FFmpeg中的关键方法及结构体（二）avformat_open_input
1、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，其中有几个关键方法：
1.1、init_input，该方法用来探测合适的IO和demux组件
1.2、调用AVInputFormat的read_header方法来尝试读取Stream的相关信息
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;
// 如果传入得AVFormatContext对象为空则在内部创建一个 if (!s && !(s = avformat_alloc_context())) return AVERROR(ENOMEM);
// 通过强转来获得一个FFFormatContext类型的指针，记录了AVFormatContext得状态信息 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;
// 如果有参数选项传递则拷贝选项 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;
// 将参数设置给AVFormatContext if ((ret = av_opt_set_dict(s, &tmp)) < 0) goto fail; // 拷贝url到AVFormatContext中 if (!(s->url = av_strdup(filename ? filename : ""))) { ret = AVERROR(ENOMEM); goto fail; }
////////////////////////////////////////////////////////////////////////// // 1.1、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;
// 探测得分记录到AVFormatContext得probe_score当中 s->probe_score = ret; //////////////////////////////////////////////////////////////////////////
// 拷贝白名单到AVFormatContext得protocol_whitelist中 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; } } // 拷贝黑名单到AVFormatContext得protocol_blacklist中 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; } } // 检查探测到得格式必须要在白名单当中 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; }
// 设置AVIOContext内部缓冲区跳过初始字节 avio_skip(s->pb, s->skip_initial_bytes);
// 检查文件名是否图片序列中的一个 /* 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; } }
// 初始化文件的时长和起始时间为AV_NOPTS_VALUE 0x8000000000000000 s->duration = s->start_time = AV_NOPTS_VALUE;
// 如果AVInputFormat带有private data，那么为AVFormatContext的priv_data分配内存 /* 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) {
// 将AVInputFormat的priv_class拷贝给AVFormatContext的priv_data *(const AVClass **) s->priv_data = s->iformat->priv_class; // 初始化AVFormatCOntext私有数据的其他成员
av_opt_set_defaults(s->priv_data);
// 给私有数据设置用户选项 if ((ret = av_opt_set_dict(s->priv_data, &tmp)) < 0) goto fail; } }
// 如果创建了AVIOContext，读取id3信息 /* 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);
// 如果AVInputFormat的read_header方法不为NULL，那么就去调用read_header读取header中的内容 if (s->iformat->read_header)
///////////////////////////////////////////////////
// 1.2、调用read_header方法读取stream相关信息 if ((ret = s->iformat->read_header(s)) < 0) { if (s->iformat->flags_internal & FF_FMT_INIT_CLEANUP) goto close; goto fail; }
///////////////////////////////////////////////////
// 处理meta与id3信息 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); }
// 封面图片处理 if ((ret = avformat_queue_attached_pictures(s)) < 0) goto close;
// 如果AVIOContext不为NULL，并且FFFormatContext记录的AVFormatContext数据读取偏移量为0，则记录下当前AVIOContext写入数据偏移量到FFFormatContext中 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; }
1.1、init_input
该方法实现位于libavformat/demux.c:150，在avformat_open_input中被调用用于找到合适的IO组件以及合适的demux组件。一般来说不指定AVIOContext以及AVInputFormat，需要自己探测拿到最合适的。这里面有2个重要的方法：
1.1.1、io_open，选择并打开IO组件
1.1.2、av_probe_input_buffer2，探测合适的demux组件
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) {
// 给AVFormatContext添加flag，表示IO由用户指定 s->flags |= AVFMT_FLAG_CUSTOM_IO;
// 如果AVInputFormat为NULL则调用方法去探测 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AVInputFormat不为空并且标志位NOFILE，或者为空并且找到了合适的demux组件，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;
// 1.1.1、否则先调用io_open查找IO组件，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;
// 1.1.2、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); }
1.1.1、io_open
该方法的实现位于libavformat/options.c，方法实现为io_open_default，用于创建并初始化AVFormatContext中的AVIOContext，创建方法很简单调用ffio_open_whitlist方法。
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，这个方法做了两件事：
1.1.1.1、ffurl_open_whitelist，该方法用于创建一个URLContext对象，URLContext中封装有最终的IO实现
