FFmpeg中函数avformat_open_input()（含URL解析文件格式）

摘要: avformat_open_input 的源码分析

版本：

FFMPEG代码为3.2 release版本， 输入为flv文件。

分析结论：

FFMPEG中函数avformat_open_input()作用为打开输入文件，并将输入文件中的数据读入到buf，以及判断输入文件的格式。例如可以判断是否为flv格式等，并将输入文件格式保存到指针AVFormatContext里面iformat成员。

函数avformat_open_input()

int avformat_open_input(AVFormatContext **ps, const char *filename,
AVInputFormat *fmt, AVDictionary **options)
{
AVFormatContext *s = *ps;
int i, ret = 0;
AVDictionary *tmp = NULL;
ID3v2ExtraMeta *id3v2_extra_meta = NULL;

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 by avformat_alloc_context() and is not NULL either\n");
return AVERROR(EINVAL);
}
if (fmt)
s->iformat = fmt;

if (options)
av_dict_copy(&tmp, *options, 0);

if (s->pb) // must be before any goto fail
s->flags |= AVFMT_FLAG_CUSTOM_IO;

if ((ret = av_opt_set_dict(s, &tmp)) < 0)
goto fail;

if ((ret = init_input(s, filename, &tmp)) < 0)
goto fail;
s->probe_score = ret;

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;
}
}

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);

/* 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;
av_strlcpy(s->filename, filename ? filename : "", sizeof(s->filename));

/* 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;
}
}

/* e.g. AVFMT_NOFILE formats will not have a AVIOContext */
if (s->pb)
ff_id3v2_read(s, ID3v2_DEFAULT_MAGIC, &id3v2_extra_meta, 0);

if (!(s->flags&AVFMT_FLAG_PRIV_OPT) && s->iformat->read_header)
if ((ret = s->iformat->read_header(s)) < 0)
goto fail;

if (id3v2_extra_meta) {
if (!strcmp(s->iformat->name, "mp3") || !strcmp(s->iformat->name, "aac") ||
!strcmp(s->iformat->name, "tta")) {
if ((ret = ff_id3v2_parse_apic(s, &id3v2_extra_meta)) < 0)
goto fail;
} 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 fail;

if (!(s->flags&AVFMT_FLAG_PRIV_OPT) && s->pb && !s->internal->data_offset)
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;

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;
}

函数avformat_open_input()调用init_input()

/* Open input file and probe the format if necessary. */
static int init_input(AVFormatContext *s, const char *filename,
AVDictionary **options)
{
int ret;
AVProbeData pd = { filename, NULL, 0 };
int score = AVPROBE_SCORE_RETRY;

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 and "
"will be ignored with AVFMT_NOFILE format.\n");
return 0;
}

if ((s->iformat && s->iformat->flags & AVFMT_NOFILE) ||
(!s->iformat && (s->iformat = av_probe_input_format2(&pd, 0, &score))))
return score;

if ((ret = s->io_open(s, &s->pb, filename, AVIO_FLAG_READ | s->avio_flags, options)) < 0)
return ret;

if (s->iformat)
return 0;
return av_probe_input_buffer2(s->pb, &s->iformat, filename,
s, 0, s->format_probesize);
}

函数init_input()调用av_probe_input_format2()分析输入文件

AVInputFormat *av_probe_input_format2(AVProbeData *pd, int is_opened, int *score_max)
{
int score_ret;
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;
}

AVInputFormat *av_probe_input_format3(AVProbeData *pd, int is_opened,
int *score_ret)
{
AVProbeData lpd = *pd;
AVInputFormat *fmt1 = NULL, *fmt;
int score, score_max = 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;

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;
}

fmt = NULL;
while ((fmt1 = av_iformat_next(fmt1))) {
if (!is_opened == !(fmt1->flags & AVFMT_NOFILE) && strcmp(fmt1->name, "image2"))
continue;
score = 0;
if (fmt1->read_probe) {
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);
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;
}
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) {
score_max = score;
fmt       = 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;
}

