FFmpeg源代码简单分析：avformat_find_stream_info()

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FFmpeg的库函数源代码分析文章列表：

【架构图】

FFmpeg源代码结构图 - 解码

FFmpeg源代码结构图 - 编码

【通用】

FFmpeg 源代码简单分析：av_register_all()

FFmpeg 源代码简单分析：avcodec_register_all()

FFmpeg 源代码简单分析：内存的分配和释放（av_malloc()、av_free()等）

FFmpeg 源代码简单分析：常见结构体的初始化和销毁（AVFormatContext，AVFrame等）

FFmpeg 源代码简单分析：avio_open2()

FFmpeg 源代码简单分析：av_find_decoder()和av_find_encoder()

FFmpeg 源代码简单分析：avcodec_open2()

FFmpeg 源代码简单分析：avcodec_close()

【解码】

图解FFMPEG打开媒体的函数avformat_open_input

FFmpeg 源代码简单分析：avformat_open_input()

FFmpeg 源代码简单分析：avformat_find_stream_info()

FFmpeg 源代码简单分析：av_read_frame()

FFmpeg 源代码简单分析：avcodec_decode_video2()

FFmpeg 源代码简单分析：avformat_close_input()

【编码】

FFmpeg 源代码简单分析：avformat_alloc_output_context2()

FFmpeg 源代码简单分析：avformat_write_header()

FFmpeg 源代码简单分析：avcodec_encode_video()

FFmpeg 源代码简单分析：av_write_frame()

FFmpeg 源代码简单分析：av_write_trailer()

【其它】

FFmpeg源代码简单分析：日志输出系统（av_log()等）

FFmpeg源代码简单分析：结构体成员管理系统-AVClass

FFmpeg源代码简单分析：结构体成员管理系统-AVOption

FFmpeg源代码简单分析：libswscale的sws_getContext()

FFmpeg源代码简单分析：libswscale的sws_scale()

FFmpeg源代码简单分析：libavdevice的avdevice_register_all()

FFmpeg源代码简单分析：libavdevice的gdigrab

【脚本】

FFmpeg源代码简单分析：makefile

FFmpeg源代码简单分析：configure

【H.264】

FFmpeg的H.264解码器源代码简单分析：概述

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本文简单分析FFmpeg中一个常用的函数：avformat_find_stream_info()。该函数可以读取一部分视音频数据并且获得一些相关的信息。avformat_find_stream_info()的声明位于libavformat\avformat.h，如下所示。

/**
* Read packets of a media file to get stream information. This
* is useful for file formats with no headers such as MPEG. This
* function also computes the real framerate in case of MPEG-2 repeat
* frame mode.
* The logical file position is not changed by this function;
* examined packets may be buffered for later processing.
*
* @param ic media file handle
* @param options  If non-NULL, an ic.nb_streams long array of pointers to
*                 dictionaries, where i-th member contains options for
*                 codec corresponding to i-th stream.
*                 On return each dictionary will be filled with options that were not found.
* @return >=0 if OK, AVERROR_xxx on error
*
* @note this function isn't guaranteed to open all the codecs, so
*       options being non-empty at return is a perfectly normal behavior.
*
* @todo Let the user decide somehow what information is needed so that
*       we do not waste time getting stuff the user does not need.
*/
int avformat_find_stream_info(AVFormatContext *ic, AVDictionary **options);

简单解释一下它的参数的含义：
函数正常执行后返回值大于等于0。
该函数最典型的例子可以参考：最简单的基于FFMPEG+SDL的视频播放器 ver2 （采用SDL2.0）

PS：由于该函数比较复杂，所以只看了一部分代码，以后有时间再进一步分析。

函数调用关系图

函数的调用关系如下图所示。

avformat_find_stream_info()

avformat_find_stream_info()的定义位于libavformat\utils.c。它的代码比较长，如下所示。

int avformat_find_stream_info(AVFormatContext *ic, AVDictionary **options)
{
int i, count, ret = 0, j;
int64_t read_size;
AVStream *st;
AVPacket pkt1, *pkt;
int64_t old_offset  = avio_tell(ic->pb);
// new streams might appear, no options for those
int orig_nb_streams = ic->nb_streams;
int flush_codecs;
int64_t max_analyze_duration = ic->max_analyze_duration2;
int64_t probesize = ic->probesize2;

