參考：
FFmpeg源代碼簡單分析：avio_open2()

示例工程：
【FFmpeg】調用ffmpeg庫實現264軟編
【FFmpeg】調用ffmpeg庫實現264軟解
【FFmpeg】調用ffmpeg庫進行RTMP推流和拉流
【FFmpeg】調用ffmpeg庫進行SDL2解碼后渲染

流程分析：
【FFmpeg】編碼鏈路上主要函數的簡單分析
【FFmpeg】解碼鏈路上主要函數的簡單分析

結構體分析：
【FFmpeg】AVCodec結構體
【FFmpeg】AVCodecContext結構體
【FFmpeg】AVStream結構體
【FFmpeg】AVFormatContext結構體
【FFmpeg】AVIOContext結構體
【FFmpeg】AVPacket結構體

函數分析：
【FFmpeg】avformat_open_input函數
【FFmpeg】avformat_find_stream_info函數
【FFmpeg】avformat_alloc_output_context2函數

avio_open2函數的內部調用關系為

1.avio_open2

avio_open2函數的定義位于libavformat\avio.c中，功能是打開URL，之后方便進行讀寫操作。這里的URL是廣義的地址，對于文件而言，就是文件的路徑，如"C:\xxx\test.flv"，也可以是地址例如"rtmp://127.0.0.1:1935/live/stream"，flag表示控制如何打開url所指示的資源的標志，如AVIO_FLAG_READ和AVIO_FLAG_WRITE

/*** Create and initialize a AVIOContext for accessing the* resource indicated by url.* @note When the resource indicated by url has been opened in* read+write mode, the AVIOContext can be used only for writing.** @param s Used to return the pointer to the created AVIOContext.* In case of failure the pointed to value is set to NULL.* @param url resource to access* @param flags flags which control how the resource indicated by url* is to be opened* @param int_cb an interrupt callback to be used at the protocols level* @param options  A dictionary filled with protocol-private options. On return* this parameter will be destroyed and replaced with a dict containing options* that were not found. May be NULL.* @return >= 0 in case of success, a negative value corresponding to an* AVERROR code in case of failure*/
// 創建并初始化一個AVIOContext，用于訪問url指定的資源
// @note：當url所指示的資源以讀+寫方式打開時，AVIOContext只能用于寫
int avio_open2(AVIOContext **s, const char *filename, int flags,const AVIOInterruptCB *int_cb, AVDictionary **options)
{return ffio_open_whitelist(s, filename, flags, int_cb, options, NULL, NULL);
}

avio_open2調用ffio_open_whitelist進行AVIOContext的創建，而ffio_open_whitelist的實現方式如下，其中主要使用了兩個函數：（1）ffurl_open_whitelist根據白名單創建URLContext；（2）ffio_fdopen根據創建的URLContext來初始化AVIOContext。其中，URLContext完成協議的讀寫操作。

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.根據whitelist創建URLContexterr = ffurl_open_whitelist(&h, filename, flags, int_cb, options, whitelist, blacklist, NULL);if (err < 0)return err;// 2.根據創建的URLContext初始化AVIOContexterr = ffio_fdopen(s, h);if (err < 0) {ffurl_close(h);return err;}return 0;
}

1.1 創建URLContext（ffurl_open_whitelist）

該函數的主要內容為兩個部分：（1）ffurl_alloc：創建URLContext；（2）ffurl_connect：打開已創建的URLContext；

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;// 創建URLContextint 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;// 打開獲得的URLProtocolret = ffurl_connect(*puc, options);if (!ret)return 0;
fail:ffurl_closep(puc);return ret;
}

1.1.1 創建URLContext（ffurl_alloc）

int ffurl_alloc(URLContext **puc, const char *filename, int flags,const AVIOInterruptCB *int_cb)
{const URLProtocol *p = NULL;// 根據filename查找合適的protocolp = url_find_protocol(filename);if (p)// 為protocol分配URLContextreturn url_alloc_for_protocol(puc, p, filename, flags, int_cb);*puc = NULL;return AVERROR_PROTOCOL_NOT_FOUND;
}

1.1.1.1 查找合適的protocol（url_find_protocol）

函數主要工作是調用ffurl_get_protocols獲取一個protocol

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");elseav_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';// 尋找一個protocolsprotocols = 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);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_protocol的定義如下，大致的流程是遍歷可選的url_protocol，檢查是否位于白名單中且不在黑名單中

const URLProtocol **ffurl_get_protocols(const char *whitelist,const char *blacklist)
{const URLProtocol **ret;int i, ret_idx = 0;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;ret[ret_idx++] = up;}return ret;
}

1.1.1.2 為查找到的URLProtocol創建URLContext（url_alloc_for_protocol）

在創建URLContext時，會先檢查flag和函數對應的關系：（1）網絡操作；（2）讀取；（3）寫入；如果有一個初始化失敗，則返回錯誤

static int url_alloc_for_protocol(URLContext **puc, const URLProtocol *up,const char *filename, int flags,const AVIOInterruptCB *int_cb)
{URLContext *uc;int err;// flag中包含NETWORK，但是網絡初始化模塊失敗，則返回錯誤
#if CONFIG_NETWORKif (up->flags & URL_PROTOCOL_FLAG_NETWORK && !ff_network_init())return AVERROR(EIO);
#endif// flag中包含AVIO_FLAG_READ，但是不包含url_read，則輸出錯誤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);}// flag中包含AVIO_FALG_WRITE，但是不包含url_write，則輸出錯誤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);}// 前續檢查結束，創建結構體，并且進行初始化賦值uc = av_mallocz(sizeof(URLContext) + strlen(filename) + 1);if (!uc) {err = AVERROR(ENOMEM);goto fail;}uc->av_class = &url_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;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);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_NETWORKif (up->flags & URL_PROTOCOL_FLAG_NETWORK)ff_network_close();
#endifreturn err;
}

