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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 getservbyport(3), but unlike those functions, get-
       nameinfo() 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 argu-
       ments 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 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.

       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), geth-
       ostbyaddr(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,  inter-
       net       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 3.53 of the Linux man-pages project.  A description of the
       project, and information about reporting bugs, can be found at

GNU                                         2013-01-15                             GETNAMEINFO(3)

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