1.1.1.2、ffio_fdopen，使用URLContext来创建一个IO上下文AVIOContext
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; // 1.1.1.1、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;
// 1.1.1.2、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; }
1.1.1.1、ffurl_open_whitelist
方法实现位于libavformat/avio.c:306，用于创建一个URLContext对象，总共分别两步：
1.1.1.1.1、ffurl_alloc，创建并初始化一个URLContext，并且要选择好合适的IO protocol
1.1.1.1.2、ffurl_connect，打开url
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; // 删减部分代码
// 调用ffurl_connect，打开url ret = ffurl_connect(*puc, options); if (!ret) return 0; fail: ffurl_closep(puc); return ret; }
1.1.1.1.1、ffurl_alloc
方法实现在libavformat/avio.c:293，该方法会先调用url_find_protocol，根据文件名来查找合适的URLProtocol，这部分部分就不单独拆分为小节了；查找到合适的URLProtocol之后就调用url_alloc_for_protocol，来为URLProtocol创建上下文URLContext
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，主要是根据文件名的前缀来判断使用什么URLProtocol，所有的protocol可以通过ffurl_get_protocols来获取
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，所有的URLProtocol的地址都组织在一个指针数组url_protocol当中，
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，URLProtocol是真正的文件读写或者网络读写的实现
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中，找到合适的protocol之后，调用url_alloc_for_protocol来创建URLProtocol的上下文URLContext
url_alloc_for_protocol实现位于libavformat/avio.c:74
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); }
// 为一个URLContext对象开辟空间，空间的大小是URLContext结构体的大小 + 文件名的字符串长度
//（因为URLCOntext中的filename是一个char指针） uc = av_mallocz(sizeof(URLContext) + strlen(filename) + 1); if (!uc) { err = AVERROR(ENOMEM); goto fail; }
// 初始化URLContext uc->av_class = &ffurl_context_class; uc->filename = (char *)&uc[1]; // 字符串的地址在整个结构体的末端，利用该地址为filename地址赋值 strcpy(uc->filename, filename); uc->prot = up; // 将URLProtocol对象地址保存到prot指针 uc->flags = flags; uc->is_streamed = 0; /* default = not streamed */ uc->max_packet_size = 0; /* default: stream file */ if (up->priv_data_size) { // 需要根据实际的URLProtocol来判断是否有priv_data_size
// 这里的priv_data指的是不同protocol特有的数据，比如本地文件打开拿到的fd uc->priv_data = av_mallocz(up->priv_data_size); // 给priv_data开辟空间，这个空间大小需要到对应的Protocol中查找 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; // 将创建的URLContext成员的地址返回 *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; }
1.1.1.1.2、ffurl_connect
再回到ffurl_open_whitelist中来，调用ffurl_alloc获取到URLContext以及URLProtocol之后会调用ffurl_connect来打开url
ffurl_connect实现位于libavformat/avio.c:166
打开url的方法很简单，调用URLContext中的URLProtocol成员的url_open方法就好了，如果打开成功那么就将URLContext中的is_connect置为1
int ffurl_connect(URLContext *uc, AVDictionary **options) { int err; AVDictionary *tmp_opts = NULL; AVDictionaryEntry *e; if (!options) options = &tmp_opts; // 删减部分代码
// 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中有一个成员priv_data，用于存储不同protocol的独有数据，ff_file_protocol的priv_data为FileContext 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;
// 初始化URLContext的is_stream状态，获取fd的状态，并判断其是否为FIFO文件，判断该文件是否为streaming 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; }
1.1.1.2、ffio_fdopen
该方法实现位于libavformat/aviobuf.c:956
做完ffurl_open_whitelist（创建URLContext，选择了合适的URLProtocol并且url_open）之后，调用ffio_fdopen方法来为URLContext方法创建更高级的上下文AVIOContext，AVIOContext的read_pause和read_seek等于URLProtocol中的方法
int ffio_fdopen(AVIOContext **s, URLContext *h) { 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; } if (!(h->flags & AVIO_FLAG_WRITE) && h->is_streamed) { if (buffer_size > INT_MAX/2) return AVERROR(EINVAL); buffer_size *= 2; }
// 为一块buffer开辟空间 buffer = av_malloc(buffer_size); if (!buffer) return AVERROR(ENOMEM); // 调用avio_alloc_context方法创建FFIOContext对象 *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); if (!*s) { av_freep(&buffer); return AVERROR(ENOMEM); } // ......
