/**
* Write a packet to an output media file.
*
* This function passes the packet directly to the muxer, without any buffering
* or reordering. The caller is responsible for correctly interleaving the
* packets if the format requires it. Callers that want libavformat to handle
* the interleaving should call av_interleaved_write_frame() instead of this
* function.
*
* @param s media file handle
* @param pkt The packet containing the data to be written. Note that unlike
* av_interleaved_write_frame(), this function does not take
* ownership of the packet passed to it (though some muxers may make
* an internal reference to the input packet).
* <br>
* This parameter can be NULL (at any time, not just at the end), in
* order to immediately flush data buffered within the muxer, for
* muxers that buffer up data internally before writing it to the
* output.
* <br>
* Packet's @ref AVPacket.stream_index "stream_index" field must be
* set to the index of the corresponding stream in @ref
* AVFormatContext.streams "s->streams".
* <br>
* The timestamps (@ref AVPacket.pts "pts", @ref AVPacket.dts "dts")
* must be set to correct values in the stream's timebase (unless the
* output format is flagged with the AVFMT_NOTIMESTAMPS flag, then
* they can be set to AV_NOPTS_VALUE).
* The dts for subsequent packets passed to this function must be strictly
* increasing when compared in their respective timebases (unless the
* output format is flagged with the AVFMT_TS_NONSTRICT, then they
* merely have to be nondecreasing). @ref AVPacket.duration
* "duration") should also be set if known.
* @return < 0 on error, = 0 if OK, 1 if flushed and there is no more data to flush
*
* @see av_interleaved_write_frame()
*/
int av_write_frame(AVFormatContext *s, AVPacket *pkt);av_write_frame()的定义位于libavformat\mux.c，如下所示。
int av_write_frame(AVFormatContext *s, AVPacket *pkt)
{
int ret;
ret = prepare_input_packet(s, pkt);
if (ret < 0)
return ret;
if (!pkt) {
if (s->oformat->flags & AVFMT_ALLOW_FLUSH) {
ret = s->oformat->write_packet(s, NULL);
flush_if_needed(s);
if (ret >= 0 && s->pb && s->pb->error < 0)
ret = s->pb->error;
return ret;
}
return 1;
}
ret = do_packet_auto_bsf(s, pkt);
if (ret <= 0)
return ret;
#if FF_API_COMPUTE_PKT_FIELDS2 && FF_API_LAVF_AVCTX
ret = compute_muxer_pkt_fields(s, s->streams[pkt->stream_index], pkt);
if (ret < 0 && !(s->oformat->flags & AVFMT_NOTIMESTAMPS))
return ret;
#endif
ret = write_packet(s, pkt);
if (ret >= 0 && s->pb && s->pb->error < 0)
ret = s->pb->error;
if (ret >= 0)
s->streams[pkt->stream_index]->nb_frames++;
return ret;
}从源代码可以看出，av_write_frame()主要完成了以下几步工作：
（1）调用prepare_input_packet()->check_packet()做一些简单的检测
（2）调用compute_pkt_fields2()设置AVPacket的一些属性值
（3）调用write_packet()写入数据
下面分别看一下这几个函数功能。
prepare_input_packet()->check_packet()
static int check_packet(AVFormatContext *s, AVPacket *pkt)
{
if (!pkt)
return 0;
if (pkt->stream_index < 0 || pkt->stream_index >= s->nb_streams) {
av_log(s, AV_LOG_ERROR, "Invalid packet stream index: %d\n",
pkt->stream_index);
return AVERROR(EINVAL);
}
if (s->streams[pkt->stream_index]->codecpar->codec_type == AVMEDIA_TYPE_ATTACHMENT) {
av_log(s, AV_LOG_ERROR, "Received a packet for an attachment stream.\n");
return AVERROR(EINVAL);
}
return 0;
}从代码中可以看出，check_packet()的功能比较简单：首先检查一下输入的AVPacket是否为空，如果为空，则是直接返回；然后检查一下AVPacket的stream_index（标记了该AVPacket所属的AVStream）设置是否正常，如果为负数或者大于AVStream的个数，则返回错误信息；最后检查AVPacket所属的AVStream是否属于attachment stream，这个地方没见过，目前还没有研究。
AVOutputFormat->write_packet()
write_packet()函数的定义位于libavformat\mux.c，如下所示。
/**
* Make timestamps non negative, move side data from payload to internal struct, call muxer, and restore
* sidedata.
*
* FIXME: this function should NEVER get undefined pts/dts beside when the
* AVFMT_NOTIMESTAMPS is set.
* Those additional safety checks should be dropped once the correct checks
* are set in the callers.
