Programming with RTnet

Programming with RTnet is pretty straightforward once you understand the concepts of how RTnet operates in relation to an RTOS. RTnet itself is really only a kernel module that communicates to a real-time Ethernet device driver.

The interface between RTnet and the RTOS is provided by RTDM (Real-Time Driver Model), which Xenomai implements. This interface is fully documented in the Xenomai documentation .

The RTDM specification is located under the Modules/Real-Time Driver Model/User API section of the Xenomai API tree.

The basics

To program in RTnet the standard Linux socket-programming method is followed. If you are unfamiliar with Linux socket-programming the links below will provide some extra information on the subject.

_Differences of native rt_dev_xxx vs. POSIX service calls (with Xenomai):

  • Independent of RTnet, Xenomai application can call into normal networking services of Linux. They just loose timing guarantees at that point.

  • The programming model of the POSIX skin allows you to use socket functions as if you are writing a normal Linux application. In case your calls address a service which RTnet provides (UDP or AF_PACKET) and RTnet is loaded, it will handle it for you under realtime constraints. Services unknown to RTnet are passed through to Linux, without timing guarantees. BTW, the same pattern works for AF_CAN.

(Both points copied from RTnet mailing-list)

The example projects in RTnet provides the basic introduction to programming in this environment. The generic steps in the examples are as follows:

  • Declare two socket descriptor structures, one for the local socket and one for the remote socket.
static struct sockaddr_in local_addr;
static struct sockaddr_in server_addr;
  • Clear these structures before populating it with values.
memset(&local_addr, 0, sizeof(struct sockaddr_in));
memset(&server_addr, 0, sizeof(struct sockaddr_in));
  • Set the address format to Internet (IP)
local_addr.sin_family = AF_INET;
  • Set the local address to the host address
local_addr.sin_addr.s_addr = INADDR_ANY;
  • Convert the sting to a port number and store it in the structure
local_addr.sin_port = htons(atoi(argv[1]));
  • For the remote port setup, again set the device format to IP.
server_addr.sin_family = AF_INET;
  • Convert the IP address sting to network format
server_addr.sin_addr.s_addr = rt_inet_aton(argv[2]);
  • Setup the remote port number
server_addr.sin_port = htons(atoi(argv[3]));
  • Create a new socket to manage the connection.
sockfd = rt_dev_socket(AF_INET, SOCK_DGRAM, 0);
  • Bind the new socket to the local port
ret = rt_dev_bind(sockfd, (struct sockaddr *)&local_addr, sizeof(struct sockaddr_in));
  • Connect the socket to the remote port
rt_dev_connect(sockfd, (struct sockaddr *)&server_addr, sizeof(struct sockaddr_in));
  • And send the message.
rt_dev_send(sockfd, msg, sizeof(msg), 0);
  • On the receive side receive the message
ret = rt_dev_recv(sockfd, msg, sizeof(msg), 0);
  • Close the socket after transmission

Further Information

  • Beej’s Guide to Network Programming , a comprehensive and well-written guide on network programming.

  • The Open Group Base Specifications is the IEEE standard used in Linux socket programming. All of the relevant Linux socket functions are listed here with detailed descriptions. Note that the RTDM interface has been adopted from this standard (and by adding a rt_dev_ to the name) but might not include all of the functionalities listed.