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


       #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);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       getnameinfo(): _POSIX_C_SOURCE >= 1 || _XOPEN_SOURCE || _POSIX_SOURCE


       The  getnameinfo()  function  is  the  inverse  of  getaddrinfo(3):  it
       converts  a  socket  address  to a corresponding host and service, in a
       protocol-independent  manner.   It  combines   the   functionality   of
       gethostbyaddr(3)  and  getservbyport(3),  but  unlike  those functions,
       getaddrinfo(3)  is  reentrant  and   allows   programs   to   eliminate
       IPv4-versus-IPv6 dependencies.

       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
       caller-allocated buffers (of size  hostlen  and  servlen  respectively)
       into  which getnameinfo() places null-terminated strings containing the
       host and service names respectively.

       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) argument.  However, at least one of  hostname  or  service
       name must be requested.

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

              If  set,  then  an  error  is returned if the hostname cannot be

              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.

              If set, return only the hostname part  of  the  fully  qualified
              domain name for local hosts.

              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 determined.)

              If  set,  then  the  numeric  form  of  the  service  address is
              returned.  (When not set, this will still  happen  in  case  the
              service’s name cannot be determined.)

   Extensions to getaddrinfo() for Internationalized Domain Names
       Starting   with   glibc  2.3.4,  getnameinfo()  has  been  extended  to
       selectively allow hostnames to be transparently converted to  and  from
       the   Internationalized   Domain  Name  (IDN)  format  (see  RFC  3490,
       Internationalizing Domain Names in  Applications  (IDNA)).   Three  new
       flags are defined:

       NI_IDN If  this flag is used, then the name found in the lookup process
              is converted  from  IDN  format  to  the  locale’s  encoding  if
              necessary.  ASCII-only names are not affected by the conversion,
              which  makes  this  flag  usable  in   existing   programs   and

              Setting these flags will enable the IDNA_ALLOW_UNASSIGNED (allow
              unassigned Unicode code  points)  and  IDNA_USE_STD3_ASCII_RULES
              (check  output  to  make  sure it is a STD3 conforming hostname)
              flags respectively to be used in the IDNA handling.


       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 nonzero  error
       codes is returned:

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

              The flags argument has an invalid value.

              A nonrecoverable 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   arguments.
              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.




       getnameinfo() is provided in glibc since version 2.1.


       RFC 2553, POSIX.1-2001.


       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

       Since  glibc  2.8,  these  definitions  are  exposed only if one of the
       feature  test  macros  _BSD_SOURCE,  _SVID_SOURCE,  or  _GNU_SOURCE  is

       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
       services 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 */
           socklen_t len;         /* input */
           char hbuf[NI_MAXHOST], sbuf[NI_MAXSERV];

           if (getnameinfo(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 */
           socklen_t len;         /* input */
           char hbuf[NI_MAXHOST];

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

       An example program using getnameinfo() can be found in  getaddrinfo(3).


       accept(2),   getpeername(2),  getsockname(2),  recvfrom(2),  socket(2),
       getaddrinfo(3), gethostbyaddr(3),  getservbyname(3),  getservbyport(3),
       inet_ntop(3), hosts(5), services(5), hostname(7), named(8)

       R.  Gilligan,  S.  Thomson,  J.  Bound  and  W.  Stevens,  Basic Socket
       Interface 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.


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