在上篇文章中的demo中，main函式的流程裡呼叫的第一個函式就是avformat_open_input()。直觀看來，其最明顯的功能就是制定了要播放的檔名了。但是除了問價名之外還有幾個結構體作為了函式的引數。那麼這個函式的功能是什麼？又是怎麼完成的？一起慢慢研究。

先貼程式碼：

int avformat_open_input(AVFormatContext **ps, const char *filename, AVInputFormat *fmt, AVDictionary **options)
{
    AVFormatContext *s = *ps;
    int ret = 0;
    AVDictionary *tmp = NULL;
    ID3v2ExtraMeta *id3v2_extra_meta = NULL;

    if (!s && !(s = avformat_alloc_context()))
        return AVERROR(ENOMEM);
    if (!s->av_class)
	{
        av_log(NULL, AV_LOG_ERROR, "Input context has not been properly allocated by avformat_alloc_context() and is not NULL either\n");
        return AVERROR(EINVAL);
    }
    if (fmt)
        s->iformat = fmt;

    if (options)
        av_dict_copy(&tmp, *options, 0);

    if ((ret = av_opt_set_dict(s, &tmp)) < 0)
        goto fail;

    if ((ret = init_input(s, filename, &tmp)) < 0)
        goto fail;
    s->probe_score = ret;
    avio_skip(s->pb, s->skip_initial_bytes);

    /* check filename in case an image number is expected */
    if (s->iformat->flags & AVFMT_NEEDNUMBER) 
	{
        if (!av_filename_number_test(filename)) 
		{
            ret = AVERROR(EINVAL);
            goto fail;
        }
    }

    s->duration = s->start_time = AV_NOPTS_VALUE;
    av_strlcpy(s->filename, filename ? filename : "", sizeof(s->filename));

    /* allocate private data */
    if (s->iformat->priv_data_size > 0) 
	{
        if (!(s->priv_data = av_mallocz(s->iformat->priv_data_size)))
		{
            ret = AVERROR(ENOMEM);
            goto fail;
        }
        if (s->iformat->priv_class) 
		{
            *(const AVClass**)s->priv_data = s->iformat->priv_class;
            av_opt_set_defaults(s->priv_data);
            if ((ret = av_opt_set_dict(s->priv_data, &tmp)) < 0)
                goto fail;
        }
    }

    /* e.g. AVFMT_NOFILE formats will not have a AVIOContext */
    if (s->pb)
        ff_id3v2_read(s, ID3v2_DEFAULT_MAGIC, &id3v2_extra_meta);

    if (!(s->flags&AVFMT_FLAG_PRIV_OPT) && s->iformat->read_header)
        if ((ret = s->iformat->read_header(s)) < 0)
            goto fail;

    if (id3v2_extra_meta)
	{
        if (!strcmp(s->iformat->name, "mp3") || !strcmp(s->iformat->name, "aac") ||
            !strcmp(s->iformat->name, "tta")) {
            if((ret = ff_id3v2_parse_apic(s, &id3v2_extra_meta)) < 0)
                goto fail;
        } else
            av_log(s, AV_LOG_DEBUG, "demuxer does not support additional id3 data, skipping\n");
    }
    ff_id3v2_free_extra_meta(&id3v2_extra_meta);

    if ((ret = avformat_queue_attached_pictures(s)) < 0)
        goto fail;

    if (!(s->flags&AVFMT_FLAG_PRIV_OPT) && s->pb && !s->data_offset)
        s->data_offset = avio_tell(s->pb);

    s->raw_packet_buffer_remaining_size = RAW_PACKET_BUFFER_SIZE;

    if (options) {
        av_dict_free(options);
        *options = tmp;
    }
    *ps = s;
    return 0;

fail:
    ff_id3v2_free_extra_meta(&id3v2_extra_meta);
    av_dict_free(&tmp);
    if (s->pb && !(s->flags & AVFMT_FLAG_CUSTOM_IO))
        avio_close(s->pb);
    avformat_free_context(s);
    *ps = NULL;
    return ret;
}
 

函式的各個引數：

1、AVFormatContext **ps：指向使用者提供的結構體，一般可以將這個引數定義指向空然後傳遞到函式中，這樣avformat_open_input函式將會分配這個結構體的記憶體空間並初始化。

2、const char *filename：開啟視訊檔案的檔名。

3、AVInputFormat *fmt：如果這個引數不為空，則指定固定的輸入格式，否則自動檢測輸入格式；一般設為空即可。

4、AVDictionary **options：由AVFormatContext和demuxer-private options組成的字典結構，可設為空。

該函式中呼叫了init_input()函式實現開啟目標檔案和檢測檔案格式等操作，程式碼如下：

static int init_input(AVFormatContext *s, const char *filename, AVDictionary **options)
{
    int ret;
    AVProbeData pd = {filename, NULL, 0};
    int score = AVPROBE_SCORE_RETRY;

    if (s->pb) 
	{
        s->flags |= AVFMT_FLAG_CUSTOM_IO;
        if (!s->iformat)
            return av_probe_input_buffer2(s->pb, &s->iformat, filename, s, 0, s->probesize);
        else if (s->iformat->flags & AVFMT_NOFILE)
            av_log(s, AV_LOG_WARNING, "Custom AVIOContext makes no sense and "
                                      "will be ignored with AVFMT_NOFILE format.\n");
        return 0;
    }

    if ( (s->iformat && s->iformat->flags & AVFMT_NOFILE) ||
        (!s->iformat && (s->iformat = av_probe_input_format2(&pd, 0, &score))))
        return score;

    if ((ret = avio_open2(&s->pb, filename, AVIO_FLAG_READ | s->avio_flags,
                          &s->interrupt_callback, options)) < 0)
        return ret;
    if (s->iformat)
        return 0;
    return av_probe_input_buffer2(s->pb, &s->iformat, filename, s, 0, s->probesize);
}
 

