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GETNAMEINFO(3)             Linux Programmer's Manual            GETNAMEINFO(3)

       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 get-
       servbyport(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-termi-
       nated 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

       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 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 environments.

              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 non-zero error codes is

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

              The flags argument 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 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


       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

           #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 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), getservby-
       name(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

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

       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 3.22 of the Linux man-pages project.  A description of the project,  and  informa-
       tion about reporting bugs, can be found at

GNU                               2009-02-23                    GETNAMEINFO(3)