.\"	$OpenBSD: inet6_option_space.3,v 1.10 2003/05/01 19:17:37 jmc Exp $
.\"	$KAME: inet6_option_space.3,v 1.7 2000/05/17 14:32:13 itojun Exp $
.\"
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.Dd December 10, 1999
.Dt INET6_OPTION_SPACE 3
.Os
.\"
.Sh NAME
.Nm inet6_option_space ,
.Nm inet6_option_init ,
.Nm inet6_option_append ,
.Nm inet6_option_alloc ,
.Nm inet6_option_next ,
.Nm inet6_option_find
.Nd IPv6 Hop-by-Hop and Destination Options manipulation
.\"
.Sh SYNOPSIS
.Fd #include <netinet/in.h>
.Ft "int"
.Fn inet6_option_space "int nbytes"
.Ft "int"
.Fn inet6_option_init "void *bp" "struct cmsghdr **cmsgp" "int type"
.Ft "int"
.Fn inet6_option_append "struct cmsghdr *cmsg" "const u_int8_t *typep" "int multx" "int plusy"
.Ft "u_int8_t *"
.Fn inet6_option_alloc "struct cmsghdr *cmsg" "int datalen" "int multx" "int plusy";
.Ft "int"
.Fn inet6_option_next "const struct cmsghdr *cmsg" "u_int8_t **tptrp"
.Ft "int"
.Fn inet6_option_find "const struct cmsghdr *cmsg" "u_int8_t **tptrp" "int type"
.\"
.Sh DESCRIPTION
.\"
Building and parsing the Hop-by-Hop and Destination options is
complicated due to alignment constraints, padding and
ancillary data manipulation.
RFC2292 defines a set of functions to help the application.
The function prototypes for
these functions are all in the
.Aq Pa netinet/in.h
header.
.\"
.Ss inet6_option_space
.Fn inet6_option_space
returns the number of bytes required to hold an option when it is stored as
ancillary data, including the
.Li cmsghdr
structure at the beginning,
and any padding at the end
.Po
to make its size a multiple of 8 bytes
.Pc .
The argument is the size of the structure defining the option,
which must include any pad bytes at the beginning
.Po
the value
.Li y
in the alignment term
.Dq Li xn + y
.Pc ,
the type byte, the length byte, and the option data.
.Pp
Note: If multiple options are stored in a single ancillary data
object, which is the recommended technique, this function
overestimates the amount of space required by the size of
.Li N-1
.Li cmsghdr
structures,
where
.Li N
is the number of options to be stored in the object.
This is of little consequence, since it is assumed that most
Hop-by-Hop option headers and Destination option headers carry only
one option
.Pq appendix B of [RFC-2460] .
.\"
.Ss inet6_option_init
.Fn inet6_option_init
is called once per ancillary data object that will
contain either Hop-by-Hop or Destination options.
It returns
.Li 0
on success or
.Li -1
on an error.
.Pp
.Fa bp
is a pointer to previously allocated space that will contain the
ancillary data object.
It must be large enough to contain all the
individual options to be added by later calls to
.Fn inet6_option_append
and
.Fn inet6_option_alloc .
.Pp
.Fa cmsgp
is a pointer to a pointer to a
.Li cmsghdr
structure.
.Fa *cmsgp
is initialized by this function to point to the
.Li cmsghdr
structure constructed by this function in the buffer pointed to by
.Fa bp .
.Pp
.Fa type
is either
.Dv IPV6_HOPOPTS
or
.Dv IPV6_DSTOPTS .
This
.Fa type
is stored in the
.Li cmsg_type
member of the
.Li cmsghdr
structure pointed to by
.Fa *cmsgp .
.\"
.Ss inet6_option_append
This function appends a Hop-by-Hop option or a Destination option
into an ancillary data object that has been initialized by
.Fn inet6_option_init .
This function returns
.Li 0
if it succeeds or
.Li -1
on an error.
.Pp
.Fa cmsg
is a pointer to the
.Li cmsghdr
structure that must have been
initialized by
.Fn inet6_option_init .
.Pp
.Fa typep
is a pointer to the 8-bit option type.
It is assumed that this
field is immediately followed by the 8-bit option data length field,
which is then followed immediately by the option data.
The caller
initializes these three fields
.Pq the type-length-value, or TLV
before calling this function.
.Pp
The option type must have a value from
.Li 2
to
.Li 255 ,
inclusive.
.Po
.Li 0
and
.Li 1
are reserved for the
.Li Pad1
and
.Li PadN
options, respectively.
.Pc
.Pp
The option data length must have a value between
.Li 0
and
.Li 255 ,
inclusive, and is the length of the option data that follows.
.Pp
.Fa multx
is the value
.Li x
in the alignment term
.Dq Li xn + y .
It must have a value of
.Li 1 ,
.Li 2 ,
.Li 4 ,
or
.Li 8 .
.Pp
.Fa plusy
is the value
.Li y
in the alignment term
.Dq Li xn + y .
It must have a value between
.Li 0
and
.Li 7 ,
inclusive.
