getservbyname, getservbyport
Prototype: 
#include <netdb.h>
struct servent *getservbyname(const char *name, const char *proto);
struct servent *getservbyport(int port, const char *proto);
General Description: These functions find the associated servent either by name or by port number. Since a servent can support more than one protocol, you need to supply the protocol as well.
Return Value: The call returns a pointer to servent (defined below) if successful. Otherwise, it returns a NULL. 
struct servent {
	char    *s_name;        /* official service name */
	char    **s_aliases;    /* alias list */
	int     s_port;         /* port number */
	char    *s_proto;       /* protocol to use */
};
The field p_proto is the protocol number you should use with the service.
Parameters
name The servent name (e.g., "daytime", "telnet").
proto Protocol name. This may be any of the recognized protocol names or aliases.
port The port number.
Possible Errors
(errno not set)
Examples 
#include <netdb.h>
struct servent *s = getservbyname("telnet", "tcp");
printf("%s %d %s ", s->s_name, ntohs(s->s_port), s->s_proto);
s = getservbyport(22, "udp");
printf("%s %d %s ", s->s_name, ntohs(s->s_port), s->s_proto);

(c)Socket Controls