参考链接

FFmpeg源代码简单分析：avio_open2()_雷霄骅的博客-CSDN博客_avio_open

avio_open2()

该函数用于打开FFmpeg的输入输出文件
avio_open2()的声明位于libavformat\avio.h文件中，如下所示。

/*** 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*/
int avio_open2(AVIOContext **s, const char *url, int flags,const AVIOInterruptCB *int_cb, AVDictionary **options);

avio_open2()函数参数的含义如下：

s：函数调用成功之后创建的AVIOContext结构体。
url：输入输出协议的地址（文件也是一种“广义”的协议，对于文件来说就是文件的路径）
flags：打开地址的方式。可以选择只读，只写，或者读写。取值如下。

/*** @name URL open modes* The flags argument to avio_open must be one of the following* constants, optionally ORed with other flags.* @{*/
#define AVIO_FLAG_READ  1                                      /**< read-only */
#define AVIO_FLAG_WRITE 2                                      /**< write-only */
#define AVIO_FLAG_READ_WRITE (AVIO_FLAG_READ|AVIO_FLAG_WRITE)  /**< read-write pseudo flag */

AVIO_FLAG_READ：只读。
AVIO_FLAG_WRITE：只写。
AVIO_FLAG_READ_WRITE：读写。
int_cb：目前还没有用过。
options：目前还没有用过
具体使用的参考链接：最简单的基于FFMPEG的视频编码器（YUV编码为H.264）_雷霄骅的博客-CSDN博客_最简单的基于ffmpeg

函数调用结构图

avio_open()

有一个和avio_open2()“长得很像”的函数avio_open()，应该是avio_open2()的早期版本。
avio_open()比avio_open2()少了最后2个参数。而它前面几个参数的含义和avio_open2()是一样的。
从源代码中可以看出，avio_open()内部调用了avio_open2()，并且把avio_open2()的后2个参数设置成了NULL，因此它的功能实际上和avio_open2()是一样的。
avio_open()源代码如下所示。

int avio_open(AVIOContext **s, const char *filename, int flags)
{return avio_open2(s, filename, flags, NULL, NULL);
}

avio_open2()

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);
}

从avio_open2()的源代码可以看出，它主要调用了：ffio_open_whitelist
AVIOContext则是在URLContext的读写函数外面加上了一层“包装”（通过retry_transfer_wrapper()函数）

ffio_open_whitelist

ffio_open_whitelist 主要调用了ffurl_open_whitelist、ffio_fdopen()和ffurl_close()
根据URLContext初始化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;err = ffurl_open_whitelist(&h, filename, flags, int_cb, options, whitelist, blacklist, NULL);if (err < 0)return err;err = ffio_fdopen(s, h);if (err < 0) {ffurl_close(h);return err;}return 0;
}

ffurl_open_whitelist

初始化URLContext
替代原有的 ffurl_open函数

int ffurl_open_whitelist(URLContext **puc, const char *filename, int flags,const AVIOInterruptCB *int_cb, AVDictionary **options,const char *whitelist, const char* blacklist,URLContext *parent)
{AVDictionary *tmp_opts = NULL;AVDictionaryEntry *e;int ret = ffurl_alloc(puc, filename, flags, int_cb);if (ret < 0)return ret;if (parent) {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;ret = ffurl_connect(*puc, options);if (!ret)return 0;
fail:ffurl_closep(puc);return ret;
}

从代码中可以看出，ffurl_open()主要调用了2个函数：ffurl_alloc()和ffurl_connect()。
ffurl_alloc()用于查找合适的URLProtocol，并创建一个URLContext；
ffurl_connect()用于打开获得的URLProtocol。
ffurl_open() 已经被弃用

ffurl_alloc()

ffurl_alloc()的定义位于libavformat\avio.c中，如下所示。

int ffurl_alloc(URLContext **puc, const char *filename, int flags,const AVIOInterruptCB *int_cb)
{const URLProtocol *p = NULL;p = url_find_protocol(filename);if (p)return url_alloc_for_protocol(puc, p, filename, flags, int_cb);*puc = NULL;return AVERROR_PROTOCOL_NOT_FOUND;
}

从代码中可以看出，ffurl_alloc()主要调用了2个函数：
url_find_protocol()根据文件路径查找合适的URLProtocol
url_alloc_for_protocol()为查找到的URLProtocol创建URLContext。

url_find_protocol()