*/
static int write_packet(AVFormatContext *s, AVPacket *pkt)
{
int ret;
int64_t pts_backup, dts_backup;
pts_backup = pkt->pts;
dts_backup = pkt->dts;
// If the timestamp offsetting below is adjusted, adjust
// ff_interleaved_peek similarly.
if (s->output_ts_offset) {
AVStream *st = s->streams[pkt->stream_index];
int64_t offset = av_rescale_q(s->output_ts_offset, AV_TIME_BASE_Q, st->time_base);
if (pkt->dts != AV_NOPTS_VALUE)
pkt->dts += offset;
if (pkt->pts != AV_NOPTS_VALUE)
pkt->pts += offset;
}
if (s->avoid_negative_ts > 0) {
AVStream *st = s->streams[pkt->stream_index];
int64_t offset = st->mux_ts_offset;
int64_t ts = s->internal->avoid_negative_ts_use_pts ? pkt->pts : pkt->dts;
if (s->internal->offset == AV_NOPTS_VALUE && ts != AV_NOPTS_VALUE &&
(ts < 0 || s->avoid_negative_ts == AVFMT_AVOID_NEG_TS_MAKE_ZERO)) {
s->internal->offset = -ts;
s->internal->offset_timebase = st->time_base;
}
if (s->internal->offset != AV_NOPTS_VALUE && !offset) {
offset = st->mux_ts_offset =
av_rescale_q_rnd(s->internal->offset,
s->internal->offset_timebase,
st->time_base,
AV_ROUND_UP);
}
if (pkt->dts != AV_NOPTS_VALUE)
pkt->dts += offset;
if (pkt->pts != AV_NOPTS_VALUE)
pkt->pts += offset;
if (s->internal->avoid_negative_ts_use_pts) {
if (pkt->pts != AV_NOPTS_VALUE && pkt->pts < 0) {
av_log(s, AV_LOG_WARNING, "failed to avoid negative "
"pts %s in stream %d.\n"
"Try -avoid_negative_ts 1 as a possible workaround.\n",
av_ts2str(pkt->pts),
pkt->stream_index
);
}
} else {
av_assert2(pkt->dts == AV_NOPTS_VALUE || pkt->dts >= 0 || s->max_interleave_delta > 0);
if (pkt->dts != AV_NOPTS_VALUE && pkt->dts < 0) {
av_log(s, AV_LOG_WARNING,
"Packets poorly interleaved, failed to avoid negative "
"timestamp %s in stream %d.\n"
"Try -max_interleave_delta 0 as a possible workaround.\n",
av_ts2str(pkt->dts),
pkt->stream_index
);
}
}
}
if ((pkt->flags & AV_PKT_FLAG_UNCODED_FRAME)) {
AVFrame *frame = (AVFrame *)pkt->data;
av_assert0(pkt->size == UNCODED_FRAME_PACKET_SIZE);
ret = s->oformat->write_uncoded_frame(s, pkt->stream_index, &frame, 0);
av_frame_free(&frame);
} else {
ret = s->oformat->write_packet(s, pkt);
}
if (s->pb && ret >= 0) {
flush_if_needed(s);
if (s->pb->error < 0)
ret = s->pb->error;
}
if (ret < 0) {
pkt->pts = pts_backup;
pkt->dts = dts_backup;
}
return ret;
}write_packet()最后调用了AVOutputFormat中的write_packet()函数。write_packet()函数最关键的地方就是调用了AVOutputFormat中写入数据的方法。如果AVPacket中的flag标记中包含AV_PKT_FLAG_UNCODED_FRAME，就会调用AVOutputFormat的write_uncoded_frame()函数；如果不包含那个标记，就会调用write_packet()函数。write_packet()实际上是一个函数指针，指向特定的AVOutputFormat中的实现函数。例如，我们看一下FLV对应的AVOutputFormat，位于libavformat\flvenc.c，如下所示。
AVOutputFormat ff_flv_muxer = {
.name = "flv",
.long_name = NULL_IF_CONFIG_SMALL("FLV (Flash Video)"),
.mime_type = "video/x-flv",
.extensions = "flv",
.priv_data_size = sizeof(FLVContext),
.audio_codec = CONFIG_LIBMP3LAME ? AV_CODEC_ID_MP3 : AV_CODEC_ID_ADPCM_SWF,
.video_codec = AV_CODEC_ID_FLV1,
.init = flv_init,
.write_header = flv_write_header,
.write_packet = flv_write_packet,
.write_trailer = flv_write_trailer,
.check_bitstream= flv_check_bitstream,
.codec_tag = (const AVCodecTag* const []) {
flv_video_codec_ids, flv_audio_codec_ids, 0
},
.flags = AVFMT_GLOBALHEADER | AVFMT_VARIABLE_FPS |
