ffmpeg中为方便对资源进行访问，定义了两个结构体，URLContext中是对具体资源文件进行操作的上下文，URLProtocol则是在将资源进行分类的基础上，对某一类资源操作的函数集，熟悉Linux设备驱动程序的话，很容易联想到file_operations结构体typedef struct URLContext {const AVClass *av_class;    /**< information for av_log(). Set by url_open(). */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 */} URLContext;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()/ffurl_connect()* for those nested protocols.*/int     (*url_open2)(URLContext *h, const char *url, int flags, AVDictionary **options);/*** 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);struct URLProtocol *next;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_shutdown)(URLContext *h, int flags);int priv_data_size;const AVClass *priv_data_class;int flags;int (*url_check)(URLContext *h, int mask);}URLProtocol;在代码中搜索url_open，很容易能够找到ffmpeg中支持的几个协议：URLProtocol ff_bluray_protocol = {.name            = "bluray",.url_close       = bluray_close,.url_open        = bluray_open,.url_read        = bluray_read,.url_seek        = bluray_seek,.priv_data_size  = sizeof(BlurayContext),.priv_data_class = &bluray_context_class,};URLProtocol ff_data_protocol = {.name           = "data",.url_open       = data_open,.url_close      = data_close,.url_read       = data_read,.priv_data_size = sizeof(DataContext),};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,.priv_data_size      = sizeof(FileContext),.priv_data_class     = &file_class,};URLProtocol ff_pipe_protocol = {.name                = "pipe",.url_open            = pipe_open,.url_read            = file_read,.url_write           = file_write,.url_get_file_handle = file_get_handle,.url_check           = file_check,.priv_data_size      = sizeof(FileContext),.priv_data_class     = &pipe_class,};URLProtocol ff_ftp_protocol = {.name                = "ftp",.url_open            = ftp_open,.url_read            = ftp_read,.url_write           = ftp_write,.url_seek            = ftp_seek,.url_close           = ftp_close,.url_get_file_handle = ftp_get_file_handle,.url_shutdown        = ftp_shutdown,.priv_data_size      = sizeof(FTPContext),.priv_data_class     = &ftp_context_class,.flags               = URL_PROTOCOL_FLAG_NETWORK,};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),};URLProtocol ff_httpproxy_protocol = {.name                = "httpproxy",.url_open            = http_proxy_open,.url_read            = http_buf_read,.url_write           = http_proxy_write,.url_close           = http_proxy_close,.url_get_file_handle = http_get_file_handle,.priv_data_size      = sizeof(HTTPContext),.flags               = URL_PROTOCOL_FLAG_NETWORK,};... 还有很多协议，不全部列出了，例如tcp, udp, ...支持这些协议而定义的结构体在av_register_all()函数被调用时而注册，例如：REGISTER_PROTOCOL(FILE,             file);#define REGISTER_PROTOCOL(X, x)                                         \{                                                                   \extern URLProtocol ff_##x##_protocol;                           \if (CONFIG_##X##_PROTOCOL)                                      \ffurl_register_protocol(&ff_##x##_protocol);                \}ffurl_register_protocol(&ff_file_protocol);int ffurl_register_protocol(URLProtocol *protocol){URLProtocol **p;p = &first_protocol;while (*p != NULL)p = &(*p)->next;*p             = protocol;protocol->next = NULL;return 0;}注册到以static URLProtocol *first_protocol = NULL;为头的一个链表中，每一个协议结构体是链表中的一项；以avio_open2为例：int avio_open2(AVIOContext **s, const char *filename, int flags,const AVIOInterruptCB *int_cb, AVDictionary **options){URLContext *h;int err;err = ffurl_open(&h, filename, flags, int_cb, options);if (err < 0)return err;err = ffio_fdopen(s, h);if (err < 0) {ffurl_close(h);return err;}return 0;}ffurl_open--->ffurl_alloc--->static struct URLProtocol *url_find_protocol(const char *filename)url_find_protocol函数会根据filename判断协议，这个函数的定义没太看懂，但是其作用，无非是先判断具体的协议，然后根据判读出来的协议，调用URLProtocol *ffurl_protocol_next(URLProtocol *prev){return prev ? prev->next : first_protocol;}函数，在之前注册的first_protocol链表上找到相应的协议结构体，然后返回给函数url_alloc_for_protocol，在这个函数中，则是根据判断的协议，生成URLContext结构体，uc = av_mallocz(sizeof(URLContext) + strlen(filename) + 1);*puc = uc;在ffurl_open函数中，ffurl_alloc函数返回之后，URLContext **puc结构体就已经被设置完毕，然后在ffurl_open函数中，再调用ret = ffurl_connect(*puc, options);int ffurl_connect(URLContext *uc, AVDictionary **options){int err =uc->prot->url_open2 ? uc->prot->url_open2(uc,uc->filename,uc->flags,options) :uc->prot->url_open(uc, uc->filename, uc->flags);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;}prot是协议，在url_alloc_for_protocol函数中设置的：uc->prot            = up;然后url_open2， url_open都是在协议结构体中定义的函数指针，以"file"协议为例：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,.priv_data_size      = sizeof(FileContext),.priv_data_class     = &file_class,
};url_open2是NULL，则调用url_open函数指针指向的函数file_open：static int file_open(URLContext *h, const char *filename, int flags){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_BINARYaccess |= O_BINARY;#endiffd = avpriv_open(filename, access, 0666);if (fd == -1)return AVERROR(errno);c->fd = fd;h->is_streamed = !fstat(fd, &st) && S_ISFIFO(st.st_mode);return 0;}