// 初始化AVIOContext中的成员 (*s)->direct = h->flags & AVIO_FLAG_DIRECT; (*s)->seekable = h->is_streamed ? 0 : AVIO_SEEKABLE_NORMAL; (*s)->max_packet_size = max_packet_size; (*s)->min_packet_size = h->min_packet_size; if(h->prot) {
// 给AVIOContext中的 read_pause、read_seek、url_read_seek赋值 (*s)->read_pause = (int (*)(void *, int))h->prot->url_read_pause; (*s)->read_seek = (int64_t (*)(void *, int, int64_t, int))h->prot->url_read_seek; if (h->prot->url_read_seek) (*s)->seekable |= AVIO_SEEKABLE_TIME; }
// 给FFIOContext中的成员方法赋值 ((FFIOContext*)(*s))->short_seek_get = (int (*)(void *))ffurl_get_short_seek; (*s)->av_class = &ff_avio_class; return 0; }
avio_aloc_context用于创建并初始化一个FFIOContext对象，里面会包含有一个AVIOContext对象
libavformat/aviobuf.c:135:
AVIOContext *avio_alloc_context( unsigned char *buffer, int buffer_size, int write_flag, void *opaque, int (*read_packet)(void *opaque, uint8_t *buf, int buf_size), int (*write_packet)(void *opaque, uint8_t *buf, int buf_size), int64_t (*seek)(void *opaque, int64_t offset, int whence)) { FFIOContext *s = av_malloc(sizeof(*s)); if (!s) return NULL; ffio_init_context(s, buffer, buffer_size, write_flag, opaque, read_packet, write_packet, seek); return &s->pub; }
ffio_init_context实现位于libavformat/aviobuf.c:81，用于初始化AVIOContext，该结构体中持有的buffer用于存储读取到的数据
其中write_packet、read_packet、seek方法对应libavformat/avio.c:401中的ffurl_read、ffurl_write、ffurl_seek，其实就是调用的URLProtocol中的方法。上面可以看到AVIOContext中还有read_pause、read_seek这两个方法，也是指向了URLProtocol中的方法，可能这两个方法是用来做读写控制的，实现pause、seek等命令的
void ffio_init_context(FFIOContext *ctx, unsigned char *buffer, int buffer_size, int write_flag, void *opaque, int (*read_packet)(void *opaque, uint8_t *buf, int buf_size), int (*write_packet)(void *opaque, uint8_t *buf, int buf_size), int64_t (*seek)(void *opaque, int64_t offset, int whence)) { AVIOContext *const s = &ctx->pub; memset(ctx, 0, sizeof(*ctx)); // 用于存储读取的数据 s->buffer = buffer;
// buffer的最大大小 ctx->orig_buffer_size = s->buffer_size = buffer_size;
// buffer的读写指针 s->buf_ptr = buffer;
// 可读写最大位置 s->buf_ptr_max = buffer;
// URLProtocol s->opaque = opaque; s->direct = 0; url_resetbuf(s, write_flag ? AVIO_FLAG_WRITE : AVIO_FLAG_READ); // 初始化write、read、seek方法，这里是直接给定的ffurl_read、ffurl_write、ffurl_seek s->write_packet = write_packet; s->read_packet = read_packet; s->seek = seek;
// 初始化AVIOContext中的其他成员 s->pos = 0; s->eof_reached = 0; s->error = 0; s->seekable = seek ? AVIO_SEEKABLE_NORMAL : 0; s->min_packet_size = 0; s->max_packet_size = 0; s->update_checksum = NULL; ctx->short_seek_threshold = SHORT_SEEK_THRESHOLD; if (!read_packet && !write_flag) { s->pos = buffer_size; s->buf_end = s->buffer + buffer_size; } s->read_pause = NULL; s->read_seek = NULL; s->write_data_type = NULL; s->ignore_boundary_point = 0; ctx->current_type = AVIO_DATA_MARKER_UNKNOWN; ctx->last_time = AV_NOPTS_VALUE; ctx->short_seek_get = NULL; #if FF_API_AVIOCONTEXT_WRITTEN FF_DISABLE_DEPRECATION_WARNINGS s->written = 0; FF_ENABLE_DEPRECATION_WARNINGS #endif }
1.1.2、av_probe_input_buffer2
我们再回到init_input中，调用io_open获取到FFIOContext / AVIOContext之后，会再调用av_probe_input_buffer2来做探测
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。
