basic_socket::native_non_blocking (2 of 3 overloads)

basic_socket::native_non_blocking (2 of 3 overloads)

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

void native_non_blocking(
    bool mode);

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 asio::error::would_block (or the equivalent system error).

Exceptions

asio::system_error: Thrown on failure. If the mode is false, but the current value of non_blocking() is true, this function fails with 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()(asio::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 = asio::error_code(n < 0 ? errno : 0,
            asio::error::get_system_category());
        total_bytes_transferred_ += ec ? 0 : n;

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

        // Check if we need to run the operation again.
        if (ec == asio::error::would_block
            || ec == asio::error::try_again)
        {
          // We have to wait for the socket to become ready again.
          sock_.async_wait(tcp::socket::wait_write, *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_wait(tcp::socket::wait_write, op);
}