service to port number like getaddrinfo

Question

In C you can use a service name to get the port number when using getaddrinfo, like this:

getaddrinfo("stackoverflow.com", "http", &hints, &results);

This will assign 80 (big endian) to the first two bytes (uint16_t) of results->ai_addr.

Is there some easy way to get the port number from ?only? giving the service name? I need this for the server side, where I want to allow the user to specify either port number or service name for the port to bind and listen to.

Answer 1

Using getaddrinfo with a null pointer for the hostname and AI_PASSIVE in the flags will give you a sockaddr you can bind on for running a server. See the documentation for getaddrinfo:

http://pubs.opengroup.org/onlinepubs/9699919799/functions/getaddrinfo.html

Answer 2

man getservbyname

  #include \netdb.h>
   struct servent *getservent(void);
   struct servent *getservbyname(const char *name, const char *proto);
   struct servent *getservbyport(int port, const char *proto);