avformat_write_header()的声明位于libavformat\avformat.h，如下所示。
/**
* Allocate the stream private data and write the stream header to
* an output media file.
*
* @param s Media file handle, must be allocated with avformat_alloc_context().
* Its oformat field must be set to the desired output format;
* Its pb field must be set to an already opened AVIOContext.
* @param options An AVDictionary filled with AVFormatContext and muxer-private options.
* On return this parameter will be destroyed and replaced with a dict containing
* options that were not found. May be NULL.
*
* @return AVSTREAM_INIT_IN_WRITE_HEADER on success if the codec had not already been fully initialized in avformat_init,
* AVSTREAM_INIT_IN_INIT_OUTPUT on success if the codec had already been fully initialized in avformat_init,
* negative AVERROR on failure.
*
* @see av_opt_find, av_dict_set, avio_open, av_oformat_next, avformat_init_output.
*/
av_warn_unused_result
int avformat_write_header(AVFormatContext *s, AVDictionary **options);简单解释一下它的参数的含义：
@ s：用于输出的AVFormatContext。
@ options：额外的选项，目前没有深入研究过，一般为NULL。
avformat_write_header()的定义位于libavformat\mux.c，如下所示。
int avformat_write_header(AVFormatContext *s, AVDictionary **options)
{
int ret = 0;
int already_initialized = s->internal->initialized;
int streams_already_initialized = s->internal->streams_initialized;
if (!already_initialized)
if ((ret = avformat_init_output(s, options)) < 0)
return ret;
if (!(s->oformat->flags & AVFMT_NOFILE) && s->pb)
avio_write_marker(s->pb, AV_NOPTS_VALUE, AVIO_DATA_MARKER_HEADER);
if (s->oformat->write_header) {
ret = s->oformat->write_header(s);
if (ret >= 0 && s->pb && s->pb->error < 0)
ret = s->pb->error;
if (ret < 0)
goto fail;
flush_if_needed(s);
}
if (!(s->oformat->flags & AVFMT_NOFILE) && s->pb)
avio_write_marker(s->pb, AV_NOPTS_VALUE, AVIO_DATA_MARKER_UNKNOWN);
if (!s->internal->streams_initialized) {
if ((ret = init_pts(s)) < 0)
goto fail;
}
return streams_already_initialized;
fail:
if (s->oformat->deinit)
s->oformat->deinit(s);
return ret;
}从源代码可以看出，avformat_write_header()完成了以下工作：
（1）调用avformat_init_output->init_muxer()初始化复用器
（2）调用AVOutputFormat的write_header()
下面看一下这两个函数。
1.1 avformat_init_output()
int avformat_init_output(AVFormatContext *s, AVDictionary **options)
{
int ret = 0;
if ((ret = init_muxer(s, options)) < 0)
return ret;
s->internal->initialized = 1;
s->internal->streams_initialized = ret;
if (s->oformat->init && ret) {
if ((ret = init_pts(s)) < 0)
return ret;
return AVSTREAM_INIT_IN_INIT_OUTPUT;
}
return AVSTREAM_INIT_IN_WRITE_HEADER;
}avformat_init_output()函数调用了init_muxer()。
1.2 init_muxer()
static int init_muxer(AVFormatContext *s, AVDictionary **options)
{
int ret = 0, i;
AVStream *st;
AVDictionary *tmp = NULL;
AVCodecParameters *par = NULL;
const AVOutputFormat *of = s->oformat;
const AVCodecDescriptor *desc;
AVDictionaryEntry *e;
if (options)
av_dict_copy(&tmp, *options, 0);
if ((ret = av_opt_set_dict(s, &tmp)) < 0)
goto fail;
if (s->priv_data && s->oformat->priv_class && *(const AVClass**)s->priv_data==s->oformat->priv_class &&
(ret = av_opt_set_dict2(s->priv_data, &tmp, AV_OPT_SEARCH_CHILDREN)) < 0)
