ffmpeg源码之xx学习笔记

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AVClass

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AVFormatContext *s;
s->av_class = &av_format_context_class;

static const AVClass av_format_context_class = {
    .class_name     = "AVFormatContext",
    .item_name      = format_to_name,
    .option         = avformat_options,
    .version        = LIBAVUTIL_VERSION_INT,
    .child_next     = format_child_next,
    .child_class_iterate = format_child_class_iterate,
    .category       = AV_CLASS_CATEGORY_MUXER,
    .get_category   = get_category,
};
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#define OFFSET(x) offsetof(AVFormatContext,x)

static const AVOption avformat_options[] = {
{"avioflags", NULL, OFFSET(avio_flags), AV_OPT_TYPE_FLAGS, {.i64 = DEFAULT }, INT_MIN, INT_MAX, D|E, "avioflags"},
{"direct", "reduce buffering", 0, AV_OPT_TYPE_CONST, {.i64 = AVIO_FLAG_DIRECT }, INT_MIN, INT_MAX, D|E, "avioflags"},
{"probesize", "set probing size", OFFSET(probesize), AV_OPT_TYPE_INT64, {.i64 = 5000000 }, 32, INT64_MAX, D},
{"formatprobesize", "number of bytes to probe file format", OFFSET(format_probesize), AV_OPT_TYPE_INT, {.i64 = PROBE_BUF_MAX}, 0, INT_MAX-1, D},
{"packetsize", "set packet size", OFFSET(packet_size), AV_OPT_TYPE_INT, {.i64 = DEFAULT }, 0, INT_MAX, E},
{"fflags", NULL, OFFSET(flags), AV_OPT_TYPE_FLAGS, {.i64 = AVFMT_FLAG_AUTO_BSF }, INT_MIN, INT_MAX, D|E, "fflags"},
{"flush_packets", "reduce the latency by flushing out packets immediately", 0, AV_OPT_TYPE_CONST, {.i64 = AVFMT_FLAG_FLUSH_PACKETS }, INT_MIN, INT_MAX, E, "fflags"},
{"ignidx", "ignore index", 0, AV_OPT_TYPE_CONST, {.i64 = AVFMT_FLAG_IGNIDX }, INT_MIN, INT_MAX, D, "fflags"},
{"genpts", "generate pts", 0, AV_OPT_TYPE_CONST, {.i64 = AVFMT_FLAG_GENPTS }, INT_MIN, INT_MAX, D, "fflags"},
{"nofillin", "do not fill in missing values that can be exactly calculated", 0, AV_OPT_TYPE_CONST, {.i64 = AVFMT_FLAG_NOFILLIN }, INT_MIN, INT_MAX, D, "fflags"},
{"noparse", "disable AVParsers, this needs nofillin too", 0, AV_OPT_TYPE_CONST, {.i64 = AVFMT_FLAG_NOPARSE }, INT_MIN, INT_MAX, D, "fflags"},
{"igndts", "ignore dts", 0, AV_OPT_TYPE_CONST, {.i64 = AVFMT_FLAG_IGNDTS }, INT_MIN, INT_MAX, D, "fflags"},
{"discardcorrupt", "discard corrupted frames", 0, AV_OPT_TYPE_CONST, {.i64 = AVFMT_FLAG_DISCARD_CORRUPT }, INT_MIN, INT_MAX, D, "fflags"},
{"sortdts", "try to interleave outputted packets by dts", 0, AV_OPT_TYPE_CONST, {.i64 = AVFMT_FLAG_SORT_DTS }, INT_MIN, INT_MAX, D, "fflags"},
{"fastseek", "fast but inaccurate seeks", 0, AV_OPT_TYPE_CONST, {.i64 = AVFMT_FLAG_FAST_SEEK }, INT_MIN, INT_MAX, D, "fflags"},
{"nobuffer", "reduce the latency introduced by optional buffering", 0, AV_OPT_TYPE_CONST, {.i64 = AVFMT_FLAG_NOBUFFER }, 0, INT_MAX, D, "fflags"},