1.1.2 打開URLContext（ffurl_connect）

函數的主要功能是打開前面創建的URLContext，首先檢查前面創建的URLContext是否被正確初始化，然后調用url_open打開URL

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// 檢查URLContext被正確的初始化// 如果設置了黑白名單需要檢查URLContext是否符合要求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 whitelistif ((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;// 使用url_open2或者url_open// 對于協議例如file，ftp，rtp，tcp，libRTMP等，使用的是url_open// 對于http協議，使用的是url_open2// 如果是file，url_open會鏈接到ftp_open()// 如果是libRTMP，url_open會鏈接到rtmp_open()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;
}

例如協議的格式為ftp，會調用ftp_open，定義如下；會調用ftp_connect實現ftp連接的打開

static int ftp_open(URLContext *h, const char *url, int flags)
{FTPContext *s = h->priv_data;int err;ff_dlog(h, "ftp protocol open\n");if ((err = ftp_connect(h, url)) < 0)goto fail;if (ftp_restart(s, 0) < 0) {h->is_streamed = 1;} else {ftp_file_size(s);if (s->write_seekable != 1 && flags & AVIO_FLAG_WRITE)h->is_streamed = 1;}return 0;fail:av_log(h, AV_LOG_ERROR, "FTP open failed\n");ftp_close(h);return err;
}

1.2 根據創建的URLContext初始化AVIOContext（ffio_fdopen）

/*** Create and initialize a AVIOContext for accessing the* resource referenced by the URLContext h.* @note When the URLContext h has been opened in read+write mode, the* AVIOContext can be used only for writing.** @param s Used to return the pointer to the created AVIOContext.* In case of failure the pointed to value is set to NULL.* @return >= 0 in case of success, a negative value corresponding to an* AVERROR code in case of failure*/
// 創建并初始化一個AVIOContext，用于訪問URLContext引用的資源
// @note 如果URLContext已經以read + write模式打開，那么AVIOContext只能被用于writing
int ffio_fdopen(AVIOContext **sp, URLContext *h)
{AVIOContext *s;uint8_t *buffer = NULL;int buffer_size, max_packet_size;// 首先初始化AVIOContext當中的buffer，如果前面配置max_packet_size，則將其配置為buffer_size// 否則將buffer_size配置為IO_BUFFER_SIZE=32768max_packet_size = h->max_packet_size;if (max_packet_size) {// buffer_size不必超過packet最大size，因為最多填充一個packet即可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;}buffer = av_malloc(buffer_size);if (!buffer)return AVERROR(ENOMEM);// 創建AVIOContext*sp = avio_alloc_context(buffer, buffer_size, h->flags & AVIO_FLAG_WRITE, h,ffurl_read2, ffurl_write2, ffurl_seek2);if (!*sp) {av_freep(&buffer);return AVERROR(ENOMEM);}s = *sp;if (h->protocol_whitelist) {s->protocol_whitelist = av_strdup(h->protocol_whitelist);if (!s->protocol_whitelist) {avio_closep(sp);return AVERROR(ENOMEM);}}if (h->protocol_blacklist) {s->protocol_blacklist = av_strdup(h->protocol_blacklist);if (!s->protocol_blacklist) {avio_closep(sp);return AVERROR(ENOMEM);}}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) {s->read_pause = h->prot->url_read_pause;s->read_seek  = h->prot->url_read_seek;if (h->prot->url_read_seek)s->seekable |= AVIO_SEEKABLE_TIME;}((FFIOContext*)s)->short_seek_get = ffurl_get_short_seek;s->av_class = &ff_avio_class;return 0;
}

1.2.1 創建AVIOContext（avio_alloc_context）

該函數首先使用av_malloc創建FFIOContext，隨后通過調用ffio_init_context來初始化，這里做了一個封裝，先通過初始化FFIOContext，隨后返回FFIOContext之中的AVIOContext

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, const 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的初始化操作為

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, const 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));s->buffer      = buffer;ctx->orig_buffer_size =s->buffer_size = buffer_size;s->buf_ptr     = buffer;s->buf_ptr_max = buffer;s->opaque      = opaque;s->direct      = 0;url_resetbuf(s, write_flag ? AVIO_FLAG_WRITE : AVIO_FLAG_READ);s->write_packet    = write_packet;s->read_packet     = read_packet;s->seek            = seek;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;
}

2.小結

avio_open2函數用于為指定的URL創建一個URLContext，并且打開它，隨后基于這個URLContext創建一個AVIOContext，從而實現對數據源的控制。在AVIOContext中，可以使用read_packet和write_packet進行packet讀寫操作，在URLProtocol中，可以使用url_read和url_write從protocol中讀寫數據。

因為還不是很理解協議層procotol的內容，下面是從別的地方看來的對于read_packet和url_read的理解
url_read和read_packet都是用于讀取數據的函數，但實現方式和應用場景有所不同：
（1）url_read
主要用于從網絡URL中讀取數據。在實際應用中，可能會涉及到更復雜的網絡協議，如http和rtmp。例如librtmp庫中的RTMP packet結構就復雜處理發送和接收過程中的協議解析、分包、合包等復雜邏輯
（2）read_packet
主要用于從數據流當中讀取特定格式的數據包。在FFmpeg中，av_read_frame()函數就是用來讀取AVPacket的。這種情況下，數據處理可能會涉及更加復雜的數據流處理，如解碼、過濾等

CSDN : https://blog.csdn.net/weixin_42877471
Github : https://github.com/DoFulangChen