avformat_open_input() is used to open the input media stream and read header information, including local files, network streams, and custom buffers. Key process: Open avio and detect the encapsulation format of the input stream. The corresponding release method is avformat_close_input().
1. Open the input media stream
The avformat_open_input method is located in libavformat/utils.c. The process includes allocating AVFormatContext, setting options, initializing input stream, copying whitelist and blacklist protocols, and reading ID3V2 parameters. The specific method is as follows:
int avformat_open_input(AVFormatContext **ps, const char *filename,
ff_const59 AVInputFormat *fmt, AVDictionary **options)
{
AVFormatContext *s = *ps;
int i, ret = 0;
AVDictionary *tmp = NULL;
ID3v2ExtraMeta *id3v2_extra_meta = NULL;
// 分配AVFormatContext
if (!s && !(s = avformat_alloc_context()))
return AVERROR(ENOMEM);
if (!s->av_class) {
av_log(NULL, AV_LOG_ERROR, "Input context has not been properly allocated\n");
return AVERROR(EINVAL);
}
if (fmt)
s->iformat = fmt;
// 拷贝options
if (options)
av_dict_copy(&tmp, *options, 0);
// 如果已经设置AVIOContext，那么flag设为自定义IO
if (s->pb) // must be before any goto fail
s->flags |= AVFMT_FLAG_CUSTOM_IO;
// 设置options
if ((ret = av_opt_set_dict(s, &tmp)) < 0)
goto fail;
if (!(s->url = av_strdup(filename ? filename : ""))) {
ret = AVERROR(ENOMEM);
goto fail;
}
// 初始化输入媒体流
if ((ret = init_input(s, filename, &tmp)) < 0)
goto fail;
s->probe_score = ret;
// 拷贝白名单协议
if (!s->protocol_whitelist && s->pb && s->pb->protocol_whitelist) {
s->protocol_whitelist = av_strdup(s->pb->protocol_whitelist);
if (!s->protocol_whitelist) {
ret = AVERROR(ENOMEM);
goto fail;
}
}
// 拷贝黑名单协议
if (!s->protocol_blacklist && s->pb && s->pb->protocol_blacklist) {
s->protocol_blacklist = av_strdup(s->pb->protocol_blacklist);
if (!s->protocol_blacklist) {
ret = AVERROR(ENOMEM);
goto fail;
}
}
if (s->format_whitelist && av_match_list(s->iformat->name, s->format_whitelist, ',') <= 0) {
av_log(s, AV_LOG_ERROR, "Format not on whitelist \'%s\'\n", s->format_whitelist);
ret = AVERROR(EINVAL);
goto fail;
}
avio_skip(s->pb, s->skip_initial_bytes);
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;
/* Allocate private data. */
if (s->iformat->priv_data_size > 0) {
if (!(s->priv_data = av_mallocz(s->iformat->priv_data_size))) {
ret = AVERROR(ENOMEM);
goto fail;
}
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;
}
}
if (s->pb)
ff_id3v2_read_dict(s->pb, &s->internal->id3v2_meta, ID3v2_DEFAULT_MAGIC, &id3v2_extra_meta);
#if FF_API_DEMUXER_OPEN
if (!(s->flags&AVFMT_FLAG_PRIV_OPT) && s->iformat->read_header)
#else
if (s->iformat->read_header)
#endif
if ((ret = s->iformat->read_header(s)) < 0)
goto fail;
if (!s->metadata) {
s->metadata = s->internal->id3v2_meta;
s->internal->id3v2_meta = NULL;
} else if (s->internal->id3v2_meta) {
av_log(s, AV_LOG_WARNING, "Discarding ID3 tags because more suitable tags were found.\n");
av_dict_free(&s->internal->id3v2_meta);
}
if (id3v2_extra_meta) {
if (!strcmp(s->iformat->name, "mp3") || !strcmp(s->iformat->name, "aac") ||
!strcmp(s->iformat->name, "tta") || !strcmp(s->iformat->name, "wav")) {
if ((ret = ff_id3v2_parse_apic(s, id3v2_extra_meta)) < 0)
