The getnameinfo() function shall translate a socket address to a node
name and service location, all of which are defined as in getaddrinfo()
The sa argument points to a socket address structure to be translated.
If the socket address structure contains an IPv4-mapped IPv6 address or an
IPv4-compatible IPv6 address, the implementation shall extract the embedded
IPv4 address and lookup the node name for that IPv4 address.
The IPv6 unspecified address ( "::" ) and
the IPv6 loopback address ( "::1" ) are not
IPv4-compatible addresses. If the address is the IPv6 unspecified address
( "::" ), a lookup is not performed, and the [EAI_NONAME]
error is returned.
If the node argument is non-NULL and the nodelen argument is
non-zero, then the node argument points to a buffer able to contain up
to nodelen characters that receives the node name as a null-terminated
string. If the node argument is NULL or the nodelen argument is
zero, the node name shall not be returned. If the node's name cannot be
located, the numeric form of the address contained in the socket address
structure pointed to by the sa argument is returned instead of its
If the service argument is non-NULL and the servicelen argument is
non-zero, then the service argument points to a buffer able to contain
up to servicelen bytes that receives the service name as a
null-terminated string. If the service argument is NULL or the
servicelen argument is zero, the service name shall not be returned. If
the service's name cannot be located, the numeric form of the service address
(for example, its port number) shall be returned instead of its name.
The flags argument is a flag that changes the default actions of the
function. By default the fully-qualified domain name (FQDN) for the host shall
be returned, but:
If the flag bit NI_NOFQDN is set, only the node name
portion of the FQDN shall be returned for local hosts.
If the flag bit NI_NUMERICHOST is set, the numeric form of
the address contained in the socket address structure pointed to by the
sa argument shall be returned instead of its name, under all
If the flag bit NI_NAMEREQD is set, an error shall be
returned if the host's name cannot be located.
If the flag bit NI_NUMERICSERV is set, the numeric form of
the service address shall be returned (for example, its port number)
instead of its name, under all circumstances.
If the flag bit NI_NUMERICSCOPE is set, the numeric form of
the scope identifier shall be returned (for example, interface index)
instead of its name. This flag shall be ignored if the sa argument
is not an IPv6 address.
If the flag bit NI_DGRAM is set, this indicates that the
service is a datagram service (SOCK_DGRAM). The default behavior shall
assume that the service is a stream service (SOCK_STREAM).
The two NI_NUMERICxxx flags are required to support the
-n flag that many commands provide.
The NI_DGRAM flag is required for the few AF_INET and
AF_INET6 port numbers (for example, [512,514]) that represent different
services for UDP and TCP.
A zero return value for getnameinfo() indicates successful completion; a
non-zero return value indicates failure. The possible values for the failures
are listed in the ERRORS section.
Upon successful completion, getnameinfo() shall return the node
and service names, if requested, in the buffers provided. The returned
names are always null-terminated strings.
If the returned values are to be used as part of any further name resolution
(for example, passed to getaddrinfo()), applications should provide
buffers large enough to store any result possible on the system.
Given the IPv4-mapped IPv6 address "::ffff:188.8.131.52" , the
implementation performs a lookup as if the socket address structure contains
the IPv4 address "184.108.40.206" .
Portions of this text are reprinted and reproduced in electronic form from IEEE
Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable
Operating System Interface (POSIX), The Open Group Base Specifications Issue
6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics
Engineers, Inc and The Open Group. In the event of any discrepancy between
this version and the original IEEE and The Open Group Standard, the original
IEEE and The Open Group Standard is the referee document. The original
Standard can be obtained online at http://www.opengroup.org/unix/online.html