{"bitexact", "do not write random/volatile data", 0, AV_OPT_TYPE_CONST, { .i64 = AVFMT_FLAG_BITEXACT }, 0, 0, E, "fflags" },
{"shortest", "stop muxing with the shortest stream", 0, AV_OPT_TYPE_CONST, { .i64 = AVFMT_FLAG_SHORTEST }, 0, 0, E, "fflags" },
{"autobsf", "add needed bsfs automatically", 0, AV_OPT_TYPE_CONST, { .i64 = AVFMT_FLAG_AUTO_BSF }, 0, 0, E, "fflags" },
{"seek2any", "allow seeking to non-keyframes on demuxer level when supported", OFFSET(seek2any), AV_OPT_TYPE_BOOL, {.i64 = 0 }, 0, 1, D},
{"analyzeduration", "specify how many microseconds are analyzed to probe the input", OFFSET(max_analyze_duration), AV_OPT_TYPE_INT64, {.i64 = 0 }, 0, INT64_MAX, D},
{"cryptokey", "decryption key", OFFSET(key), AV_OPT_TYPE_BINARY, {.dbl = 0}, 0, 0, D},
{"indexmem", "max memory used for timestamp index (per stream)", OFFSET(max_index_size), AV_OPT_TYPE_INT, {.i64 = 1<<20 }, 0, INT_MAX, D},
{"rtbufsize", "max memory used for buffering real-time frames", OFFSET(max_picture_buffer), AV_OPT_TYPE_INT, {.i64 = 3041280 }, 0, INT_MAX, D}, /* defaults to 1s of 15fps 352x288 YUYV422 video */
{"fdebug", "print specific debug info", OFFSET(debug), AV_OPT_TYPE_FLAGS, {.i64 = DEFAULT }, 0, INT_MAX, E|D, "fdebug"},
{"ts", NULL, 0, AV_OPT_TYPE_CONST, {.i64 = FF_FDEBUG_TS }, INT_MIN, INT_MAX, E|D, "fdebug"},
{"max_delay", "maximum muxing or demuxing delay in microseconds", OFFSET(max_delay), AV_OPT_TYPE_INT, {.i64 = -1 }, -1, INT_MAX, E|D},
{"start_time_realtime", "wall-clock time when stream begins (PTS==0)", OFFSET(start_time_realtime), AV_OPT_TYPE_INT64, {.i64 = AV_NOPTS_VALUE}, INT64_MIN, INT64_MAX, E},
{"fpsprobesize", "number of frames used to probe fps", OFFSET(fps_probe_size), AV_OPT_TYPE_INT, {.i64 = -1}, -1, INT_MAX-1, D},
{"audio_preload", "microseconds by which audio packets should be interleaved earlier", OFFSET(audio_preload), AV_OPT_TYPE_INT, {.i64 = 0}, 0, INT_MAX-1, E},
{"chunk_duration", "microseconds for each chunk", OFFSET(max_chunk_duration), AV_OPT_TYPE_INT, {.i64 = 0}, 0, INT_MAX-1, E},
{"chunk_size", "size in bytes for each chunk", OFFSET(max_chunk_size), AV_OPT_TYPE_INT, {.i64 = 0}, 0, INT_MAX-1, E},
/* this is a crutch for avconv, since it cannot deal with identically named options in different contexts.
 * to be removed when avconv is fixed */
{"f_err_detect", "set error detection flags (deprecated; use err_detect, save via avconv)", OFFSET(error_recognition), AV_OPT_TYPE_FLAGS, {.i64 = AV_EF_CRCCHECK }, INT_MIN, INT_MAX, D, "err_detect"},
{"err_detect", "set error detection flags", OFFSET(error_recognition), AV_OPT_TYPE_FLAGS, {.i64 = AV_EF_CRCCHECK }, INT_MIN, INT_MAX, D, "err_detect"},
{"crccheck", "verify embedded CRCs", 0, AV_OPT_TYPE_CONST, {.i64 = AV_EF_CRCCHECK }, INT_MIN, INT_MAX, D, "err_detect"},