goto close;
if ((ret = ff_id3v2_parse_chapters(s, id3v2_extra_meta)) < 0)
goto close;
if ((ret = ff_id3v2_parse_priv(s, id3v2_extra_meta)) < 0)
goto close;
} else
av_log(s, AV_LOG_DEBUG, "demuxer does not support additional id3 data, skipping\n");
}
ff_id3v2_free_extra_meta(&id3v2_extra_meta);
if ((ret = avformat_queue_attached_pictures(s)) < 0)
goto close;
#if FF_API_DEMUXER_OPEN
if (!(s->flags&AVFMT_FLAG_PRIV_OPT) && s->pb && !s->internal->data_offset)
#else
if (s->pb && !s->internal->data_offset)
#endif
s->internal->data_offset = avio_tell(s->pb);
s->internal->raw_packet_buffer_remaining_size = RAW_PACKET_BUFFER_SIZE;
update_stream_avctx(s);
for (i = 0; i < s->nb_streams; i++)
s->streams[i]->internal->orig_codec_id = s->streams[i]->codecpar->codec_id;
if (options) {
av_dict_free(options);
*options = tmp;
}
*ps = s;
return 0;
close:
if (s->iformat->read_close)
s->iformat->read_close(s);
fail:
ff_id3v2_free_extra_meta(&id3v2_extra_meta);
av_dict_free(&tmp);
if (s->pb && !(s->flags & AVFMT_FLAG_CUSTOM_IO))
avio_closep(&s->pb);
avformat_free_context(s);
*ps = NULL;
return ret;
}2. Initialize the input media stream
The core method is init_input(), which is used to open the input media stream and detect the encapsulation format. The code is as follows:
static int init_input(AVFormatContext *s, const char *filename,
AVDictionary **options)
{
int ret;
AVProbeData pd = { filename, NULL, 0 };
int score = AVPROBE_SCORE_RETRY;
// 探测输入缓冲区
if (s->pb) {
s->flags |= AVFMT_FLAG_CUSTOM_IO;
if (!s->iformat)
return av_probe_input_buffer2(s->pb, &s->iformat, filename,
s, 0, s->format_probesize);
else if (s->iformat->flags & AVFMT_NOFILE)
av_log(s, AV_LOG_WARNING, "Custom AVIOContext makes no sense.\n");
return 0;
}
// 探测输入格式
if ((s->iformat && s->iformat->flags & AVFMT_NOFILE) ||
(!s->iformat && (s->iformat = av_probe_input_format2(&pd, 0, &score))))
return score;
// 打开avio
if ((ret = s->io_open(s, &s->pb, filename, AVIO_FLAG_READ | s->avio_flags, options)) < 0)
return ret;
if (s->iformat)
return 0;
// 探测输入缓冲区
return av_probe_input_buffer2(s->pb, &s->iformat, filename,
s, 0, s->format_probesize);
}3. Detection input format
av_probe_input_format2() is located in libavformat/format.c, internally calls av_probe_input_format3(), and av_probe_input_format() internally calls av_probe_input_format2(). The specific calling relationship is as follows:
ff_const59 AVInputFormat *av_probe_input_format2(ff_const59 AVProbeData *pd, int is_opened, int *score_max)
{
int score_ret;
ff_const59 AVInputFormat *fmt = av_probe_input_format3(pd, is_opened, &score_ret);
if (score_ret > *score_max) {
*score_max = score_ret;
return fmt;
} else
return NULL;
}
ff_const59 AVInputFormat *av_probe_input_format(ff_const59 AVProbeData *pd, int is_opened)
{
int score = 0;
return av_probe_input_format2(pd, is_opened, &score);
}3.1 Detection process
We mainly look at the av_probe_input_format3 method. The detection process is divided into three steps: read_probe, av_match_ext, and av_match_name. The matching result of each step has a score, and the highest score is used as the format. The specific detection matching process is as follows:
ff_const59 AVInputFormat *av_probe_input_format3(ff_const59 AVProbeData *pd, int is_opened,
