【FFmpeg】avio_open2函数

【FFmpeg】avio_open2函数

  • 1.avio_open2
    • [1.1 创建URLContext(ffurl_open_whitelist)](#1.1 创建URLContext(ffurl_open_whitelist))
      • [1.1.1 创建URLContext(ffurl_alloc)](#1.1.1 创建URLContext(ffurl_alloc))
        • [1.1.1.1 查找合适的protocol(url_find_protocol)](#1.1.1.1 查找合适的protocol(url_find_protocol))
        • [1.1.1.2 为查找到的URLProtocol创建URLContext(url_alloc_for_protocol)](#1.1.1.2 为查找到的URLProtocol创建URLContext(url_alloc_for_protocol))
      • [1.1.2 打开URLContext(ffurl_connect)](#1.1.2 打开URLContext(ffurl_connect))
    • [1.2 根据创建的URLContext初始化AVIOContext(ffio_fdopen)](#1.2 根据创建的URLContext初始化AVIOContext(ffio_fdopen))
      • [1.2.1 创建AVIOContext(avio_alloc_context)](#1.2.1 创建AVIOContext(avio_alloc_context))
  • 2.小结

参考:
FFmpeg源代码简单分析:avio_open2()

示例工程:
【FFmpeg】调用ffmpeg库实现264软编
【FFmpeg】调用ffmpeg库实现264软解
【FFmpeg】调用ffmpeg库进行RTMP推流和拉流
【FFmpeg】调用ffmpeg库进行SDL2解码后渲染

流程分析:
【FFmpeg】编码链路上主要函数的简单分析
【FFmpeg】解码链路上主要函数的简单分析

结构体分析:
【FFmpeg】AVCodec结构体
【FFmpeg】AVCodecContext结构体
【FFmpeg】AVStream结构体
【FFmpeg】AVFormatContext结构体
【FFmpeg】AVIOContext结构体
【FFmpeg】AVPacket结构体

函数分析:
【FFmpeg】avformat_open_input函数
【FFmpeg】avformat_find_stream_info函数
【FFmpeg】avformat_alloc_output_context2函数

avio_open2函数的内部调用关系为

1.avio_open2

avio_open2函数的定义位于libavformat\avio.c中,功能是打开URL,之后方便进行读写操作。这里的URL是广义的地址,对于文件而言,就是文件的路径,如"C:\xxx\test.flv",也可以是地址例如"rtmp://127.0.0.1:1935/live/stream",flag表示控制如何打开url所指示的资源的标志,如AVIO_FLAG_READ和AVIO_FLAG_WRITE

c 复制代码
/**
 * 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
 */
// 创建并初始化一个AVIOContext,用于访问url指定的资源
// @note:当url所指示的资源以读+写方式打开时,AVIOContext只能用于写
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);
}

avio_open2调用ffio_open_whitelist进行AVIOContext的创建,而ffio_open_whitelist的实现方式如下,其中主要使用了两个函数:(1)ffurl_open_whitelist根据白名单创建URLContext;(2)ffio_fdopen根据创建的URLContext来初始化AVIOContext。其中,URLContext完成协议的读写操作。

c 复制代码
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;
	// 1.根据whitelist创建URLContext
    err = ffurl_open_whitelist(&h, filename, flags, int_cb, options, whitelist, blacklist, NULL);
    if (err < 0)
        return err;
    // 2.根据创建的URLContext初始化AVIOContext
    err = ffio_fdopen(s, h);
    if (err < 0) {
        ffurl_close(h);
        return err;
    }
    return 0;
}

1.1 创建URLContext(ffurl_open_whitelist)

该函数的主要内容为两个部分:(1)ffurl_alloc:创建URLContext;(2)ffurl_connect:打开已创建的URLContext;

c 复制代码
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;
    // 创建URLContext
    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;
	// 打开获得的URLProtocol
    ret = ffurl_connect(*puc, options);

    if (!ret)
        return 0;
fail:
    ffurl_closep(puc);
    return ret;
}

1.1.1 创建URLContext(ffurl_alloc)

c 复制代码
int ffurl_alloc(URLContext **puc, const char *filename, int flags,
                const AVIOInterruptCB *int_cb)
{
    const URLProtocol *p = NULL;
	// 根据filename查找合适的protocol
    p = url_find_protocol(filename);
    if (p)
       // 为protocol分配URLContext
       return url_alloc_for_protocol(puc, p, filename, flags, int_cb);

    *puc = NULL;
    return AVERROR_PROTOCOL_NOT_FOUND;
}
1.1.1.1 查找合适的protocol(url_find_protocol)

函数主要工作是调用ffurl_get_protocols获取一个protocol

c 复制代码
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';
	// 寻找一个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);
    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;
}

ffurl_get_protocol的定义如下,大致的流程是遍历可选的url_protocol,检查是否位于白名单中且不在黑名单中

c 复制代码
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;
}
1.1.1.2 为查找到的URLProtocol创建URLContext(url_alloc_for_protocol)

在创建URLContext时,会先检查flag和函数对应的关系:(1)网络操作;(2)读取;(3)写入;如果有一个初始化失败,则返回错误

c 复制代码
static int url_alloc_for_protocol(URLContext **puc, const URLProtocol *up,
                                  const char *filename, int flags,
                                  const AVIOInterruptCB *int_cb)
{
    URLContext *uc;
    int err;
	// flag中包含NETWORK,但是网络初始化模块失败,则返回错误
#if CONFIG_NETWORK
    if (up->flags & URL_PROTOCOL_FLAG_NETWORK && !ff_network_init())
        return AVERROR(EIO);
#endif
	// flag中包含AVIO_FLAG_READ,但是不包含url_read,则输出错误
    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);
    }
    // flag中包含AVIO_FALG_WRITE,但是不包含url_write,则输出错误
    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);
    }
   	// 前续检查结束,创建结构体,并且进行初始化赋值
    uc = av_mallocz(sizeof(URLContext) + strlen(filename) + 1);
    if (!uc) {
        err = AVERROR(ENOMEM);
        goto fail;
    }
    uc->av_class = &url_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;
                    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);
                    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;
}