先来看一下url_find_protocol()函数，定义如下所示。

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

#define URL_SCHEME_CHARS                        \"abcdefghijklmnopqrstuvwxyz"                \"ABCDEFGHIJKLMNOPQRSTUVWXYZ"                \"0123456789+-."

url_find_protocol()函数表明了FFmpeg根据文件路径猜测协议的方法。
该函数首先根据strspn()函数查找字符串中第一个“非字母或数字”的字符的位置，并保存在proto_len中。
一般情况下，协议URL中都是包含“:”的，比如说RTMP的URL格式是“rtmp://xxx…”，UDP的URL格式是“udp://…”，HTTP的URL格式是“http://...”。因此，一般情况下proto_len的数值就是“:”的下标（代表了“:”前面的协议名称的字符的个数，例如rtmp://的proto_len为4）。
接下来函数将filename的前proto_len个字节拷贝至proto_str字符串中。
这个地方比较纠结，源代码中av_strlcpy()函数的第3个参数size写的字符串的长度是（proto_len+1），但是查了一下av_strlcpy()的定义，发现该函数至多拷贝（size-1）个字符。这么一涨一消，最终还是拷贝了proto_len个字节。例如RTMP协议就拷贝了“rtmp”，UDP协议就拷贝了“udp”。
av_strlcpy()是FFMpeg的一个工具函数，声明位于libavutil\avstring.h，如下所示。

size_t av_strlcpy(char *dst, const char *src, size_t size)
{size_t len = 0;while (++len < size && *src)*dst++ = *src++;if (len <= size)*dst = 0;return len + strlen(src) - 1;
}

这里有一种例外，那就是文件路径。“文件”在FFmpeg中也是一种“协议”，并且前缀是“file”。也就是标准的文件路径应该是“file://...”格式的。但是这太不符合我们一般人的使用习惯，我们一般是不会在文件路径前面加上“file”协议名称的。所以该函数采取的方法是：一旦检测出来输入的URL是文件路径而不是网络协议，就自动向proto_str中拷贝“file”。
strcpy(proto_str, "file");
其中判断文件路径那里有一个很复杂的if()语句。根据我的理解，“||”前面的语句用于判断是否是相对文件路径，“||”后面的语句用于判断是否是绝对路径。
判断绝对路径的时候用到了一个函数is_dos_path()，定义位于libavformat\os_support.h，如下所示。

static inline int is_dos_path(const char *path)
{
#if HAVE_DOS_PATHSif (path[0] && path[1] == ':')return 1;
#endifreturn 0;
}

注意“&&”优先级低于“==”。如果文件路径第1个字符不为空（一般情况下是盘符）而且第2个字符为“:”，就认为它是绝对文件路径。

url_alloc_for_protocol()

url_alloc_for_protocol()的定义位于libavformat\avio.c中，如下所示。

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_NETWORKif (up->flags & URL_PROTOCOL_FLAG_NETWORK && !ff_network_init())return AVERROR(EIO);
#endifif ((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;} elseret= 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_NETWORKif (up->flags & URL_PROTOCOL_FLAG_NETWORK)ff_network_close();
#endifreturn err;
}

url_alloc_for_protocol()完成了以下步骤：首先，检查输入的URLProtocol是否支持指定的flag。
比如flag中如果指定了AVIO_FLAG_READ，则URLProtocol中必须包含url_read()；
如果指定了AVIO_FLAG_WRITE，则URLProtocol中必须包含url_write()。
在检查无误之后，接着就可以调用av_mallocz()为即将创建的URLContext分配内存了。接下来基本上就是各种赋值工作，在这里不再详细记录。

ffurl_connect()

ffurl_connect()用于打开获得的URLProtocol。该函数的定义位于libavformat\avio.c中，如下所示。

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 setav_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;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;
}

URLProtocol中是否包含url_open2()？如果包含的话，就调用url_open2()，否则就调用url_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);

url_open()本身是URLProtocol的一个函数指针，这个地方根据不同的协议调用的url_open()具体实现函数也是不一样的，例如file协议的url_open()对应的是file_open()，而file_open()最终调用了_wsopen()，_sopen()（Windows下）或者open()（Linux下，类似于fopen()）这样的系统中打开文件的API函数；
而libRTMP的url_open()对应的是rtmp_open()，而rtmp_open()最终调用了libRTMP的API函数RTMP_Init()，RTMP_SetupURL()，RTMP_Connect() 以及RTMP_ConnectStream()。
通用转向与偏特化

ffio_fdopen()

ffio_fdopen()使用已经获得的URLContext初始化AVIOContext。
它的函数定义位于libavformat\aviobuf.c中，如下所示。