int *score_ret)
{
AVProbeData lpd = *pd;
const AVInputFormat *fmt1 = NULL;
ff_const59 AVInputFormat *fmt = NULL;
int score, score_max = 0;
void *i = 0;
const static uint8_t zerobuffer[AVPROBE_PADDING_SIZE];
enum nodat {
NO_ID3,
ID3_ALMOST_GREATER_PROBE,
ID3_GREATER_PROBE,
ID3_GREATER_MAX_PROBE,
} nodat = NO_ID3;
if (!lpd.buf)
lpd.buf = (unsigned char *) zerobuffer;
// 判断是否匹配到ID3V2，存在于mp3、aac、wav、tta
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) {
if (lpd.buf_size < 2LL*id3len + 16)
nodat = ID3_ALMOST_GREATER_PROBE;
lpd.buf += id3len;
lpd.buf_size -= id3len;
} else if (id3len >= PROBE_BUF_MAX) {
nodat = ID3_GREATER_MAX_PROBE;
} else
nodat = ID3_GREATER_PROBE;
}
while ((fmt1 = av_demuxer_iterate(&i))) {
if (!is_opened == !(fmt1->flags & AVFMT_NOFILE) && strcmp(fmt1->name, "image2"))
continue;
score = 0;
if (fmt1->read_probe) {
// 匹配格式的特定标识符，比如flv格式头部为"FLV"
score = fmt1->read_probe(&lpd);
if (score)
av_log(NULL, AV_LOG_TRACE, "Probing %s score:%d size:%d\n", fmt1->name, score, lpd.buf_size);
// 匹配外部格式
if (fmt1->extensions && av_match_ext(lpd.filename, fmt1->extensions)) {
switch (nodat) {
case NO_ID3:
score = FFMAX(score, 1);
break;
case ID3_GREATER_PROBE:
case ID3_ALMOST_GREATER_PROBE:
score = FFMAX(score, AVPROBE_SCORE_EXTENSION / 2 - 1);
break;
case ID3_GREATER_MAX_PROBE:
score = FFMAX(score, AVPROBE_SCORE_EXTENSION);
break;
}
}
} else if (fmt1->extensions) {
if (av_match_ext(lpd.filename, fmt1->extensions))
score = AVPROBE_SCORE_EXTENSION;
}
// 匹配mimetype
if (av_match_name(lpd.mime_type, fmt1->mime_type)) {
if (AVPROBE_SCORE_MIME > score) {
av_log(NULL, AV_LOG_DEBUG, "Probing %s score:%d increased to %d due to MIME type\n", fmt1->name, score, AVPROBE_SCORE_MIME);
score = AVPROBE_SCORE_MIME;
}
}
if (score > score_max) { // 取最高得分作为format
score_max = score;
fmt = (AVInputFormat*)fmt1;
} else if (score == score_max)
fmt = NULL;
}
if (nodat == ID3_GREATER_PROBE)
score_max = FFMIN(AVPROBE_SCORE_EXTENSION / 2 - 1, score_max);
*score_ret = score_max;
return fmt;
}3.2 Match Score
Each matching step has different scores, the highest score is 100, the retry is 25, the extension is 50, and the mimetype is 75. The specific AVProbeData structure and scoring rules are as follows:
typedef struct AVProbeData {
const char *filename;
unsigned char *buf; /**< Buffer must have AVPROBE_PADDING_SIZE of extra allocated bytes filled with zero. */
int buf_size; /**< Size of buf except extra allocated bytes */
const char *mime_type; /**< mime_type, when known. */
} AVProbeData;
#define AVPROBE_SCORE_RETRY (AVPROBE_SCORE_MAX/4)
#define AVPROBE_SCORE_STREAM_RETRY (AVPROBE_SCORE_MAX/4-1)
#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 score3.3 Detect mp4 format
The mp4/mov/3gp formats are all in libavformat/mov.c, and read_probe points to the mov_probe method. First, detect the tags of key boxes such as ftyp, moov, and mdat, and then the tags of hdlr, mhlr, and MPEG. The specific detection methods are as follows:
static int mov_probe(const AVProbeData *p)
{
int64_t offset = 0;
uint32_t tag;
int score = 0;
int moov_offset = -1;
for (;;) {
tag = AV_RL32(p->buf + offset + 4);
switch(tag) {
case MKTAG('m','o','o','v'):
moov_offset = offset + 4;
