Section (3) getnameinfo
getnameinfo — address-to-name translation in protocol-independent manner
#include <sys/socket.h> #include <netdb.h>
||const struct sockaddr *addr,|
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
unlike those functions,
getnameinfo() is reentrant and allows
programs to eliminate IPv4-versus-IPv6 dependencies.
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
serv are pointers to
caller-allocated buffers (of size
servlen respectively) into
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
serv) argument or a zero
However, at least one of hostname or service name must be
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 hosts.
If set, then the numeric form of the hostname is returned. (When not set, this will still happen in case the node_zsingle_quotesz_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_zsingle_quotesz_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
If this flag is used, then the name found in the lookup process is converted from IDN format to the locale_zsingle_quotesz_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.
flagsargument 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_NAMEREQDis set and the host_zsingle_quotesz_s name cannot be located, or neither hostname nor service name were requested.
The buffer pointed to by
servwas 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.
For an explanation of the terms used in this section, see attributes(7).
||Thread safety||MT-Safe env locale|
In order to assist the programmer in choosing reasonable
sizes for the supplied buffers,
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:
_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_zsingle_quotesz_s
> header file. The latter is a guess
based on the services listed in the current Assigned Numbers
Before glibc version 2.2, the
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 , 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); else printf(host=%s , hbuf);
An example program using
getnameinfo() can be found in getaddrinfo(3).
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 ftp://ftp.ietf.org/internet-drafts/draft-ietf-ipngwg-scopedaddr-format-02.txt
Craig Metz, Protocol Independence Using the Sockets API, Proceedings of the freenix track: 2000 USENIX annual technical conference, June 2000 http://www.usenix.org/publications/library/proceedings/usenix2000/freenix/metzprotocol.html
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Almost all details are from RFC 2553.
2004-12-14, mtk, Added EAI_OVERFLOW error
2004-12-14 Fixed description of error return