AVFMT_TS_NONSTRICT,
.priv_class = &flv_muxer_class,
};从ff_flv_muxer的定义可以看出，write_packet()指向的是flv_write_packet()函数。在看flv_write_packet()函数的定义之前，我们先回顾一下FLV封装格式的结构，《xxx》。
flv_write_packet()
下面我们看一下FLV格式中write_packet()对应的实现函数flv_write_packet()的定义，位于libavformat\flvenc.c，如下所示。
static int flv_write_packet(AVFormatContext *s, AVPacket *pkt)
{
AVIOContext *pb = s->pb;
AVCodecParameters *par = s->streams[pkt->stream_index]->codecpar;
FLVContext *flv = s->priv_data;
FLVStreamContext *sc = s->streams[pkt->stream_index]->priv_data;
unsigned ts;
int size = pkt->size;
uint8_t *data = NULL;
int flags = -1, flags_size, ret;
int64_t cur_offset = avio_tell(pb);
if (par->codec_type == AVMEDIA_TYPE_AUDIO && !pkt->size) {
av_log(s, AV_LOG_WARNING, "Empty audio Packet\n");
return AVERROR(EINVAL);
}
if (par->codec_id == AV_CODEC_ID_VP6F || par->codec_id == AV_CODEC_ID_VP6A ||
par->codec_id == AV_CODEC_ID_VP6 || par->codec_id == AV_CODEC_ID_AAC)
flags_size = 2;
else if (par->codec_id == AV_CODEC_ID_H264 || par->codec_id == AV_CODEC_ID_MPEG4)
flags_size = 5;
else
flags_size = 1;
if (par->codec_id == AV_CODEC_ID_AAC || par->codec_id == AV_CODEC_ID_H264
|| par->codec_id == AV_CODEC_ID_MPEG4) {
int side_size = 0;
uint8_t *side = av_packet_get_side_data(pkt, AV_PKT_DATA_NEW_EXTRADATA, &side_size);
if (side && side_size > 0 && (side_size != par->extradata_size || memcmp(side, par->extradata, side_size))) {
av_free(par->extradata);
par->extradata = av_mallocz(side_size + AV_INPUT_BUFFER_PADDING_SIZE);
if (!par->extradata) {
par->extradata_size = 0;
return AVERROR(ENOMEM);
}
memcpy(par->extradata, side, side_size);
par->extradata_size = side_size;
flv_write_codec_header(s, par, pkt->dts);
}
}
if (flv->delay == AV_NOPTS_VALUE)
flv->delay = -pkt->dts;
if (pkt->dts < -flv->delay) {
av_log(s, AV_LOG_WARNING,
"Packets are not in the proper order with respect to DTS\n");
return AVERROR(EINVAL);
}
ts = pkt->dts;
if (s->event_flags & AVSTREAM_EVENT_FLAG_METADATA_UPDATED) {
write_metadata(s, ts);
s->event_flags &= ~AVSTREAM_EVENT_FLAG_METADATA_UPDATED;
}
avio_write_marker(pb, av_rescale(ts, AV_TIME_BASE, 1000),
pkt->flags & AV_PKT_FLAG_KEY && (flv->video_par ? par->codec_type == AVMEDIA_TYPE_VIDEO : 1) ? AVIO_DATA_MARKER_SYNC_POINT : AVIO_DATA_MARKER_BOUNDARY_POINT);
switch (par->codec_type) {
case AVMEDIA_TYPE_VIDEO:
avio_w8(pb, FLV_TAG_TYPE_VIDEO);
flags = ff_codec_get_tag(flv_video_codec_ids, par->codec_id);
flags |= pkt->flags & AV_PKT_FLAG_KEY ? FLV_FRAME_KEY : FLV_FRAME_INTER;
break;
case AVMEDIA_TYPE_AUDIO:
flags = get_audio_flags(s, par);
av_assert0(size);
avio_w8(pb, FLV_TAG_TYPE_AUDIO);
break;
case AVMEDIA_TYPE_SUBTITLE:
case AVMEDIA_TYPE_DATA:
avio_w8(pb, FLV_TAG_TYPE_META);
break;
default:
return AVERROR(EINVAL);
}
if (par->codec_id == AV_CODEC_ID_H264 || par->codec_id == AV_CODEC_ID_MPEG4) {
/* check if extradata looks like mp4 formatted */
if (par->extradata_size > 0 && *(uint8_t*)par->extradata != 1)
if ((ret = ff_avc_parse_nal_units_buf(pkt->data, &data, &size)) < 0)
return ret;
} else if (par->codec_id == AV_CODEC_ID_AAC && pkt->size > 2 &&
(AV_RB16(pkt->data) & 0xfff0) == 0xfff0) {
if (!s->streams[pkt->stream_index]->nb_frames) {