这个方法主要内容分为两步：
1.1.2.1、调用avio_read读取数据，为什么要看这个方法呢？是因为读取的数据后续可能还会再用，所以看下数据是如何存储的
1.1.2.2、调用av_probe_input_format2来探测input format，直到有一个合适的demux组件返回，就会停止读取了
1.1.2.3、调用ffio_rewind_with_probe_data复用已经读取的数据
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;
// 检查是否设定最大探测长度，如果没有定义则使用PROBE_BUF_MAX （1 << 20） if (!max_probe_size) max_probe_size = PROBE_BUF_MAX; else if (max_probe_size < PROBE_BUF_MIN) { // 探测值不能小于 2048 av_log(logctx, AV_LOG_ERROR, "Specified probe size value %u cannot be < %u\n", max_probe_size, PROBE_BUF_MIN); return AVERROR(EINVAL); }
// 检查探测的初始便宜是否合法 if (offset >= max_probe_size) return AVERROR(EINVAL); if (pb->av_class) { uint8_t *mime_type_opt = NULL; char *semi;
// 从AVIOContext中搜索mime_type av_opt_get(pb, "mime_type", AV_OPT_SEARCH_CHILDREN, &mime_type_opt); pd.mime_type = (const char *)mime_type_opt;
// 选项信息中存在多少个，以分号分隔，截取第一个；如果没有mime这置为null semi = pd.mime_type ? strchr(pd.mime_type, ';') : NULL; if (semi) { *semi = '\0'; } }
// probe_size至少读2048，直到到达上限或者AVInputFormat被找到 for (probe_size = PROBE_BUF_MIN; probe_size <= max_probe_size && !*fmt; // probe_size x 2
probe_size = FFMIN(probe_size << 1, FFMAX(max_probe_size, probe_size + 1))) {
// 设置score为25 score = probe_size < max_probe_size ? AVPROBE_SCORE_RETRY : 0; /* Read probe data. */
// 重新分配一块 probe_size + padding_size大小的buf，缓冲区中仍然保存有上次读取的数据 if ((ret = av_reallocp(&buf, probe_size + AVPROBE_PADDING_SIZE)) < 0) goto fail;
// 1.1.2.1、调用avio_read来读取buffer，存储到从buf_offset开始，大小为probe_size - buf_offset的数据 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 */ }
// 记录buf中数据的长度 buf_offset += ret;
// 如果数据没有到达offset的位置 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]; // 多余的padding_size设置为0 memset(pd.buf + pd.buf_size, 0, AVPROBE_PADDING_SIZE); /* Guess file format. */ // 1.1.2.2、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. */
// 如果找到了合适的demux组件，那么让AVIOContext内部的缓冲区归位，保留文件探测过程中读取到的数据 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; }
1.1.2.1、avio_read
avio_read实现位于libavformat/aviobuf.c:641
这个方法是对AVIOContext的read_packet的又一层封装，将数据读到AVIOContext中，然后从中拷贝到外部buf中。如果buf大小大于了AVIOContext的最大大小，就直接读到外部buffer中
int avio_read(AVIOContext *s, unsigned char *buf, int size) { int len, size1; size1 = size; while (size > 0) {
// 检查AVIOContext中的buf的大小 len = FFMIN(s->buf_end - s->buf_ptr, size);
// 如果AVIOContext没有空间或者要读取的size为0（其实一般不会出现size为0，因为这时候直接退出循环了
// 也就是读满AVIOContext之后就不再往里面读了，直接读到buffer中） if (len == 0 || s->write_flag) {
// 如果AVIOContext的direct flag为 1 或者要读取的大小大于AVIOContext的最大buf大小 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方法直接将读取数据到buf当中
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 {
// 从AVIOContext中拷贝buffer出来，并且移动读写指针 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; }
libavformat/aviobuf.c:527
read_packet_wrapper方法会调用AVIOContext的read_packet方法
static int read_packet_wrapper(AVIOContext *s, uint8_t *buf, int size) { int ret; if (!s->read_packet) return AVERROR(EINVAL); ret = s->read_packet(s->opaque, buf, size); av_assert2(ret || s->max_packet_size); return ret; }
1.1.2.2、av_probe_input_format2
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) {