{"bitstream", "detect bitstream specification deviations", 0, AV_OPT_TYPE_CONST, {.i64 = AV_EF_BITSTREAM }, INT_MIN, INT_MAX, D, "err_detect"},
{"buffer", "detect improper bitstream length", 0, AV_OPT_TYPE_CONST, {.i64 = AV_EF_BUFFER }, INT_MIN, INT_MAX, D, "err_detect"},
{"explode", "abort decoding on minor error detection", 0, AV_OPT_TYPE_CONST, {.i64 = AV_EF_EXPLODE }, INT_MIN, INT_MAX, D, "err_detect"},
{"ignore_err", "ignore errors", 0, AV_OPT_TYPE_CONST, {.i64 = AV_EF_IGNORE_ERR }, INT_MIN, INT_MAX, D, "err_detect"},
{"careful",    "consider things that violate the spec, are fast to check and have not been seen in the wild as errors", 0, AV_OPT_TYPE_CONST, {.i64 = AV_EF_CAREFUL }, INT_MIN, INT_MAX, D, "err_detect"},
{"compliant",  "consider all spec non compliancies as errors", 0, AV_OPT_TYPE_CONST, {.i64 = AV_EF_COMPLIANT | AV_EF_CAREFUL }, INT_MIN, INT_MAX, D, "err_detect"},
{"aggressive", "consider things that a sane encoder shouldn't do as an error", 0, AV_OPT_TYPE_CONST, {.i64 = AV_EF_AGGRESSIVE | AV_EF_COMPLIANT | AV_EF_CAREFUL}, INT_MIN, INT_MAX, D, "err_detect"},
{"use_wallclock_as_timestamps", "use wallclock as timestamps", OFFSET(use_wallclock_as_timestamps), AV_OPT_TYPE_BOOL, {.i64 = 0}, 0, 1, D},
{"skip_initial_bytes", "set number of bytes to skip before reading header and frames", OFFSET(skip_initial_bytes), AV_OPT_TYPE_INT64, {.i64 = 0}, 0, INT64_MAX-1, D},
{"correct_ts_overflow", "correct single timestamp overflows", OFFSET(correct_ts_overflow), AV_OPT_TYPE_BOOL, {.i64 = 1}, 0, 1, D},
{"flush_packets", "enable flushing of the I/O context after each packet", OFFSET(flush_packets), AV_OPT_TYPE_INT, {.i64 = -1}, -1, 1, E},
{"metadata_header_padding", "set number of bytes to be written as padding in a metadata header", OFFSET(metadata_header_padding), AV_OPT_TYPE_INT, {.i64 = -1}, -1, INT_MAX, E},
{"output_ts_offset", "set output timestamp offset", OFFSET(output_ts_offset), AV_OPT_TYPE_DURATION, {.i64 = 0}, -INT64_MAX, INT64_MAX, E},
{"max_interleave_delta", "maximum buffering duration for interleaving", OFFSET(max_interleave_delta), AV_OPT_TYPE_INT64, { .i64 = 10000000 }, 0, INT64_MAX, E },
{"f_strict", "how strictly to follow the standards (deprecated; use strict, save via avconv)", OFFSET(strict_std_compliance), AV_OPT_TYPE_INT, {.i64 = DEFAULT }, INT_MIN, INT_MAX, D|E, "strict"},
{"strict", "how strictly to follow the standards", OFFSET(strict_std_compliance), AV_OPT_TYPE_INT, {.i64 = DEFAULT }, INT_MIN, INT_MAX, D|E, "strict"},
{"very", "strictly conform to a older more strict version of the spec or reference software", 0, AV_OPT_TYPE_CONST, {.i64 = FF_COMPLIANCE_VERY_STRICT }, INT_MIN, INT_MAX, D|E, "strict"},
{"strict", "strictly conform to all the things in the spec no matter what the consequences", 0, AV_OPT_TYPE_CONST, {.i64 = FF_COMPLIANCE_STRICT }, INT_MIN, INT_MAX, D|E, "strict"},