av_log(s, AV_LOG_ERROR, "Malformed AAC bitstream detected: "
"use the audio bitstream filter 'aac_adtstoasc' to fix it "
"('-bsf:a aac_adtstoasc' option with ffmpeg)\n");
return AVERROR_INVALIDDATA;
}
av_log(s, AV_LOG_WARNING, "aac bitstream error\n");
}
/* check Speex packet duration */
if (par->codec_id == AV_CODEC_ID_SPEEX && ts - sc->last_ts > 160)
av_log(s, AV_LOG_WARNING, "Warning: Speex stream has more than "
"8 frames per packet. Adobe Flash "
"Player cannot handle this!\n");
if (sc->last_ts < ts)
sc->last_ts = ts;
if (size + flags_size >= 1<<24) {
av_log(s, AV_LOG_ERROR, "Too large packet with size %u >= %u\n",
size + flags_size, 1<<24);
return AVERROR(EINVAL);
}
avio_wb24(pb, size + flags_size);
put_timestamp(pb, ts);
avio_wb24(pb, flv->reserved);
if (par->codec_type == AVMEDIA_TYPE_DATA ||
par->codec_type == AVMEDIA_TYPE_SUBTITLE ) {
int data_size;
int64_t metadata_size_pos = avio_tell(pb);
if (par->codec_id == AV_CODEC_ID_TEXT) {
// legacy FFmpeg magic?
avio_w8(pb, AMF_DATA_TYPE_STRING);
put_amf_string(pb, "onTextData");
avio_w8(pb, AMF_DATA_TYPE_MIXEDARRAY);
avio_wb32(pb, 2);
put_amf_string(pb, "type");
avio_w8(pb, AMF_DATA_TYPE_STRING);
put_amf_string(pb, "Text");
put_amf_string(pb, "text");
avio_w8(pb, AMF_DATA_TYPE_STRING);
put_amf_string(pb, pkt->data);
put_amf_string(pb, "");
avio_w8(pb, AMF_END_OF_OBJECT);
} else {
// just pass the metadata through
avio_write(pb, data ? data : pkt->data, size);
}
/* write total size of tag */
data_size = avio_tell(pb) - metadata_size_pos;
avio_seek(pb, metadata_size_pos - 10, SEEK_SET);
avio_wb24(pb, data_size);
avio_seek(pb, data_size + 10 - 3, SEEK_CUR);
avio_wb32(pb, data_size + 11);
} else {
av_assert1(flags>=0);
avio_w8(pb,flags);
if (par->codec_id == AV_CODEC_ID_VP6)
avio_w8(pb,0);
if (par->codec_id == AV_CODEC_ID_VP6F || par->codec_id == AV_CODEC_ID_VP6A) {
if (par->extradata_size)
avio_w8(pb, par->extradata[0]);
else
avio_w8(pb, ((FFALIGN(par->width, 16) - par->width) << 4) |
(FFALIGN(par->height, 16) - par->height));
} else if (par->codec_id == AV_CODEC_ID_AAC)
avio_w8(pb, 1); // AAC raw
else if (par->codec_id == AV_CODEC_ID_H264 || par->codec_id == AV_CODEC_ID_MPEG4) {
avio_w8(pb, 1); // AVC NALU
avio_wb24(pb, pkt->pts - pkt->dts);
}
avio_write(pb, data ? data : pkt->data, size);
avio_wb32(pb, size + flags_size + 11); // previous tag size
flv->duration = FFMAX(flv->duration,
pkt->pts + flv->delay + pkt->duration);
}
if (flv->flags & FLV_ADD_KEYFRAME_INDEX) {
switch (par->codec_type) {
case AVMEDIA_TYPE_VIDEO:
flv->videosize += (avio_tell(pb) - cur_offset);
flv->lasttimestamp = flv->acurframeindex / flv->framerate;
if (pkt->flags & AV_PKT_FLAG_KEY) {
double ts = flv->acurframeindex / flv->framerate;
int64_t pos = cur_offset;
flv->lastkeyframetimestamp = flv->acurframeindex / flv->framerate;
flv->lastkeyframelocation = pos;
flv_append_keyframe_info(s, flv, ts, pos);
}
flv->acurframeindex++;
break;
case AVMEDIA_TYPE_AUDIO:
flv->audiosize += (avio_tell(pb) - cur_offset);
break;
default:
av_log(s, AV_LOG_WARNING, "par->codec_type is type = [%d]\n", par->codec_type);
break;
}
}
av_free(data);
return pb->error;
}
后面单独研究flv格式的时候再分析这个函数。
参考文献：
https://blog.csdn.net/leixiaohua1020/article/details/44199673