FFMPEG函数fill_buffer()

最新推荐文章于 2024-08-23 02:15:00 发布

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最新推荐文章于 2024-08-23 02:15:00 发布

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文章标签： ffmpeg python

原文链接：https://my.oschina.net/u/2326611/blog/805347

本文详细解析了FFMPEG中fill_buffer函数的工作原理及流程，包括如何从文件或网络流读取数据到AVIOContext缓冲区，以及涉及的相关函数如io_read_packet、ffurl_read和retry_transfer_wrapper等。

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FFMPEG 版本3.2 release。

函数fill_buffer(AVIOContext *s)

static void fill_buffer(AVIOContext *s)
{
    int max_buffer_size = s->max_packet_size ?
                          s->max_packet_size : IO_BUFFER_SIZE;
    uint8_t *dst        = s->buf_end - s->buffer + max_buffer_size < s->buffer_size ?
                          s->buf_end : s->buffer;
    int len             = s->buffer_size - (dst - s->buffer);

    /* can't fill the buffer without read_packet, just set EOF if appropriate */
    if (!s->read_packet && s->buf_ptr >= s->buf_end)
        s->eof_reached = 1;

    /* no need to do anything if EOF already reached */
    if (s->eof_reached)
        return;

    if (s->update_checksum && dst == s->buffer) {
        if (s->buf_end > s->checksum_ptr)
            s->checksum = s->update_checksum(s->checksum, s->checksum_ptr,
                                             s->buf_end - s->checksum_ptr);
        s->checksum_ptr = s->buffer;
    }

    /* make buffer smaller in case it ended up large after probing */
    if (s->read_packet && s->orig_buffer_size && s->buffer_size > s->orig_buffer_size) {
        if (dst == s->buffer) {
            int ret = ffio_set_buf_size(s, s->orig_buffer_size);
            if (ret < 0)
                av_log(s, AV_LOG_WARNING, "Failed to decrease buffer size\n");

            s->checksum_ptr = dst = s->buffer;
        }
        av_assert0(len >= s->orig_buffer_size);
        len = s->orig_buffer_size;
    }

    if (s->read_packet)
        len = s->read_packet(s->opaque, dst, len);
    else
        len = 0;
    if (len <= 0) {
        /* do not modify buffer if EOF reached so that a seek back can
           be done without rereading data */
        s->eof_reached = 1;
        if (len < 0)
            s->error = len;
    } else {
        s->pos += len;
        s->buf_ptr = dst;
        s->buf_end = dst + len;
        s->bytes_read += len;
    }
}

函数fill_buffer()的作用

从file或者网络流等读数据到AVIOContext *s的buf。

如果先调用函数ffio_fdopen()，再调用函数fill_buffer()为例，并且假设read_packet()指向函数io_read_packet()，则执行过程如下：

执行函数s->read_packet(s->opaque, dst, len)

该函数的作用为通过指针s->opaque指向的结构体中的变量，获取数据流并复制到dst。

例如：read_packet()指向函数io_read_packet().

函数io_read_packet()位于libavformat/aviobuf.c。

函数ffurl_read()位于libavformat/avio.c

函数retry_transfer_wrapper()位于libavformat/avio.c

如果输入的是文件，例如test.flv，则函数transfer_func(h, buf + len, size - len)指向file_read()。

static int io_read_packet(void *opaque, uint8_t *buf, int buf_size)
{
    AVIOInternal *internal = opaque;
    return ffurl_read(internal->h, buf, buf_size);
}

int ffurl_read(URLContext *h, unsigned char *buf, int size)
{
    if (!(h->flags & AVIO_FLAG_READ))
        return AVERROR(EIO);
    return retry_transfer_wrapper(h, buf, size, 1, h->prot->url_read);
}

static inline int retry_transfer_wrapper(URLContext *h, uint8_t *buf,
                                         int size, int size_min,
                                         int (*transfer_func)(URLContext *h,
                                                              uint8_t *buf,
                                                              int size))
{
    int ret, len;
    int fast_retries = 5;
    int64_t wait_since = 0;

    len = 0;
    while (len < size_min) {
        if (ff_check_interrupt(&h->interrupt_callback))
            return AVERROR_EXIT;
        ret = transfer_func(h, buf + len, size - len);
        if (ret == AVERROR(EINTR))
            continue;
        if (h->flags & AVIO_FLAG_NONBLOCK)
            return ret;
        if (ret == AVERROR(EAGAIN)) {
            ret = 0;
            if (fast_retries) {
                fast_retries--;
            } else {
                if (h->rw_timeout) {
                    if (!wait_since)
                        wait_since = av_gettime_relative();
                    else if (av_gettime_relative() > wait_since + h->rw_timeout)
                        return AVERROR(EIO);
                }
                av_usleep(1000);
            }
        } else if (ret < 1)
            return (ret < 0 && ret != AVERROR_EOF) ? ret : len;
        if (ret) {
            fast_retries = FFMAX(fast_retries, 2);
            wait_since = 0;
        }
        len += ret;
    }
    return len;
}

输入文件transfer_func()回调函数file_read()

static int file_read(URLContext *h, unsigned char *buf, int size)
{
    FileContext *c = h->priv_data;
    int ret;
    size = FFMIN(size, c->blocksize);
    ret = read(c->fd, buf, size);
    if (ret == 0 && c->follow)
        return AVERROR(EAGAIN);
    return (ret == -1) ? AVERROR(errno) : ret;
}

输入是rtmp协议流transfer_func()回调函数rtmp_read()

libavformat/rtmpproto.c

static int rtmp_read(URLContext *s, uint8_t *buf, int size)
{
    RTMPContext *rt = s->priv_data;
    int orig_size = size;
    int ret;

    while (size > 0) {
        int data_left = rt->flv_size - rt->flv_off;

        if (data_left >= size) {
            memcpy(buf, rt->flv_data + rt->flv_off, size);
            rt->flv_off += size;
            return orig_size;
        }
        if (data_left > 0) {
            memcpy(buf, rt->flv_data + rt->flv_off, data_left);
            buf  += data_left;
            size -= data_left;
            rt->flv_off = rt->flv_size;
            return data_left;
        }
        if ((ret = get_packet(s, 0)) < 0)
           return ret;
    }
    return orig_size;
}