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.Dd May 9, 2025
.Dt LINK_ADDR 3
.Os
.Sh NAME
.Nm link_addr ,
.Nm link_ntoa ,
.Nm link_ntoa_r
.Nd elementary address specification routines for link level access
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In sys/types.h
.In sys/socket.h
.In net/if_dl.h
.Ft int
.Fn link_addr "const char *addr" "struct sockaddr_dl *sdl"
.Ft char *
.Fn link_ntoa "const struct sockaddr_dl *sdl"
.Ft int
.Fn link_ntoa_r "const struct sockaddr_dl *sdl" "char *obuf" "size_t *buflen"
.Sh DESCRIPTION
The routine
.Fn link_addr
parses a character string
.Fa addr
representing a link-level address,
and stores the resulting address in the structure pointed to by
.Fa sdl .
A link-level address consists of an optional interface name, followed by
a colon (which is required in all cases), followed by an address
consisting of either a string of hexadecimal digits, or a series of
hexadecimal octets separated by one of the characters
.Sq "." ,
.Sq ":" ,
or
.Sq - .
.Pp
The routine
.Fn link_ntoa
takes
a link-level
address and returns an
.Tn ASCII
string representing some of the information present,
including the link level address itself, and the interface name
or number, if present.
The returned string is stored in a static buffer.
This facility is experimental and is
still subject to change.
.Pp
The routine
.Fn link_ntoa_r
behaves like
.Fn link_ntoa ,
except the string is placed in the provided buffer instead of a static
buffer.
The caller should initialize
.Fa buflen
to the number of bytes available in
.Fa obuf .
On return,
.Fa buflen
is set to the actual number of bytes required for the output buffer,
including the NUL terminator.
If
.Fa obuf
is NULL, then
.Fa buflen
is set as described, but nothing is written.
This may be used to determine the required length of the buffer before
calling
.Fn link_ntoa_r
a second time.
.Pp
For
.Fn link_addr ,
the string
.Fa addr
may contain
an optional network interface identifier of the form
.Dq "name unit-number" ,
suitable for the first argument to
.Xr ifconfig 8 ,
followed in all cases by a colon and
an interface address in the form of
groups of hexadecimal digits
separated by periods.
Each group represents a byte of address;
address bytes are filled left to right from
low order bytes through high order bytes.
.Pp
.\" A regular expression may make this format clearer:
.\" .Bd -literal -offset indent
.\" ([a-z]+[0-9]+:)?[0-9a-f]+(\e.[0-9a-f]+)*
.\" .Ed
.\" .Pp
Thus
.Li le0:8.0.9.13.d.30
represents an ethernet address
to be transmitted on the first Lance ethernet interface.
.Sh RETURN VALUES
The
.Fn link_ntoa
function
always returns a null terminated string.
.Pp
The
.Fn link_ntoa_r
function returns 0 on success, or -1 if the provided buffer was not
large enough; in the latter case, the contents of the buffer are
indeterminate, but a trailing NUL will always be written if the buffer
was at least one byte in size.
.Pp
The
.Fn link_addr
function returns 0 on success.
If the address did not appear to be a valid link-level address, -1 is
returned and
.Va errno
is set to indicate the error.
.Sh SEE ALSO
.Xr getnameinfo 3
.Sh HISTORY
The
.Fn link_addr
and
.Fn link_ntoa
functions appeared in
.Bx 4.3 Reno .
The
.Fn link_ntoa_r
function appeared in
.Fx 15.0 .
.Sh BUGS
The returned values for link_ntoa
reside in a static memory area.
.Pp
If the
.Va sdl_len
field of the link socket address
.Fa sdl
is 0,
.Fn link_ntoa
will not insert a colon before the interface address bytes.
If this translated address is given to
.Fn link_addr
without inserting an initial colon,
the latter will not interpret it correctly.