ffmpeg源码分析之open_input_file
杜曜灿
open_input_file函数是被open_files函数调用，在调用之前open_files函数将解析得到的输入相关参数存入到 OptionsContext o中，以ffmpeg -f v4l2 -i /dev/video0 test.mp4为例，其实就是-f v4l2 -i /dev/video0，
其中 -f v4l2通过parse_optgroup函数来解析，这里主要还是跟options中对f参数的定义相关的，会去根据这些定义来进行相关的操作（write_option函数），write_option部分代码，这里我们其实就是获取了一个v4l2转存了一下，需要注意的就是-f 定义中的 ｛ .off = OFFSET(format) ｝，其实就是格式了，这个会在后面的时候会用到
if (po->flags & OPT_STRING) { //
char *str;
str = av_strdup(arg);
av_freep(dst);
if (!str)
return AVERROR(ENOMEM);
*(char **)dst = str;
} else if (po->flags & OPT_BOOL || po->flags & OPT_INT) {
*(int *)dst = parse_number_or_die(opt, arg, OPT_INT64, INT_MIN, INT_MAX);
} else if (po->flags & OPT_INT64) {
*(int64_t *)dst = parse_number_or_die(opt, arg, OPT_INT64, INT64_MIN, INT64_MAX);
} else if (po->flags & OPT_TIME) {
*(int64_t *)dst = parse_time_or_die(opt, arg, 1);
} else if (po->flags & OPT_FLOAT) {
*(float *)dst = parse_number_or_die(opt, arg, OPT_FLOAT, -INFINITY, INFINITY);
} else if (po->flags & OPT_DOUBLE) {
*(double *)dst = parse_number_or_die(opt, arg, OPT_DOUBLE, -INFINITY, INFINITY);
} else if (po->u.func_arg) {
int ret = po->u.func_arg(optctx, opt, arg);
if (ret < 0) {
av_log(NULL, AV_LOG_ERROR,
"Failed to set value '%s' for option '%s': %s\n",
arg, opt, av_err2str(ret));
return ret;
}
}
if (po->flags & OPT_EXIT)
exit_program(0);然后调用了真正的open_input_file函数 传入两个参数一个是上面的 o就是转存的数据，另一个就是g->arg 就是filename 这里就是/dev/video0，open_input_file函数相对复杂，定义在ffmpeg_opt.c中。open_input_file函数中有很多if判断，看起来就很迷糊，都是针对一些具体的参数用的，这里我是参考了别人的文章简单看了一下
//输入参数o里面存放着文件的对应参数，filename表示文件名
static int open_input_file(OptionsContext *o, const char *filename)
{
InputFile *f;//存放此文件的所有信息，之后会放到input_files中
AVFormatContext *ic;//每个文件对应一个这样的结构体
AVInputFormat *file_iformat = NULL;
int err, i, ret;
int64_t timestamp;
AVDictionary **opts;
AVDictionary *unused_opts = NULL;
AVDictionaryEntry *e = NULL;
int orig_nb_streams; // number of streams before avformat_find_stream_info
char * video_codec_name = NULL;
char * audio_codec_name = NULL;
char *subtitle_codec_name = NULL;
char * data_codec_name = NULL;
int scan_all_pmts_set = 0;
/*在options[]中对应“f”，指定输入文件的格式“-f”*/
if (o->format) {
if (!(file_iformat = av_find_input_format(o->format))) {
av_log(NULL, AV_LOG_FATAL, "Unknown input format: '%s'\n", o->format);
exit_program(1);
}
}
//如果是管道输入文件名为“-”
if (!strcmp(filename, "-"))
