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 *restrict addr, socklen_t addrlen,
                       char *restrict host, socklen_t hostlen,
                       char *restrict serv, socklen_t servlen, int flags);

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

           Since glibc 2.22:
               _POSIX_C_SOURCE >= 200112L
           Glibc 2.21 and earlier:

       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

              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.

       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 attributes(7).

       │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) _BSD_SOURCE
       or _SVID_SOURCE.

       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

       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

           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 5.13 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                                2021-03-22                     GETNAMEINFO(3)