GETNAMEINFO(3P) POSIX Programmer's Manual GETNAMEINFO(3P)PROLOG
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.
NAMEgetnameinfo — get name information
int getnameinfo(const struct sockaddr *restrict sa, socklen_t salen,
char *restrict node, socklen_t nodelen, char *restrict service,
socklen_t servicelen, int flags);
The getnameinfo() function shall translate a socket address to a node
name and service location, all of which are defined as in freead‐
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 address.
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 exam‐
ple, 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 ser‐
vice 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 rep‐
resent 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 reso‐
lution (for example, passed to getaddrinfo()), applications should pro‐
vide buffers large enough to store any result possible on the system.
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
The IPv6 unspecified address ("::") and the IPv6 loopback address
("::1") are not IPv4-compatible addresses.
SEE ALSOendservent(), freeaddrinfo(), gai_strerror(), inet_ntop(), socket()
The Base Definitions volume of POSIX.1‐2008, <netdb.h>, <sys_socket.h>
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 Electri‐
cal 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 .
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IEEE/The Open Group 2013 GETNAMEINFO(3P)