if (!max_analyze_duration)
max_analyze_duration = ic->max_analyze_duration;
if (ic->probesize)
probesize = ic->probesize;
flush_codecs = probesize > 0;

av_opt_set(ic, "skip_clear", "1", AV_OPT_SEARCH_CHILDREN);

if (!max_analyze_duration) {
if (!strcmp(ic->iformat->name, "flv") && !(ic->ctx_flags & AVFMTCTX_NOHEADER)) {
max_analyze_duration = 10*AV_TIME_BASE;
} else
max_analyze_duration = 5*AV_TIME_BASE;
}

if (ic->pb)
av_log(ic, AV_LOG_DEBUG, "Before avformat_find_stream_info() pos: %"PRId64" bytes read:%"PRId64" seeks:%d\n",
avio_tell(ic->pb), ic->pb->bytes_read, ic->pb->seek_count);

for (i = 0; i < ic->nb_streams; i++) {
const AVCodec *codec;
AVDictionary *thread_opt = NULL;
st = ic->streams[i];

if (st->codec->codec_type == AVMEDIA_TYPE_VIDEO ||
st->codec->codec_type == AVMEDIA_TYPE_SUBTITLE) {
/*            if (!st->time_base.num)
st->time_base = */
if (!st->codec->time_base.num)
st->codec->time_base = st->time_base;
}
// only for the split stuff
if (!st->parser && !(ic->flags & AVFMT_FLAG_NOPARSE)) {
st->parser = av_parser_init(st->codec->codec_id);
if (st->parser) {
if (st->need_parsing == AVSTREAM_PARSE_HEADERS) {
st->parser->flags |= PARSER_FLAG_COMPLETE_FRAMES;
} else if (st->need_parsing == AVSTREAM_PARSE_FULL_RAW) {
st->parser->flags |= PARSER_FLAG_USE_CODEC_TS;
}
} else if (st->need_parsing) {
av_log(ic, AV_LOG_VERBOSE, "parser not found for codec "
"%s, packets or times may be invalid.\n",
avcodec_get_name(st->codec->codec_id));
}
}
codec = find_decoder(ic, st, st->codec->codec_id);

/* Force thread count to 1 since the H.264 decoder will not extract
* SPS and PPS to extradata during multi-threaded decoding. */
av_dict_set(options ? &options[i] : &thread_opt, "threads", "1", 0);

if (ic->codec_whitelist)
av_dict_set(options ? &options[i] : &thread_opt, "codec_whitelist", ic->codec_whitelist, 0);

/* Ensure that subtitle_header is properly set. */
if (st->codec->codec_type == AVMEDIA_TYPE_SUBTITLE
&& codec && !st->codec->codec) {
if (avcodec_open2(st->codec, codec, options ? &options[i] : &thread_opt) < 0)
av_log(ic, AV_LOG_WARNING,
"Failed to open codec in av_find_stream_info\n");
}

// Try to just open decoders, in case this is enough to get parameters.
if (!has_codec_parameters(st, NULL) && st->request_probe <= 0) {
if (codec && !st->codec->codec)
if (avcodec_open2(st->codec, codec, options ? &options[i] : &thread_opt) < 0)
av_log(ic, AV_LOG_WARNING,
"Failed to open codec in av_find_stream_info\n");
}
if (!options)
av_dict_free(&thread_opt);
}

for (i = 0; i < ic->nb_streams; i++) {
#if FF_API_R_FRAME_RATE
ic->streams[i]->info->last_dts = AV_NOPTS_VALUE;
#endif
ic->streams[i]->info->fps_first_dts = AV_NOPTS_VALUE;
ic->streams[i]->info->fps_last_dts  = AV_NOPTS_VALUE;
}

count     = 0;
read_size = 0;
for (;;) {
if (ff_check_interrupt(&ic->interrupt_callback)) {
ret = AVERROR_EXIT;
av_log(ic, AV_LOG_DEBUG, "interrupted\n");
break;
}