.\"
.Ss inet6_option_alloc
This function appends a Hop-by-Hop option or a Destination option
into an ancillary data object that has been initialized by
.Fn inet6_option_init .
This function returns a pointer to the 8-bit
option type field that starts the option on success, or
.Dv NULL
on an error.
.Pp
The difference between this function and
.Fn inet6_option_append
is that the latter copies the contents of a previously built option into
the ancillary data object while the current function returns a
pointer to the space in the data object where the option's TLV must
then be built by the caller.
.Pp
.Fa cmsg
is a pointer to the
.Li cmsghdr
structure that must have been
initialized by
.Fn inet6_option_init .
.Pp
.Fa datalen
is the value of the option data length byte for this option.
This value is required as an argument to allow the function to
determine if padding must be appended at the end of the option.
(The
.Fn inet6_option_append
function does not need a data length argument
since the option data length must already be stored by the caller.)
.Pp
.Fa multx
is the value
.Li x
in the alignment term
.Dq Li xn + y .
It must have a value of
.Li 1 ,
.Li 2 ,
.Li 4 ,
or
.Li 8 .
.Pp
.Fa plusy
is the value
.Li y
in the alignment term
.Dq Li xn + y .
It must have a value between
.Li 0
and
.Li 7 ,
inclusive.
.\"
.Ss inet6_option_next
This function processes the next Hop-by-Hop option or Destination
option in an ancillary data object.
If another option remains to be
processed, the return value of the function is
.Li 0
and
.Fa *tptrp
points to
the 8-bit option type field
(which is followed by the 8-bit option
data length, followed by the option data).
If no more options remain
to be processed, the return value is
.Li -1
and
.Fa *tptrp
is
.Dv NULL .
If an error occurs, the return value is
.Li -1
and
.Fa *tptrp
is not
.Dv NULL .
.Pp
.Fa cmsg
is a pointer to
.Li cmsghdr
structure of which
.Li cmsg_level
equals
.Dv IPPROTO_IPV6
and
.Li cmsg_type
equals either
.Dv IPV6_HOPOPTS
or
.Dv IPV6_DSTOPTS .
.Pp
.Fa tptrp
is a pointer to a pointer to an 8-bit byte and
.Fa *tptrp
is used
by the function to remember its place in the ancillary data object
each time the function is called.
The first time this function is
called for a given ancillary data object,
.Fa *tptrp
must be set to
.Dv NULL .
.Pp
Each time this function returns success,
.Fa *tptrp
points to the 8-bit
option type field for the next option to be processed.
.\"
.Ss inet6_option_find
This function is similar to the previously described
.Fn inet6_option_next
function, except this function lets the caller
specify the option type to be searched for, instead of always
returning the next option in the ancillary data object.
.Fa cmsg
is a
pointer to
.Li cmsghdr
structure of which
.Li cmsg_level
equals
.Dv IPPROTO_IPV6
and
.Li cmsg_type
equals either
.Dv IPV6_HOPOPTS
or
.Dv IPV6_DSTOPTS .
.Pp
.Fa tptrp
is a pointer to a pointer to an 8-bit byte and
.Fa *tptrp
is used
by the function to remember its place in the ancillary data object
each time the function is called.
The first time this function is
called for a given ancillary data object,
.Fa *tptrp
must be set to
.Dv NULL .
.Pp
This function starts searching for an option of the specified type
beginning after the value of
.Fa *tptrp .
If an option of the specified
type is located, this function returns
.Li 0
and
.Fa *tptrp
points to the 8-
bit option type field for the option of the specified type.
If an
option of the specified type is not located, the return value is
.Li -1
and
.Fa *tptrp
is
.Dv NULL .
If an error occurs, the return value is
.Li -1
and
.Fa *tptrp
is not
.Dv NULL .
.\"
.Sh DIAGNOSTICS
.Fn inet6_option_init
and
.Fn inet6_option_append
return
.Li 0
on success or
.Li -1
on an error.
.Pp
.Fn inet6_option_alloc
returns
.Dv NULL
on an error.
.Pp
On errors,
.Fn inet6_option_next
and
.Fn inet6_option_find
return
.Li -1
setting
.Fa *tptrp
to non
.Dv NULL
value.
.\"
.Sh EXAMPLES
RFC2292 gives comprehensive examples in chapter 6.
.\"
.Sh SEE ALSO
.Rs
.%A W. Stevens
.%A M. Thomas
.%T "Advanced Sockets API for IPv6"
.%N RFC2292
.%D February 1998
.Re
.Rs
.%A S. Deering
.%A R. Hinden
.%T "Internet Protocol, Version 6 (IPv6) Specification"
.%N RFC2460
.%D December 1998
.Re
.\"
.Sh HISTORY
The implementation first appeared in KAME advanced networking kit.
.\"
.Sh STANDARDS
The functions
are documented in
.Dq Advanced Sockets API for IPv6
.Pq RFC2292 .
.\"
.Sh BUGS
The text was shamelessly copied from RFC2292.