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Information regarding services is a bit more complicated. A service is expected to reside at a specific port and use a particular communication protocol. This view is consistent with the Internet domain, but inconsistent with other network architectures. Further, a service may reside on multiple ports. If this occurs, the higher level library routines will have to be bypassed or extended.
A service mapping is described by the servent structure:
struct servent {
char *s_name; /* official service name */
char **s_aliases; /* alias list */
int s_port; /* port number, network byte order */
char *s_proto; /* protocol to use */
};
The routine
getservbyname
maps service names to a
servent
structure by specifying a service name and,
optionally, a qualifying protocol.
Thus the call
sp = getservbyname("telnet", (char *) 0);
returns the service specification for a telnet server using
any protocol, while the call
sp = getservbyname("telnet", "tcp");
returns only that telnet server that uses the
TCP
protocol.
The routines
getservbyport
and
getservent(3N)
are
also provided.
The getservbyport
routine has an interface similar
to that provided by
getservbyname;
an optional protocol name may
be specified to qualify lookups.