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;}buffer = av_malloc(buffer_size);if (!buffer)return AVERROR(ENOMEM);*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);}(*s)->protocol_whitelist = av_strdup(h->protocol_whitelist);if (!(*s)->protocol_whitelist && h->protocol_whitelist) {avio_closep(s);return AVERROR(ENOMEM);}(*s)->protocol_blacklist = av_strdup(h->protocol_blacklist);if (!(*s)->protocol_blacklist && h->protocol_blacklist) {avio_closep(s);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 = (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;}((FFIOContext*)(*s))->short_seek_get = (int (*)(void *))ffurl_get_short_seek;(*s)->av_class = &ff_avio_class;return 0;
}

ffio_fdopen()函数首先初始化AVIOContext中的Buffer。
如果URLContext中设置了max_packet_size，则将Buffer的大小设置为max_packet_size。如果没有设置的话（似乎大部分URLContext都没有设置该值），则会分配IO_BUFFER_SIZE个字节给Buffer。
IO_BUFFER_SIZE取值为32768。

avio_alloc_context()

ffio_fdopen()接下来会调用avio_alloc_context()初始化一个AVIOContext。
avio_alloc_context()本身是一个FFmpeg的API函数。它的声明位于libavformat\avio.h中，如下所示。

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;
}

avio_alloc_context()看上去参数很多，但实际上并不复杂。先简单解释一下它各个参数的含义：
- buffer：AVIOContext中的Buffer。
- buffer_size：AVIOContext中的Buffer的大小。
- write_flag：设置为1则Buffer可写；否则Buffer只可读。
- opaque：用户自定义数据。
- read_packet()：读取外部数据，填充Buffer的函数。
- write_packet()：向Buffer中写入数据的函数。
- seek()：用于Seek的函数。
该函数成功执行的话则会返回一个创建好的AVIOContext。
该函数代码很简单：首先调用av_mallocz()为AVIOContext分配一块内存空间，然后基本上将所有输入参数传递给ffio_init_context()。

ffio_init_context()

ffio_init_context()的定义如下。

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));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;
#if FF_API_AVIOCONTEXT_WRITTEN
FF_DISABLE_DEPRECATION_WARNINGSs->written               = 0;
FF_ENABLE_DEPRECATION_WARNINGS
#endif
}

ffurl_read()，ffurl_write()，ffurl_seek()

现在我们再回到ffio_fdopen()，会发现它初始化AVIOContext的结构体的时候，首先将自己分配的Buffer设置为该AVIOContext的Buffer；
然后将URLContext作为用户自定义数据（对应AVIOContext的opaque变量）提供给该AVIOContext；
最后分别将3个函数作为该AVIOContext的读，写，跳转函数：
ffurl_read()，ffurl_write()，ffurl_seek()。
下面我们选择一个ffurl_read()看看它的定义。
ffurl_read()的定义位于libavformat\avio.c，如下所示。

int ffurl_read(URLContext *h, unsigned char *buf, int size)
{if (!(h->flags & AVIO_FLAG_READ))return AVERROR(EIO);return retry_transfer_wrapper(h, buf, size, 1, h->prot->url_read);
}

该函数先判断了一下输入的URLContext是否支持“读”操作，接着调用了一个函数：retry_transfer_wrapper()。
如果我们看ffurl_write()的代码，如下所示。

int ffurl_write(URLContext *h, const unsigned char *buf, int size)
{if (!(h->flags & AVIO_FLAG_WRITE))return AVERROR(EIO);/* avoid sending too big packets */if (h->max_packet_size && size > h->max_packet_size)return AVERROR(EIO);return retry_transfer_wrapper(h, (unsigned char *)buf, size, size,(int (*)(struct URLContext *, uint8_t *, int))h->prot->url_write);
}

