参考：
FFmpeg源代码简单分析：avio_open2()

示例工程：
【FFmpeg】调用ffmpeg库实现264软编
【FFmpeg】调用ffmpeg库实现264软解
【FFmpeg】调用ffmpeg库进行RTMP推流和拉流
【FFmpeg】调用ffmpeg库进行SDL2解码后渲染

流程分析：
【FFmpeg】编码链路上主要函数的简单分析
【FFmpeg】解码链路上主要函数的简单分析

结构体分析：
【FFmpeg】AVCodec结构体
【FFmpeg】AVCodecContext结构体
【FFmpeg】AVStream结构体
【FFmpeg】AVFormatContext结构体
【FFmpeg】AVIOContext结构体
【FFmpeg】AVPacket结构体

函数分析：
【FFmpeg】avformat_open_input函数
【FFmpeg】avformat_find_stream_info函数
【FFmpeg】avformat_alloc_output_context2函数

avio_open2函数的内部调用关系为

1.avio_open2

avio_open2函数的定义位于libavformat\avio.c中，功能是打开URL，之后方便进行读写操作。这里的URL是广义的地址，对于文件而言，就是文件的路径，如"C:\xxx\test.flv"，也可以是地址例如"rtmp://127.0.0.1:1935/live/stream"，flag表示控制如何打开url所指示的资源的标志，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*/
// 创建并初始化一个AVIOContext，用于访问url指定的资源
// @note：当url所指示的资源以读+写方式打开时，AVIOContext只能用于写
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调用ffio_open_whitelist进行AVIOContext的创建，而ffio_open_whitelist的实现方式如下，其中主要使用了两个函数：（1）ffurl_open_whitelist根据白名单创建URLContext；（2）ffio_fdopen根据创建的URLContext来初始化AVIOContext。其中，URLContext完成协议的读写操作。

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

1.1 创建URLContext（ffurl_open_whitelist）

该函数的主要内容为两个部分：（1）ffurl_alloc：创建URLContext；（2）ffurl_connect：打开已创建的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;// 创建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;// 打开获得的URLProtocolret = ffurl_connect(*puc, options);if (!ret)return 0;
fail:ffurl_closep(puc);return ret;
}

1.1.1 创建URLContext（ffurl_alloc）

int ffurl_alloc(URLContext **puc, const char *filename, int flags,const AVIOInterruptCB *int_cb)
{const URLProtocol *p = NULL;// 根据filename查找合适的protocolp = url_find_protocol(filename);if (p)// 为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 查找合适的protocol（url_find_protocol）

函数主要工作是调用ffurl_get_protocols获取一个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';// 寻找一个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的定义如下，大致的流程是遍历可选的url_protocol，检查是否位于白名单中且不在黑名单中

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 为查找到的URLProtocol创建URLContext（url_alloc_for_protocol）

在创建URLContext时，会先检查flag和函数对应的关系：（1）网络操作；（2）读取；（3）写入；如果有一个初始化失败，则返回错误

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，但是网络初始化模块失败，则返回错误
#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，则输出错误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，则输出错误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);}// 前续检查结束，创建结构体，并且进行初始化赋值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 打开URLContext（ffurl_connect）

函数的主要功能是打开前面创建的URLContext，首先检查前面创建的URLContext是否被正确初始化，然后调用url_open打开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// 检查URLContext被正确的初始化// 如果设置了黑白名单需要检查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// 对于协议例如file，ftp，rtp，tcp，libRTMP等，使用的是url_open// 对于http协议，使用的是url_open2// 如果是file，url_open会链接到ftp_open()// 如果是libRTMP，url_open会链接到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;
}

例如协议的格式为ftp，会调用ftp_open，定义如下；会调用ftp_connect实现ftp连接的打开

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 根据创建的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*/
// 创建并初始化一个AVIOContext，用于访问URLContext引用的资源
// @note 如果URLContext已经以read + write模式打开，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当中的buffer，如果前面配置max_packet_size，则将其配置为buffer_size// 否则将buffer_size配置为IO_BUFFER_SIZE=32768max_packet_size = h->max_packet_size;if (max_packet_size) {// buffer_size不必超过packet最大size，因为最多填充一个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);// 创建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 创建AVIOContext（avio_alloc_context）

该函数首先使用av_malloc创建FFIOContext，随后通过调用ffio_init_context来初始化，这里做了一个封装，先通过初始化FFIOContext，随后返回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的初始化操作为

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.小结

avio_open2函数用于为指定的URL创建一个URLContext，并且打开它，随后基于这个URLContext创建一个AVIOContext，从而实现对数据源的控制。在AVIOContext中，可以使用read_packet和write_packet进行packet读写操作，在URLProtocol中，可以使用url_read和url_write从protocol中读写数据。

因为还不是很理解协议层procotol的内容，下面是从别的地方看来的对于read_packet和url_read的理解
url_read和read_packet都是用于读取数据的函数，但实现方式和应用场景有所不同：
（1）url_read
主要用于从网络URL中读取数据。在实际应用中，可能会涉及到更复杂的网络协议，如http和rtmp。例如librtmp库中的RTMP packet结构就复杂处理发送和接收过程中的协议解析、分包、合包等复杂逻辑
（2）read_packet
主要用于从数据流当中读取特定格式的数据包。在FFmpeg中，av_read_frame()函数就是用来读取AVPacket的。这种情况下，数据处理可能会涉及更加复杂的数据流处理，如解码、过滤等

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