Java教程
FFmpeg中的关键方法及结构体（二）avformat_open_input
本文主要是介绍FFmpeg中的关键方法及结构体（二）avformat_open_input，对大家解决编程问题具有一定的参考价值，需要的程序猿们随着小编来一起学习吧！
avformat_open_input
该方法声明在libavformat/avformat.h:2093
int avformat_open_input(AVFormatContext **ps, const char *url,
const AVInputFormat *fmt, AVDictionary **options);
方法实现位于libavformat/demux.c:207，该方法主要用来选择IO以及解复用组件
int avformat_open_input(AVFormatContext **ps, const char *filename,
const AVInputFormat *fmt, AVDictionary **options)
{
AVFormatContext *s = *ps;
FFFormatContext *si;
AVDictionary *tmp = NULL;
ID3v2ExtraMeta *id3v2_extra_meta = NULL;
int ret = 0;
if (!s && !(s = avformat_alloc_context()))
return AVERROR(ENOMEM);
// 通过强转来获得一个FFFormatContext类型的指针
si = ffformatcontext(s);
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);
}
// 是否指定AVInputFormat，如果指定的话就使用指定的Format
if (fmt)
s->iformat = fmt;
if (options)
av_dict_copy(&tmp, *options, 0);
// 这个pb的类型为AVIOContext，pb不为null表示使用自定义的IO
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;
// 拷贝url
if (!(s->url = av_strdup(filename ? filename : ""))) {
ret = AVERROR(ENOMEM);
goto fail;
}
// 调用init_input方法来初始化，主要做了两件事选择AVIOContext、选择AVInputFormat
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;
/* 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 an AVIOContext */
if (s->pb)
ff_id3v2_read_dict(s->pb, &si->id3v2_meta, ID3v2_DEFAULT_MAGIC, &id3v2_extra_meta);
if (s->iformat->read_header)
if ((ret = s->iformat->read_header(s)) < 0) {
if (s->iformat->flags_internal & FF_FMT_INIT_CLEANUP)
goto close;
goto fail;
}
if (!s->metadata) {
s->metadata = si->id3v2_meta;
si->id3v2_meta = NULL;
} else if (si->id3v2_meta) {
av_log(s, AV_LOG_WARNING, "Discarding ID3 tags because more suitable tags were found.\n");
av_dict_free(&si->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 (s->pb && !si->data_offset)
si->data_offset = avio_tell(s->pb);
si->raw_packet_buffer_size = 0;
update_stream_avctx(s);
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;
}
init_input方法会调用AVFormatContext的io_open方法
static int init_input(AVFormatContext *s, const char *filename,
AVDictionary **options)
{
int ret;
// 创建一个AVProbeData
AVProbeData pd = { filename, NULL, 0 };
int score = AVPROBE_SCORE_RETRY;
// 默认不使用自己的IO
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;
}
// 如果我们指定了format，那么这里直接返回score
if ((s->iformat && s->iformat->flags & AVFMT_NOFILE) ||
(!s->iformat && (s->iformat = av_probe_input_format2(&pd, 0, &score))))
return score;
// 否则调用io_open，io_open的实现在libavformat/options.c中，pb的类型为AVIOContext
if ((ret = s->io_open(s, &s->pb, filename, AVIO_FLAG_READ | s->avio_flags, options)) < 0)
return ret;
if (s->iformat)
return 0;
// 调用av_probe_input_buffer2去探测IO类型
return av_probe_input_buffer2(s->pb, &s->iformat, filename,
s, 0, s->format_probesize);
}
io_open方法的实现位于libavformat/options.c，方法实现为io_open_default，用于创建并初始化AVFormatContext中的AVIOContext
static int io_open_default(AVFormatContext *s, AVIOContext **pb,
const char *url, int flags, AVDictionary **options)
{
int loglevel;
if (!strcmp(url, s->url) ||
s->iformat && !strcmp(s->iformat->name, "image2") ||
s->oformat && !strcmp(s->oformat->name, "image2")
) {
loglevel = AV_LOG_DEBUG;
} else
loglevel = AV_LOG_INFO;
av_log(s, loglevel, "Opening \'%s\' for %s\n", url, flags & AVIO_FLAG_WRITE ? "writing" : "reading");
// 这里的url就是前面的filename
