getnameinfo(3)             UNIX Programmer's Manual             getnameinfo(3)

       getnameinfo  - address-to-name translation in protocol-independent man-

       #include <sys/socket.h>
       #include <netdb.h>

       int getnameinfo(const struct sockaddr *sa, socklen_t salen,
                       char *host, size_t hostlen,
                       char *serv, size_t servlen, int flags);

       The  getnameinfo(3)  function  is  defined   for   protocol-independent
       address-to-nodename  translation.   It  combines  the  functionality of
       gethostbyaddr(3) and getservbyport(3) and  is  the  inverse  of  getad-
       drinfo(3).   The  sa  argument is a pointer to a generic socket address
       structure (of type sockaddr_in or  sockaddr_in6)  of  size  salen  that
       holds  the  input  IP  address and port number.  The arguments host and
       serv are pointers to buffers (of size hostlen and servlen respectively)
       to hold the return values.

       The  caller  can  specify  that  no  hostname  (or  no service name) is
       required by providing a NULL host (or serv) argument or a zero  hostlen
       (or  servlen)  parameter.  However, at least one of hostname or service
       name must be requested.

       The flags argument modifies the behaviour of getnameinfo(3) as follows:

              If  set,  return  only  the  hostname part of the FQDN for local

              If set, then the numeric  form  of  the  hostname  is  returned.
              (When  not  set,  this will still happen in case the node's name
              cannot be looked up.)

              If set, then a error is  returned  if  the  hostname  cannot  be
              looked up.

              If  set,  then  the service address is returned in numeric form,
              for example by its port number.

              If set, then the service is datagram  (UDP)  based  rather  than
              stream (TCP) based. This is required for the few ports (512-514)
              that have different services for UDP and TCP.

       On success 0 is returned, and node and service names, if requested, are
       filled  with  null-terminated  strings,  possibly  truncated to fit the
       specified buffer lengths.  On error one of the following non-zero error
       codes is returned:

              The name could not be resolved at this time. Try again later.

              The flags parameter has an invalid value.

              A non-recoverable error occurred.

              The address family was not recognized, or the address length was
              invalid for the specified family.

              Out of memory.

              The  name  does  not  resolve  for  the   supplied   parameters.
              NI_NAMEREQD  is  set  and  the host's name cannot be located, or
              neither hostname nor service name were requested.

              The buffer pointed to by host or serv was too small.

              A system error occurred. The error code can be found in errno.

       The gai_strerror(3) function translates these error codes  to  a  human
       readable string, suitable for error reporting.


       In  order to assist the programmer in choosing reasonable sizes for the
       supplied buffers, <netdb.h> defines the constants
              # define NI_MAXHOST      1025
              # define NI_MAXSERV      32
       The former is the  constant  MAXDNAME  in  recent  versions  of  BIND's
       <arpa/nameser.h>  header  file. The latter is a guess based on the ser-
       vices listed in the current Assigned Numbers RFC.

       The following code tries to get the numeric hostname and service  name,
       for  a  given socket address. Note that there is no hardcoded reference
       to a particular address family.

                struct sockaddr *sa;    /* input */
                char hbuf[NI_MAXHOST], sbuf[NI_MAXSERV];

                if (getnameinfo(sa, sa->sa_len, hbuf, sizeof(hbuf), sbuf,
                    sizeof(sbuf), NI_NUMERICHOST | NI_NUMERICSERV) == 0)
                        printf("host=%s, serv=%s\n", hbuf, sbuf);

       The following version checks  if  the  socket  address  has  a  reverse
       address mapping.

                struct sockaddr *sa;    /* input */
                char hbuf[NI_MAXHOST];

                if (getnameinfo(sa, sa->sa_len, hbuf, sizeof(hbuf),
                    NULL, 0, NI_NAMEREQD))
                       printf("could not resolve hostname");
                       printf("host=%s\n", hbuf);

       RFC 2553, POSIX.1-2001.

       getaddrinfo(3),  gethostbyaddr(3),  getservbyname(3), getservbyport(3),
       inet_ntop(3), socket(3), hosts(5), services(5), hostname(7), named(8)

       R. Gilligan, S. Thomson, J. Bound and W. Stevens, Basic  Socket  Inter-
       face Extensions for IPv6, RFC 2553, March 1999.

       Tatsuya Jinmei and Atsushi Onoe, An Extension of Format for IPv6 Scoped
       Addresses, internet draft, work in progress.

       Craig Metz, Protocol Independence Using the Sockets API, Proceedings of
       the freenix track: 2000 USENIX annual technical conference, June  2000.

Linux Man Page                    2000-12-11                    getnameinfo(3)

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