AVInputFormat *av_probe_input_format2(AVProbeData *pd, int is_opened, int *score_max)
{
int score_ret;
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;
}

因为文件后缀为flv格式，在函数av_probe_input_format3()中匹配到ff_flv_demuxer后，跳出循环，score_ret=0,fmt=null。所以需要继续解析文件。

 

io_open()调用的是io_open_default()

static int io_open_default(AVFormatContext *s, AVIOContext **pb,
const char *url, int flags, AVDictionary **options)
{
#if FF_API_OLD_OPEN_CALLBACKS
FF_DISABLE_DEPRECATION_WARNINGS
if (s->open_cb)
return s->open_cb(s, pb, url, flags, &s->interrupt_callback, options);
FF_ENABLE_DEPRECATION_WARNINGS
#endif

return ffio_open_whitelist(pb, url, flags, &s->interrupt_callback, options, s->protocol_whitelist, s->protocol_blacklist);
}

通过FFMPEG的IO函数ffio_open_whitelist()打开文件

int ffio_open_whitelist(AVIOContext **s, const char *filename, int flags,
const AVIOInterruptCB *int_cb, AVDictionary **options,
const char *whitelist, const char *blacklist
)
{
URLContext *h;
int err;

err = ffurl_open_whitelist(&h, filename, flags, int_cb, options, whitelist, blacklist, NULL);
if (err < 0)
return err;
err = ffio_fdopen(s, h);
if (err < 0) {
ffurl_close(h);
return err;
}
return 0;
}

根据文件名打开函数

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)
{
AVDictionary *tmp_opts = NULL;
AVDictionaryEntry *e;
int ret = ffurl_alloc(puc, filename, flags, int_cb);
if (ret < 0)
return ret;
if (parent)
av_opt_copy(*puc, parent);
if (options &&
(ret = av_opt_set_dict(*puc, options)) < 0)
goto fail;
if (options && (*puc)->prot->priv_data_class &&
(ret = av_opt_set_dict((*puc)->priv_data, options)) < 0)
goto fail;

if (!options)
options = &tmp_opts;

av_assert0(!whitelist ||
!(e=av_dict_get(*options, "protocol_whitelist", NULL, 0)) ||
!strcmp(whitelist, e->value));
av_assert0(!blacklist ||
!(e=av_dict_get(*options, "protocol_blacklist", NULL, 0)) ||
!strcmp(blacklist, e->value));

if ((ret = av_dict_set(options, "protocol_whitelist", whitelist, 0)) < 0)
goto fail;

if ((ret = av_dict_set(options, "protocol_blacklist", blacklist, 0)) < 0)
goto fail;

if ((ret = av_opt_set_dict(*puc, options)) < 0)
goto fail;

ret = ffurl_connect(*puc, options);

if (!ret)
return 0;
fail:
ffurl_close(*puc);
*puc = NULL;
return ret;
}

首先需要找到输入文件的协议，并且为该协议对于的URL分配空间，即得到结构体指针URLContext分配空间。

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;
if (av_strstart(filename, "https:", NULL))
av_log(NULL, AV_LOG_WARNING, "https protocol not found, recompile FFmpeg with "
"openssl, gnutls "
"or securetransport enabled.\n");
return AVERROR_PROTOCOL_NOT_FOUND;
}

