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.\" Copyright 1996 Massachusetts Institute of Technology
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.\"     $ANA: addr2ascii.3,v 1.1 1996/06/13 18:41:46 wollman Exp $
.\"     $Id: addr2ascii.3,v 1.2 2001-09-27 12:01:53 chris Exp $
.\"
.Dd June 13, 1996
.Dt ADDR2ASCII 3
.Os
.Sh NAME
.Nm addr2ascii ,
.Nm ascii2addr
.Nd Generic address formatting routines
.Sh SYNOPSIS
.Fd #include <sys/types.h>
.Fd #include <netinet/in.h>
.Fd #include <arpa/inet.h>
.Ft "char *"
.Fn addr2ascii "int af" "const void *addrp" "int len" "char *buf"
.Ft int
.Fn ascii2addr "int af" "const char *ascii" "void *result"
.Sh DESCRIPTION
The routines
.Fn addr2ascii
and
.Fn ascii2addr
are used to convert network addresses between binary form and a
printable form appropriate to the address family.  Both functions take
an
.Fa af
argument, specifying the address family to be used in the conversion
process.
(Currently, only the
.Dv AF_INET
and
.Dv AF_LINK
address families are supported.)
.Pp
The
.Fn addr2ascii
function
is used to convert binary, network-format addresses into printable
form.  In addition to
.Fa af ,
there are three other arguments.  The
.Fa addrp
argument is a pointer to the network address to be converted.
The
.Fa len
argument is the length of the address.  The
.Fa buf
argument is an optional pointer to a caller-allocated buffer to hold
the result; if a null pointer is passed,
.Fn addr2ascii
uses a statically-allocated buffer.
.Pp
The
.Fn ascii2addr
function performs the inverse operation to
.Fn addr2ascii .
In addition to
.Fa af ,
it takes two parameters,
.Fa ascii
and
.Fa result .
The
.Fa ascii
parameter is a pointer to the string which is to be converted into
binary.  The
.Fa result
parameter is a pointer to an appropriate network address structure for
the specified family.
.Pp
The following gives the appropriate structure to use for binary
addresses in the specified family:
.Pp
.Bl -tag -width AF_INETxxxx -compact
.It Dv AF_INET
.Li struct in_addr
.Pq in Aq Pa netinet/in.h
.It Dv AF_LINK
.Li struct sockaddr_dl
.Pq in Aq Pa net/if_dl.h
.\" .It Dv AF_INET6
.\" .Li struct in6_addr
.\" .Pq in Aq Pa netinet6/in6.h
.El
.Sh RETURN VALUES
The
.Fn addr2ascii
function returns the address of the buffer it was passed, or a static
buffer if the a null pointer was passed; on failure, it returns a null
pointer.
The
.Fn ascii2addr
function returns the length of the binary address in bytes, or -1 on
failure.
.Sh EXAMPLES
The
.Xr inet 3
functions
.Fn inet_ntoa
and
.Fn inet_aton
could be implemented thusly:
.Bd -literal -offset indent
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>

char *
inet_ntoa(struct in_addr addr)
{
        return addr2ascii(AF_INET, &addr, sizeof addr, 0);
}

int
inet_aton(const char *ascii, struct in_addr *addr)
{
        return (ascii2addr(AF_INET, ascii, addr) 
            == sizeof(*addr));
}
.Ed
.Pp
In actuality, this cannot be done because
.Fn addr2ascii
and
.Fn ascii2addr
are implemented in terms of the
.Xr inet 3
functions, rather than the other way around.
.Sh ERRORS
When a failure is returned,
.Li errno
is set to one of the following values:
.Bl -tag -width [EPROTONOSUPPORT]
.It Bq Er ENAMETOOLONG
The
.Fn addr2ascii
routine was passed a
.Fa len
parameter which was inappropriate for the address family given by
.Fa af .
.It Bq Er EPROTONOSUPPORT
Either routine was passed an
.Fa af
parameter other than
.Dv AF_INET
or
.Dv AF_LINK .
.It Bq Er EINVAL
The string passed to
.Fn ascii2addr
was improperly formatted for address family
.Fa af .
.El
.Sh SEE ALSO
.Xr inet 3 ,
.Xr linkaddr 3 ,
.Xr inet 4
.Sh HISTORY
An interface close to this one was originally suggested by Craig
Partridge.  This particular interface originally appeared in the
.Tn INRIA
.Tn IPv6
implementation.
.Sh AUTHORS
Code and documentation by
.An Garrett A. Wollman ,
MIT Laboratory for Computer Science.
.Sh BUGS
The original implementations supported IPv6.  This support should
eventually be resurrected.  The
.Tn NRL
implementation also included support for the
.Dv AF_ISO
and
.Dv AF_NS
address families.
.Pp
The genericity of this interface is somewhat questionable.  A truly
generic interface would provide a means for determining the length of
the buffer to be used so that it could be dynamically allocated, and
would always require a
.Dq Li "struct sockaddr"
to hold the binary address.  Unfortunately, this is incompatible with existing
practice.  This limitation means that a routine for printing network
addresses from arbitrary address families must still have internal
knowledge of the maximum buffer length needed and the appropriate part
of the address to use as the binary address.