getnameinfo(3) — Linux manual page


getnameinfo(3)          Library Functions Manual          getnameinfo(3)

NAME         top

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

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

       #include <sys/socket.h>
       #include <netdb.h>

       int getnameinfo(const struct sockaddr *restrict addr, socklen_t addrlen,
                       char host[_Nullable restrict .hostlen],
                       socklen_t hostlen,
                       char serv[_Nullable restrict .servlen],
                       socklen_t servlen,
                       int flags);

   Feature Test Macro Requirements for glibc (see

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

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

       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

              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

              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

              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


ATTRIBUTES         top

       For an explanation of the terms used in this section, see
       │ Interface                Attribute     Value              │
       │ getnameinfo()            │ Thread safety │ MT-Safe env locale │

STANDARDS         top

       POSIX.1-2008.  RFC 2553.

HISTORY         top

       glibc 2.1.  POSIX.1-2001.

       Before glibc 2.2, the hostlen and servlen arguments were typed as

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
       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.

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
       address 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 2000 

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Linux man-pages 6.9.1          2024-06-15                 getnameinfo(3)

Pages that refer to this page: getaddrinfo(3)gethostbyname(3)getifaddrs(3)getipnodebyname(3)inet(3)inet_ntop(3)sockaddr(3type)