GETNAMEINFO(P) POSIX Programmer's Manual GETNAMEINFO(P)
getnameinfo - 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 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.
Note: 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 argu-
ment 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 name.
If the service argument is non-NULL and the servicelen argument is non-zero, then the ser-
vice 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 service-
len 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 circumstances.
* If the flag bit NI_NAMEREQD is set, an error shall be returned if the host's name can-
not 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
1. The two NI_NUMERICxxx flags are required to support the -n flag that many com-
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
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 value if:
The name could not be resolved at this time. Future attempts may succeed.
The flags had an invalid value.
A non-recoverable error occurred.
The address family was not recognized or the address length was invalid for the
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
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 errno.
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 the system.
Given the IPv4-mapped IPv6 address "::ffff:220.127.116.11" , the implementation performs a lookup
as if the socket address structure contains the IPv4 address "18.104.22.168" .
gai_strerror() , getaddrinfo() , getservbyname() , inet_ntop() , socket() , the Base Defi-
nitions volume of IEEE Std 1003.1-2001, <netdb.h>, <sys/socket.h>
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 orig-
inal Standard can be obtained online at http://www.opengroup.org/unix/online.html .
IEEE/The Open Group 2003 GETNAMEINFO(P)