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#1 2002-07-24 05:32 PM

From: Colombia
Registered: 2002-06-12
Posts: 353

Re: 2.3 - How do I get the port number for a given service?

Taken from the original FAQ.


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 > 65535) )
      return -1; /* Invalid port address */
    port = htons(lport);
  return port;

Matthias Urlichs has pointed out that the old max value of 5000 is wrong, and that the number should be 65535.  Quite right, and I don't recall where the original number came from.  Thanks for picking up on that.


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