return ffio_open_whitelist(pb, url, flags, &s->interrupt_callback, options, s->protocol_whitelist, s->protocol_blacklist);
}
ffio_open_whitelist声明在libavformat/avio_internal.h:225，实现位于libavformat/aviobuf.c:1225
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;
*s = NULL;
// 调用该方法返回一个URLContext
err = ffurl_open_whitelist(&h, filename, flags, int_cb, options, whitelist, blacklist, NULL);
if (err < 0)
return err;
// 用URLContext来创建并初始化一个AVIOContext
err = ffio_fdopen(s, h);
if (err < 0) {
ffurl_close(h);
return err;
}
return 0;
}
ffurl_open_whitelist实现位于libavformat/avio.c:306
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;
// 创建并且初始化一个URLContext对象
int ret = ffurl_alloc(puc, filename, flags, int_cb);
if (ret < 0)
return ret;
if (parent) {
ret = av_opt_copy(*puc, parent);
if (ret < 0)
goto fail;
}
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;
// 调用ffurl_connect，打开url
ret = ffurl_connect(*puc, options);
if (!ret)
return 0;
fail:
ffurl_closep(puc);
return ret;
}
ffurl_alloc实现在libavformat/avio.c:293
int ffurl_alloc(URLContext **puc, const char *filename, int flags,
const AVIOInterruptCB *int_cb)
{
const URLProtocol *p = NULL;
// 找到合适的protocol
p = url_find_protocol(filename);
// 利用找到的protocol来创建一个URLContext
if (p)
return url_alloc_for_protocol(puc, p, filename, flags, int_cb);
*puc = NULL;
return AVERROR_PROTOCOL_NOT_FOUND;
}
url_find_protocol实现位于libavformat/avio.c:251
static const struct URLProtocol *url_find_protocol(const char *filename)
{
const URLProtocol **protocols;
char proto_str[128], proto_nested[128], *ptr;
// 这个方法比较有意思，是用于查找filename中第一个没有出现在URL_SCHEME_CHARS中的字符的下表，起始就是查找":"
size_t proto_len = strspn(filename, URL_SCHEME_CHARS);
int i;
// 如果filename中没有":"，并且以subfile开头或者没有":"，或者是路径，protocol类型为file
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)));
av_strlcpy(proto_nested, proto_str, sizeof(proto_nested));
if ((ptr = strchr(proto_nested, '+')))
*ptr = '\0';
// 获取所有的protocal
protocols = ffurl_get_protocols(NULL, NULL);
if (!protocols)
return NULL；
// 遍历protocals列表，对比protocol name
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;
}
}
// 释放protocols指针
av_freep(&protocols);
if (av_strstart(filename, "https:", NULL) || av_strstart(filename, "tls:", NULL))
av_log(NULL, AV_LOG_WARNING, "https protocol not found, recompile FFmpeg with "
"openssl, gnutls or securetransport enabled.\n");
return NULL;
}
ffurl_get_protocols实现位于libavformat/protocols.c
const URLProtocol **ffurl_get_protocols(const char *whitelist,
const char *blacklist)
{
const URLProtocol **ret;
int i, ret_idx = 0;
// 开辟一块内存空间
ret = av_calloc(FF_ARRAY_ELEMS(url_protocols), sizeof(*ret));
if (!ret)
return NULL;
for (i = 0; url_protocols[i]; i++) {
const URLProtocol *up = url_protocols[i];
if (whitelist && *whitelist && !av_match_name(up->name, whitelist))
continue;
if (blacklist && *blacklist && av_match_name(up->name, blacklist))
continue;
// 将url_protocals中的地址拷贝到返回值中，它这里做拷贝主要是为了使用白名单和黑名单的筛选功能
ret[ret_idx++] = up;
}
return ret;
}
url_protocols定义在libavformat/protocol_list.c，是一个二级指针，保存着各个protocol的地址
static const URLProtocol * const url_protocols[] = {
&ff_async_protocol,
&ff_cache_protocol,
&ff_concat_protocol,
&ff_concatf_protocol,
&ff_crypto_protocol,
&ff_data_protocol,
&ff_ffrtmphttp_protocol,
&ff_file_protocol,
&ff_ftp_protocol,
&ff_gopher_protocol,
&ff_hls_protocol,
&ff_http_protocol,