会发现他也调用了同样的一个函数retry_transfer_wrapper()。
唯一的不同在于ffurl_read()调用retry_transfer_wrapper()的时候，最后一个参数是URLProtocol的url_read()，而ffurl_write()调用retry_transfer_wrapper()的时候，最后一个参数是URLProtocol的url_write()。
下面我们看一下retry_transfer_wrapper()的定义，位于libavformat\avio.c，如下所示。

static inline int retry_transfer_wrapper(URLContext *h, uint8_t *buf,int size, int size_min,int (*transfer_func)(URLContext *h,uint8_t *buf,int size))
{int ret, len;int fast_retries = 5;int64_t wait_since = 0;len = 0;while (len < size_min) {if (ff_check_interrupt(&h->interrupt_callback))return AVERROR_EXIT;ret = transfer_func(h, buf + len, size - len);if (ret == AVERROR(EINTR))continue;if (h->flags & AVIO_FLAG_NONBLOCK)return ret;if (ret == AVERROR(EAGAIN)) {ret = 0;if (fast_retries) {fast_retries--;} else {if (h->rw_timeout) {if (!wait_since)wait_since = av_gettime_relative();else if (av_gettime_relative() > wait_since + h->rw_timeout)return AVERROR(EIO);}av_usleep(1000);}} else if (ret == AVERROR_EOF)return (len > 0) ? len : AVERROR_EOF;else if (ret < 0)return ret;if (ret) {fast_retries = FFMAX(fast_retries, 2);wait_since = 0;}len += ret;}return len;
}

从代码中可以看出，它的核心实际上是调用了一个名称为transfer_func()的函数。而该函数就是retry_transfer_wrapper()的第四个参数。该函数实际上是对URLProtocol的读写操作中的错误进行了一些“容错”处理，可以让数据的读写更加的稳定
avio_alloc_context()执行完毕后，ffio_fdopen()函数的工作就基本完成了，avio_open2()的工作也就做完了。

URLProtocol和URLContext

这两个结构体在FFmpeg的早期版本的SDK中是定义在头文件中可以直接使用的。但是近期的FFmpeg的SDK中已经找不到这两个结构体的定义了。FFmpeg把这两个结构体移动到了源代码的内部，变成了内部结构体。
URLProtocol的定义位于libavformat\url.h，如下所示。

typedef struct URLProtocol {const char *name;int     (*url_open)( URLContext *h, const char *url, int flags);/*** This callback is to be used by protocols which open further nested* protocols. options are then to be passed to ffurl_open_whitelist()* or ffurl_connect() for those nested protocols.*/int     (*url_open2)(URLContext *h, const char *url, int flags, AVDictionary **options);int     (*url_accept)(URLContext *s, URLContext **c);int     (*url_handshake)(URLContext *c);/*** Read data from the protocol.* If data is immediately available (even less than size), EOF is* reached or an error occurs (including EINTR), return immediately.* Otherwise:* In non-blocking mode, return AVERROR(EAGAIN) immediately.* In blocking mode, wait for data/EOF/error with a short timeout (0.1s),* and return AVERROR(EAGAIN) on timeout.* Checking interrupt_callback, looping on EINTR and EAGAIN and until* enough data has been read is left to the calling function; see* retry_transfer_wrapper in avio.c.*/int     (*url_read)( URLContext *h, unsigned char *buf, int size);int     (*url_write)(URLContext *h, const unsigned char *buf, int size);int64_t (*url_seek)( URLContext *h, int64_t pos, int whence);int     (*url_close)(URLContext *h);int (*url_read_pause)(URLContext *h, int pause);int64_t (*url_read_seek)(URLContext *h, int stream_index,int64_t timestamp, int flags);int (*url_get_file_handle)(URLContext *h);int (*url_get_multi_file_handle)(URLContext *h, int **handles,int *numhandles);int (*url_get_short_seek)(URLContext *h);int (*url_shutdown)(URLContext *h, int flags);const AVClass *priv_data_class;int priv_data_size;int flags;int (*url_check)(URLContext *h, int mask);int (*url_open_dir)(URLContext *h);int (*url_read_dir)(URLContext *h, AVIODirEntry **next);int (*url_close_dir)(URLContext *h);int (*url_delete)(URLContext *h);int (*url_move)(URLContext *h_src, URLContext *h_dst);const char *default_whitelist;
} URLProtocol;

从URLProtocol的定义可以看出，其中包含了用于协议读写的函数指针。例如：
- url_open()：打开协议。
- url_read()：读数据。
- url_write()：写数据。
- url_close()：关闭协议。
每种具体的协议都包含了一个URLProtocol结构体，例如：