filename = "pipe:";
/*stdin_interaction针对参数-stdin*/
stdin_interaction &= strncmp(filename, "pipe:", 5) &&
strcmp(filename, "/dev/stdin");
/* get default parameters from command line */
ic = avformat_alloc_context();
if (!ic) {
print_error(filename, AVERROR(ENOMEM));
exit_program(1);
}
/*对应参数“-ar”设置音频采样率*/
if (o->nb_audio_sample_rate) {
av_dict_set_int(&o->g->format_opts, "sample_rate", o->audio_sample_rate[o->nb_audio_sample_rate - 1].u.i, 0);
}
/*对应参数"-ac"*/
if (o->nb_audio_channels) {
/* because we set audio_channels based on both the "ac" and
* "channel_layout" options, we need to check that the specified
* demuxer actually has the "channels" option before setting it */
if (file_iformat && file_iformat->priv_class &&
av_opt_find(&file_iformat->priv_class, "channels", NULL, 0,
AV_OPT_SEARCH_FAKE_OBJ)) {
av_dict_set_int(&o->g->format_opts, "channels", o->audio_channels[o->nb_audio_channels - 1].u.i, 0);
}
}
/*对应参数"r"*/
if (o->nb_frame_rates) {
/* set the format-level framerate option;
* this is important for video grabbers, e.g. x11 */
if (file_iformat && file_iformat->priv_class &&
av_opt_find(&file_iformat->priv_class, "framerate", NULL, 0,
AV_OPT_SEARCH_FAKE_OBJ)) {
av_dict_set(&o->g->format_opts, "framerate",
o->frame_rates[o->nb_frame_rates - 1].u.str, 0);
}
}
//对应参数"s"
if (o->nb_frame_sizes) {
av_dict_set(&o->g->format_opts, "video_size", o->frame_sizes[o->nb_frame_sizes - 1].u.str, 0);
}
//对应参数"pix_fmt"
if (o->nb_frame_pix_fmts)
av_dict_set(&o->g->format_opts, "pixel_format", o->frame_pix_fmts[o->nb_frame_pix_fmts - 1].u.str, 0);
//对应参数"c"和"codec" 或"c:[v/a/s/d]"和"codec:[v/a/s/d]"
MATCH_PER_TYPE_OPT(codec_names, str, video_codec_name, ic, "v");
MATCH_PER_TYPE_OPT(codec_names, str, audio_codec_name, ic, "a");
MATCH_PER_TYPE_OPT(codec_names, str, subtitle_codec_name, ic, "s");
MATCH_PER_TYPE_OPT(codec_names, str, data_codec_name, ic, "d");
//根据命令行参数设置对应的编码格式
ic->video_codec_id = video_codec_name ?
find_codec_or_die(video_codec_name , AVMEDIA_TYPE_VIDEO , 0)->id : AV_CODEC_ID_NONE;
ic->audio_codec_id = audio_codec_name ?
find_codec_or_die(audio_codec_name , AVMEDIA_TYPE_AUDIO , 0)->id : AV_CODEC_ID_NONE;
ic->subtitle_codec_id= subtitle_codec_name ?
find_codec_or_die(subtitle_codec_name, AVMEDIA_TYPE_SUBTITLE, 0)->id : AV_CODEC_ID_NONE;
ic->data_codec_id = data_codec_name ?
