FFmpeg源代码简单学习- -avformat_open_input
部分转载自:https://blog.csdn.net/leixiaohua1020/article/details/44064715
//参数ps包含一切媒体相关的上下文结构,有它就有了一切,本函数如果打开媒体成功,
//会返回一个AVFormatContext的实例.
//参数filename是媒体文件名或URL. 文件=url
//参数fmt是要打开的媒体格式的操作结构,因为是读,所以是inputFormat.此处可以
//传入一个调用者定义的inputFormat,对应命令行中的 -f xxx段,如果指定了它,
//在打开文件中就不会探测文件的实际格式了,以它为准了.
//参数options是对某种格式的一些操作,是为了在命令行中可以对不同的格式传入
//特殊的操作参数而建的, 为了了解流程,完全可以无视它.
int avformat_open_input(AVFormatContext **ps,
const char *filename,
AVInputFormat *fmt,
AVDictionary **options)
{
AVFormatContext *s = *ps;9
int ret = 0;
AVFormatParameters ap = { { 0 } };
AVDictionary *tmp = NULL;
//创建上下文结构
//若未创建成功,则报错
if (!s && !(s = avformat_alloc_context()))
return AVERROR(ENOMEM);
//如果用户指定了输入格式,直接使用它
//-f xxx
//对应于用户指定的格式
if (fmt)
s->iformat = fmt;
//忽略
if (options)
av_dict_copy(&tmp, *options, 0);
if ((ret = av_opt_set_dict(s, &tmp)) < 0)
goto fail;
//打开输入媒体(如果需要的话),初始化所有与媒体读写有关的结构们,比如
//AVIOContext,AVInputFormat等等
//init_input包含结构体的初始化函数
if ((ret = init_input(s, filename)) < 0)
goto fail;
//执行完此函数后,s->pb和s->iformat都已经指向了有效实例.pb是用于读写数据的,它
//把媒体数据当做流来读写,不管是什么媒体格式,而iformat把pb读出来的流按某种媒体格
//式进行分析,也就是说pb在底层,iformat在上层.
//很多静态图像文件格式,都被当作一个格式处理,比如要打开.jpeg文件,需要的格式
//名为image2.此处还不是很了解具体细节,作不得准哦.
/* check filename in case an image number is expected */
if (s->iformat->flags & AVFMT_NEEDNUMBER) {
if (!av_filename_number_test(filename)) {
ret = AVERROR(EINVAL);
goto fail;
}
}
s->duration = s->start_time = AV_NOPTS_VALUE;
//上下文中保存下文件名
av_strlcpy(s->filename, filename, sizeof(s->filename));
/* allocate private data */
//为当前格式分配私有数据,主要用于某格式的读写操作时所用的私有结构.
//此结构的大小在定义AVInputFormat时已指定了.
if (s->iformat->priv_data_size > 0) {
if (!(s->priv_data = av_mallocz(s->iformat->priv_data_size))) {
ret = AVERROR(ENOMEM);
goto fail;
}
//这个可以先不必管它
if (s->iformat->priv_class) {
*(const AVClass**) s->priv_data = s->iformat->priv_class;
av_opt_set_defaults(s->priv_data);
if ((ret = av_opt_set_dict(s->priv_data, &tmp)) < 0)
goto fail;
}
}
/* e.g. AVFMT_NOFILE formats will not have a AVIOContext */
//从mp3文件中读ID3数据并保存之.
if (s->pb)
ff_id3v2_read(s, ID3v2_DEFAULT_MAGIC);
//读一下媒体的头部,在read_header()中主要是做某种格式的初始化工作,比如填充自己的
//私有结构,根据流的数量分配流结构并初始化,把文件指针指向数据区开始处等.