/* check if one codec still needs to be handled */
for (i = 0; i < ic->nb_streams; i++) {
int fps_analyze_framecount = 20;

st = ic->streams[i];
if (!has_codec_parameters(st, NULL))
break;
/* If the timebase is coarse (like the usual millisecond precision
* of mkv), we need to analyze more frames to reliably arrive at
* the correct fps. */
if (av_q2d(st->time_base) > 0.0005)
fps_analyze_framecount *= 2;
if (!tb_unreliable(st->codec))
fps_analyze_framecount = 0;
if (ic->fps_probe_size >= 0)
fps_analyze_framecount = ic->fps_probe_size;
if (st->disposition & AV_DISPOSITION_ATTACHED_PIC)
fps_analyze_framecount = 0;
/* variable fps and no guess at the real fps */
if (!(st->r_frame_rate.num && st->avg_frame_rate.num) &&
st->info->duration_count < fps_analyze_framecount &&
st->codec->codec_type == AVMEDIA_TYPE_VIDEO)
break;
if (st->parser && st->parser->parser->split &&
!st->codec->extradata)
break;
if (st->first_dts == AV_NOPTS_VALUE &&
!(ic->iformat->flags & AVFMT_NOTIMESTAMPS) &&
st->codec_info_nb_frames < ic->max_ts_probe &&
(st->codec->codec_type == AVMEDIA_TYPE_VIDEO ||
st->codec->codec_type == AVMEDIA_TYPE_AUDIO))
break;
}
if (i == ic->nb_streams) {
/* NOTE: If the format has no header, then we need to read some
* packets to get most of the streams, so we cannot stop here. */
if (!(ic->ctx_flags & AVFMTCTX_NOHEADER)) {
/* If we found the info for all the codecs, we can stop. */
ret = count;
av_log(ic, AV_LOG_DEBUG, "All info found\n");
flush_codecs = 0;
break;
}
}
/* We did not get all the codec info, but we read too much data. */
if (read_size >= probesize) {
ret = count;
av_log(ic, AV_LOG_DEBUG,
"Probe buffer size limit of %"PRId64" bytes reached\n", probesize);
for (i = 0; i < ic->nb_streams; i++)
if (!ic->streams[i]->r_frame_rate.num &&
ic->streams[i]->info->duration_count <= 1 &&
ic->streams[i]->codec->codec_type == AVMEDIA_TYPE_VIDEO &&
strcmp(ic->iformat->name, "image2"))
av_log(ic, AV_LOG_WARNING,
"Stream #%d: not enough frames to estimate rate; "
"consider increasing probesize\n", i);
break;
}

/* NOTE: A new stream can be added there if no header in file
* (AVFMTCTX_NOHEADER). */
ret = read_frame_internal(ic, &pkt1);
if (ret == AVERROR(EAGAIN))
continue;

if (ret < 0) {
/* EOF or error*/
break;
}

if (ic->flags & AVFMT_FLAG_NOBUFFER)
free_packet_buffer(&ic->packet_buffer, &ic->packet_buffer_end);
{
pkt = add_to_pktbuf(&ic->packet_buffer, &pkt1,
&ic->packet_buffer_end);
if (!pkt) {
ret = AVERROR(ENOMEM);
goto find_stream_info_err;
}
if ((ret = av_dup_packet(pkt)) < 0)
goto find_stream_info_err;
}

st = ic->streams[pkt->stream_index];
if (!(st->disposition & AV_DISPOSITION_ATTACHED_PIC))
read_size += pkt->size;

if (pkt->dts != AV_NOPTS_VALUE && st->codec_info_nb_frames > 1) {
/* check for non-increasing dts */
if (st->info->fps_last_dts != AV_NOPTS_VALUE &&
st->info->fps_last_dts >= pkt->dts) {
av_log(ic, AV_LOG_DEBUG,
"Non-increasing DTS in stream %d: packet %d with DTS "
"%"PRId64", packet %d with DTS %"PRId64"\n",
st->index, st->info->fps_last_dts_idx,
st->info->fps_last_dts, st->codec_info_nb_frames,
pkt->dts);
st->info->fps_first_dts =
st->info->fps_last_dts  = AV_NOPTS_VALUE;
}
/* Check for a discontinuity in dts. If the difference in dts
* is more than 1000 times the average packet duration in the
* sequence, we treat it as a discontinuity. */
if (st->info->fps_last_dts != AV_NOPTS_VALUE &&
st->info->fps_last_dts_idx > st->info->fps_first_dts_idx &&
(pkt->dts - st->info->fps_last_dts) / 1000 >
(st->info->fps_last_dts     - st->info->fps_first_dts) /
(st->info->fps_last_dts_idx - st->info->fps_first_dts_idx)) {
av_log(ic, AV_LOG_WARNING,
"DTS discontinuity in stream %d: packet %d with DTS "
"%"PRId64", packet %d with DTS %"PRId64"\n",
st->index, st->info->fps_last_dts_idx,
st->info->fps_last_dts, st->codec_info_nb_frames,
pkt->dts);
st->info->fps_first_dts =
st->info->fps_last_dts  = AV_NOPTS_VALUE;
}