{"normal", NULL, 0, AV_OPT_TYPE_CONST, {.i64 = FF_COMPLIANCE_NORMAL }, INT_MIN, INT_MAX, D|E, "strict"},
{"unofficial", "allow unofficial extensions", 0, AV_OPT_TYPE_CONST, {.i64 = FF_COMPLIANCE_UNOFFICIAL }, INT_MIN, INT_MAX, D|E, "strict"},
{"experimental", "allow non-standardized experimental variants", 0, AV_OPT_TYPE_CONST, {.i64 = FF_COMPLIANCE_EXPERIMENTAL }, INT_MIN, INT_MAX, D|E, "strict"},
{"max_ts_probe", "maximum number of packets to read while waiting for the first timestamp", OFFSET(max_ts_probe), AV_OPT_TYPE_INT, { .i64 = 50 }, 0, INT_MAX, D },
{"avoid_negative_ts", "shift timestamps so they start at 0", OFFSET(avoid_negative_ts), AV_OPT_TYPE_INT, {.i64 = -1}, -1, 2, E, "avoid_negative_ts"},
{"auto",              "enabled when required by target format",    0, AV_OPT_TYPE_CONST, {.i64 = AVFMT_AVOID_NEG_TS_AUTO },              INT_MIN, INT_MAX, E, "avoid_negative_ts"},
{"disabled",          "do not change timestamps",                  0, AV_OPT_TYPE_CONST, {.i64 = AVFMT_AVOID_NEG_TS_DISABLED },          INT_MIN, INT_MAX, E, "avoid_negative_ts"},
{"make_non_negative", "shift timestamps so they are non negative", 0, AV_OPT_TYPE_CONST, {.i64 = AVFMT_AVOID_NEG_TS_MAKE_NON_NEGATIVE }, INT_MIN, INT_MAX, E, "avoid_negative_ts"},
{"make_zero",         "shift timestamps so they start at 0",       0, AV_OPT_TYPE_CONST, {.i64 = AVFMT_AVOID_NEG_TS_MAKE_ZERO },         INT_MIN, INT_MAX, E, "avoid_negative_ts"},
{"dump_separator", "set information dump field separator", OFFSET(dump_separator), AV_OPT_TYPE_STRING, {.str = ", "}, 0, 0, D|E},
{"codec_whitelist", "List of decoders that are allowed to be used", OFFSET(codec_whitelist), AV_OPT_TYPE_STRING, { .str = NULL },  0, 0, D },
{"format_whitelist", "List of demuxers that are allowed to be used", OFFSET(format_whitelist), AV_OPT_TYPE_STRING, { .str = NULL },  0, 0, D },
{"protocol_whitelist", "List of protocols that are allowed to be used", OFFSET(protocol_whitelist), AV_OPT_TYPE_STRING, { .str = NULL },  0, 0, D },
{"protocol_blacklist", "List of protocols that are not allowed to be used", OFFSET(protocol_blacklist), AV_OPT_TYPE_STRING, { .str = NULL },  0, 0, D },
{"max_streams", "maximum number of streams", OFFSET(max_streams), AV_OPT_TYPE_INT, { .i64 = 1000 }, 0, INT_MAX, D },
{"skip_estimate_duration_from_pts", "skip duration calculation in estimate_timings_from_pts", OFFSET(skip_estimate_duration_from_pts), AV_OPT_TYPE_BOOL, {.i64 = 0}, 0, 1, D},
{"max_probe_packets", "Maximum number of packets to probe a codec", OFFSET(max_probe_packets), AV_OPT_TYPE_INT, { .i64 = 2500 }, 0, INT_MAX, D },
{NULL},
};
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int avformat_open_input(AVFormatContext **ps, const char *filename,
                        const AVInputFormat *fmt, AVDictionary **options)