FILE

FILE协议（“文件”在FFmpeg中也被当做一种协议）的结构体ff_file_protocol的定义如下所示（位于libavformat\file.c）

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"
};

在使用FILE协议进行读写的时候，调用url_open()实际上就是调用了file_open()函数，这里限于篇幅不再对file_open()的源代码进行分析。
file_open()函数实际上调用了系统的打开文件函数open()。
同理，调用url_read()实际上就是调用了file_read()函数；file_read()函数实际上调用了系统的读取文件函数read()。
url_write()，url_seek()等函数的道理都是一样的。

LibRTMP

LibRTMP协议的结构体ff_librtmp_protocol的定义如下所示（位于libavformat\librtmp.c）

RTMP_CLASS(rtmp)
const URLProtocol ff_librtmp_protocol = {.name                = "rtmp",.url_open            = rtmp_open,.url_read            = rtmp_read,.url_write           = rtmp_write,.url_close           = rtmp_close,.url_read_pause      = rtmp_read_pause,.url_read_seek       = rtmp_read_seek,.url_get_file_handle = rtmp_get_file_handle,.priv_data_size      = sizeof(LibRTMPContext),.priv_data_class     = &librtmp_class,.flags               = URL_PROTOCOL_FLAG_NETWORK,
};RTMP_CLASS(rtmpt)
const URLProtocol ff_librtmpt_protocol = {.name                = "rtmpt",.url_open            = rtmp_open,.url_read            = rtmp_read,.url_write           = rtmp_write,.url_close           = rtmp_close,.url_read_pause      = rtmp_read_pause,.url_read_seek       = rtmp_read_seek,.url_get_file_handle = rtmp_get_file_handle,.priv_data_size      = sizeof(LibRTMPContext),.priv_data_class     = &librtmpt_class,.flags               = URL_PROTOCOL_FLAG_NETWORK,
};RTMP_CLASS(rtmpe)
const URLProtocol ff_librtmpe_protocol = {.name                = "rtmpe",.url_open            = rtmp_open,.url_read            = rtmp_read,.url_write           = rtmp_write,.url_close           = rtmp_close,.url_read_pause      = rtmp_read_pause,.url_read_seek       = rtmp_read_seek,.url_get_file_handle = rtmp_get_file_handle,.priv_data_size      = sizeof(LibRTMPContext),.priv_data_class     = &librtmpe_class,.flags               = URL_PROTOCOL_FLAG_NETWORK,
};RTMP_CLASS(rtmpte)
const URLProtocol ff_librtmpte_protocol = {.name                = "rtmpte",.url_open            = rtmp_open,.url_read            = rtmp_read,.url_write           = rtmp_write,.url_close           = rtmp_close,.url_read_pause      = rtmp_read_pause,.url_read_seek       = rtmp_read_seek,.url_get_file_handle = rtmp_get_file_handle,.priv_data_size      = sizeof(LibRTMPContext),.priv_data_class     = &librtmpte_class,.flags               = URL_PROTOCOL_FLAG_NETWORK,
};RTMP_CLASS(rtmps)
const URLProtocol ff_librtmps_protocol = {.name                = "rtmps",.url_open            = rtmp_open,.url_read            = rtmp_read,.url_write           = rtmp_write,.url_close           = rtmp_close,.url_read_pause      = rtmp_read_pause,.url_read_seek       = rtmp_read_seek,.url_get_file_handle = rtmp_get_file_handle,.priv_data_size      = sizeof(LibRTMPContext),.priv_data_class     = &librtmps_class,.flags               = URL_PROTOCOL_FLAG_NETWORK,
};

URLContext

URLContext的定义也位于libavformat\url.h，如下所示。

typedef struct URLContext {const AVClass *av_class;    /**< information for av_log(). Set by url_open(). */const struct URLProtocol *prot;void *priv_data;char *filename;             /**< specified URL */int flags;int max_packet_size;        /**< if non zero, the stream is packetized with this max packet size */int is_streamed;            /**< true if streamed (no seek possible), default = false */int is_connected;AVIOInterruptCB interrupt_callback;int64_t rw_timeout;         /**< maximum time to wait for (network) read/write operation completion, in mcs */const char *protocol_whitelist;const char *protocol_blacklist;int min_packet_size;        /**< if non zero, the stream is packetized with this min packet size */
} URLContext;