/* update stored dts values */
if (st->info->fps_first_dts == AV_NOPTS_VALUE) {
st->info->fps_first_dts     = pkt->dts;
st->info->fps_first_dts_idx = st->codec_info_nb_frames;
}
st->info->fps_last_dts     = pkt->dts;
st->info->fps_last_dts_idx = st->codec_info_nb_frames;
}
if (st->codec_info_nb_frames>1) {
int64_t t = 0;

if (st->time_base.den > 0)
t = av_rescale_q(st->info->codec_info_duration, st->time_base, AV_TIME_BASE_Q);
if (st->avg_frame_rate.num > 0)
t = FFMAX(t, av_rescale_q(st->codec_info_nb_frames, av_inv_q(st->avg_frame_rate), AV_TIME_BASE_Q));

if (   t == 0
&& st->codec_info_nb_frames>30
&& st->info->fps_first_dts != AV_NOPTS_VALUE
&& st->info->fps_last_dts  != AV_NOPTS_VALUE)
t = FFMAX(t, av_rescale_q(st->info->fps_last_dts - st->info->fps_first_dts, st->time_base, AV_TIME_BASE_Q));

if (t >= max_analyze_duration) {
av_log(ic, AV_LOG_VERBOSE, "max_analyze_duration %"PRId64" reached at %"PRId64" microseconds\n",
max_analyze_duration,
t);
if (ic->flags & AVFMT_FLAG_NOBUFFER)
av_packet_unref(pkt);
break;
}
if (pkt->duration) {
st->info->codec_info_duration        += pkt->duration;
st->info->codec_info_duration_fields += st->parser && st->need_parsing && st->codec->ticks_per_frame ==2 ? st->parser->repeat_pict + 1 : 2;
}
}
#if FF_API_R_FRAME_RATE
if (st->codec->codec_type == AVMEDIA_TYPE_VIDEO)
ff_rfps_add_frame(ic, st, pkt->dts);
#endif
if (st->parser && st->parser->parser->split && !st->codec->extradata) {
int i = st->parser->parser->split(st->codec, pkt->data, pkt->size);
if (i > 0 && i < FF_MAX_EXTRADATA_SIZE) {
if (ff_alloc_extradata(st->codec, i))
return AVERROR(ENOMEM);
memcpy(st->codec->extradata, pkt->data,
st->codec->extradata_size);
}
}

/* If still no information, we try to open the codec and to
* decompress the frame. We try to avoid that in most cases as
* it takes longer and uses more memory. For MPEG-4, we need to
* decompress for QuickTime.
*
* If CODEC_CAP_CHANNEL_CONF is set this will force decoding of at
* least one frame of codec data, this makes sure the codec initializes
* the channel configuration and does not only trust the values from
* the container. */
try_decode_frame(ic, st, pkt,
(options && i < orig_nb_streams) ? &options[i] : NULL);

if (ic->flags & AVFMT_FLAG_NOBUFFER)
av_packet_unref(pkt);

st->codec_info_nb_frames++;
count++;
}

if (flush_codecs) {
AVPacket empty_pkt = { 0 };
int err = 0;
av_init_packet(&empty_pkt);

for (i = 0; i < ic->nb_streams; i++) {

st = ic->streams[i];

/* flush the decoders */
if (st->info->found_decoder == 1) {
do {
err = try_decode_frame(ic, st, &empty_pkt,
(options && i < orig_nb_streams)
? &options[i] : NULL);
} while (err > 0 && !has_codec_parameters(st, NULL));

if (err < 0) {
av_log(ic, AV_LOG_INFO,
"decoding for stream %d failed\n", st->index);
}
}
}
}

// close codecs which were opened in try_decode_frame()
for (i = 0; i < ic->nb_streams; i++) {
st = ic->streams[i];
avcodec_close(st->codec);
}

ff_rfps_calculate(ic);

for (i = 0; i < ic->nb_streams; i++) {
st = ic->streams[i];
if (st->codec->codec_type == AVMEDIA_TYPE_VIDEO) {
if (st->codec->codec_id == AV_CODEC_ID_RAWVIDEO && !st->codec->codec_tag && !st->codec->bits_per_coded_sample) {
uint32_t tag= avcodec_pix_fmt_to_codec_tag(st->codec->pix_fmt);
if (avpriv_find_pix_fmt(avpriv_get_raw_pix_fmt_tags(), tag) == st->codec->pix_fmt)
st->codec->codec_tag= tag;
}

