Provided by: manpages-dev_2.77-1_all
getnameinfo - address-to-name translation in protocol-independent
int getnameinfo(const struct sockaddr *sa, socklen_t salen,
char *host, size_t hostlen,
char *serv, size_t servlen, int flags);
The getnameinfo() function is defined for protocol-independent address-
to-nodename translation. It combines the functionality of
gethostbyaddr(3) and getservbyport(3) and is the inverse of
getaddrinfo(3). 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 buffers (of size hostlen and servlen
respectively) to hold the return values.
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) parameter. However, at least one of hostname or service
name must be requested.
The flags argument modifies the behavior of getnameinfo() as follows:
If set, return only the hostname part of the FQDN 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 looked up.)
If set, then an error is returned if the hostname cannot be
If set, then the service address is returned in numeric form,
for example by its port number.
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.
Extensions to getaddrinfo() for Internationalized Domain Names
Starting with glibc 2.3.4, getnameinfo() has been extended to
selectively allow host names 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 host name)
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 parameter 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 parameters.
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.
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
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 */
char hbuf[NI_MAXHOST], sbuf[NI_MAXSERV];
if (getnameinfo(sa, sa->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
struct sockaddr *sa; /* input */
if (getnameinfo(sa, sa->sa_len, hbuf, sizeof(hbuf),
NULL, 0, NI_NAMEREQD))
printf("could not resolve hostname");
An example program using getnameinfo() can be found in getaddrinfo(3).
socket(2), getaddrinfo(3), gethostbyaddr(3), getservbyname(3),
getservbyport(3), inet_ntop(3), hosts(5), services(5), hostname(7),
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 2.77 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.