man-pages/man3/getnameinfo.3

.\" This page is in the public domain.
.\" Almost all details are from RFC 2553.
.\"
.\" 2004-12-14, mtk, Added EAI_OVERFLOW error
.\" 2004-12-14 Fixed description of error return
.\"
.TH getnameinfo 3 2000-12-11 "Linux Man Page" "UNIX Programmer's Manual"
.SH NAME
getnameinfo \- address-to-name translation in protocol-independent manner
.SH SYNOPSIS
.nf
.B #include <sys/socket.h>
.B #include <netdb.h>
.sp
.BI "int getnameinfo(const struct sockaddr *" "sa" ", socklen_t " "salen" ,
.BI "                char *" "host" ", size_t " "hostlen" ,
.BI "                char *" "serv" ", size_t " "servlen" ", int " "flags" );
.fi
.SH DESCRIPTION
The
.BR getnameinfo (3)
function is defined for protocol-independent address-to-nodename translation.
It combines the functionality of
.BR gethostbyaddr (3)
and
.BR getservbyport (3)
and is the inverse of
.BR getaddrinfo (3).
The
.I sa
argument is a pointer to a generic socket address structure
(of type
.I sockaddr_in
or
.IR sockaddr_in6 )
of size
.IR salen
that holds the input IP address and port number.
The arguments
.I host
and
.I serv
are pointers to buffers (of size
.I hostlen
and
.I servlen
respectively) to hold the return values.

The caller can specify that no hostname (or no service name)
is required by providing a NULL
.I host
(or
.IR serv )
argument or a zero
.I hostlen
(or
.IR servlen )
parameter. However, at least one of hostname or service name
must be requested.

The
.I flags
argument modifies the behaviour of
.BR getnameinfo (3)
as follows:
.TP
.B NI_NOFQDN
If set, return only the hostname part of the FQDN for local hosts.
.TP
.B NI_NUMERICHOST
If set, then the numeric form of the hostname is returned.
.\" For example, by calling
.\" .I inet_ntop()
.\" instead of
.\" .IR gethostbyaddr() .
(When not set, this will still happen in case the node's name
cannot be looked up.)
.TP
.B NI_NAMEREQD
If set, then a error is returned if the hostname cannot be looked up.
.TP
.B NI_NUMERICSERV
If set, then the service address is returned in numeric form,
for example by its port number.
.TP
.B NI_DGRAM
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.
.SH "RETURN VALUE"
On success 0 is returned, and node and service names, if requested,
are filled with NUL-terminated strings, possibly truncated to fit
the specified buffer lengths.
On error one of the following non-zero error codes is returned:
.TP
.B EAI_AGAIN
The name could not be resolved at this time. Try again later.
.TP
.B EAI_BADFLAGS
The
.I flags
parameter has an invalid value.
.TP
.B EAI_FAIL
A non-recoverable error occurred.
.TP
.B EAI_FAMILY
The address family was not recognized,
or the address length was invalid for the specified family.
.TP
.B EAI_MEMORY
Out of memory.
.TP
.B EAI_NONAME
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.
.TP
.B EAI_OVERFLOW
The buffer pointed to by
.I host
or
.I serv
was too small.
.TP
.B EAI_SYSTEM
A system error occurred. The error code can be found in
.IR errno .
.SH FILES
/etc/hosts
.br
/etc/nsswitch.conf
.br
/etc/resolv.conf
The
.BR gai_strerror (3)
function translates these error codes to a human readable string,
suitable for error reporting.
.SH NOTE
In order to assist the programmer in choosing reasonable sizes
for the supplied buffers,
.I <netdb.h>
defines the constants
.RS
.nf
# define NI_MAXHOST      1025
.br
# define NI_MAXSERV      32
.fi
.RE
The former is the constant MAXDNAME in recent versions of BIND's
.I <arpa/nameser.h>
header file. The latter is a guess based on the services listed
in the current Assigned Numbers RFC.
.SH EXAMPLES
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.

.RS
.nf
  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\en", hbuf, sbuf);
.fi
.RE

The following version checks if the socket address has a
reverse address mapping.

.RS
.fi
  struct sockaddr *sa;    /* input */
  char hbuf[NI_MAXHOST];

  if (getnameinfo(sa, sa->sa_len, hbuf, sizeof(hbuf),
      NULL, 0, NI_NAMEREQD))
         printf("could not resolve hostname");
  else
         printf("host=%s\en", hbuf);
.fi
.RE
.SH "CONFORMING TO"
RFC 2553. (See also XNS, issue 5.2.)
.SH "SEE ALSO"
.BR getaddrinfo (3),
.BR gethostbyaddr (3),
.BR getservbyname (3),
.BR getservbyport (3),
.BR inet_ntop (3),
.BR socket (3),
.BR hosts (5),
.BR services (5),
.BR hostname (7),
.BR named (8)
.LP
R. Gilligan, S. Thomson, J. Bound and W. Stevens,
.IR "Basic Socket Interface Extensions for IPv6" ,
RFC 2553, March 1999.
.LP
Tatsuya Jinmei and Atsushi Onoe,
.IR "An Extension of Format for IPv6 Scoped Addresses" ,
internet draft, work in progress.
ftp://ftp.ietf.org/internet-drafts/draft-ietf-ipngwg-scopedaddr-format-02.txt
.LP
Craig Metz,
.IR "Protocol Independence Using the Sockets API" ,
Proceedings of the freenix track:
2000 USENIX annual technical conference, June 2000.
http://www.usenix.org/publications/library/proceedings/usenix2000/freenix/metzprotocol.html