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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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GETNAMEINFO(3P) POSIX Programmer's Manual GETNAMEINFO(3P)
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.
getnameinfo — get name information
#include <sys/socket.h>
#include <netdb.h>
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
freeaddrinfo(3p).
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 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).
Notes:
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
value if:
[EAI_AGAIN] The name could not be resolved at this time. Future
attempts may succeed.
[EAI_BADFLAGS]
The flags had an invalid value.
[EAI_FAIL] A non-recoverable error occurred.
[EAI_FAMILY]
The address family was not recognized or the address
length was invalid for the specified family.
[EAI_MEMORY]
There was a memory allocation failure.
[EAI_NONAME]
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.
[EAI_OVERFLOW]
An argument buffer overflowed. The buffer pointed to
by the node argument or the service argument was too
small.
[EAI_SYSTEM]
A system error occurred. The error code can be found
in errno.
The following sections are informative.
None.
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:1.2.3.4", the
implementation performs a lookup as if the socket address
structure contains the IPv4 address "1.2.3.4".
The IPv6 unspecified address ("::") and the IPv6 loopback address
("::1") are not IPv4-compatible addresses.
None.
None.
endservent(3p), freeaddrinfo(3p), gai_strerror(3p), inet_ntop(3p),
socket(3p)
The Base Definitions volume of POSIX.1‐2017, netdb.h(0p),
sys_socket.h(0p)
Portions of this text are reprinted and reproduced in electronic
form from IEEE Std 1003.1-2017, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The
Open Group Base Specifications Issue 7, 2018 Edition, Copyright
(C) 2018 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 .
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
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2017 GETNAMEINFO(3P)
Pages that refer to this page: netdb.h(0p), freeaddrinfo(3p)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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