// 如果得分大于25则返回该demux组件 *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，分析前10个字节中是否存在id3 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);
// 综合考虑计算得分与扩展名得分 if (fmt1->extensions && av_match_ext(lpd.filename, fmt1->extensions)) { switch (nodat) { case NO_ID3: score = FFMAX(score, 1); // 如果没有id3，取计算得分 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) { // 如果没有read_probe，存在扩展名，根据扩展名来判断 if (av_match_ext(lpd.filename, fmt1->extensions)) score = AVPROBE_SCORE_EXTENSION; }
// 再次根据mime类型来判断 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
1.1.2.3、调用ffio_rewind_with_probe_data
ffio_rewind_with_probe_data这个方法用于复用已经读取的用于探测demux组件的buffer，我的理解如下：因为读取buffer时可能超过了AVIOContext最大缓存值，也就是bufp中的大小会超过该最大缓存值，
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); }
// 获取buffer中数据的长度 buffer_size = s->buf_end - s->buffer; /* the buffers must touch or overlap */
// buffer_start表示bufp比AVIOContext多出的数据长度
// 如果该多出的长度大于总长度说明有错误，始放掉buffer if ((buffer_start = s->pos - buffer_size) > buf_size) { av_freep(bufp); return AVERROR(EINVAL); } // 计算出重合的长度 overlap = buf_size - buffer_start;
// 计算出新长度buffer_size + buffer_start，我觉得好像就和buf_size大小是相同的 new_size = buf_size + buffer_size - overlap;
// 如果new_size(buf_size)大于原来的AVIOContext size，那么alloc_size就赋大值 alloc_size = FFMAX(s->buffer_size, new_size);
// 我觉得这两个值是相等的，所以不需要重新分配空间 if (alloc_size > buf_size) if (!(buf = (*bufp) = av_realloc_f(buf, 1, alloc_size))) return AVERROR(ENOMEM); // 这里应该也不用重新拷贝数据 if (new_size > buf_size) { memcpy(buf + buf_size, s->buffer + overlap, buffer_size - overlap); buf_size = new_size; }
// 释放掉AVIOContext中原来的buffer av_free(s->buffer);
// 将buf指针指向新的可能更大的buffer s->buf_ptr = s->buffer = buf;
// 修改buffer_size s->buffer_size = alloc_size;
// 修改读写位置 s->pos = buf_size;
// 修改数据结束位置 s->buf_end = s->buf_ptr + buf_size; s->eof_reached = 0; return 0; }
1.2、read_header
read_header其实是AVInputFormat中的方法，在所有的准备工作都做完了之后就调用该方法读取meta data，获取stream等信息。这个方法比较适合flv、mov这种有header的封装格式，对于mpeg2这种没有header的可能会无法提取出steam info。
下面以ff_mov_demuxer举个例子，代码实现路径 libavformat/mov.c:8566
代码很长，这里只看比较关键的节点，主要就是调用mov_read_default，读取box（atom）
static int mov_read_header(AVFormatContext *s) { MOVContext *mov = s->priv_data; AVIOContext *pb = s->pb; int j, err; MOVAtom atom = { AV_RL32("root") }; int i; if (mov->decryption_key_len != 0 && mov->decryption_key_len != AES_CTR_KEY_SIZE) { av_log(s, AV_LOG_ERROR, "Invalid decryption key len %d expected %d\n", mov->decryption_key_len, AES_CTR_KEY_SIZE); return AVERROR(EINVAL); } mov->fc = s; mov->trak_index = -1; /* .mov and .mp4 aren't streamable anyway (only progressive download if moov is before mdat) */ if (pb->seekable & AVIO_SEEKABLE_NORMAL) atom.size = avio_size(pb); else atom.size = INT64_MAX; /* check MOV header */ do { if (mov->moov_retry) avio_seek(pb, 0, SEEK_SET); if ((err = mov_read_default(mov, pb, atom)) < 0) { av_log(s, AV_LOG_ERROR, "error reading header\n"); return err; } } while ((pb->seekable & AVIO_SEEKABLE_NORMAL) && !mov->found_moov && !mov->moov_retry++); if (!mov->found_moov) { av_log(s, AV_LOG_ERROR, "moov atom not found\n"); return AVERROR_INVALIDDATA; } av_log(mov->fc, AV_LOG_TRACE, "on_parse_exit_offset=%"PRId64"\n", avio_tell(pb)); // ...... }