1.1.2 打开URLContext(ffurl_connect)

函数的主要功能是打开前面创建的URLContext,首先检查前面创建的URLContext是否被正确初始化,然后调用url_open打开URL

c 复制代码
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 set
    // 检查URLContext被正确的初始化
    // 如果设置了黑白名单需要检查URLContext是否符合要求
    av_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 whitelist

    if ((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;
	// 使用url_open2或者url_open
	// 对于协议例如file,ftp,rtp,tcp,libRTMP等,使用的是url_open
	// 对于http协议,使用的是url_open2
	// 如果是file,url_open会链接到ftp_open()
	// 如果是libRTMP,url_open会链接到rtmp_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);

    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;
}

例如协议的格式为ftp,会调用ftp_open,定义如下;会调用ftp_connect实现ftp连接的打开

c 复制代码
static int ftp_open(URLContext *h, const char *url, int flags)
{
    FTPContext *s = h->priv_data;
    int err;

    ff_dlog(h, "ftp protocol open\n");

    if ((err = ftp_connect(h, url)) < 0)
        goto fail;

    if (ftp_restart(s, 0) < 0) {
        h->is_streamed = 1;
    } else {
        ftp_file_size(s);
        if (s->write_seekable != 1 && flags & AVIO_FLAG_WRITE)
            h->is_streamed = 1;
    }

    return 0;

  fail:
    av_log(h, AV_LOG_ERROR, "FTP open failed\n");
    ftp_close(h);
    return err;
}

1.2 根据创建的URLContext初始化AVIOContext(ffio_fdopen)

c 复制代码
/**
 * Create and initialize a AVIOContext for accessing the
 * resource referenced by the URLContext h.
 * @note When the URLContext h 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.
 * @return >= 0 in case of success, a negative value corresponding to an
 * AVERROR code in case of failure
 */
// 创建并初始化一个AVIOContext,用于访问URLContext引用的资源
// @note 如果URLContext已经以read + write模式打开,那么AVIOContext只能被用于writing
int ffio_fdopen(AVIOContext **sp, URLContext *h)
{
    AVIOContext *s;
    uint8_t *buffer = NULL;
    int buffer_size, max_packet_size;
	// 首先初始化AVIOContext当中的buffer,如果前面配置max_packet_size,则将其配置为buffer_size
	// 否则将buffer_size配置为IO_BUFFER_SIZE=32768
    max_packet_size = h->max_packet_size;
    if (max_packet_size) {
    	// buffer_size不必超过packet最大size,因为最多填充一个packet即可
        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);
	// 创建AVIOContext
    *sp = avio_alloc_context(buffer, buffer_size, h->flags & AVIO_FLAG_WRITE, h,
                             ffurl_read2, ffurl_write2, ffurl_seek2);
    if (!*sp) {
        av_freep(&buffer);
        return AVERROR(ENOMEM);
    }
    s = *sp;
    if (h->protocol_whitelist) {
        s->protocol_whitelist = av_strdup(h->protocol_whitelist);
        if (!s->protocol_whitelist) {
            avio_closep(sp);
            return AVERROR(ENOMEM);
        }
    }
    if (h->protocol_blacklist) {
        s->protocol_blacklist = av_strdup(h->protocol_blacklist);
        if (!s->protocol_blacklist) {
            avio_closep(sp);
            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 = h->prot->url_read_pause;
        s->read_seek  = h->prot->url_read_seek;

        if (h->prot->url_read_seek)
            s->seekable |= AVIO_SEEKABLE_TIME;
    }
    ((FFIOContext*)s)->short_seek_get = ffurl_get_short_seek;
    s->av_class = &ff_avio_class;
    return 0;
}

1.2.1 创建AVIOContext(avio_alloc_context)

该函数首先使用av_malloc创建FFIOContext,随后通过调用ffio_init_context来初始化,这里做了一个封装,先通过初始化FFIOContext,随后返回FFIOContext之中的AVIOContext

c 复制代码
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, const 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;
}

ffio_init_context的初始化操作为

c 复制代码
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, const 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;
}

2.小结

avio_open2函数用于为指定的URL创建一个URLContext,并且打开它,随后基于这个URLContext创建一个AVIOContext,从而实现对数据源的控制。在AVIOContext中,可以使用read_packet和write_packet进行packet读写操作,在URLProtocol中,可以使用url_read和url_write从protocol中读写数据。

因为还不是很理解协议层procotol的内容,下面是从别的地方看来的对于read_packet和url_read的理解

url_read和read_packet都是用于读取数据的函数,但实现方式和应用场景有所不同:

(1)url_read

主要用于从网络URL中读取数据。在实际应用中,可能会涉及到更复杂的网络协议,如http和rtmp。例如librtmp库中的RTMP packet结构就复杂处理发送和接收过程中的协议解析、分包、合包等复杂逻辑

(2)read_packet

主要用于从数据流当中读取特定格式的数据包。在FFmpeg中,av_read_frame()函数就是用来读取AVPacket的。这种情况下,数据处理可能会涉及更加复杂的数据流处理,如解码、过滤等

CSDN : https://blog.csdn.net/weixin_42877471
Github : https://github.com/DoFulangChen

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