&ff_httpproxy_protocol,
&ff_icecast_protocol,
&ff_mmsh_protocol,
&ff_mmst_protocol,
&ff_md5_protocol,
&ff_pipe_protocol,
&ff_prompeg_protocol,
&ff_rtmp_protocol,
&ff_rtmpt_protocol,
&ff_rtp_protocol,
&ff_srtp_protocol,
&ff_subfile_protocol,
&ff_tee_protocol,
&ff_tcp_protocol,
&ff_udp_protocol,
&ff_udplite_protocol,
&ff_unix_protocol,
NULL };
下面以 ff_hls_protocol 和 ff_file_protocol举个例子
URLProtocol这个结构体中有很多函数指针，用于实现多态，在这里不同的protocol会为结构体中的函数指针赋不同的值
ff_hls_protocol 定义在libavformat/hlsproto.c:311
const URLProtocol ff_hls_protocol = {
.name = "hls",
.url_open = hls_open,
.url_read = hls_read,
.url_close = hls_close,
.flags = URL_PROTOCOL_FLAG_NESTED_SCHEME,
.priv_data_size = sizeof(HLSContext),
};
ff_file_protocol定义在libavformat/file.c:357
const URLProtocol ff_file_protocol = {
.name = "file",
.url_open = file_open,
.url_read = file_read,
.url_write = file_write,
.url_seek = file_seek,
.url_close = file_close,
.url_get_file_handle = file_get_handle,
.url_check = file_check,
.url_delete = file_delete,
.url_move = file_move,
.priv_data_size = sizeof(FileContext),
.priv_data_class = &file_class,
.url_open_dir = file_open_dir,
.url_read_dir = file_read_dir,
.url_close_dir = file_close_dir,
.default_whitelist = "file,crypto,data"
};
再回到ffurl_alloc，找到合适的protocol之后，调用url_alloc_for_protocol来创建URLContext
url_alloc_for_protocol实现位于libavformat/avio.c:74
URLContext这个上下文保存有URL相关信息，并保存了获取到的URLProtocol
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) {
char *start;
*(const AVClass **)uc->priv_data = up->priv_data_class;
av_opt_set_defaults(uc->priv_data);
if (av_strstart(uc->filename, up->name, (const char**)&start) && *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;
}
再回到ffurl_open_whitelist中来，调用ffurl_alloc获取到URLContext之后会调用ffurl_connect来打开url
ffurl_connect实现位于libavformat/avio.c:166
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 '%s' not on whitelist '%s'!\n", uc->prot->name, 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 '%s' on blacklist '%s'!\n", uc->prot->name, 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;
// 调用URLContext的prot url_open方法，如果有url_open2就调用，没有就调用url_open，该方法定义在URLProtocol中，不同的protocol会有不同的实现
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;
// 将URLContext中的连接flag置1
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;
}
下面以ff_file_protocol的file_open为例子，这是用来打开本地文件的protocol
方法实现位于libavformat/file.c:208
static int file_open(URLContext *h, const char *filename, int flags)
{
// URLContext中还有一个FileContext，用于保存和文件相关的内容
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
// 调用系统函数open来打开文件，获取文件描述符
fd = avpriv_open(filename, access, 0666);
if (fd == -1)
return AVERROR(errno);
// 将文件描述符保存到FileContext中
c->fd = fd;
h->is_streamed = !fstat(fd, &st) && S_ISFIFO(st.st_mode);
/* Buffer writes more than the default 32k to improve throughput especially
* with networked file systems */
if (!h->is_streamed && flags & AVIO_FLAG_WRITE)
h->min_packet_size = h->max_packet_size = 262144;
if (c->seekable >= 0)
h->is_streamed = !c->seekable;
return 0;
}
我们再回到init_input中，调用io_open获取到AVIOContext之后，会再调用av_probe_input_buffer2来做探测
av_probe_input_buffer2实现位于libavformat/format.c:225，这里开始就是来探测demux的组件了，也就是AVInputFomat
int av_probe_input_buffer2(AVIOContext *pb, const 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';
}
}
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来读取buffer
