This manual page is part of the POSIX Programmer's Manual. The Linux
implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior), or
the interface may not be implemented on Linux.
The getnameinfo() function shall translate a socket address to a node
name and service location, all of which are defined as in
The sa argument points to a socket address structure to be
translated. The salen argument contains the length of the address
pointed to by sa.
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
If the address is the IPv6 unspecified address ("::"), a lookup shall
not be performed and the behavior shall be the same as when the
node's name cannot be located.
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 bytes 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 name.
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.
* 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.
* 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).
1. The two NI_NUMERICxxx flags are required to support the
−n flag that many commands provide.
2. 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.
The getnameinfo() function shall be thread-safe.
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.
The getnameinfo() function shall fail and return the corresponding
[EAI_AGAIN] The name could not be resolved at this time. Future
attempts may succeed.
The flags had an invalid value.
[EAI_FAIL] A non-recoverable error occurred.
The address family was not recognized or the address
length was invalid for the specified family.
There was a memory allocation failure.
The name does not resolve for the supplied parameters.
NI_NAMEREQD is set and the host's name cannot be located,
or both nodename and servname were null.
An argument buffer overflowed. The buffer pointed to by
the node argument or the service argument was too small.
A system error occurred. The error code can be found in
The following sections are informative.
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
Given the IPv4-mapped IPv6 address "::ffff:22.214.171.124", the
implementation performs a lookup as if the socket address structure
contains the IPv4 address "126.96.36.199".
The IPv6 unspecified address ("::") and the IPv6 loopback address
("::1") are not IPv4-compatible addresses.
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The Open
Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open
Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
applied.) 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.unix.org/online.html .
Any typographical or formatting errors that appear in this page are
most likely to have been introduced during the conversion of the
source files to man page format. To report such errors, see
IEEE/The Open Group 2013 GETNAMEINFO(3P)