Unix Socket Programming Question:

How do I get the port number for a given service?


Use the getservbyname() routine. This will return a pointer to a servent structure. You are interested in the s_port field, which contains the port number, with correct byte ordering (so you don't need to call htons() on it). Here is a sample routine:

/* Take a service name, and a service type, and return a port number. If the service name is not found, it tries it as a decimal number. The number returned is byte ordered for the network. */

int atoport(char *service, char *proto)
int port;
long int lport;
struct servent *serv;
char *errpos;

/* First try to read it from /etc/services */
serv = getservbyname(service, proto);
if (serv != NULL)
port = serv->s_port;
else {
/* Not in services, maybe a number? */
lport = strtol(service,&errpos,0);
if ( (errpos[0] != 0) || (lport < 1)
|| (lport > 5000) )

return -1;
/* Invalid port address */
port = htons(lport);
return port;

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