函數avio_read()執行流程

時間 2019-11-13

標籤函數 avio read 執行流程简体版

原文原文鏈接

以讀取flv文件爲例，查看avio_read()函數的執行流程。app

在av_probe_input_buffer2()函數會調用avio_read()

函數avio_read()

int avio_read(AVIOContext *s, unsigned char *buf, int size)
{
    int len, size1;

    size1 = size;
    while (size > 0) {
        len = FFMIN(s->buf_end - s->buf_ptr, size);
        if (len == 0 || s->write_flag) {
            if((s->direct || size > s->buffer_size) && !s->update_checksum) {
                // bypass the buffer and read data directly into buf
                if(s->read_packet)
                    len = s->read_packet(s->opaque, buf, size);

                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;
                    break;
                } else {
                    s->pos += len;
                    s->bytes_read += len;
                    size -= len;
                    buf += len;
                    // reset the buffer
                    s->buf_ptr = s->buffer;
                    s->buf_end = s->buffer/* + len*/;
                }
            } else {
                fill_buffer(s);
                len = s->buf_end - s->buf_ptr;
                if (len == 0)
                    break;
            }
        } else {
            memcpy(buf, s->buf_ptr, len);
            buf += len;
            s->buf_ptr += len;
            size -= len;
        }
    }
    if (size1 == size) {
        if (s->error)      return s->error;
        if (avio_feof(s))  return AVERROR_EOF;
    }
    return size1 - size;
}

開始size爲2048，s->buf_end和s->buf_ptr相等，並且s->direct爲0，s->buffer_size爲32768，因此執行fill_buffer()函數

函數fill_buffer()

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;
    }
}

其中max_buffer_size爲32768.ui

剛進入函數fill_buffer()時s內容：url

(gdb) p *s
$1 = {av_class = 0x1884b80, buffer = 0x2768740 "", buffer_size = 32768, buf_ptr = 0x2768740 "", buf_end = 0x2768740 "", opaque = 0x2768620, read_packet = 0x5f5190 <io_read_packet>, write_packet = 0x5f5180 <io_write_packet>, 
  seek = 0x5f5170 <io_seek>, pos = 0, must_flush = 0, eof_reached = 0, write_flag = 0, max_packet_size = 0, checksum = 0, checksum_ptr = 0x0, update_checksum = 0, error = 0, read_pause = 0x5f4670 <io_read_pause>, 
  read_seek = 0x5f4690 <io_read_seek>, seekable = 1, maxsize = 0, direct = 0, bytes_read = 0, seek_count = 0, writeout_count = 0, orig_buffer_size = 32768, short_seek_threshold = 4096, protocol_whitelist = 0x2768380 "file,crypto", 
  protocol_blacklist = 0x0, write_data_type = 0, ignore_boundary_point = 0, current_type = AVIO_DATA_MARKER_UNKNOWN, last_time = -9223372036854775808}

其中s->read_packet()回調函數io_read_packet()spa

函數io_read_packet()

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

其中h->prot內容以下：code

(gdb) p *(h->prot)
$3 = {name = 0x189f6fe "file", url_open = 0x728790 <file_open>, url_open2 = 0, url_accept = 0, url_handshake = 0, url_read = 0x728740 <file_read>, url_write = 0x728710 <file_write>, url_seek = 0x728690 <file_seek>, 
  url_close = 0x728680 <file_close>, url_read_pause = 0, url_read_seek = 0, url_get_file_handle = 0x728190 <file_get_handle>, url_get_multi_file_handle = 0, url_shutdown = 0, priv_data_size = 32, priv_data_class = 0x18c5160, flags = 0, 
  url_check = 0x7285c0 <file_check>, url_open_dir = 0x728590 <file_open_dir>, url_read_dir = 0x728320 <file_read_dir>, url_close_dir = 0x728300 <file_close_dir>, url_delete = 0x728290 <file_delete>, url_move = 0x728230 <file_move>, 
  default_whitelist = 0x18c4fc0 "file,crypto"}

即url_read() 回調函數file_read()。get

函數retry_transfer_wrapper()

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;
}

其中h->interrupt_callback內容
$6 = {callback = 0x4861d0 <decode_interrupt_cb>, opaque = 0x0}input

函數transfer_func()回調函數file_read()回調函數

函數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;
}

從文件讀取數據後，在函數fill_buffer()末尾打印sit

(gdb) p *s
$10 = {av_class = 0x1884b80, buffer = 0x2768740 "FLV\001\005", buffer_size = 32768, buf_ptr = 0x2768740 "FLV\001\005", buf_end = 0x2770740 "", opaque = 0x2768620, read_packet = 0x5f5190 <io_read_packet>, 
  write_packet = 0x5f5180 <io_write_packet>, seek = 0x5f5170 <io_seek>, pos = 32768, must_flush = 0, eof_reached = 0, write_flag = 0, max_packet_size = 0, checksum = 0, checksum_ptr = 0x0, update_checksum = 0, error = 0, 
  read_pause = 0x5f4670 <io_read_pause>, read_seek = 0x5f4690 <io_read_seek>, seekable = 1, maxsize = 0, direct = 0, bytes_read = 32768, seek_count = 0, writeout_count = 0, orig_buffer_size = 32768, short_seek_threshold = 4096, 
  protocol_whitelist = 0x2768380 "file,crypto", protocol_blacklist = 0x0, write_data_type = 0, ignore_boundary_point = 0, current_type = AVIO_DATA_MARKER_UNKNOWN, last_time = -9223372036854775808}

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