根据文件名找到对应的协议

static const struct URLProtocol *url_find_protocol(const char *filename)
{
const URLProtocol **protocols;
char proto_str[128], proto_nested[128], *ptr;
size_t proto_len = strspn(filename, URL_SCHEME_CHARS);
int i;

if (filename[proto_len] != ':' &&
(strncmp(filename, "subfile,", 8) || !strchr(filename + proto_len + 1, ':')) ||
is_dos_path(filename))
strcpy(proto_str, "file");
else
av_strlcpy(proto_str, filename,
FFMIN(proto_len + 1, sizeof(proto_str)));

if ((ptr = strchr(proto_str, ',')))
*ptr = '\0';
av_strlcpy(proto_nested, proto_str, sizeof(proto_nested));
if ((ptr = strchr(proto_nested, '+')))
*ptr = '\0';

protocols = ffurl_get_protocols(NULL, NULL);
if (!protocols)
return NULL;
for (i = 0; protocols[i]; i++) {
const URLProtocol *up = protocols[i];
if (!strcmp(proto_str, up->name)) {
av_freep(&protocols);
return up;
}
if (up->flags & URL_PROTOCOL_FLAG_NESTED_SCHEME &&
!strcmp(proto_nested, up->name)) {
av_freep(&protocols);
return up;
}
}
av_freep(&protocols);

return NULL;
}

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;
if (av_strstart(filename, "https:", NULL))
av_log(NULL, AV_LOG_WARNING, "https protocol not found, recompile FFmpeg with "
"openssl, gnutls "
"or securetransport enabled.\n");
return AVERROR_PROTOCOL_NOT_FOUND;
}

函数url_find_protocol()根据输入文件名找到对应的协议，其中通过函数ffurl_get_protocols(NULL, NULL);获取所有支持的协议。即得到输入文件属于file类型：

(gdb) p *p
$6 = {name = 0x189f6fe "file", url_open = 0x728790 <file_open>, url_open2 = 0, url_accept = 0, url_handshake = 0, url_read = 0x728740 <file_read>, url_write = 0x728710 <file_write>, url_seek = 0x728690 <file_seek>,
url_close = 0x728680 <file_close>, url_read_pause = 0, url_read_seek = 0, url_get_file_handle = 0x728190 <file_get_handle>, url_get_multi_file_handle = 0, url_shutdown = 0, priv_data_size = 32, priv_data_class = 0x18c5160, flags = 0,
url_check = 0x7285c0 <file_check>, url_open_dir = 0x728590 <file_open_dir>, url_read_dir = 0x728320 <file_read_dir>, url_close_dir = 0x728300 <file_close_dir>, url_delete = 0x728290 <file_delete>, url_move = 0x728230 <file_move>,
default_whitelist = 0x18c4fc0 "file,crypto"}

为协议分配空间

static int url_alloc_for_protocol(URLContext **puc, const URLProtocol *up,
const char *filename, int flags,
const AVIOInterruptCB *int_cb)
{
URLContext *uc;
int err;

#if CONFIG_NETWORK
if (up->flags & URL_PROTOCOL_FLAG_NETWORK && !ff_network_init())
return AVERROR(EIO);
#endif
if ((flags & AVIO_FLAG_READ) && !up->url_read) {
av_log(NULL, AV_LOG_ERROR,
"Impossible to open the '%s' protocol for reading\n", up->name);
return AVERROR(EIO);
}
if ((flags & AVIO_FLAG_WRITE) && !up->url_write) {
av_log(NULL, AV_LOG_ERROR,
"Impossible to open the '%s' protocol for writing\n", up->name);
return AVERROR(EIO);
}
uc = av_mallocz(sizeof(URLContext) + strlen(filename) + 1);
if (!uc) {
err = AVERROR(ENOMEM);
goto fail;
}
uc->av_class = &ffurl_context_class;
uc->filename = (char *)&uc[1];
strcpy(uc->filename, filename);
uc->prot            = up;
uc->flags           = flags;
uc->is_streamed     = 0; /* default = not streamed */
uc->max_packet_size = 0; /* default: stream file */
if (up->priv_data_size) {
uc->priv_data = av_mallocz(up->priv_data_size);
if (!uc->priv_data) {
err = AVERROR(ENOMEM);
goto fail;
}
if (up->priv_data_class) {
int proto_len= strlen(up->name);
char *start = strchr(uc->filename, ',');
*(const AVClass **)uc->priv_data = up->priv_data_class;
av_opt_set_defaults(uc->priv_data);
if(!strncmp(up->name, uc->filename, proto_len) && uc->filename + proto_len == start){
int ret= 0;
char *p= start;
char sep= *++p;
char *key, *val;
p++;