find_codec_or_die(data_codec_name, AVMEDIA_TYPE_DATA, 0)->id : AV_CODEC_ID_NONE;
/*对ic中的下列项进行设置
AVCodec *video_codec;
AVCodec *audio_codec;
AVCodec *subtitle_codec;
AVCodec *data_codec;*/
if (video_codec_name)
av_format_set_video_codec (ic, find_codec_or_die(video_codec_name , AVMEDIA_TYPE_VIDEO , 0));
if (audio_codec_name)
av_format_set_audio_codec (ic, find_codec_or_die(audio_codec_name , AVMEDIA_TYPE_AUDIO , 0));
if (subtitle_codec_name)
av_format_set_subtitle_codec(ic, find_codec_or_die(subtitle_codec_name, AVMEDIA_TYPE_SUBTITLE, 0));
if (data_codec_name)
av_format_set_data_codec(ic, find_codec_or_die(data_codec_name, AVMEDIA_TYPE_DATA, 0));
ic->flags |= AVFMT_FLAG_NONBLOCK;
ic->interrupt_callback = int_cb;
//设置"scan_all_pmts"，应该只是针对avformat_open_input()函数用的。之后就又设为NULL了
if (!av_dict_get(o->g->format_opts, "scan_all_pmts", NULL, AV_DICT_MATCH_CASE)) {
av_dict_set(&o->g->format_opts, "scan_all_pmts", "1", AV_DICT_DONT_OVERWRITE);
scan_all_pmts_set = 1;
}
/* open the input file with generic avformat function */
err = avformat_open_input(&ic, filename, file_iformat, &o->g->format_opts);
if (err < 0) {
print_error(filename, err);
exit_program(1);
}
if (scan_all_pmts_set)
av_dict_set(&o->g->format_opts, "scan_all_pmts", NULL, AV_DICT_MATCH_CASE);
//删除o->g->format_opts中和o->g->codec_opts中相同的参数项
remove_avoptions(&o->g->format_opts, o->g->codec_opts);
assert_avoptions(o->g->format_opts);
/* apply forced codec ids 找到解码器，如果命令行里有指定强制解码类型就用之，否则根据流中的st->codec->codec_id找*/
for (i = 0; i < ic->nb_streams; i++)
choose_decoder(o, ic, ic->streams[i]);
/* Set AVCodecContext options for avformat_find_stream_info
返回的opts为一个链表，它的成员个数和本输入文件的流个数相同，opts[0]表示stream[0]对应的参数，来源是o->g->codec_opts
也就是将一个文件中的视频流参数、音频流参数、字幕流参数和数据流参数分开，之后提供给avformat_find_stream_info()函数用*/
opts = setup_find_stream_info_opts(ic, o->g->codec_opts);
orig_nb_streams = ic->nb_streams;
/* If not enough info to get the stream parameters, we decode the
first frames to get it. (used in mpeg case for example)
获取各路流的信息*/
ret = avformat_find_stream_info(ic, opts);
if (ret < 0) {
av_log(NULL, AV_LOG_FATAL, "%s: could not find codec parameters\n", filename);
if (ic->nb_streams == 0) {
avformat_close_input(&ic);
exit_program(1);
}
}
//对应参数"sseof",设置相对于结束的开始时间
if (o->start_time_eof != AV_NOPTS_VALUE) {
if (ic->duration>0) {
o->start_time = o->start_time_eof + ic->duration;
} else
av_log(NULL, AV_LOG_WARNING, "Cannot use -sseof, duration of %s not known\n", filename);
}
timestamp = (o->start_time == AV_NOPTS_VALUE) ? 0 : o->start_time;
/* add the stream start time */
if (!o->seek_timestamp && ic->start_time != AV_NOPTS_VALUE)
timestamp += ic->start_time;
/* if seeking requested, we execute it
如果开始时间指定的话需要做seek*/
if (o->start_time != AV_NOPTS_VALUE) {
int64_t seek_timestamp = timestamp;
if (!(ic->iformat->flags & AVFMT_SEEK_TO_PTS)) {
int dts_heuristic = 0;
for (i=0; i<ic->nb_streams; i++) {
AVCodecContext *avctx = ic->streams[i]->codec;
if (avctx->has_b_frames)
dts_heuristic = 1;
}
if (dts_heuristic) {
seek_timestamp -= 3*AV_TIME_BASE / 23;
}
}
ret = avformat_seek_file(ic, -1, INT64_MIN, seek_timestamp, seek_timestamp, 0);
if (ret < 0) {
av_log(NULL, AV_LOG_WARNING, "%s: could not seek to position %0.3f\n",
filename, (double)timestamp / AV_TIME_BASE);
}
}
/* update the current parameters so that they match the one of the input stream */
/* Add all the streams from the given input file to the global
* list of input streams.
