux添加到链表的尾部；