Linux Network Programming, Part 1

Listing 4 shows a connectionless server using UDP. While UDP applications are similar to their TCP cousins, they have some important differences. Foremost, UDP does not guarantee reliable delivery—if you require reliability and are using UDP, you either have to implement it yourself in your application logic or switch to TCP.

Like TCP applications, with UDP you create a socket and bind an address to it. (Some UDP servers do not need to call bind(), but it does no harm and will save you from making a mistake.) UDP servers do not listen or accept incoming connections, and clients do not explicitly connect to servers. In fact, there is very little difference between UDP clients and servers. The server must be bound to a known port and address only so that the client knows where to send messages. Additionally, the order of expected data transmissions is reversed, i.e., when you send data using send() in the server, your client should expect to receive data using recv().

It is common for UDP clients to fill in the sockaddr_in structure with a sin_port value of 0. (Note that 0 in either byte-order is 0.) The system then automatically assigns an unused port number (between 1024 and 5000) to the client. I'm leaving it as an exercise to the reader to convert the server in Listing 4 into a UDP client.

In order to connect to a server, you must first know both the address and port number on which it is listening. Many common services (FTP, TELNET, etc.) are listed in a text database file called /etc/services. An interface exists to request a service by name and to receive the port number (correctly formatted in network byte-order) for that service. The function is getservbyname(), and its prototype is in the header file /usr/include/netdb.h. This example takes a service name and protocol type and returns a pointer to struct servent.

struct servent
{
        char *s_name;   /* official service name */
        char **s_aliases;       /* alias list */
        int s_port;     /* port number, network<\n>
                         * byte-order--so do not
                         * use host-to-network macros */
        char *s_proto;  /* protocol to use */
};

This article has introduced network programming in Linux, using C and the BSD Socket API. In general, coding with this API tends to be quite laborious, especially when compared to some of the other techniques available. In future articles, I will compare two alternatives to the BSD Socket API for Linux: the use of Remote Procedure Calls (RPCs) and the Common Object Request Broker Architecture (CORBA). RPCs were introduced in Ed Petron's article “Remote Procedure Calls” in Linux Journal Issue #42 (October, 1997).

Resources

TCP listen() Backlog

Major System Calls The next article in this series will cover the issues involved in developing long-lived network services (daemons) in Linux.

All listings referred to in this article are available by anonymous download in the file ftp://ftp.linuxjournal.com/lj/listings/issue46/2333.tgz.