getnameinfo(3) UNIX Programmer's Manual getnameinfo(3)
getnameinfo - address-to-name translation in protocol-independent man-
int getnameinfo(const struct sockaddr *sa, socklen_t salen,
char *host, size_t hostlen,
char *serv, size_t servlen, int flags);
The getnameinfo(3) function is defined for protocol-independent
address-to-nodename translation. It combines the functionality of
gethostbyaddr(3) and getservbyport(3) and is the inverse of getad-
drinfo(3). The sa argument is a pointer to a generic socket address
structure (of type sockaddr_in or sockaddr_in6) of size salen that
holds the input IP address and port number. The arguments host and
serv are pointers to buffers (of size hostlen and servlen respectively)
to hold the return values.
The caller can specify that no hostname (or no service name) is
required by providing a NULL host (or serv) argument or a zero hostlen
(or servlen) parameter. However, at least one of hostname or service
name must be requested.
The flags argument modifies the behaviour of getnameinfo(3) as follows:
If set, return only the hostname part of the FQDN for local
If set, then the numeric form of the hostname is returned.
(When not set, this will still happen in case the node's name
cannot be looked up.)
If set, then a error is returned if the hostname cannot be
If set, then the service address is returned in numeric form,
for example by its port number.
If set, then the service is datagram (UDP) based rather than
stream (TCP) based. This is required for the few ports (512-514)
that have different services for UDP and TCP.
On success 0 is returned, and node and service names, if requested, are
filled with null-terminated strings, possibly truncated to fit the
specified buffer lengths. On error one of the following non-zero error
codes is returned:
The name could not be resolved at this time. Try again later.
The flags parameter has an invalid value.
A non-recoverable error occurred.
The address family was not recognized, or the address length was
invalid for the specified family.
Out of memory.
The name does not resolve for the supplied parameters.
NI_NAMEREQD is set and the host's name cannot be located, or
neither hostname nor service name were requested.
The buffer pointed to by host or serv was too small.
A system error occurred. The error code can be found in errno.
The gai_strerror(3) function translates these error codes to a human
readable string, suitable for error reporting.
In order to assist the programmer in choosing reasonable sizes for the
supplied buffers, <netdb.h> defines the constants
# define NI_MAXHOST 1025
# define NI_MAXSERV 32
The former is the constant MAXDNAME in recent versions of BIND's
<arpa/nameser.h> header file. The latter is a guess based on the ser-
vices listed in the current Assigned Numbers RFC.
The following code tries to get the numeric hostname and service name,
for a given socket address. Note that there is no hardcoded reference
to a particular address family.
struct sockaddr *sa; /* input */
char hbuf[NI_MAXHOST], sbuf[NI_MAXSERV];
if (getnameinfo(sa, sa->sa_len, hbuf, sizeof(hbuf), sbuf,
sizeof(sbuf), NI_NUMERICHOST | NI_NUMERICSERV) == 0)
printf("host=%s, serv=%s\n", hbuf, sbuf);
The following version checks if the socket address has a reverse
struct sockaddr *sa; /* input */
if (getnameinfo(sa, sa->sa_len, hbuf, sizeof(hbuf),
NULL, 0, NI_NAMEREQD))
printf("could not resolve hostname");
RFC 2553, POSIX.1-2001.
getaddrinfo(3), gethostbyaddr(3), getservbyname(3), getservbyport(3),
inet_ntop(3), socket(3), hosts(5), services(5), hostname(7), named(8)
R. Gilligan, S. Thomson, J. Bound and W. Stevens, Basic Socket Inter-
face Extensions for IPv6, RFC 2553, March 1999.
Tatsuya Jinmei and Atsushi Onoe, An Extension of Format for IPv6 Scoped
Addresses, internet draft, work in progress. ftp://ftp.ietf.org/inter-
Craig Metz, Protocol Independence Using the Sockets API, Proceedings of
the freenix track: 2000 USENIX annual technical conference, June 2000.
Linux Man Page 2000-12-11 getnameinfo(3)
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