getnameinfo(3) — Linux manual page


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

NAME         top

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

SYNOPSIS         top

       #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

DESCRIPTION         top

       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 addr 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 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.

RETURN VALUE         top

       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

       The gai_strerror(3) function translates these error codes to a human
       readable string, suitable for error reporting.

FILES         top


VERSIONS         top

       getnameinfo() is provided in glibc since version 2.1.

ATTRIBUTES         top

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

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

CONFORMING TO         top

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

NOTES         top

       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 fea‐
       ture 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.

EXAMPLES         top

       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 ad‐
       dress 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

SEE ALSO         top

       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

COLOPHON         top

       This page is part of release 5.09 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

GNU                              2020-06-09                   GETNAMEINFO(3)

Pages that refer to this page: endhostent(3)freeaddrinfo(3)freehostent(3)freeifaddrs(3)gai_strerror(3)getaddrinfo(3)gethostbyaddr(3)gethostbyaddr_r(3)gethostbyname2(3)gethostbyname2_r(3)gethostbyname(3)gethostbyname_r(3)gethostent(3)gethostent_r(3)getifaddrs(3)getipnodebyaddr(3)getipnodebyname(3)h_errno(3)herror(3)hstrerror(3)inet(3)inet_addr(3)inet_aton(3)inet_lnaof(3)inet_makeaddr(3)inet_netof(3)inet_network(3)inet_ntoa(3)inet_ntop(3)sethostent(3)