mov_read_default主要是解析出box type，然后找到对应的方法来解析box，比如trak box，那就会找到根据box type找到mov_default_parse_table中对应的处理方法mov_read_trak
static int mov_read_default(MOVContext *c, AVIOContext *pb, MOVAtom atom) { int64_t total_size = 0; MOVAtom a; int i; if (c->atom_depth > 10) { av_log(c->fc, AV_LOG_ERROR, "Atoms too deeply nested\n"); return AVERROR_INVALIDDATA; } c->atom_depth ++; if (atom.size < 0) atom.size = INT64_MAX; while (total_size <= atom.size - 8 && !avio_feof(pb)) { int (*parse)(MOVContext*, AVIOContext*, MOVAtom) = NULL; a.size = atom.size; a.type=0; if (atom.size >= 8) { a.size = avio_rb32(pb); a.type = avio_rl32(pb); if (((a.type == MKTAG('f','r','e','e') && c->moov_retry) || a.type == MKTAG('h','o','o','v')) && a.size >= 8 && c->fc->strict_std_compliance < FF_COMPLIANCE_STRICT) { uint32_t type; avio_skip(pb, 4); type = avio_rl32(pb); if (avio_feof(pb)) break; avio_seek(pb, -8, SEEK_CUR); if (type == MKTAG('m','v','h','d') || type == MKTAG('c','m','o','v')) { av_log(c->fc, AV_LOG_ERROR, "Detected moov in a free or hoov atom.\n"); a.type = MKTAG('m','o','o','v'); } } if (atom.type != MKTAG('r','o','o','t') && atom.type != MKTAG('m','o','o','v')) { if (a.type == MKTAG('t','r','a','k') || a.type == MKTAG('m','d','a','t')) { av_log(c->fc, AV_LOG_ERROR, "Broken file, trak/mdat not at top-level\n"); avio_skip(pb, -8); c->atom_depth --; return 0; } } total_size += 8; if (a.size == 1 && total_size + 8 <= atom.size) { /* 64 bit extended size */ a.size = avio_rb64(pb) - 8; total_size += 8; } } av_log(c->fc, AV_LOG_TRACE, "type:'%s' parent:'%s' sz: %"PRId64" %"PRId64" %"PRId64"\n", av_fourcc2str(a.type), av_fourcc2str(atom.type), a.size, total_size, atom.size); if (a.size == 0) { a.size = atom.size - total_size + 8; } if (a.size < 0) break; a.size -= 8; if (a.size < 0) break; a.size = FFMIN(a.size, atom.size - total_size); // 遍历查找box type对应的处理方法 for (i = 0; mov_default_parse_table[i].type; i++) if (mov_default_parse_table[i].type == a.type) { parse = mov_default_parse_table[i].parse; break; } // ...... }
mov_read_trak
该方法会读取box中的track信息，并且创建一个AVStream，将这些信息存储到AVSTream当中
1.2.1、调用avformat_new_stream创建一个AVStream，传入参数为AVFormatContext
static int mov_read_trak(MOVContext *c, AVIOContext *pb, MOVAtom atom) { AVStream *st; MOVStreamContext *sc; int ret; // 1.2.1、创建一个AVStream st = avformat_new_stream(c->fc, NULL); if (!st) return AVERROR(ENOMEM);
// 默认id为-1 st->id = -1;
// 创建一个MOVSTreamContext休息 sc = av_mallocz(sizeof(MOVStreamContext)); if (!sc) return AVERROR(ENOMEM); // AVStream中的priv_data存放的每个InputFormat独有的Stream info组织形式 st->priv_data = sc;
// 默认置codecpar->codec_type为data st->codecpar->codec_type = AVMEDIA_TYPE_DATA;