* 将本文件的各个流放入input_streams链表中，并对解码器进行设置
* InputStream中的项在此函数中填值 */
add_input_streams(o, ic);
/* dump the file content */
av_dump_format(ic, nb_input_files, filename, 0);
/*将这个文件放到input_files链表中，并对其中的变量做设置，如下*/
GROW_ARRAY(input_files, nb_input_files);
f = av_mallocz(sizeof(*f));
if (!f)
exit_program(1);
input_files[nb_input_files - 1] = f;
f->ctx = ic;
f->ist_index = nb_input_streams - ic->nb_streams;/* index of first stream in input_streams */
f->start_time = o->start_time;
f->recording_time = o->recording_time;
f->input_ts_offset = o->input_ts_offset;
f->ts_offset = o->input_ts_offset - (copy_ts ? (start_at_zero && ic->start_time != AV_NOPTS_VALUE ? ic->start_time : 0) : timestamp);
f->nb_streams = ic->nb_streams;
f->rate_emu = o->rate_emu;
f->accurate_seek = o->accurate_seek;
f->loop = o->loop;
f->duration = 0;
f->time_base = (AVRational){ 1, 1 };
#if HAVE_PTHREADS
f->thread_queue_size = o->thread_queue_size > 0 ? o->thread_queue_size : 8;
#endif
/* check if all codec options have been used */
unused_opts = strip_specifiers(o->g->codec_opts);
for (i = f->ist_index; i < nb_input_streams; i++) {
e = NULL;
while ((e = av_dict_get(input_streams[i]->decoder_opts, "", e,
AV_DICT_IGNORE_SUFFIX)))
av_dict_set(&unused_opts, e->key, NULL, 0);
}
e = NULL;
while ((e = av_dict_get(unused_opts, "", e, AV_DICT_IGNORE_SUFFIX))) {
const AVClass *class = avcodec_get_class();
const AVOption *option = av_opt_find(&class, e->key, NULL, 0,
AV_OPT_SEARCH_CHILDREN | AV_OPT_SEARCH_FAKE_OBJ);
const AVClass *fclass = avformat_get_class();
const AVOption *foption = av_opt_find(&fclass, e->key, NULL, 0,
AV_OPT_SEARCH_CHILDREN | AV_OPT_SEARCH_FAKE_OBJ);
if (!option || foption)
continue;
if (!(option->flags & AV_OPT_FLAG_DECODING_PARAM)) {
av_log(NULL, AV_LOG_ERROR, "Codec AVOption %s (%s) specified for "
"input file #%d (%s) is not a decoding option.\n", e->key,
option->help ? option->help : "", nb_input_files - 1,
filename);
exit_program(1);
}
av_log(NULL, AV_LOG_WARNING, "Codec AVOption %s (%s) specified for "
"input file #%d (%s) has not been used for any stream. The most "
"likely reason is either wrong type (e.g. a video option with "
"no video streams) or that it is a private option of some decoder "
"which was not actually used for any stream.\n", e->key,
option->help ? option->help : "", nb_input_files - 1, filename);
}
av_dict_free(&unused_opts);
//对应参数"dump_attachment"
for (i = 0; i < o->nb_dump_attachment; i++) {
int j;
for (j = 0; j < ic->nb_streams; j++) {
AVStream *st = ic->streams[j];
if (check_stream_specifier(ic, st, o->dump_attachment[i].specifier) == 1)
dump_attachment(st, o->dump_attachment[i].u.str);//把st->codec->extradata输出到文件
}
}
for (i = 0; i < orig_nb_streams; i++)
av_dict_free(&opts[i]);
av_freep(&opts);
input_stream_potentially_available = 1;
return 0;
}整个函数流程：
1：av_find_input_format 查找输入的格式，返回的的是一个AVInputFormat结构体类型，我这里是ff_v4l2_demuxer，定义在v4l2.c中
2：avformat_alloc_context 申请上下文，分配内存
3：一些参数的设置，参考上图，这里是参考了https://blog.csdn.net/sidumqz/article/details/53007923
4：avformat_open_input 打开输入，这个比较重要，具体的可以参考雷神 https://blog.csdn.net/leixiaohua1020/article/details/44064715
5：avformat_find_stream_info ，还是参考雷神吧 https://blog.csdn.net/leixiaohua1020/article/details/44084321
后面就没啥了，在下面就是打开输出了
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