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


       #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

       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 determined.

              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

              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 defined.

       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


       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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