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basic_socket_streambuf::native_non_blocking (3 of 3 overloads)

Inherited from basic_socket.

Sets the non-blocking mode of the native socket implementation.

boost::system::error_code native_non_blocking(
    bool mode,
    boost::system::error_code & ec);

This function is used to modify the non-blocking mode of the underlying native socket. It has no effect on the behaviour of the socket object's synchronous operations.

Parameters

mode

If true, the underlying socket is put into non-blocking mode and direct system calls may fail with boost::asio::error::would_block (or the equivalent system error).

ec

Set to indicate what error occurred, if any. If the mode is false, but the current value of non_blocking() is true, this function fails with boost::asio::error::invalid_argument, as the combination does not make sense.

Example

This function is intended to allow the encapsulation of arbitrary non-blocking system calls as asynchronous operations, in a way that is transparent to the user of the socket object. The following example illustrates how Linux's sendfile system call might be encapsulated:

template <typename Handler>
struct sendfile_op
{
  tcp::socket& sock_;
  int fd_;
  Handler handler_;
  off_t offset_;
  std::size_t total_bytes_transferred_;

  // Function call operator meeting WriteHandler requirements.
  // Used as the handler for the async_write_some operation.
  void operator()(boost::system::error_code ec, std::size_t)
  {
    // Put the underlying socket into non-blocking mode.
    if (!ec)
      if (!sock_.native_non_blocking())
        sock_.native_non_blocking(true, ec);

    if (!ec)
    {
      for (;;)
      {
        // Try the system call.
        errno = 0;
        int n = ::sendfile(sock_.native_handle(), fd_, &offset_, 65536);
        ec = boost::system::error_code(n < 0 ? errno : 0,
            boost::asio::error::get_system_category());
        total_bytes_transferred_ += ec ? 0 : n;

        // Retry operation immediately if interrupted by signal.
        if (ec == boost::asio::error::interrupted)
          continue;

        // Check if we need to run the operation again.
        if (ec == boost::asio::error::would_block
            || ec == boost::asio::error::try_again)
        {
          // We have to wait for the socket to become ready again.
          sock_.async_write_some(boost::asio::null_buffers(), *this);
          return;
        }

        if (ec || n == 0)
        {
          // An error occurred, or we have reached the end of the file.
          // Either way we must exit the loop so we can call the handler.
          break;
        }

        // Loop around to try calling sendfile again.
      }
    }

    // Pass result back to user's handler.
    handler_(ec, total_bytes_transferred_);
  }
};

template <typename Handler>
void async_sendfile(tcp::socket& sock, int fd, Handler h)
{
  sendfile_op<Handler> op = { sock, fd, h, 0, 0 };
  sock.async_write_some(boost::asio::null_buffers(), op);
}

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