BSD Sockets Interface Programmer's Guide

90 Chapter 4

Using Internet Datagram Sockets

Preparing Address Variables

#include <netdb.h>

struct hostent *hp; /* point to host info for name server host */

...

servaddr.sin_family = AF_INET;

hp = gethostbyname (argv[1]);

servaddr.sin_addr.s_addr = ((struct in_addr *)(hp->h_addr))->s_addr;

The argv[1] parameter is the host name speciﬁed in the client program

command line. Refer to the gethostent(3N) man page for more

information on gethostbyname.

Getting the Port Address for the Desired

Service

When a client process needs to use a service that is offered by some

server process, it must send a message to the server's socket. The client

process must know the port address for that socket. If the service is not

in /etc/services, you must add it.

getservbyname obtains the port address of the speciﬁed service from

/etc/services. getservbyname and its parameters are described in

the following table.

Include ﬁles: #include <netdb.h>

System call: struct servent *getservbyname(name, proto)

char *name, *proto;

Function result: pointer to struct servent containing port address,

NULL pointer (0) if failure occurs.

Example:

#include <netdb.h>

struct servent *sp; /* pointer to service info */

...

sp = getservbyname (“example”, “udp”);

servaddr.sin_port = sp->s_port;

Parameter Description of Contents INPUT Value

name pointer to a valid service

name

service name

proto pointer to the protocol to be

used

udp or 0 if UDP is the

only protocol for the

service