ffmpeg解析mpeg2ts时的数据结构分析

FFmpeg对输入数据的接收是通过FIFO实现的。

下面将对UDP实时TS流进行数据结构和流程分析

1. 相关的数据结构的定义

顶层的所有URL数据结构定义

typedef struct URLContext {
  const AVClass *av_class; ///< information for av_log(). Set by url_open().
  struct URLProtocol *prot;
  int flags;
  int is_streamed; /**< true if streamed (no seek possible), default = false */
  int max_packet_size;
         /**< if non zero, the stream is packetized with this max packet size */

  void *priv_data;

  char *filename; /**< specified URL */
  int is_connected;
} URLContext;

FIFO的定义:

typedef struct AVFifoBuffer {
    uint8_t *buffer;
    uint8_t *rptr, *wptr, *end;
    uint32_t rndx, wndx;
} AVFifoBuffer;

UDP数据结构定义

typedef struct {
  int udp_fd;
  int ttl;
  int buffer_size;
  int is_multicast;

  int local_port;
  int reuse_socket;
  struct sockaddr_storage dest_addr;
  int dest_addr_len;
  int is_connected;

  /* Circular Buffer variables for use in UDP receive code */
  int circular_buffer_size;
  AVFifoBuffer *fifo;
  int circular_buffer_error;

#if HAVE_PTHREADS
    pthread_t circular_buffer_thread;
#endif
} UDPContext;

2. 初始化UDP的socket，并接收数据

/* put it in UDP context ,return non zero if error */
static int udp_open(URLContext *h, const char *uri, int flags)
{
  ...
  UDPContext *s = NULL;

  s = av_mallocz(sizeof(UDPContext));

  h->priv_data = s;

  进行UDP的相关设置;

  s->circular_buffer_size = 7*188*65536;

  if (!is_output && s->circular_buffer_size)
  {
    /* start the task going */
    s->fifo = av_fifo_alloc(s->circular_buffer_size);

    /*
     * 启动UDP数据接收线程,
     * 使得实时数据的接收和转码并行进行。
     */
    if (pthread_create(&s->circular_buffer_thread, NULL, circular_buffer_task, h))
    {
      av_log(h, AV_LOG_ERROR, "pthread_create failed\n");
      goto fail;
    }
  }
}

/* 初始化输入数据的FIFO */
void av_fifo_reset(AVFifoBuffer *f)
{
  f->wptr = f->rptr = f->buffer;
  f->wndx = f->rndx = 0;
}


/*
 * 函数功能:
 * 从UDP对应的socket中接收数据，
 * 并存储到FIFO中
 */
void *circular_buffer_task( void *_URLContext)
{
  URLContext *h = _URLContext;
  UDPContext *s = h->priv_data;

  for ( ; ; )
  {
    ...

    /*
     * 将从socket s->udp_fd接收到的数据
     * 存储到由s->fifo->wptr指定的s->fifo->buffer的位置
     */
    len = recv(s->udp_fd, s->fifo->wptr, left, 0);
    s->fifo->wptr += len;

    /* FIFO的循环使用 */
    if (s->fifo->wptr >= s->fifo->end)
      s->fifo->wptr = s->fifo->buffer;
    s->fifo->wndx += len;
  }
}

3. mpeg2-TS解析时的数据使用

函数调用关系图与从FIFO中获取数据的代码

av_demuxer_open()
  |
  V
mpegts_read_header()
  |
  V
avio_read()
  |
  V
ffurl_read()
  |
  V
int udp_read(URLContext *h, uint8_t *buf, int size)
{
  UDPContext *s = h->priv_data;
  int ret;
  int avail;
  fd_set rfds;
  struct timeval tv;

  if (s->fifo)
  {
    do
    {
      /* 查看FIFO中现有多少字节数据可用 */
      avail = av_fifo_size(s->fifo);
      if (avail)
      {
        // Maximum amount available
        size = FFMIN( avail, size);

        /* 将FIFO s->fifo中的数据复制size个字节到buf中 */
        av_fifo_generic_read(s->fifo, buf, size, NULL);
        return size;
      }
      else
      { ... }
    } while( 1);
  }
  ...
}

4. 总的来说,

UDP的数据(UDPContext)都是通过URLContext的"void *priv_data"以指针的方式来传递;