if (strcmp(up->name, "subfile"))
ret = AVERROR(EINVAL);

while(ret >= 0 && (key= strchr(p, sep)) && p<key && (val = strchr(key+1, sep))){
*val= *key= 0;
if (strcmp(p, "start") && strcmp(p, "end")) {
ret = AVERROR_OPTION_NOT_FOUND;
} else
ret= av_opt_set(uc->priv_data, p, key+1, 0);
if (ret == AVERROR_OPTION_NOT_FOUND)
av_log(uc, AV_LOG_ERROR, "Key '%s' not found.\n", p);
*val= *key= sep;
p= val+1;
}
if(ret<0 || p!=key){
av_log(uc, AV_LOG_ERROR, "Error parsing options string %s\n", start);
av_freep(&uc->priv_data);
av_freep(&uc);
err = AVERROR(EINVAL);
goto fail;
}
memmove(start, key+1, strlen(key));
}
}
}
if (int_cb)
uc->interrupt_callback = *int_cb;

*puc = uc;
return 0;
fail:
*puc = NULL;
if (uc)
av_freep(&uc->priv_data);
av_freep(&uc);
#if CONFIG_NETWORK
if (up->flags & URL_PROTOCOL_FLAG_NETWORK)
ff_network_close();
#endif
return err;
}

连接输入文件

int ffurl_connect(URLContext *uc, AVDictionary **options){
int err;
AVDictionary *tmp_opts = NULL;
AVDictionaryEntry *e;

if (!options)
options = &tmp_opts;

// Check that URLContext was initialized correctly and lists are matching if set
av_assert0(!(e=av_dict_get(*options, "protocol_whitelist", NULL, 0)) ||
(uc->protocol_whitelist && !strcmp(uc->protocol_whitelist, e->value)));
av_assert0(!(e=av_dict_get(*options, "protocol_blacklist", NULL, 0)) ||
(uc->protocol_blacklist && !strcmp(uc->protocol_blacklist, e->value)));

if (uc->protocol_whitelist && av_match_list(uc->prot->name, uc->protocol_whitelist, ',') <= 0) {
av_log(uc, AV_LOG_ERROR, "Protocol not on whitelist \'%s\'!\n", uc->protocol_whitelist);
return AVERROR(EINVAL);
}

if (uc->protocol_blacklist && av_match_list(uc->prot->name, uc->protocol_blacklist, ',') > 0) {
av_log(uc, AV_LOG_ERROR, "Protocol blacklisted \'%s\'!\n", uc->protocol_blacklist);
return AVERROR(EINVAL);
}

if (!uc->protocol_whitelist && uc->prot->default_whitelist) {
av_log(uc, AV_LOG_DEBUG, "Setting default whitelist '%s'\n", uc->prot->default_whitelist);
uc->protocol_whitelist = av_strdup(uc->prot->default_whitelist);
if (!uc->protocol_whitelist) {
return AVERROR(ENOMEM);
}
} else if (!uc->protocol_whitelist)
av_log(uc, AV_LOG_DEBUG, "No default whitelist set\n"); // This should be an error once all declare a default whitelist

if ((err = av_dict_set(options, "protocol_whitelist", uc->protocol_whitelist, 0)) < 0)
return err;
if ((err = av_dict_set(options, "protocol_blacklist", uc->protocol_blacklist, 0)) < 0)
return err;

err =
uc->prot->url_open2 ? uc->prot->url_open2(uc,
uc->filename,
uc->flags,
options) :
uc->prot->url_open(uc, uc->filename, uc->flags);

av_dict_set(options, "protocol_whitelist", NULL, 0);
av_dict_set(options, "protocol_blacklist", NULL, 0);

if (err)
return err;
uc->is_connected = 1;
/* We must be careful here as ffurl_seek() could be slow,
* for example for http */
if ((uc->flags & AVIO_FLAG_WRITE) || !strcmp(uc->prot->name, "file"))
if (!uc->is_streamed && ffurl_seek(uc, 0, SEEK_SET) < 0)
uc->is_streamed = 1;
return 0;
}

