FFmpeg源码分析：avcodec_find_decoder

查找解码器

/**
 * Find a registered decoder with a matching codec ID.
 *
 * @param id AVCodecID of the requested decoder
 * @return A decoder if one was found, NULL otherwise.
 */
AVCodec *avcodec_find_decoder(enum AVCodecID id);

分析

AVCodec *avcodec_find_encoder(enum AVCodecID id)
{
    return find_codec(id, av_codec_is_encoder);
}

static AVCodec *find_codec(enum AVCodecID id, int (*x)(const AVCodec *))
{
    const AVCodec *p, *experimental = NULL;
    void *i = 0;

    id = remap_deprecated_codec_id(id);

    while ((p = av_codec_iterate(&i))) {
        if (!x(p))
            continue;
        if (p->id == id) {
            if (p->capabilities & AV_CODEC_CAP_EXPERIMENTAL && !experimental) {
                experimental = p;
            } else
                return (AVCodec*)p;
        }
    }

    return (AVCodec*)experimental;
}

const AVCodec *av_codec_iterate(void **opaque)
{
    uintptr_t i = (uintptr_t)*opaque;
    const AVCodec *c = codec_list[i];

    ff_thread_once(&av_codec_static_init, av_codec_init_static);

    if (c)
        *opaque = (void*)(i + 1);

    return c;
}

可以发现解码器最终从codec_list数组里获取，不过这个是编译的时候生成的，比如ff_aac_decoder

AVCodec ff_aac_decoder = {
    .name            = "aac",
    .long_name       = NULL_IF_CONFIG_SMALL("AAC (Advanced Audio Coding)"),
    .type            = AVMEDIA_TYPE_AUDIO,
    .id              = AV_CODEC_ID_AAC,
    .priv_data_size  = sizeof(AACContext),
    .init            = aac_decode_init,
    .close           = aac_decode_close,
    .decode          = aac_decode_frame,
    .sample_fmts     = (const enum AVSampleFormat[]) {
        AV_SAMPLE_FMT_FLTP, AV_SAMPLE_FMT_NONE
    },
    .capabilities    = AV_CODEC_CAP_CHANNEL_CONF | AV_CODEC_CAP_DR1,
    .caps_internal   = FF_CODEC_CAP_INIT_THREADSAFE | FF_CODEC_CAP_INIT_CLEANUP,
    .channel_layouts = aac_channel_layout,
    .flush = flush,
    .priv_class      = &aac_decoder_class,
    .profiles        = NULL_IF_CONFIG_SMALL(ff_aac_profiles),
};