LINK_ADDR(3)           FreeBSD Library Functions Manual           LINK_ADDR(3)

NAME

     link_addr, link_ntoa, link_ntoa_r - elementary address specification
     routines for link level access

LIBRARY

     Standard C Library (libc, -lc)

SYNOPSIS

     #include <sys/types.h>
     #include <sys/socket.h>
     #include <net/if_dl.h>

     int
     link_addr(const char *addr, struct sockaddr_dl *sdl);

     char *
     link_ntoa(const struct sockaddr_dl *sdl);

     int
     link_ntoa_r(const struct sockaddr_dl *sdl, char *obuf, size_t *buflen);

DESCRIPTION

     The routine link_addr() parses a character string addr representing a
     link-level address, and stores the resulting address in the structure
     pointed to by 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 `.', `:', or `-'.

     The routine link_ntoa() takes a link-level address and returns an 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.

     The routine link_ntoa_r() behaves like link_ntoa(), except the string is
     placed in the provided buffer instead of a static buffer.  The caller
     should initialize buflen to the number of bytes available in obuf.  On
     return, buflen is set to the actual number of bytes required for the
     output buffer, including the NUL terminator.  If obuf is NULL, then
     buflen is set as described, but nothing is written.  This may be used to
     determine the required length of the buffer before calling link_ntoa_r()
     a second time.

     For link_addr(), the string addr may contain an optional network
     interface identifier of the form "name unit-number", suitable for the
     first argument to 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.

     Thus le0:8.0.9.13.d.30 represents an ethernet address to be transmitted
     on the first Lance ethernet interface.

RETURN VALUES

     The link_ntoa() function always returns a null terminated string.

     The 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.

     The link_addr() function returns 0 on success.  If the address did not
     appear to be a valid link-level address, -1 is returned and errno is set
     to indicate the error.

SEE ALSO

     getnameinfo(3)

HISTORY

     The link_addr() and link_ntoa() functions appeared in 4.3BSD-Reno.  The
     link_ntoa_r() function appeared in FreeBSD 15.0.

BUGS

     The returned values for link_ntoa reside in a static memory area.

     If the sdl_len field of the link socket address sdl is 0, link_ntoa()
     will not insert a colon before the interface address bytes.  If this
     translated address is given to link_addr() without inserting an initial
     colon, the latter will not interpret it correctly.

FreeBSD 15.1-STABLE-HBSD          May 9, 2025                     LINK_ADDR(3)