调用uc->prot->url_open(uc, uc->filename, uc->flags);

其中uc->prot内容：

(gdb) p *(uc->prot)
$18 = {name = 0x189f6fe "file", url_open = 0x728790 <file_open>, url_open2 = 0, url_accept = 0, url_handshake = 0, url_read = 0x728740 <file_read>, url_write = 0x728710 <file_write>, url_seek = 0x728690 <file_seek>,
url_close = 0x728680 <file_close>, url_read_pause = 0, url_read_seek = 0, url_get_file_handle = 0x728190 <file_get_handle>, url_get_multi_file_handle = 0, url_shutdown = 0, priv_data_size = 32, priv_data_class = 0x18c5160, flags = 0,
url_check = 0x7285c0 <file_check>, url_open_dir = 0x728590 <file_open_dir>, url_read_dir = 0x728320 <file_read_dir>, url_close_dir = 0x728300 <file_close_dir>, url_delete = 0x728290 <file_delete>, url_move = 0x728230 <file_move>,
default_whitelist = 0x18c4fc0 "file,crypto"}

uc->prot->url_open调用的是file_open()

根据回调函数打开文件：

static int file_open(URLContext *h, const char *filename, int flags)
{
FileContext *c = h->priv_data;
int access;
int fd;
struct stat st;

av_strstart(filename, "file:", &filename);

if (flags & AVIO_FLAG_WRITE && flags & AVIO_FLAG_READ) {
access = O_CREAT | O_RDWR;
if (c->trunc)
access |= O_TRUNC;
} else if (flags & AVIO_FLAG_WRITE) {
access = O_CREAT | O_WRONLY;
if (c->trunc)
access |= O_TRUNC;
} else {
access = O_RDONLY;
}
#ifdef O_BINARY
access |= O_BINARY;
#endif
fd = avpriv_open(filename, access, 0666);
if (fd == -1)
return AVERROR(errno);
c->fd = fd;

h->is_streamed = !fstat(fd, &st) && S_ISFIFO(st.st_mode);

return 0;
}

函数ffio_open_whitelist()调用ffio_fdopen().

其中#define IO_BUFFER_SIZE 32768

int ffio_fdopen(AVIOContext **s, URLContext *h)
{
AVIOInternal *internal = NULL;
uint8_t *buffer = NULL;
int buffer_size, max_packet_size;

max_packet_size = h->max_packet_size;
if (max_packet_size) {
buffer_size = max_packet_size; /* no need to bufferize more than one packet */
} else {
buffer_size = IO_BUFFER_SIZE;
}
buffer = av_malloc(buffer_size);
if (!buffer)
return AVERROR(ENOMEM);

internal = av_mallocz(sizeof(*internal));
if (!internal)
goto fail;

internal->h = h;

*s = avio_alloc_context(buffer, buffer_size, h->flags & AVIO_FLAG_WRITE,
internal, io_read_packet, io_write_packet, io_seek);
if (!*s)
goto fail;

(*s)->protocol_whitelist = av_strdup(h->protocol_whitelist);
if (!(*s)->protocol_whitelist && h->protocol_whitelist) {
avio_closep(s);
goto fail;
}
(*s)->protocol_blacklist = av_strdup(h->protocol_blacklist);
if (!(*s)->protocol_blacklist && h->protocol_blacklist) {
avio_closep(s);
goto fail;
}
(*s)->direct = h->flags & AVIO_FLAG_DIRECT;