if (!(s->flags & AVFMT_FLAG_PRIV_OPT) && s->iformat->read_header)
if ((ret = s->iformat->read_header(s, &ap)) < 0)
goto fail;
//保存数据区开始的位置
if (!(s->flags & AVFMT_FLAG_PRIV_OPT) && s->pb && !s->data_offset)
s->data_offset = avio_tell(s->pb);
s->raw_packet_buffer_remaining_size = RAW_PACKET_BUFFER_SIZE;
if (options) {
av_dict_free(options);
*options = tmp;
}
*ps = s;
//执行成功
return 0;
//执行失败
fail: av_dict_free(&tmp);
if (s->pb && !(s->flags & AVFMT_FLAG_CUSTOM_IO))
avio_close(s->pb);
avformat_free_context(s);
*ps = NULL;
return ret;
}
#define AVPROBE_PADDING_SIZE 32 ///< extra allocated bytes at the end of the probe buffer
#define AVPROBE_SCORE_EXTENSION 50 ///< score for file extension
#define AVPROBE_SCORE_MIME 75 ///< score for file mime type
#define AVPROBE_SCORE_MAX 100 ///< maximum score
AVInputFormat *av_probe_input_format3(AVProbeData *pd, int is_opened,
int *score_ret)
{
AVProbeData lpd = *pd;
AVInputFormat *fmt1 = NULL, *fmt;
int score, nodat = 0, score_max = 0;
const static uint8_t zerobuffer[AVPROBE_PADDING_SIZE];
if (!lpd.buf)
lpd.buf = zerobuffer;
if (lpd.buf_size > 10 && ff_id3v2_match(lpd.buf, ID3v2_DEFAULT_MAGIC)) {
int id3len = ff_id3v2_tag_len(lpd.buf);
if (lpd.buf_size > id3len + 16) {
lpd.buf += id3len;
lpd.buf_size -= id3len;
} else if (id3len >= PROBE_BUF_MAX) {
nodat = 2;
} else
nodat = 1;
}
fmt = NULL;
//循环调用函数av_iformat_next
while ((fmt1 = av_iformat_next(fmt1))) {
if (!is_opened == !(fmt1->flags & AVFMT_NOFILE) && strcmp(fmt1->name, "image2"))
continue;
score = 0;
if (fmt1->read_probe) {//如果包含read_probe,则调用
score = fmt1->read_probe(&lpd);//获取分数
if (fmt1->extensions && av_match_ext(lpd.filename, fmt1->extensions)) {
//如果扩展名存在,并且使用av_match_ext来比较扩展名和lpd的文件名
//FFMAX分数取较大值
if (nodat == 0) score = FFMAX(score, 1);
else if (nodat == 1) score = FFMAX(score, AVPROBE_SCORE_EXTENSION / 2 - 1);
else score = FFMAX(score, AVPROBE_SCORE_EXTENSION);
}
} else if (fmt1->extensions) {
if (av_match_ext(lpd.filename, fmt1->extensions))
score = AVPROBE_SCORE_EXTENSION;
}
if (av_match_name(lpd.mime_type, fmt1->mime_type))
score = FFMAX(score, AVPROBE_SCORE_MIME);
if (score > score_max) {
score_max = score;
fmt = fmt1;
} else if (score == score_max)
fmt = NULL;
}
if (nodat == 1)
score_max = FFMIN(AVPROBE_SCORE_EXTENSION / 2 - 1, score_max);
*score_ret = score_max;
return fmt;
}
int av_probe_input_buffer2(AVIOContext *pb, AVInputFormat **fmt,
const char *filename, void *logctx,
unsigned int offset, unsigned int max_probe_size)
{
AVProbeData pd = { filename ? filename : "" };
uint8_t *buf = NULL;
int ret = 0, probe_size, buf_offset = 0;
int score = 0;
int ret2;
//max_probe_size的处理
if (!max_probe_size)
max_probe_size = PROBE_BUF_MAX;
else if (max_probe_size < PROBE_BUF_MIN) {
av_log(logctx, AV_LOG_ERROR,
"Specified probe size value %u cannot be < %u\n", max_probe_size, PROBE_BUF_MIN);
return AVERROR(EINVAL);
}
if (offset >= max_probe_size)
return AVERROR(EINVAL);
if (pb->av_class) {
uint8_t *mime_type_opt = NULL;
av_opt_get(pb, "mime_type", AV_OPT_SEARCH_CHILDREN, &mime_type_opt);
pd.mime_type = (const char *)mime_type_opt;
}
//忽略
#if 0
if (!*fmt && pb->av_class && av_opt_get(pb, "mime_type", AV_OPT_SEARCH_CHILDREN, &mime_type) >= 0 && mime_type) {
if (!av_strcasecmp(mime_type, "audio/aacp")) {
*fmt = av_find_input_format("aac");
}
av_freep(&mime_type);
}
#endif
//逐步增大probe_size
//如果直到max时仍然未读取到fmt,则退出
for (probe_size = PROBE_BUF_MIN; probe_size <= max_probe_size && !*fmt;
probe_size = FFMIN(probe_size << 1,
