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_BINARY
access |= O_BINARY;
#endif
fd = 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;
}