case MKTAG('m','d','a','t'):
case MKTAG('p','n','o','t'):
case MKTAG('u','d','t','a'):
case MKTAG('f','t','y','p'):
if (tag == MKTAG('f','t','y','p') &&
( AV_RL32(p->buf + offset + 8) == MKTAG('j','p','2',' ')
|| AV_RL32(p->buf + offset + 8) == MKTAG('j','p','x',' ')
)) {
score = FFMAX(score, 5);
} else {
score = AVPROBE_SCORE_MAX;
}
break;
case MKTAG('e','d','i','w'): /* xdcam files have reverted first tags */
case MKTAG('w','i','d','e'):
case MKTAG('f','r','e','e'):
case MKTAG('j','u','n','k'):
case MKTAG('p','i','c','t'):
score = FFMAX(score, AVPROBE_SCORE_MAX - 5);
break;
case MKTAG(0x82,0x82,0x7f,0x7d):
case MKTAG('s','k','i','p'):
case MKTAG('u','u','i','d'):
case MKTAG('p','r','f','l'):
score = FFMAX(score, AVPROBE_SCORE_EXTENSION);
break;
}
if (size > INT64_MAX - offset)
break;
offset += size;
}
if (score > AVPROBE_SCORE_MAX - 50 && moov_offset != -1) {
offset = moov_offset;
while (offset < (p->buf_size - 16)) {
if (AV_RL32(p->buf + offset ) == MKTAG('h','d','l','r') &&
AV_RL32(p->buf + offset + 8) == MKTAG('m','h','l','r') &&
AV_RL32(p->buf + offset + 12) == MKTAG('M','P','E','G')) {
return 5;
} else {
/* Keep looking */
offset += 2;
}
}
}
return score;
}4. Open avio
The avio_open2 method opens ffurl_open_whitelist() and fifo_fdopen(). The specific call chain is shown in the following figure:
4.1 open_whiltelist
Among them, ffurl_open_whitelist() located in avio.c performs two-step operations: ffurl_alloc and ffurl_connect. The specific codes are as follows:
int ffurl_open_whitelist(URLContext **puc, const char *filename, int flags,
const AVIOInterruptCB *int_cb, AVDictionary **options,
const char *whitelist, const char* blacklist,
URLContext *parent)
{
int ret = ffurl_alloc(puc, filename, flags, int_cb);
if (ret < 0)
return ret;
......
ret = ffurl_connect(*puc, options);
if (!ret)
return 0;
fail:
ffurl_closep(puc);
return ret;
}ffurl_alloc mainly implements two functions: search protocol and allocation protocol. The search process is to traverse the protocol array and match according to the scheme. code show as below:
int ffurl_alloc(URLContext **puc, const char *filename, int flags,
const AVIOInterruptCB *int_cb)
{
const URLProtocol *p = NULL;
p = url_find_protocol(filename);
if (p)
return url_alloc_for_protocol(puc, p, filename, flags, int_cb);
*puc = NULL;
return AVERROR_PROTOCOL_NOT_FOUND;
}ffurl_connect mainly judges whether to use url_oepn2 or url_open to open:
int ffurl_connect(URLContext *uc, AVDictionary **options)
{
......
int err =
uc->prot->url_open2 ? uc->prot->url_open2(uc, uc->filename, uc->flags, options) :
uc->prot->url_open(uc, uc->filename, uc->flags);
......
return 0;
}
4.2 Open fifo
Call the fifo_fdopen method of aviobuf.c to open the fifo, mainly to allocate buffer buffers and allocate AVIOContext:
int ffio_fdopen(AVIOContext **s, URLContext *h)
{
// malloc buffer
buffer = av_malloc(buffer_size);
if (!buffer)
return AVERROR(ENOMEM);
// malloc context
*s = avio_alloc_context(buffer, buffer_size, h->flags & AVIO_FLAG_WRITE, h,
(int (*)(void *, uint8_t *, int)) ffurl_read,
(int (*)(void *, uint8_t *, int)) ffurl_write,
(int64_t (*)(void *, int64_t, int))ffurl_seek);
fail:
av_freep(&buffer);
return AVERROR(ENOMEM);
}