以AVFormatContext来分析，av_format_context_class.option 是一个列表，基本对应着 AVFormatContext 内部成员。option 通过offset宏 来找到对应的成员的偏移位置。
在avformat_open_input方法中，会遍历外部传入的options参数，对AVFormatContext实例进行属性成员赋值。

类似的这种用法 在ffmpeg中很常见，比如 AVIOContext、URLContext、AVCodecContext …

priv_data

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typedef struct URLContext {
    const AVClass *av_class;    /**< information for av_log(). Set by url_open(). */
    const struct URLProtocol *prot;
    void *priv_data;
    char *filename;             /**< specified URL */
    int flags;
    int max_packet_size;        /**< if non zero, the stream is packetized with this max packet size */
    int is_streamed;            /**< true if streamed (no seek possible), default = false */
    int is_connected;
    AVIOInterruptCB interrupt_callback;
    int64_t rw_timeout;         /**< maximum time to wait for (network) read/write operation completion, in mcs */
    const char *protocol_whitelist;
    const char *protocol_blacklist;
    int min_packet_size;        /**< if non zero, the stream is packetized with this min packet size */
} URLContext;

const URLProtocol ff_https_protocol = {
    .name                = "https",
    .url_open2           = http_open,
    .url_read            = http_read,
    .url_write           = http_write,
    .url_seek            = http_seek,
    .url_close           = http_close,
    .url_get_file_handle = http_get_file_handle,
    .url_get_short_seek  = http_get_short_seek,
    .url_shutdown        = http_shutdown,
    .priv_data_size      = sizeof(HTTPContext),
    .priv_data_class     = &https_context_class,
    .flags               = URL_PROTOCOL_FLAG_NETWORK,
    .default_whitelist   = "http,https,tls,rtp,tcp,udp,crypto,httpproxy"
};

#define HTTP_CLASS(flavor)                          \
static const AVClass flavor ## _context_class = {   \
    .class_name = # flavor,                         \
    .item_name  = av_default_item_name,             \
    .option     = options,                          \
    .version    = LIBAVUTIL_VERSION_INT,            \
}

#if CONFIG_HTTP_PROTOCOL
HTTP_CLASS(http);

typedef struct HTTPContext {
    const AVClass *class;
    URLContext *hd;
    unsigned char buffer[BUFFER_SIZE], *buf_ptr, *buf_end;
    int line_count;
    int http_code;
    /* Used if "Transfer-Encoding: chunked" otherwise -1. */
    uint64_t chunksize;
    int chunkend;
    uint64_t off, end_off, filesize;
    char *uri;
    char *location;
    HTTPAuthState auth_state;
    HTTPAuthState proxy_auth_state;
    char *http_proxy;
    char *headers;
    char *mime_type;
    char *http_version;
    char *user_agent;
    char *referer;
    char *content_type;
    /* Set if the server correctly handles Connection: close and will close
     * the connection after feeding us the content. */
    int willclose;
    int seekable;           /**< Control seekability, 0 = disable, 1 = enable, -1 = probe. */
    int chunked_post;
    /* A flag which indicates if the end of chunked encoding has been sent. */
    int end_chunked_post;
    /* A flag which indicates we have finished to read POST reply. */
    int end_header;
    /* A flag which indicates if we use persistent connections. */
    int multiple_requests;
    uint8_t *post_data;
    int post_datalen;
    int is_akamai;
    int is_mediagateway;
    char *cookies;          ///< holds newline (\n) delimited Set-Cookie header field values (without the "Set-Cookie: " field name)
    /* A dictionary containing cookies keyed by cookie name */
    AVDictionary *cookie_dict;
    int icy;
    /* how much data was read since the last ICY metadata packet */
    uint64_t icy_data_read;
    /* after how many bytes of read data a new metadata packet will be found */
    uint64_t icy_metaint;
    char *icy_metadata_headers;
    char *icy_metadata_packet;
    AVDictionary *metadata;
#if CONFIG_ZLIB
    int compressed;
    z_stream inflate_stream;
    uint8_t *inflate_buffer;
#endif /* CONFIG_ZLIB */
    AVDictionary *chained_options;
    /* -1 = try to send if applicable, 0 = always disabled, 1 = always enabled */
    int send_expect_100;
    char *method;
    int reconnect;
    int reconnect_at_eof;
    int reconnect_on_network_error;
    int reconnect_streamed;
    int reconnect_delay_max;
    char *reconnect_on_http_error;
    int listen;
    char *resource;
    int reply_code;
    int is_multi_client;
    HandshakeState handshake_step;
    int is_connected_server;
    int short_seek_size;
    int64_t expires;
    char *new_location;
    AVDictionary *redirect_cache;
    uint64_t filesize_from_content_range;
} HTTPContext;


static int url_alloc_for_protocol(URLContext **puc, const URLProtocol *up,
                                  const char *filename, int flags,
                                  const AVIOInterruptCB *int_cb) {