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;
// 将读到的数据封装到AVProbeData中
pd.buf_size = buf_offset - offset;
pd.buf = &buf[offset];
memset(pd.buf + pd.buf_size, 0, AVPROBE_PADDING_SIZE);
/* Guess file format. */
// 调用该方法来创建一个最为合适的 AVInputFormat
*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;
}
avio_read实现位于libavformat/aviobuf.c:641
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 && s->read_packet) {
// bypass the buffer and read data directly into buf
// 调用AVIOContext的read_packet方法读取数据
len = read_packet_wrapper(s, buf, size);
if (len == AVERROR_EOF) {
/* do not modify buffer if EOF reached so that a seek back can
be done without rereading data */
s->eof_reached = 1;
break;
} else if (len < 0) {
s->eof_reached = 1;
s->error= len;
break;
} else {
s->pos += len;
ffiocontext(s)->bytes_read += len;
s->bytes_read = ffiocontext(s)->bytes_read;
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;
}
av_probe_input_format2实现位于libavformat/format.c:128
主要内容就是调用了av_probe_input_format3
const AVInputFormat *av_probe_input_format2(const AVProbeData *pd,
int is_opened, int *score_max)
{
int score_ret;
const 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;
}
av_probe_input_format3 实现于 libavformat/format.c:207
const AVInputFormat *av_probe_input_format3(const AVProbeData *pd,
int is_opened, int *score_ret)
{
AVProbeData lpd = *pd;
const AVInputFormat *fmt1 = NULL;
const 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;
// 这是是用来判断是否有id3信息
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;
}
// 调用av_demuxer_iterate来遍历获取demuxer
while ((fmt1 = av_demuxer_iterate(&i))) {
if (fmt1->flags & AVFMT_EXPERIMENTAL)
continue;
if (!is_opened == !(fmt1->flags & AVFMT_NOFILE) && strcmp(fmt1->name, "image2"))
continue;
score = 0;
if (fmt1->read_probe) {
// 调用AVInputFormat的read_probe方法来探测并且获取分数
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;
}
}
// 记录下最高分数和对应的AVInputFormat
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;
}
av_demuxer_iterate的实现位于libavformat/allformats.c:564，它和前面遍历获取URLContext时很类似
const AVInputFormat *av_demuxer_iterate(void **opaque)
{
static const uintptr_t size = sizeof(demuxer_list)/sizeof(demuxer_list[0]) - 1;
uintptr_t i = (uintptr_t)*opaque;
const AVInputFormat *f = NULL;
uintptr_t tmp;
if (i < size) {
// 从demuxer_list中获取指定项
f = demuxer_list[i];
} else if (tmp = atomic_load_explicit(&indev_list_intptr, memory_order_relaxed)) {
const AVInputFormat *const *indev_list = (const AVInputFormat *const *)tmp;
f = indev_list[i - size];
}
// 将索引加一
if (f)
*opaque = (void*)(i + 1);
return f;
}
demuxer_list声明于libavformat/demuxer_list.c中，由于实在太长，这里就不贴代码了，这是一个二级指针，保存了各个demuxer（AVInputFormat）的地址。下面以ff_mpegts_demuxer为例子看看代码
ff_mpegts_demuxer声明在libavformat/mpegts.c:3422中
const AVInputFormat ff_mpegts_demuxer = {
.name = "mpegts",
.long_name = NULL_IF_CONFIG_SMALL("MPEG-TS (MPEG-2 Transport Stream)"),
.priv_data_size = sizeof(MpegTSContext),
.read_probe = mpegts_probe,
.read_header = mpegts_read_header,
.read_packet = mpegts_read_packet,
.read_close = mpegts_read_close,
.read_timestamp = mpegts_get_dts,
.flags = AVFMT_SHOW_IDS | AVFMT_TS_DISCONT,
.priv_class = &mpegts_class,
};
read_probe方法就是用来探测的
这篇关于FFmpeg中的关键方法及结构体（二）avformat_open_input的文章就介绍到这儿，希望我们推荐的文章对大家有所帮助，也希望大家多多支持为之网！
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