/* estimate average framerate if not set by demuxer */
if (st->info->codec_info_duration_fields &&
!st->avg_frame_rate.num &&
st->info->codec_info_duration) {
int best_fps      = 0;
double best_error = 0.01;

if (st->info->codec_info_duration        >= INT64_MAX / st->time_base.num / 2||
st->info->codec_info_duration_fields >= INT64_MAX / st->time_base.den ||
st->info->codec_info_duration        < 0)
continue;
av_reduce(&st->avg_frame_rate.num, &st->avg_frame_rate.den,
st->info->codec_info_duration_fields * (int64_t) st->time_base.den,
st->info->codec_info_duration * 2 * (int64_t) st->time_base.num, 60000);

/* Round guessed framerate to a "standard" framerate if it's
* within 1% of the original estimate. */
for (j = 0; j < MAX_STD_TIMEBASES; j++) {
AVRational std_fps = { get_std_framerate(j), 12 * 1001 };
double error       = fabs(av_q2d(st->avg_frame_rate) /
av_q2d(std_fps) - 1);

if (error < best_error) {
best_error = error;
best_fps   = std_fps.num;
}
}
if (best_fps)
av_reduce(&st->avg_frame_rate.num, &st->avg_frame_rate.den,
best_fps, 12 * 1001, INT_MAX);
}

if (!st->r_frame_rate.num) {
if (    st->codec->time_base.den * (int64_t) st->time_base.num
<= st->codec->time_base.num * st->codec->ticks_per_frame * (int64_t) st->time_base.den) {
st->r_frame_rate.num = st->codec->time_base.den;
st->r_frame_rate.den = st->codec->time_base.num * st->codec->ticks_per_frame;
} else {
st->r_frame_rate.num = st->time_base.den;
st->r_frame_rate.den = st->time_base.num;
}
}
} else if (st->codec->codec_type == AVMEDIA_TYPE_AUDIO) {
if (!st->codec->bits_per_coded_sample)
st->codec->bits_per_coded_sample =
av_get_bits_per_sample(st->codec->codec_id);
// set stream disposition based on audio service type
switch (st->codec->audio_service_type) {
case AV_AUDIO_SERVICE_TYPE_EFFECTS:
st->disposition = AV_DISPOSITION_CLEAN_EFFECTS;
break;
case AV_AUDIO_SERVICE_TYPE_VISUALLY_IMPAIRED:
st->disposition = AV_DISPOSITION_VISUAL_IMPAIRED;
break;
case AV_AUDIO_SERVICE_TYPE_HEARING_IMPAIRED:
st->disposition = AV_DISPOSITION_HEARING_IMPAIRED;
break;
case AV_AUDIO_SERVICE_TYPE_COMMENTARY:
st->disposition = AV_DISPOSITION_COMMENT;
break;
case AV_AUDIO_SERVICE_TYPE_KARAOKE:
st->disposition = AV_DISPOSITION_KARAOKE;
break;
}
}
}

if (probesize)
estimate_timings(ic, old_offset);

av_opt_set(ic, "skip_clear", "0", AV_OPT_SEARCH_CHILDREN);

if (ret >= 0 && ic->nb_streams)
/* We could not have all the codec parameters before EOF. */
ret = -1;
for (i = 0; i < ic->nb_streams; i++) {
const char *errmsg;
st = ic->streams[i];
if (!has_codec_parameters(st, &errmsg)) {
char buf[256];
avcodec_string(buf, sizeof(buf), st->codec, 0);
av_log(ic, AV_LOG_WARNING,
"Could not find codec parameters for stream %d (%s): %s\n"
"Consider increasing the value for the 'analyzeduration' and 'probesize' options\n",
i, buf, errmsg);
} else {
ret = 0;
}
}

compute_chapters_end(ic);

find_stream_info_err:
for (i = 0; i < ic->nb_streams; i++) {
st = ic->streams[i];
if (ic->streams[i]->codec->codec_type != AVMEDIA_TYPE_AUDIO)
ic->streams[i]->codec->thread_count = 0;
if (st->info)
av_freep(&st->info->duration_error);
av_freep(&ic->streams[i]->info);
}
if (ic->pb)
av_log(ic, AV_LOG_DEBUG, "After avformat_find_stream_info() pos: %"PRId64" bytes read:%"PRId64" seeks:%d frames:%d\n",
avio_tell(ic->pb), ic->pb->bytes_read, ic->pb->seek_count, count);
return ret;
}