(*s)->seekable = h->is_streamed ? 0 : AVIO_SEEKABLE_NORMAL;
(*s)->max_packet_size = max_packet_size;
if(h->prot) {
(*s)->read_pause = io_read_pause;
(*s)->read_seek  = io_read_seek;
}
(*s)->av_class = &ff_avio_class;
return 0;
fail:
av_freep(&internal);
av_freep(&buffer);
return AVERROR(ENOMEM);
}

const AVClass ff_avio_class = {
.class_name = "AVIOContext",
.item_name  = av_default_item_name,
.version    = LIBAVUTIL_VERSION_INT,
.option     = ff_avio_options,
.child_next = ff_avio_child_next,
.child_class_next = ff_avio_child_class_next,
};

分析输入buffer内容，其中

/** size of probe buffer, for guessing file type from file contents */
#define PROBE_BUF_MIN 2048
#define PROBE_BUF_MAX (1 << 20)

#define MAX_PROBE_PACKETS 2500

max_probe_size = PROBE_BUF_MAX

{"formatprobesize", "number of bytes to probe file format", OFFSET(format_probesize), AV_OPT_TYPE_INT, {.i64 = PROBE_BUF_MAX}, 0, INT_MAX-1, D},

函数init_input()调用函数av_probe_input_buffer2()

打开文件后，需要解析文件内容，即调用函数av_probe_input_buffer2()

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;

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;
char *semi;
av_opt_get(pb, "mime_type", AV_OPT_SEARCH_CHILDREN, &mime_type_opt);
pd.mime_type = (const char *)mime_type_opt;
semi = pd.mime_type ? strchr(pd.mime_type, ';') : NULL;
if (semi) {
*semi = '\0';
}
}
#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

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;
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. */
*fmt = av_probe_input_format2(&pd, 1, &score);
if (*fmt) {
/* This can only be true in the last iteration. */
if (score <= AVPROBE_SCORE_RETRY) {
av_log(logctx, AV_LOG_WARNING,
"Format %s detected only with low score of %d, "
"misdetection possible!\n", (*fmt)->name, score);
} else
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)
ret = AVERROR_INVALIDDATA;

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;
}

从文件中读取数据到buffer

int avio_read(AVIOContext *s, unsigned char *buf, int size)
{
int len, size1;

size1 = size;
while (size > 0) {
len = FFMIN(s->buf_end - s->buf_ptr, size);
if (len == 0 || s->write_flag) {
if((s->direct || size > s->buffer_size) && !s->update_checksum) {
// bypass the buffer and read data directly into buf
if(s->read_packet)
len = s->read_packet(s->opaque, buf, size);

if (len <= 0) {
/* do not modify buffer if EOF reached so that a seek back can
be done without rereading data */
s->eof_reached = 1;
if(len<0)
s->error= len;
break;
} else {
s->pos += len;
s->bytes_read += len;
size -= len;
buf += len;
// reset the buffer
s->buf_ptr = s->buffer;
s->buf_end = s->buffer/* + len*/;
}
} else {
fill_buffer(s);
len = s->buf_end - s->buf_ptr;
if (len == 0)
break;
}
} else {
memcpy(buf, s->buf_ptr, len);
buf += len;
s->buf_ptr += len;
size -= len;
}
}
if (size1 == size) {
if (s->error)      return s->error;
if (avio_feof(s))  return AVERROR_EOF;
}
return size1 - size;
}

填充缓冲区的函数fill_buffer()

static void fill_buffer(AVIOContext *s)
{
int max_buffer_size = s->max_packet_size ?
s->max_packet_size : IO_BUFFER_SIZE;
uint8_t *dst        = s->buf_end - s->buffer + max_buffer_size < s->buffer_size ?
s->buf_end : s->buffer;
int len             = s->buffer_size - (dst - s->buffer);

/* can't fill the buffer without read_packet, just set EOF if appropriate */
if (!s->read_packet && s->buf_ptr >= s->buf_end)
s->eof_reached = 1;

