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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