由于avformat_find_stream_info()代码比较长，难以全部分析，在这里只能简单记录一下它的要点。该函数主要用于给每个媒体流（音频/视频）的AVStream结构体赋值。我们大致浏览一下这个函数的代码，会发现它其实已经实现了解码器的查找，解码器的打开，视音频帧的读取，视音频帧的解码等工作。换句话说，该函数实际上已经“走通”的解码的整个流程。下面看一下除了成员变量赋值之外，该函数的几个关键流程。
下面选择上述流程中几个关键函数的代码简单看一下。

find_decoder()

find_decoder()用于找到合适的解码器，它的定义如下所示。

static const AVCodec *find_decoder(AVFormatContext *s, AVStream *st, enum AVCodecID codec_id)
{
if (st->codec->codec)
return st->codec->codec;

switch (st->codec->codec_type) {
case AVMEDIA_TYPE_VIDEO:
if (s->video_codec)    return s->video_codec;
break;
case AVMEDIA_TYPE_AUDIO:
if (s->audio_codec)    return s->audio_codec;
break;
case AVMEDIA_TYPE_SUBTITLE:
if (s->subtitle_codec) return s->subtitle_codec;
break;
}

return avcodec_find_decoder(codec_id);
}

从代码中可以看出，如果指定的AVStream已经包含了解码器，则函数什么也不做直接返回。否则调用avcodec_find_decoder()获取解码器。avcodec_find_decoder()是一个FFmpeg的API函数，在这里不做详细分析。

read_frame_internal()

read_frame_internal()的功能是读取一帧压缩码流数据。FFmpeg的API函数av_read_frame()内部调用的就是read_frame_internal()。有关这方面的知识可以参考文章：

ffmpeg 源代码简单分析 ： av_read_frame()
因此，可以认为read_frame_internal()和av_read_frame()的功能基本上是等同的。

try_decode_frame()

try_decode_frame()的功能可以从字面上的意思进行理解：“尝试解码一些帧”，它的定义如下所示。

/* returns 1 or 0 if or if not decoded data was returned, or a negative error */
static int try_decode_frame(AVFormatContext *s, AVStream *st, AVPacket *avpkt,
AVDictionary **options)
{
const AVCodec *codec;
int got_picture = 1, ret = 0;
AVFrame *frame = av_frame_alloc();
AVSubtitle subtitle;
AVPacket pkt = *avpkt;

if (!frame)
return AVERROR(ENOMEM);

if (!avcodec_is_open(st->codec) &&
st->info->found_decoder <= 0 &&
(st->codec->codec_id != -st->info->found_decoder || !st->codec->codec_id)) {
AVDictionary *thread_opt = NULL;

codec = find_decoder(s, st, st->codec->codec_id);

if (!codec) {
st->info->found_decoder = -st->codec->codec_id;
ret                     = -1;
goto fail;
}

/* Force thread count to 1 since the H.264 decoder will not extract
* SPS and PPS to extradata during multi-threaded decoding. */
av_dict_set(options ? options : &thread_opt, "threads", "1", 0);
if (s->codec_whitelist)
av_dict_set(options ? options : &thread_opt, "codec_whitelist", s->codec_whitelist, 0);
ret = avcodec_open2(st->codec, codec, options ? options : &thread_opt);
if (!options)
av_dict_free(&thread_opt);
if (ret < 0) {
st->info->found_decoder = -st->codec->codec_id;
goto fail;
}
st->info->found_decoder = 1;
} else if (!st->info->found_decoder)
st->info->found_decoder = 1;