/* no need to do anything if EOF already reached */
if (s->eof_reached)
return;

if (s->update_checksum && dst == s->buffer) {
if (s->buf_end > s->checksum_ptr)
s->checksum = s->update_checksum(s->checksum, s->checksum_ptr,
s->buf_end - s->checksum_ptr);
s->checksum_ptr = s->buffer;
}

/* make buffer smaller in case it ended up large after probing */
if (s->read_packet && s->orig_buffer_size && s->buffer_size > s->orig_buffer_size) {
if (dst == s->buffer) {
int ret = ffio_set_buf_size(s, s->orig_buffer_size);
if (ret < 0)
av_log(s, AV_LOG_WARNING, "Failed to decrease buffer size\n");

s->checksum_ptr = dst = s->buffer;
}
av_assert0(len >= s->orig_buffer_size);
len = s->orig_buffer_size;
}

if (s->read_packet)
len = s->read_packet(s->opaque, dst, len);
else
len = 0;
if (len <= 0) {
/* do not modify buffer if EOF reached so that a seek back can
be done without rereading data */
s->eof_reached = 1;
if (len < 0)
s->error = len;
} else {
s->pos += len;
s->buf_ptr = dst;
s->buf_end = dst + len;
s->bytes_read += len;
}
}

s->read_packet()实际调用io_read_packet()

static int io_read_packet(void *opaque, uint8_t *buf, int buf_size)
{
AVIOInternal *internal = opaque;
return ffurl_read(internal->h, buf, buf_size);
}

static inline int retry_transfer_wrapper(URLContext *h, uint8_t *buf,
int size, int size_min,
int (*transfer_func)(URLContext *h,
uint8_t *buf,
int size))
{
int ret, len;
int fast_retries = 5;
int64_t wait_since = 0;

len = 0;
while (len < size_min) {
if (ff_check_interrupt(&h->interrupt_callback))
return AVERROR_EXIT;
ret = transfer_func(h, buf + len, size - len);
if (ret == AVERROR(EINTR))
continue;
if (h->flags & AVIO_FLAG_NONBLOCK)
return ret;
if (ret == AVERROR(EAGAIN)) {
ret = 0;
if (fast_retries) {
fast_retries--;
} else {
if (h->rw_timeout) {
if (!wait_since)
wait_since = av_gettime_relative();
else if (av_gettime_relative() > wait_since + h->rw_timeout)
return AVERROR(EIO);
}
av_usleep(1000);
}
} else if (ret < 1)
return (ret < 0 && ret != AVERROR_EOF) ? ret : len;
if (ret) {
fast_retries = FFMAX(fast_retries, 2);
wait_since = 0;
}
len += ret;
}
return len;
}

其中transfer_func()调用函数file_read()

static int file_read(URLContext *h, unsigned char *buf, int size)
{
FileContext *c = h->priv_data;
int ret;
size = FFMIN(size, c->blocksize);
ret = read(c->fd, buf, size);
if (ret == 0 && c->follow)
return AVERROR(EAGAIN);
return (ret == -1) ? AVERROR(errno) : ret;
}

从文件中读取32768数据到AVIOContext->buffer。在函数av_probe_input_buffer2()中定义的buf，大小为2048 + 32，从AVIOContext->buffer复制2048个数据到该buf。

 

解析输入文件的格式

AVInputFormat *av_probe_input_format2(AVProbeData *pd, int is_opened, int *score_max)
{
int score_ret;
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;
}

AVInputFormat *av_probe_input_format3(AVProbeData *pd, int is_opened,
int *score_ret)
{
AVProbeData lpd = *pd;
AVInputFormat *fmt1 = NULL, *fmt;
int score, score_max = 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;

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;
}

fmt = NULL;
while ((fmt1 = av_iformat_next(fmt1))) {
if (!is_opened == !(fmt1->flags & AVFMT_NOFILE) && strcmp(fmt1->name, "image2"))
continue;
score = 0;
if (fmt1->read_probe) {
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);
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;
}
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) {
score_max = score;
fmt       = 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;
}

