December 10, 1999 INET6_OPTION_SPACE 3

### NAME

inet6_option_space, inet6_option_init, inet6_option_append, inet6_option_alloc, inet6_option_next, inet6_option_find - IPv6 Hop-by-Hop and Destination Options manipulation

### SYNOPSIS

```int` inet6_option_space(```int nbytes`)
```int` inet6_option_init(```void *bp``, struct cmsghdr **cmsgp``, int type`)
```int` inet6_option_append(```struct cmsghdr *cmsg``, const uint8_t *typep``, int multx``, int plusy`)
```uint8_t *` inet6_option_alloc(```struct cmsghdr *cmsg``, int datalen``, int multx``, int plusy`)
```int` inet6_option_next(```const struct cmsghdr *cmsg``, uint8_t **tptrp`)
```int` inet6_option_find(```const struct cmsghdr *cmsg``, uint8_t **tptrp``, int type`)

### DESCRIPTION

Building and parsing the Hop-by-Hop and Destination options is complicated due to alignment constraints, padding and ancillary data manipulation. RFC 2292 defines a set of functions to help the application. The function prototypes for these functions are all in the <```netinet/in.h`> header.

#### inet6_option_space

inet6_option_space(``) returns the number of bytes required to hold an option when it is stored as ancillary data, including the ```cmsghdr` structure at the beginning, and any padding at the end (to make its size a multiple of 8 bytes.) The argument is the size of the structure defining the option, which must include any pad bytes at the beginning ```y` in the alignment term `````xn``+``y`'' (the value,) the type byte, the length byte, and the option data.

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 ```N-1` ```cmsghdr` structures, where ```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 (appendix B of [RFC 2460]).

#### inet6_option_init

inet6_option_init(``) is called once per ancillary data object that will contain either Hop-by-Hop or Destination options. It returns ```0` on success or ```-1` on an error.

```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 inet6_option_append(``) and inet6_option_alloc(``).

```cmsgp` is a pointer to a pointer to a ```cmsghdr` structure. ```*cmsgp` is initialized by this function to point to the ```cmsghdr` structure constructed by this function in the buffer pointed to by ```bp`.

```type` is either ```IPV6_HOPOPTS` or ```IPV6_DSTOPTS`. This ```type` is stored in the ```cmsg_type` member of the ```cmsghdr` structure pointed to by ```*cmsgp`.

#### 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 inet6_option_init(``). This function returns ```0` if it succeeds or ```-1` on an error.

```cmsg` is a pointer to the ```cmsghdr` structure that must have been initialized by inet6_option_init(``).

```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 (the type-length-value, or TLV) before calling this function.

The option type must have a value from ```2` to ```255`, inclusive. and ```1` are reserved for the ```Pad1` and ```PadN` options, respectively. (```0`)

The option data length must have a value between ```0` and ```255`, inclusive, and is the length of the option data that follows.

```multx` is the value ```x` in the alignment term `````xn``+``y`''. It must have a value of ```1`, ```2`, ```4`, or ```8`.

```plusy` is the value ```y` in the alignment term `````xn``+``y`''. It must have a value between ```0` and ```7`, inclusive.

#### 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 inet6_option_init(``). This function returns a pointer to the 8-bit option type field that starts the option on success, or ```NULL` on an error.

The difference between this function and 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.

```cmsg` is a pointer to the ```cmsghdr` structure that must have been initialized by inet6_option_init(``).

```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. inet6_option_append(``) function does not need a data length argument since the option data length must already be stored by the caller. (The)

```multx` is the value ```x` in the alignment term `````xn``+``y`''. It must have a value of ```1`, ```2`, ```4`, or ```8`.

```plusy` is the value ```y` in the alignment term `````xn``+``y`''. It must have a value between ```0` and ```7`, inclusive.

#### 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 ```0` and ```*tptrp` points to the 8-bit option type field data length, followed by the option data (which is followed by the 8-bit option.) If no more options remain to be processed, the return value is ```-1` and ```*tptrp` is ```NULL`. If an error occurs, the return value is ```-1` and ```*tptrp` is not ```NULL`.

```cmsg` is a pointer to ```cmsghdr` structure of which ```cmsg_level` equals ```IPPROTO_IPV6` and ```cmsg_type` equals either ```IPV6_HOPOPTS` or ```IPV6_DSTOPTS`.

```tptrp` is a pointer to a pointer to an 8-bit byte and ```*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, ```*tptrp` must be set to ```NULL`.

Each time this function returns success, ```*tptrp` points to the 8-bit option type field for the next option to be processed.

#### inet6_option_find

This function is similar to the previously described 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. ```cmsg` is a pointer to ```cmsghdr` structure of which ```cmsg_level` equals ```IPPROTO_IPV6` and ```cmsg_type` equals either ```IPV6_HOPOPTS` or ```IPV6_DSTOPTS`.

```tptrp` is a pointer to a pointer to an 8-bit byte and ```*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, ```*tptrp` must be set to ```NULL`. `` This function starts searching for an option of the specified type beginning after the value of ```*tptrp`. If an option of the specified type is located, this function returns ```0` and ```*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 ```-1` and ```*tptrp` is ```NULL`. If an error occurs, the return value is ```-1` and ```*tptrp` is not ```NULL`.

### EXAMPLES

RFC 2292 gives comprehensive examples in chapter 6.

### DIAGNOSTICS

inet6_option_init(``) and inet6_option_append(``) return ```0` on success or ```-1` on an error.

inet6_option_alloc(``) returns ```NULL` on an error.

On errors, inet6_option_next(``) and inet6_option_find(``) return ```-1` setting ```*tptrp` to non ```NULL` value.

### SEE ALSO

### STANDARDS

The functions are documented in ``Advanced Sockets API for IPv6'' (RFC 2292).

### HISTORY

The implementation first appeared in KAME advanced networking kit.

### BUGS

The text was shamelessly copied from RFC 2292.