FFmpeg源码：url_find_protocol函数分析

一、url_find_protocol函数的定义

url_find_protocol函数定义在FFmpeg源码（本文演示用的FFmpeg源码版本为7.0.1）的源文件libavformat/avio.c中：

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"); else av_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; }

该函数在avformat_open_input函数内部被调用。其作用是：根据形参filename查找并返回匹配的流媒体协议类型 ；如果没有匹配的协议但filename符合“file”协议的url格式，那就返回“file”协议类型（在FFmpeg源码中，“文件”也被当作一种协议类型）；如果都不符合则返回NULL。

形参filename：输入型参数，包含流媒体协议的名称信息。比如RTP对应的filename为：rtp://XXX，UDP对应的filename为：udp://XXX。

返回值：匹配到的流媒体协议类型，为URLProtocol类型。该结构体声明在libavformat/url.h中：

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)(void *urlcontext, int pause); int64_t (*url_read_seek)(void *urlcontext, 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;

URLProtocol是FFmpeg协议解析器的注册结构体。每个流媒体协议都有一个与之对应的URLProtocol结构，其定义了协议的名称、流媒体url的打开、读取、写入等操作的函数指针。

比如RTP这种流媒体协议，其对应的URLProtocol结构为：

const URLProtocol ff_rtp_protocol = { .name = "rtp", .url_open = rtp_open, .url_read = rtp_read, .url_write = rtp_write, .url_close = rtp_close, .url_get_file_handle = rtp_get_file_handle, .url_get_multi_file_handle = rtp_get_multi_file_handle, .priv_data_size = sizeof(RTPContext), .flags = URL_PROTOCOL_FLAG_NETWORK, .priv_data_class = &rtp_class, };

“file”协议对应的URLProtocol结构为：

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" };

“rtmp”协议对应的URLProtocol结构为：

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, };

二、url_find_protocol函数的内部实现分析

url_find_protocol函数中首先通过下面代码块将形参filename中的流媒体协议名称拷贝到数组proto_str中。如果没有匹配的协议但filename符合“file”协议的url格式，那就拷贝“file”，表示是“文件”协议类型：

if (filename[proto_len] != ':' && (strncmp(filename, "subfile,", 8) || !strchr(filename + proto_len + 1, ':')) || is_dos_path(filename)) strcpy(proto_str, "file"); else av_strlcpy(proto_str, filename, FFMIN(proto_len + 1, sizeof(proto_str)));

拷贝流媒体协议名称到数组proto_nested中。关于av_strlcpy函数的用法可以参考：《FFmpeg源码：av_strlcpy函数分析》：

av_strlcpy(proto_nested, proto_str, sizeof(proto_nested)); if ((ptr = strchr(proto_nested, '+'))) *ptr = '\0';

返回指向全局数组url_protocols开头的二级指针：

protocols = ffurl_get_protocols(NULL, NULL); if (!protocols) return NULL;

ffurl_get_protocols函数的定义如下。可以看到其内部遍历了全局数组url_protocols：

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; }

全局数组url_protocols定义在libavformat/protocol_list.c中，该数组中的每个元素都对应一种URLProtocol结构，也就是说数组中的每个元素都对应一种流媒体协议（RTP、FILE、RTMP等)：

static const URLProtocol * const url_protocols[] = { &ff_async_protocol, &ff_cache_protocol, &ff_concat_protocol, &ff_concatf_protocol, &ff_crypto_protocol, &ff_data_protocol, &ff_fd_protocol, &ff_ffrtmphttp_protocol, &ff_file_protocol, &ff_ftp_protocol, &ff_gopher_protocol, &ff_hls_protocol, &ff_http_protocol, &ff_httpproxy_protocol, &ff_icecast_protocol, &ff_mmsh_protocol, &ff_mmst_protocol, &ff_md5_protocol, &ff_pipe_protocol, &ff_prompeg_protocol, &ff_rtmp_protocol, &ff_rtmpt_protocol, &ff_rtp_protocol, &ff_srtp_protocol, &ff_subfile_protocol, &ff_tee_protocol, &ff_tcp_protocol, &ff_udp_protocol, &ff_udplite_protocol, &ff_unix_protocol, NULL };

url_find_protocol函数中，通过遍历全局数组url_protocols，比较协议名称，查找出该流媒体协议匹配的URLProtocol结构。如果查找到了，那就赋值给指针up，作为url_find_protocol函数的返回值返回：

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; } }

如果查找不到匹配的流媒体协议，那就打印警报日志，返回NULL：

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;