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


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

   int getnameinfo(const struct sockaddr *addr, socklen_t addrlen,
                   char *host, socklen_t hostlen,
                   char *serv, socklen_t servlen, int flags);

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

       Since glibc 2.22: _POSIX_C_SOURCE >= 201112L
       Glibc 2.21 and earlier: _POSIX_C_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,
   getnameinfo()  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 addrlen 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 getnameinfo() 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.


   For   an   explanation   of   the  terms  used  in  this  section,  see

   Interface      Attribute      Value              
   getnameinfo()  Thread safety  MT-Safe env locale 


   POSIX.1-2001, POSIX.1-2008, RFC 2553.


   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 suitable feature
   test macros are defined, namely:  _GNU_SOURCE,  _DEFAULT_SOURCE  (since
   glibc   2.19),  or  (in  glibc  versions  up  to  and  including  2.19)

   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.

   Before glibc version 2.2, the hostlen and servlen arguments were  typed
   as size_t.


   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 *addr;     /* input */
       socklen_t addrlen;         /* input */
       char hbuf[NI_MAXHOST], sbuf[NI_MAXSERV];

       if (getnameinfo(addr, addrlen, 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 *addr;     /* input */
       socklen_t addrlen;         /* input */
       char hbuf[NI_MAXHOST];

       if (getnameinfo(addr, addrlen, 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


   This page is part of release 4.09 of the Linux man-pages project.  A
   description of the project, information about reporting bugs, and the
   latest version of this page, can be found at


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