sc->ffindex = st->index; c->trak_index = st->index; // 继续读取track box的子box，这时候就把相关track meta读到AVStream当中去了 if ((ret = mov_read_default(c, pb, atom)) < 0) return ret; c->trak_index = -1; // Here stsc refers to a chunk not described in stco. This is technically invalid, // but we can overlook it (clearing stsc) whenever stts_count == 0 (indicating no samples). if (!sc->chunk_count && !sc->stts_count && sc->stsc_count) { sc->stsc_count = 0; av_freep(&sc->stsc_data); } /* sanity checks */ if ((sc->chunk_count && (!sc->stts_count || !sc->stsc_count || (!sc->sample_size && !sc->sample_count))) || (!sc->chunk_count && sc->sample_count)) { av_log(c->fc, AV_LOG_ERROR, "stream %d, missing mandatory atoms, broken header\n", st->index); return 0; } if (sc->stsc_count && sc->stsc_data[ sc->stsc_count - 1 ].first > sc->chunk_count) { av_log(c->fc, AV_LOG_ERROR, "stream %d, contradictionary STSC and STCO\n", st->index); return AVERROR_INVALIDDATA; } fix_timescale(c, sc); avpriv_set_pts_info(st, 64, 1, sc->time_scale); mov_build_index(c, st); if (sc->dref_id-1 < sc->drefs_count && sc->drefs[sc->dref_id-1].path) { MOVDref *dref = &sc->drefs[sc->dref_id - 1]; if (c->enable_drefs) { if (mov_open_dref(c, &sc->pb, c->fc->url, dref) < 0) av_log(c->fc, AV_LOG_ERROR, "stream %d, error opening alias: path='%s', dir='%s', " "filename='%s', volume='%s', nlvl_from=%d, nlvl_to=%d\n", st->index, dref->path, dref->dir, dref->filename, dref->volume, dref->nlvl_from, dref->nlvl_to); } else { av_log(c->fc, AV_LOG_WARNING, "Skipped opening external track: " "stream %d, alias: path='%s', dir='%s', " "filename='%s', volume='%s', nlvl_from=%d, nlvl_to=%d." "Set enable_drefs to allow this.\n", st->index, dref->path, dref->dir, dref->filename, dref->volume, dref->nlvl_from, dref->nlvl_to); } } else { sc->pb = c->fc->pb; sc->pb_is_copied = 1; } if (st->codecpar->codec_type == AVMEDIA_TYPE_VIDEO) { if (!st->sample_aspect_ratio.num && st->codecpar->width && st->codecpar->height && sc->height && sc->width && (st->codecpar->width != sc->width || st->codecpar->height != sc->height)) { st->sample_aspect_ratio = av_d2q(((double)st->codecpar->height * sc->width) / ((double)st->codecpar->width * sc->height), INT_MAX); } #if FF_API_R_FRAME_RATE if (sc->stts_count == 1 || (sc->stts_count == 2 && sc->stts_data[1].count == 1)) av_reduce(&st->r_frame_rate.num, &st->r_frame_rate.den, sc->time_scale, sc->stts_data[0].duration, INT_MAX); #endif } // done for ai5q, ai52, ai55, ai1q, ai12 and ai15. if (!st->codecpar->extradata_size && st->codecpar->codec_id == AV_CODEC_ID_H264 && TAG_IS_AVCI(st->codecpar->codec_tag)) { ret = ff_generate_avci_extradata(st); if (ret < 0) return ret; } switch (st->codecpar->codec_id) { #if CONFIG_H261_DECODER case AV_CODEC_ID_H261: #endif #if CONFIG_H263_DECODER case AV_CODEC_ID_H263: #endif #if CONFIG_MPEG4_DECODER case AV_CODEC_ID_MPEG4: #endif st->codecpar->width = 0; /* let decoder init width/height */ st->codecpar->height= 0; break; } // If the duration of the mp3 packets is not constant, then they could need a parser if (st->codecpar->codec_id == AV_CODEC_ID_MP3 && sc->stts_count > 3 && sc->stts_count*10 > st->nb_frames && sc->time_scale == st->codecpar->sample_rate) { ffstream(st)->need_parsing = AVSTREAM_PARSE_FULL; } /* Do not need those anymore. */ av_freep(&sc->chunk_offsets); av_freep(&sc->sample_sizes); av_freep(&sc->keyframes); av_freep(&sc->stts_data); av_freep(&sc->stps_data); av_freep(&sc->elst_data); av_freep(&sc->rap_group); return 0; }
1.2.1、avformat_new_stream
下面看下关键的创建stream的方法avformat_new_stream