if (st->info->found_decoder < 0) {
ret = -1;
goto fail;
}

while ((pkt.size > 0 || (!pkt.data && got_picture)) &&
ret >= 0 &&
(!has_codec_parameters(st, NULL) || !has_decode_delay_been_guessed(st) ||
(!st->codec_info_nb_frames &&
st->codec->codec->capabilities & CODEC_CAP_CHANNEL_CONF))) {
got_picture = 0;
switch (st->codec->codec_type) {
case AVMEDIA_TYPE_VIDEO:
ret = avcodec_decode_video2(st->codec, frame,
&got_picture, &pkt);
break;
case AVMEDIA_TYPE_AUDIO:
ret = avcodec_decode_audio4(st->codec, frame, &got_picture, &pkt);
break;
case AVMEDIA_TYPE_SUBTITLE:
ret = avcodec_decode_subtitle2(st->codec, &subtitle,
&got_picture, &pkt);
ret = pkt.size;
break;
default:
break;
}
if (ret >= 0) {
if (got_picture)
st->nb_decoded_frames++;
pkt.data += ret;
pkt.size -= ret;
ret       = got_picture;
}
}

if (!pkt.data && !got_picture)
ret = -1;

fail:
av_frame_free(&frame);
return ret;
}

从try_decode_frame()的定义可以看出，该函数首先判断视音频流的解码器是否已经打开，如果没有打开的话，先打开相应的解码器。接下来根据视音频流类型的不同，调用不同的解码函数进行解码：视频流调用avcodec_decode_video2()，音频流调用avcodec_decode_audio4()，字幕流调用avcodec_decode_subtitle2()。解码的循环会一直持续下去直到满足了while()的所有条件。
while()语句的条件中有一个has_codec_parameters()函数，用于判断AVStream中的成员变量是否都已经设置完毕。该函数在avformat_find_stream_info()中的多个地方被使用过。下面简单看一下该函数。

has_codec_parameters()

has_codec_parameters()用于检查AVStream中的成员变量是否都已经设置完毕。函数的定义如下。

static int has_codec_parameters(AVStream *st, const char **errmsg_ptr)
{
AVCodecContext *avctx = st->codec;

#define FAIL(errmsg) do {                                         \
if (errmsg_ptr)                                           \
*errmsg_ptr = errmsg;                                 \
return 0;                                                 \
} while (0)

if (   avctx->codec_id == AV_CODEC_ID_NONE
&& avctx->codec_type != AVMEDIA_TYPE_DATA)
FAIL("unknown codec");
switch (avctx->codec_type) {
case AVMEDIA_TYPE_AUDIO:
if (!avctx->frame_size && determinable_frame_size(avctx))
FAIL("unspecified frame size");
if (st->info->found_decoder >= 0 &&
avctx->sample_fmt == AV_SAMPLE_FMT_NONE)
FAIL("unspecified sample format");
if (!avctx->sample_rate)
FAIL("unspecified sample rate");
if (!avctx->channels)
FAIL("unspecified number of channels");
if (st->info->found_decoder >= 0 && !st->nb_decoded_frames && avctx->codec_id == AV_CODEC_ID_DTS)
FAIL("no decodable DTS frames");
break;
case AVMEDIA_TYPE_VIDEO:
if (!avctx->width)
FAIL("unspecified size");
if (st->info->found_decoder >= 0 && avctx->pix_fmt == AV_PIX_FMT_NONE)
FAIL("unspecified pixel format");
if (st->codec->codec_id == AV_CODEC_ID_RV30 || st->codec->codec_id == AV_CODEC_ID_RV40)
if (!st->sample_aspect_ratio.num && !st->codec->sample_aspect_ratio.num && !st->codec_info_nb_frames)
FAIL("no frame in rv30/40 and no sar");
break;
case AVMEDIA_TYPE_SUBTITLE:
if (avctx->codec_id == AV_CODEC_ID_HDMV_PGS_SUBTITLE && !avctx->width)
FAIL("unspecified size");
break;
case AVMEDIA_TYPE_DATA:
if (avctx->codec_id == AV_CODEC_ID_NONE) return 1;
}

return 1;
}

estimate_timings()

estimate_timings()位于avformat_find_stream_info()最后面，用于估算AVFormatContext以及AVStream的时长duration。它的代码如下所示。

static void estimate_timings(AVFormatContext *ic, int64_t old_offset)
{
int64_t file_size;