FFMAX(max_probe_size, probe_size + 1))) {
score = probe_size < max_probe_size ? AVPROBE_SCORE_RETRY : 0;
/* Read probe data. */
if ((ret = av_reallocp(&buf, probe_size + AVPROBE_PADDING_SIZE)) < 0)
goto fail;
//调用avio_read函数进行
if ((ret = avio_read(pb, buf + buf_offset,
probe_size - buf_offset)) < 0) {
/* Fail if error was not end of file, otherwise, lower score. */
if (ret != AVERROR_EOF)
goto fail;
score = 0;
ret = 0; /* error was end of file, nothing read */
}
buf_offset += ret;
if (buf_offset < offset)
continue;
pd.buf_size = buf_offset - offset;
pd.buf = &buf[offset];
memset(pd.buf + pd.buf_size, 0, AVPROBE_PADDING_SIZE);
/* Guess file format. */
//判断格式
*fmt = av_probe_input_format2(&pd, 1, &score);
if (*fmt) {
/* This can only be true in the last iteration. */
if (score <= AVPROBE_SCORE_RETRY) {//如果分数过低
av_log(logctx, AV_LOG_WARNING,
"Format %s detected only with low score of %d, "
"misdetection possible!\n", (*fmt)->name, score);
} else
//输出格式、probe_size、分数
av_log(logctx, AV_LOG_DEBUG,
"Format %s probed with size=%d and score=%d\n",
(*fmt)->name, probe_size, score);
#if 0
FILE *f = fopen("probestat.tmp", "ab");
fprintf(f, "probe_size:%d format:%s score:%d filename:%s\n", probe_size, (*fmt)->name, score, filename);
fclose(f);
#endif
}
}
if (!*fmt)//无效数据
ret = AVERROR_INVALIDDATA;
//fail处理
fail:
/* Rewind. Reuse probe buffer to avoid seeking. */
ret2 = ffio_rewind_with_probe_data(pb, &buf, buf_offset);
if (ret >= 0)
ret = ret2;
av_freep(&pd.mime_type);
return ret < 0 ? ret : score;
}
//flv头部文件读取
static int flv_read_header(AVFormatContext *s)
{
int flags;
FLVContext *flv = s->priv_data;
int offset;
int pre_tag_size = 0;
/* Actual FLV data at 0xe40000 in KUX file */
//如果为kux文件
if(!strcmp(s->iformat->name, "kux"))
avio_skip(s->pb, 0xe40000);
avio_skip(s->pb, 4);
flags = avio_r8(s->pb);
//
flv->missing_streams = flags & (FLV_HEADER_FLAG_HASVIDEO | FLV_HEADER_FLAG_HASAUDIO);
s->ctx_flags |= AVFMTCTX_NOHEADER;
offset = avio_rb32(s->pb);
avio_seek(s->pb, offset, SEEK_SET);
/* Annex E. The FLV File Format
* E.3 TheFLVFileBody
* Field Type Comment
* PreviousTagSize0 UI32 Always 0
* */
pre_tag_size = avio_rb32(s->pb);
if (pre_tag_size) {
av_log(s, AV_LOG_WARNING, "Read FLV header error, input file is not a standard flv format, first PreviousTagSize0 always is 0\n");
}
s->start_time = 0;
flv->sum_flv_tag_size = 0;
flv->last_keyframe_stream_index = -1;
return 0;
}
static AVStream *create_stream(AVFormatContext *s, int codec_type)
{
FLVContext *flv = s->priv_data;
//调用avformat_new_stream函数创建音视频流
AVStream *st = avformat_new_stream(s, NULL);
if (!st)//未成功创建
return NULL;
st->codecpar->codec_type = codec_type;
if (s->nb_streams>=3 ||( s->nb_streams==2
&& s->streams[0]->codecpar->codec_type != AVMEDIA_TYPE_SUBTITLE
&& s->streams[1]->codecpar->codec_type != AVMEDIA_TYPE_SUBTITLE
&& s->streams[0]->codecpar->codec_type != AVMEDIA_TYPE_DATA
&& s->streams[1]->codecpar->codec_type != AVMEDIA_TYPE_DATA))
s->ctx_flags &= ~AVFMTCTX_NOHEADER;
if (codec_type == AVMEDIA_TYPE_AUDIO) {
st->codecpar->bit_rate = flv->audio_bit_rate;
flv->missing_streams &= ~FLV_HEADER_FLAG_HASAUDIO;
}
if (codec_type == AVMEDIA_TYPE_VIDEO) {
st->codecpar->bit_rate = flv->video_bit_rate;
flv->missing_streams &= ~FLV_HEADER_FLAG_HASVIDEO;
st->avg_frame_rate = flv->framerate;
}
avpriv_set_pts_info(st, 32, 1, 1000); /* 32 bit pts in ms */
flv->last_keyframe_stream_index = s->nb_streams - 1;
add_keyframes_index(s);
return st;
}