...

if (up->priv_data_size) {
        uc->priv_data = av_mallocz(up->priv_data_size);
        if (!uc->priv_data) {
            err = AVERROR(ENOMEM);
            goto fail;
        }
        if (up->priv_data_class) {
            char *start;
            *(const AVClass **)uc->priv_data = up->priv_data_class;
            av_opt_set_defaults(uc->priv_data);

...

}

以 URLContext 来分析，在 url_alloc_for_protocol 方法中，URLContext.priv_data 赋值的其实就是 对用的 URLProtocol的 priv_data_class实例，以http-protocol来举例，
URLContext.priv_data 就是 httpcontext， 而 httpcontext 也符合上面avclass那一套，通过option对 httpcontext实例的成员变量赋值。

这里有个问题？URLContext 持有对应的 prot，为什么还要 弄一个 priv_data呢？ protocol里面有priv_data_class 、priv_data_size，也可以搞一个 priv_data，URLContext用过 prot来访问priv_data就好了。

类似的用法在ffmpeg中 也有很多，比如AVFormatContext，AVFormatContext.priv_data是处理 封装解封装的上下文，mp4格式对应的就是MOVContext. 比如 mp4格式的AVStream,AVStream.priv_data是 MOVStreamContext

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typedef struct AVFormatContext {
    ...
    /**
     * Format private data. This is an AVOptions-enabled struct
     * if and only if iformat/oformat.priv_class is not NULL.
     *
     * - muxing: set by avformat_write_header()
     * - demuxing: set by avformat_open_input()
     */
    void *priv_data;
    ...
}

internal

在 libavformat/internal.h 文件中， FFFormatContext & FFStream 定义如下：

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typedef struct FFFormatContext {
    /**
     * The public context.
     */
    AVFormatContext pub;

    ...

}

typedef struct FFStream {
    /**
     * The public context.
     */
    AVStream pub;

    ...

}

AVFormatContext & AVStream 都是作为第一个成员存在于FFFormatContext & FFStream 结构体中。

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AVFormatContext *avformat_alloc_context(void)
{
    FFFormatContext *const si = av_mallocz(sizeof(*si));
    AVFormatContext *s;

    if (!si)
        return NULL;

    s = &si->pub;

    ...

}

AVStream *avformat_new_stream(AVFormatContext *s, const AVCodec *c)
{

    FFStream *sti;
    sti = av_mallocz(sizeof(*sti));

    AVStream *st;

    st = &sti->pub;

    ...

}

初始化逻辑都是通过创建 FFFormatContext & FFStream 实例，然后取成员pub

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static av_always_inline FFFormatContext *ffformatcontext(AVFormatContext *s)
{
    return (FFFormatContext*)s;
}

static av_always_inline FFStream *ffstream(AVStream *st)
{
    return (FFStream*)st;
}

static av_always_inline const FFStream *cffstream(const AVStream *st)
{
    return (FFStream*)st;
}

av_always_inline 是编译器优化，强制内敛

由于 AVFormatContext & AVStream 作为第一个成员，所以二者可以跟FFFormatContext & FFStream 做强制类型转换
从定义上看， 好像AVFormatContext & AVStream 是应该作为internal，但是实际用法上，FFFormatContext & FFStream才是作为internal 来辅助 AVFormatContext & AVStream存储相关信息

FFIOContext -> AVIOContext , FFCodec -> AVCodec 也是这种形式

ffmepg中也不是 所有的internal 都是这种用法， AVCodecInternal、AVFilterInternal 就是 常规的 作为 AVCodecContext & AVFilterContext 的 internal 成员存在的

个人目前使用的ffmepg 是 5.x的version，早期4.x的时候，avformat & avstream 也是类似 codec & filter 这样常规的用法

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