/* get the file size, if possible */
if (ic->iformat->flags & AVFMT_NOFILE) {
file_size = 0;
} else {
file_size = avio_size(ic->pb);
file_size = FFMAX(0, file_size);
}

if ((!strcmp(ic->iformat->name, "mpeg") ||
!strcmp(ic->iformat->name, "mpegts")) &&
file_size && ic->pb->seekable) {
/* get accurate estimate from the PTSes */
estimate_timings_from_pts(ic, old_offset);
ic->duration_estimation_method = AVFMT_DURATION_FROM_PTS;
} else if (has_duration(ic)) {
/* at least one component has timings - we use them for all
* the components */
fill_all_stream_timings(ic);
ic->duration_estimation_method = AVFMT_DURATION_FROM_STREAM;
} else {
/* less precise: use bitrate info */
estimate_timings_from_bit_rate(ic);
ic->duration_estimation_method = AVFMT_DURATION_FROM_BITRATE;
}
update_stream_timings(ic);

{
int i;
AVStream av_unused *st;
for (i = 0; i < ic->nb_streams; i++) {
st = ic->streams[i];
av_dlog(ic, "%d: start_time: %0.3f duration: %0.3f\n", i,
(double) st->start_time / AV_TIME_BASE,
(double) st->duration   / AV_TIME_BASE);
}
av_dlog(ic,
"stream: start_time: %0.3f duration: %0.3f bitrate=%d kb/s\n",
(double) ic->start_time / AV_TIME_BASE,
(double) ic->duration   / AV_TIME_BASE,
ic->bit_rate / 1000);
}
}

从estimate_timings()的代码中可以看出，有3种估算方法：
（1）通过pts（显示时间戳）。该方法调用estimate_timings_from_pts()。它的基本思想就是读取视音频流中的结束位置AVPacket的PTS和起始位置AVPacket的PTS，两者相减得到时长信息。
（2）通过已知流的时长。该方法调用fill_all_stream_timings()。它的代码没有细看，但从函数的注释的意思来说，应该是当有些视音频流有时长信息的时候，直接赋值给其他视音频流。
（3）通过bitrate（码率）。该方法调用estimate_timings_from_bit_rate()。它的基本思想就是获得整个文件大小，以及整个文件的bitrate，两者相除之后得到时长信息。

estimate_timings_from_bit_rate()

在这里附上上述几种方法中最简单的函数estimate_timings_from_bit_rate()的代码。

static void estimate_timings_from_bit_rate(AVFormatContext *ic)
{
int64_t filesize, duration;
int i, show_warning = 0;
AVStream *st;

/* if bit_rate is already set, we believe it */
if (ic->bit_rate <= 0) {
int bit_rate = 0;
for (i = 0; i < ic->nb_streams; i++) {
st = ic->streams[i];
if (st->codec->bit_rate > 0) {
if (INT_MAX - st->codec->bit_rate < bit_rate) {
bit_rate = 0;
break;
}
bit_rate += st->codec->bit_rate;
}
}
ic->bit_rate = bit_rate;
}

/* if duration is already set, we believe it */
if (ic->duration == AV_NOPTS_VALUE &&
ic->bit_rate != 0) {
filesize = ic->pb ? avio_size(ic->pb) : 0;
if (filesize > ic->data_offset) {
filesize -= ic->data_offset;
for (i = 0; i < ic->nb_streams; i++) {
st      = ic->streams[i];
if (   st->time_base.num <= INT64_MAX / ic->bit_rate
&& st->duration == AV_NOPTS_VALUE) {
duration = av_rescale(8 * filesize, st->time_base.den,
ic->bit_rate *
(int64_t) st->time_base.num);
st->duration = duration;
show_warning = 1;
}
}
}
}
if (show_warning)
av_log(ic, AV_LOG_WARNING,
"Estimating duration from bitrate, this may be inaccurate\n");
}

从代码中可以看出，该函数做了两步工作：
（1）如果AVFormatContext中没有bit_rate信息，就把所有AVStream的bit_rate加起来作为AVFormatContext的bit_rate信息。
（2）使用文件大小filesize除以bitrate得到时长信息。具体的方法是：
AVStream->duration=(filesize*8/bit_rate)/time_basePS：
1）filesize乘以8是因为需要把Byte转换为Bit
2）具体的实现函数是那个av_rescale()函数。x=av_rescale(a,b,c)的含义是x=a*b/c。
3）之所以要除以time_base，是因为AVStream中的duration的单位是time_base，注意这和AVFormatContext中的duration的单位（单位是AV_TIME_BASE，固定取值为1000000）是不一样的。

至此，avformat_find_stream_info()主要的函数就分析完了。

雷霄骅
leixiaohua1020@126.com http://blog.csdn.net/leixiaohua1020