方法实现位于 libavformat/utils.c:768，主要有以下几个步骤：
1.2.1.1、给AVStream开辟空间
1.2.1.2、给AVStream创建一个上下文FFStream
1.2.1.3、调用avcodec_parameters_alloc创建AVStream中的codecpar
1.2.1.4、调用avcodec_alloc_context3给FFStream创建一个AVCodecContext，FFStream作为AVStream的上下文，起到一个管理作用，保存有Stream对应的信息，也保存有stream对应的Codec
1.2.1.5、给FFStream中的info成员分配内存并初始化
这里比较重要的是1.2.1.4
AVStream *avformat_new_stream(AVFormatContext *s, const AVCodec *c) { FFFormatContext *const si = ffformatcontext(s); FFStream *sti; AVStream *st; AVStream **streams; // 检查stream的个数 if (s->nb_streams >= s->max_streams) { av_log(s, AV_LOG_ERROR, "Number of streams exceeds max_streams parameter" " (%d), see the documentation if you wish to increase it\n", s->max_streams); return NULL; }
// 1.2.1.1、重新给AVFormatContext中的stream数组开辟空间 streams = av_realloc_array(s->streams, s->nb_streams + 1, sizeof(*streams)); if (!streams) return NULL; s->streams = streams; // 1.2.1.2、创建一个AVStream的上下文 FFStream sti = av_mallocz(sizeof(*sti)); if (!sti) return NULL;
// 将FFStream和AVStrem相关联 st = &sti->pub; #if FF_API_AVSTREAM_CLASS st->av_class = &stream_class; #endif // 1.2.1.3、调用avcodec_parameters_alloc来初始化codecpar st->codecpar = avcodec_parameters_alloc(); if (!st->codecpar) goto fail; // 1.2.1.4、调用avcodec_alloc_context3来创建一个AVCodecContext对象 sti->avctx = avcodec_alloc_context3(NULL); if (!sti->avctx) goto fail; if (s->iformat) {
// 1.2.1.5、给info结构体分配内存，存储的是codec相关信息 sti->info = av_mallocz(sizeof(*sti->info)); if (!sti->info) goto fail; #if FF_API_R_FRAME_RATE sti->info->last_dts = AV_NOPTS_VALUE; #endif sti->info->fps_first_dts = AV_NOPTS_VALUE; sti->info->fps_last_dts = AV_NOPTS_VALUE; /* default pts setting is MPEG-like */ avpriv_set_pts_info(st, 33, 1, 90000); /* we set the current DTS to 0 so that formats without any timestamps * but durations get some timestamps, formats with some unknown * timestamps have their first few packets buffered and the * timestamps corrected before they are returned to the user */ sti->cur_dts = RELATIVE_TS_BASE; } else { sti->cur_dts = AV_NOPTS_VALUE; } // AVStream的index对应于AVFormatContext中的nb_streams索引 st->index = s->nb_streams;
// 初始化AVStream和FFStream中的相关内容 st->start_time = AV_NOPTS_VALUE; st->duration = AV_NOPTS_VALUE; sti->first_dts = AV_NOPTS_VALUE; sti->probe_packets = s->max_probe_packets; sti->pts_wrap_reference = AV_NOPTS_VALUE; sti->pts_wrap_behavior = AV_PTS_WRAP_IGNORE; sti->last_IP_pts = AV_NOPTS_VALUE; sti->last_dts_for_order_check = AV_NOPTS_VALUE;
// 初始化pts_buffer for (int i = 0; i < MAX_REORDER_DELAY + 1; i++) sti->pts_buffer[i] = AV_NOPTS_VALUE; st->sample_aspect_ratio = (AVRational) { 0, 1 }; sti->inject_global_side_data = si->inject_global_side_data; sti->need_context_update = 1; // 将创建的AVStream保存到AVFormatContext中 s->streams[s->nb_streams++] = st; return st; fail: free_stream(&st); return NULL; }
1.2.1.4、avcodec_alloc_context3
该方法用于创建一个AVCodecContext对象，方法实现位于libavcodec/options.c:141
AVCodecContext *avcodec_alloc_context3(const AVCodec *codec) { AVCodecContext *avctx= av_malloc(sizeof(AVCodecContext)); if (!avctx) return NULL; if (init_context_defaults(avctx, codec) < 0) { av_free(avctx); return NULL; } return avctx; }