根據條件的不同，該函式內部呼叫了av_probe_input_buffer2()或avio_open2()兩個函式。av_probe_input_buffer2()函式通過分析bitstream來確定輸入的格式，程式碼如下：

int av_probe_input_buffer2(AVIOContext *pb, AVInputFormat **fmt, const char *filename, void *logctx, unsigned int offset, unsigned int max_probe_size)
{
    AVProbeData pd = { filename ? filename : "", NULL, -offset };
    unsigned char *buf = NULL;
    uint8_t *mime_type;
    int ret = 0, probe_size, buf_offset = 0;
    int score = 0;

    if (!max_probe_size) {
        max_probe_size = PROBE_BUF_MAX;
    } else if (max_probe_size > PROBE_BUF_MAX) {
        max_probe_size = PROBE_BUF_MAX;
    } else if (max_probe_size < PROBE_BUF_MIN) {
        av_log(logctx, AV_LOG_ERROR,
               "Specified probe size value %u cannot be < %u\n", max_probe_size, PROBE_BUF_MIN);
        return AVERROR(EINVAL);
    }

    if (offset >= max_probe_size) {
        return AVERROR(EINVAL);
    }

    if (!*fmt && pb->av_class && av_opt_get(pb, "mime_type", AV_OPT_SEARCH_CHILDREN, &mime_type) >= 0 && mime_type) {
        if (!av_strcasecmp(mime_type, "audio/aacp")) {
            *fmt = av_find_input_format("aac");
        }
        av_freep(&mime_type);
    }

    for(probe_size= PROBE_BUF_MIN; probe_size<=max_probe_size && !*fmt;
        probe_size = FFMIN(probe_size<<1, FFMAX(max_probe_size, probe_size+1))) {

        if (probe_size < offset) {
            continue;
        }
        score = probe_size < max_probe_size ? AVPROBE_SCORE_RETRY : 0;

        /* read probe data */
        if ((ret = av_reallocp(&buf, probe_size + AVPROBE_PADDING_SIZE)) < 0)
            return ret;
        if ((ret = avio_read(pb, buf + buf_offset, probe_size - buf_offset)) < 0) {
            /* fail if error was not end of file, otherwise, lower score */
            if (ret != AVERROR_EOF) {
                av_free(buf);
                return ret;
            }
            score = 0;
            ret = 0;            /* error was end of file, nothing read */
        }
        pd.buf_size = buf_offset += ret;
        pd.buf = &buf[offset];

        memset(pd.buf + pd.buf_size, 0, AVPROBE_PADDING_SIZE);

        /* guess file format */
        *fmt = av_probe_input_format2(&pd, 1, &score);
        if(*fmt){
            if(score <= AVPROBE_SCORE_RETRY){ //this can only be true in the last iteration
                av_log(logctx, AV_LOG_WARNING, "Format %s detected only with low score of %d, misdetection possible!\n", (*fmt)->name, score);
            }else
                av_log(logctx, AV_LOG_DEBUG, "Format %s probed with size=%d and score=%d\n", (*fmt)->name, probe_size, score);
        }
    }

    if (!*fmt) {
        av_free(buf);
        return AVERROR_INVALIDDATA;
    }

    /* rewind. reuse probe buffer to avoid seeking */
    ret = ffio_rewind_with_probe_data(pb, &buf, pd.buf_size);

    return ret < 0 ? ret : score;
}

其中呼叫了av_probe_input_format2()函式確定輸入格式，其具體實現在函式av_probe_input_format3()中。

avio_open2()的程式碼如下：

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;
}

該函式用於建立和初始化一個AVIOContext結構體，用於開啟指定的url指向的資源。各個引數的含義：

1、AVIOContext **s：用於指向返回的AVIOContext結構體的指標，呼叫時應設為NULL；

2、const char *url：指定資源的地址；考慮本地播放時，就是檔名的字串的地址；

3、int flags：標誌位，控制檔案開啟的方式；

4、const AVIOInterruptCB *int_cb：在協議層使用的中斷回撥指標；

5、AVDictionary **options：protocol-private options組成的字典元素，設為空即可。

avio_open2()呼叫了ffurl_open()函式，建立一個URLContext結構體用於獲取並開啟url指向的資源（分別呼叫ffurl_alloc()和ffurl_connect()建立和連結URLContext結構體）；呼叫ffio_fdopen()函式建立AVIOContext()結構體並獲取URLContext結構體引用的資源（呼叫avio_alloc_context()實現）；

整個函式的大致框圖如下：

在實際執行的過程中，url_read、url_write等函式都由一個URLProtocal實現，程式碼如下：

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;

針對不同的協議，分別定義了不同的URLPrococol物件來進行具體的操作，如：

檔案協議：

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,
};

ftp協議：

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,
};

http協議：

URLProtocol ff_http_protocol = {
    .name                = "http",
    .url_open2           = http_open,
    .url_read            = http_read,
    .url_write           = http_write,
    .url_seek            = http_seek,
    .url_close           = http_close,
    .url_get_file_handle = http_get_file_handle,
    .url_shutdown        = http_shutdown,
    .priv_data_size      = sizeof(HTTPContext),
    .priv_data_class     = &http_context_class,
    .flags               = URL_PROTOCOL_FLAG_NETWORK,
};

rtmp協議：

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,
};