goto fail;
#if FF_API_FORMAT_FILENAME
FF_DISABLE_DEPRECATION_WARNINGS
if (!s->url && !(s->url = av_strdup(s->filename))) {
FF_ENABLE_DEPRECATION_WARNINGS
#else
if (!s->url && !(s->url = av_strdup(""))) {
#endif
ret = AVERROR(ENOMEM);
goto fail;
}
#if FF_API_LAVF_AVCTX
FF_DISABLE_DEPRECATION_WARNINGS
if (s->nb_streams && s->streams[0]->codec->flags & AV_CODEC_FLAG_BITEXACT) {
if (!(s->flags & AVFMT_FLAG_BITEXACT)) {
av_log(s, AV_LOG_WARNING,
"The AVFormatContext is not in set to bitexact mode, only "
"the AVCodecContext. If this is not intended, set "
"AVFormatContext.flags |= AVFMT_FLAG_BITEXACT.\n");
}
}
FF_ENABLE_DEPRECATION_WARNINGS
#endif
// some sanity checks
if (s->nb_streams == 0 && !(of->flags & AVFMT_NOSTREAMS)) {
av_log(s, AV_LOG_ERROR, "No streams to mux were specified\n");
ret = AVERROR(EINVAL);
goto fail;
}
for (i = 0; i < s->nb_streams; i++) {
st = s->streams[i];
par = st->codecpar;
#if FF_API_LAVF_AVCTX
FF_DISABLE_DEPRECATION_WARNINGS
if (st->codecpar->codec_type == AVMEDIA_TYPE_UNKNOWN &&
st->codec->codec_type != AVMEDIA_TYPE_UNKNOWN) {
av_log(s, AV_LOG_WARNING, "Using AVStream.codec to pass codec "
"parameters to muxers is deprecated, use AVStream.codecpar "
"instead.\n");
ret = avcodec_parameters_from_context(st->codecpar, st->codec);
if (ret < 0)
goto fail;
}
FF_ENABLE_DEPRECATION_WARNINGS
#endif
if (!st->time_base.num) {
/* fall back on the default timebase values */
if (par->codec_type == AVMEDIA_TYPE_AUDIO && par->sample_rate)
avpriv_set_pts_info(st, 64, 1, par->sample_rate);
else
avpriv_set_pts_info(st, 33, 1, 90000);
}
switch (par->codec_type) {
case AVMEDIA_TYPE_AUDIO:
if (par->sample_rate <= 0) {
av_log(s, AV_LOG_ERROR, "sample rate not set\n");
ret = AVERROR(EINVAL);
goto fail;
}
if (!par->block_align)
par->block_align = par->channels *
av_get_bits_per_sample(par->codec_id) >> 3;
break;
case AVMEDIA_TYPE_VIDEO:
if ((par->width <= 0 || par->height <= 0) &&
!(of->flags & AVFMT_NODIMENSIONS)) {
av_log(s, AV_LOG_ERROR, "dimensions not set\n");
ret = AVERROR(EINVAL);
goto fail;
}
if (av_cmp_q(st->sample_aspect_ratio, par->sample_aspect_ratio)
&& fabs(av_q2d(st->sample_aspect_ratio) - av_q2d(par->sample_aspect_ratio)) > 0.004*av_q2d(st->sample_aspect_ratio)
) {
if (st->sample_aspect_ratio.num != 0 &&
st->sample_aspect_ratio.den != 0 &&
par->sample_aspect_ratio.num != 0 &&
par->sample_aspect_ratio.den != 0) {
av_log(s, AV_LOG_ERROR, "Aspect ratio mismatch between muxer "
"(%d/%d) and encoder layer (%d/%d)\n",
st->sample_aspect_ratio.num, st->sample_aspect_ratio.den,
par->sample_aspect_ratio.num,
par->sample_aspect_ratio.den);
ret = AVERROR(EINVAL);
goto fail;
}
}
break;
}
desc = avcodec_descriptor_get(par->codec_id);
if (desc && desc->props & AV_CODEC_PROP_REORDER)
st->internal->reorder = 1;
if (of->codec_tag) {
if ( par->codec_tag
&& par->codec_id == AV_CODEC_ID_RAWVIDEO
&& ( av_codec_get_tag(of->codec_tag, par->codec_id) == 0
|| av_codec_get_tag(of->codec_tag, par->codec_id) == MKTAG('r', 'a', 'w', ' '))
&& !validate_codec_tag(s, st)) {
// the current rawvideo encoding system ends up setting
