ffmpeg-URLContext和URLProtocol详解

ffmpeg支持多种输入视频协议，通过URLContext和URLProtocol的高度封装来支持多协议输入。本文的目标：搞清楚输入视频协议的自动适配原理及实现方法。

URLContext和URLProtocol关系图

URLContext

老规矩，不会介绍结构体的所有成员，只关注重点成员。

typedef struct URLContext {
    const AVClass *av_class;    /**< information for av_log(). Set by url_open(). */
    const struct URLProtocol *prot; // 指向某种UROProtocol
    void *priv_data;		   // 一般用来指向某种具体协议的上下文，如，FileContext
    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;

ffurl_context_class

av_class 指向ffurl_context_class，将URLContext的与AVOption联系起来，以支持配置选项。可参考ffmpeg-AVOption详解一文。

url_protocols

prot指向 url_protocols，url_protocols是一个静态的URLProtocol数组，定义了多种协议，如下图：

看到这里，我们似乎可以猜测出来，ffmpeg的多输入协议的支持，就是因为有url_protocols数组。但是，怎样判断输入的url是哪一种协议呢？prot是在哪儿被赋值为url_protocols？在解决以上两个疑问之前，我们还是要先看看URLProtocol这个结构体的定义。

URLProtocol

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);
    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);
    int priv_data_size;
    const AVClass *priv_data_class;
    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;

在libavformat/avio.c文件中，解决了怎样判断输入的url是哪一种协议呢？prot是在哪儿被赋值为url_protocols？这两个问题。

ffurl_alloc

该函数的作用是，根据filename找到匹配的URLProtocol，并分配URLContext。

int ffurl_alloc(URLContext **puc, const char *filename, int flags,
                const AVIOInterruptCB *int_cb)
{
    const URLProtocol *p = NULL;

    p = url_find_protocol(filename); // 找到匹配的url protocol
    if (p)
       return url_alloc_for_protocol(puc, p, filename, flags, int_cb);

    *puc = NULL;
    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 AVERROR_PROTOCOL_NOT_FOUND;
}

url_find_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;
	// 通过filename来判断获取proto_str，以便后面自动适配正确的协议
    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';
	// 获取所有支持的url protocols
    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);

    return NULL;
}

本函数就是输入视频协议自动适配的实现方法。解决了我们前面提到的问题怎样判断输入的url是哪一种协议呢？

ffurl_get_protocols

const URLProtocol **ffurl_get_protocols(const char *whitelist,
                                        const char *blacklist)
{
    const URLProtocol **ret;
    int i, ret_idx = 0;

    ret = av_mallocz_array(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;
}

第7行，创建一个URLProtocol指针数组ret，用来指向url_protocols；

第11~20行的for 循环，使ret中保存url_protocols中的所有格式的协议；

我们再回到url_find_protocol函数，假设filename是一个本地的mp4文件的路径，url_find_protocol会将ff_file_protocol返回。

依据 filename找个匹配的url protocol后，接下来，会调用url_alloc_for_protocol。

url_alloc_for_protocol

本函数会分配一个URLContext。

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_NETWORK
    if (up->flags & URL_PROTOCOL_FLAG_NETWORK && !ff_network_init())
        return AVERROR(EIO);
#endif
    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);
    }
    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);
    }
    // 分配URLContext
    // 这里也有一个小技巧：为filename申请的内存，紧跟着URLContext本身的内存之后
    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);
    //`prot`是在哪儿被赋值为`url_protocols`？就是在这里，up为url_find_protocol自动适配的URLProtocol，例如，ff_file_protocol
    uc->prot            = up;
    uc->flags           = flags;
    uc->is_streamed     = 0; /* default = not streamed */
    uc->max_packet_size = 0; /* default: stream file */
    // 分配具体协议格式的上下文，如FileContext,请看后面的图示
    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) {
            int proto_len= strlen(up->name);
            char *start = strchr(uc->filename, ',');
            *(const AVClass **)uc->priv_data = up->priv_data_class;
            av_opt_set_defaults(uc->priv_data);
            if(!strncmp(up->name, uc->filename, proto_len) && uc->filename + proto_len == 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;
                    } else
                        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);
                    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_NETWORK
    if (up->flags & URL_PROTOCOL_FLAG_NETWORK)
        ff_network_close();
#endif
    return err;
}

有一个小细节要注意的，FileContext的实际内存是在url_alloc_for_protocol函数中分配，其内存地址保存在URLContext->priv_data成员中；而FileContext的大小保存在URLProtocol->priv_data_size成员中。

看到这里，怎样判断输入的url是哪一种协议呢？ prot是在哪儿被赋值为url_protocols？这两个问题已经完美解决。ffmpeg通过URLContext和URLProtocol的高度封装，支持多种输入视频协议的自动适配。

一句话总结就是，URLContext和URLProtocol负责ffmpeg对多种输入协议格式的支撑，解决的是输入兼容性问题。