其中fmt1->read_probe(&lpd)调用函数flv_probe()

static int probe(AVProbeData *p, int live)
{
const uint8_t *d = p->buf;
unsigned offset = AV_RB32(d + 5);

if (d[0] == 'F' &&
d[1] == 'L' &&
d[2] == 'V' &&
d[3] < 5 && d[5] == 0 &&
offset + 100 < p->buf_size &&
offset > 8) {
int is_live = !memcmp(d + offset + 40, "NGINX RTMP", 10);

if (live == is_live)
return AVPROBE_SCORE_MAX;
}
return 0;
}

static int flv_probe(AVProbeData *p)
{
return probe(p, 0);
}

 Rewind. Reuse probe buffer to avoid seeking.

int ffio_rewind_with_probe_data(AVIOContext *s, unsigned char **bufp, int buf_size)
{
int64_t buffer_start;
int buffer_size;
int overlap, new_size, alloc_size;
uint8_t *buf = *bufp;

if (s->write_flag) {
av_freep(bufp);
return AVERROR(EINVAL);
}

buffer_size = s->buf_end - s->buffer;

/* the buffers must touch or overlap */
if ((buffer_start = s->pos - buffer_size) > buf_size) {
av_freep(bufp);
return AVERROR(EINVAL);
}

overlap = buf_size - buffer_start;
new_size = buf_size + buffer_size - overlap;

alloc_size = FFMAX(s->buffer_size, new_size);
if (alloc_size > buf_size)
if (!(buf = (*bufp) = av_realloc_f(buf, 1, alloc_size)))
return AVERROR(ENOMEM);

if (new_size > buf_size) {
memcpy(buf + buf_size, s->buffer + overlap, buffer_size - overlap);
buf_size = new_size;
}

av_free(s->buffer);
s->buf_ptr = s->buffer = buf;
s->buffer_size = alloc_size;
s->pos = buf_size;
s->buf_end = s->buf_ptr + buf_size;
s->eof_reached = 0;
s->must_flush = 0;

return 0;
} 

(gdb) p *(s->pb)
$49 = {av_class = 0x1884b80, buffer = 0x2770890 "FLV\001\005", buffer_size = 32768, buf_ptr = 0x2770890 "FLV\001\005", buf_end = 0x2778890 "", opaque = 0x2768620, read_packet = 0x5f5190 <io_read_packet>,
write_packet = 0x5f5180 <io_write_packet>, seek = 0x5f5170 <io_seek>, pos = 32768, must_flush = 0, eof_reached = 0, write_flag = 0, max_packet_size = 0, checksum = 0, checksum_ptr = 0x0, update_checksum = 0, error = 0,
read_pause = 0x5f4670 <io_read_pause>, read_seek = 0x5f4690 <io_read_seek>, seekable = 1, maxsize = 0, direct = 0, bytes_read = 32768, seek_count = 0, writeout_count = 0, orig_buffer_size = 32768, short_seek_threshold = 4096,
protocol_whitelist = 0x2768380 "file,crypto", protocol_blacklist = 0x0, write_data_type = 0, ignore_boundary_point = 0, current_type = AVIO_DATA_MARKER_UNKNOWN, last_time = -9223372036854775808}

avformat_open_input()函数会调用read_header(s)

s->iformat->read_header(s)回调函数flv_read_header()获取FLV文件头的信息

static int flv_read_header(AVFormatContext *s)
{
FLVContext *flv = s->priv_data;
int offset;

avio_skip(s->pb, 4);
avio_r8(s->pb); // flags

s->ctx_flags |= AVFMTCTX_NOHEADER;

offset = avio_rb32(s->pb);
avio_seek(s->pb, offset, SEEK_SET);
avio_skip(s->pb, 4);

s->start_time = 0;
flv->sum_flv_tag_size = 0;
flv->last_keyframe_stream_index = -1;

return 0;
}