// the wrong codec_tag for avi/mov, we override it here
par->codec_tag = 0;
}
if (par->codec_tag) {
if (!validate_codec_tag(s, st)) {
const uint32_t otag = av_codec_get_tag(s->oformat->codec_tag, par->codec_id);
av_log(s, AV_LOG_ERROR,
"Tag %s incompatible with output codec id '%d' (%s)\n",
av_fourcc2str(par->codec_tag), par->codec_id, av_fourcc2str(otag));
ret = AVERROR_INVALIDDATA;
goto fail;
}
} else
par->codec_tag = av_codec_get_tag(of->codec_tag, par->codec_id);
}
if (par->codec_type != AVMEDIA_TYPE_ATTACHMENT)
s->internal->nb_interleaved_streams++;
}
if (!s->priv_data && of->priv_data_size > 0) {
s->priv_data = av_mallocz(of->priv_data_size);
if (!s->priv_data) {
ret = AVERROR(ENOMEM);
goto fail;
}
if (of->priv_class) {
*(const AVClass **)s->priv_data = of->priv_class;
av_opt_set_defaults(s->priv_data);
if ((ret = av_opt_set_dict2(s->priv_data, &tmp, AV_OPT_SEARCH_CHILDREN)) < 0)
goto fail;
}
}
/* set muxer identification string */
if (!(s->flags & AVFMT_FLAG_BITEXACT)) {
av_dict_set(&s->metadata, "encoder", LIBAVFORMAT_IDENT, 0);
} else {
av_dict_set(&s->metadata, "encoder", NULL, 0);
}
for (e = NULL; e = av_dict_get(s->metadata, "encoder-", e, AV_DICT_IGNORE_SUFFIX); ) {
av_dict_set(&s->metadata, e->key, NULL, 0);
}
if (options) {
av_dict_free(options);
*options = tmp;
}
if (s->oformat->init) {
if ((ret = s->oformat->init(s)) < 0) {
if (s->oformat->deinit)
s->oformat->deinit(s);
return ret;
}
return ret == 0;
}
return 0;
fail:
av_dict_free(&tmp);
return ret;
}init_muxer()代码很长，但是它所做的工作比较简单，可以概括成两个字：检查。函数的流程可以概括成以下几步：
（1）将传入的AVDictionary形式的选项设置到AVFormatContext
（2）遍历AVFormatContext中的每个AVStream，并作如下检查：
a)AVStream的time_base是否正确设置。如果发现AVStream的time_base没有设置，则会调用avpriv_set_pts_info()进行设置。
b)对于音频，检查采样率设置是否正确；对于视频，检查宽、高、宽高比。
c)其他一些检查，不再详述。
1.3 AVOutputFormat->write_header()
avformat_write_header()中最关键的地方就是调用了AVOutputFormat的write_header()。write_header()是AVOutputFormat中的一个函数指针，指向写文件头的函数。不同的AVOutputFormat有不同的write_header()的实现方法。在这里我们举例子看一下FLV封装格式对应的AVOutputFormat，它的定义位于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的定义中可以看出，write_header()指向的函数为flv_write_header()。我们继续看一下flv_write_header()函数。flv_write_header()的定义同样位于libavformat\flvenc.c，如下所示。
static int flv_write_header(AVFormatContext *s)
{
int i;
AVIOContext *pb = s->pb;
FLVContext *flv = s->priv_data;
avio_write(pb, "FLV", 3);
avio_w8(pb, 1);
avio_w8(pb, FLV_HEADER_FLAG_HASAUDIO * !!flv->audio_par +
FLV_HEADER_FLAG_HASVIDEO * !!flv->video_par);
avio_wb32(pb, 9);
avio_wb32(pb, 0);
for (i = 0; i < s->nb_streams; i++)
if (s->streams[i]->codecpar->codec_tag == 5) {
avio_w8(pb, 8); // message type
avio_wb24(pb, 0); // include flags
avio_wb24(pb, 0); // time stamp
avio_wb32(pb, 0); // reserved
avio_wb32(pb, 11); // size
flv->reserved = 5;
}
if (flv->flags & FLV_NO_METADATA) {
pb->seekable = 0;
} else {
write_metadata(s, 0);
}
for (i = 0; i < s->nb_streams; i++) {
flv_write_codec_header(s, s->streams[i]->codecpar, 0);
}
flv->datastart_offset = avio_tell(pb);
return 0;
}从源代码可以看出，flv_write_header()完成了FLV文件头的写入工作。该函数的工作可以大体分为以下两部分：
（1）给FLVContext设置参数
（2）写文件头，以及相关的Tag。
参考文献：
https://blog.csdn.net/leixiaohua1020/article/details/44116215