libraw1394/doc/libraw1394.sgml

<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook V3.1//EN"[]>

<book>
  <bookinfo>

    <title>libraw1394</title>
    <subtitle>version 0.9</subtitle>

    <copyright>
      <year>2001</year>
      <holder>Andreas Bombe</holder>
    </copyright>

  </bookinfo>

  <toc></toc>

  <chapter id="introduction">
    <title>Introduction</title>

    <para>
      The Linux kernel's IEEE 1394 subsystem provides access to the raw 1394 bus
      through the raw1394 module.  This includes the standard 1394 transactions
      (read, write, lock) on the active side, isochronous stream receiving and
      sending and dumps of data written to the FCP_COMMAND and FCP_RESPONSE
      registers.  raw1394 uses a character device to communicate to user
      programs using a special protocol.
    </para>

    <para>
      libraw1394 was created with the intent to hide that protocol from
      applications so that
      <orderedlist numeration="arabic">
	<listitem>
	  <para>
	    the protocol has to be implemented correctly only once.
	  </para>
	</listitem>
	<listitem>
	  <para>
	    all work can be done using easy to understand functions instead of
	    handling a complicated command structure.
	  </para>
	</listitem>
	<listitem>
	  <para>
	    only libraw1394 has to be changed when raw1394's interface changes.
	  </para>
	</listitem>
      </orderedlist>
    </para>

    <para>
      To fully achieve the goals (especially 3) libraw1394 is distributed under
      the LGPL (Lesser General Public License - see file COPYING.LIB for more
      information.) to allow linking with any program, be it open source or
      binary only.  The requirements are that the libraw1394 part can be
      replaced (relinked) with another version of the library and that changes
      to libraw1394 itself fall under LGPL again.  Refer to the LGPL text for
      details.
    </para>
  </chapter>

  <chapter id="intro1394">
    <title>Short Introduction into IEEE 1394</title>

    <para>
      IEEE 1394 in fact defines two types of hardware implementations for this
      bus system, cable and backplane.  The only one described here and
      supported by the Linux subsystem is the cable implementation.  Most people
      not familiar with the standard probably don't even know that there is
      something else than the 1394 cable specification.
    </para>

    <para>
      If you are familiar with CSR architectures (as defined in --FIXME--), then
      you already know most of 1394, which is a CSR implementation.
    </para>

    <sect1>
      <title>Bus Structure</title>

      <para>
	The basic data structures defined in the standard and used in this
	document are the quadlet (32 bit quantity) and the octlet (64 bit
	quantity) and blocks (any quantity of bytes).  The bus byte ordering is
	big endian.  A transmission can be sent at one of multiple possible
	speeds, from the slowest 100 Mbit/s over 200 and 400 Mbit/s up to 3.2
	Gbit/s in the future (these speeds are also referred to as S100, S200,
	...).
      </para>

      <para>
	A 1394 bus consists of up to 64 nodes (with multiple buses possibly
	being connected, but that is outside of the scope of this document and
	not completely standardized yet), each having a local address space with
	48 bit wide addressing.  Each node is addressed with a 16 bit address,
	which is further divided into a 10 bit bus ID and a 6 bit local node ID.
	The highest value for both is a special value.  Bus ID equal to 1023
	means "local bus" (the bus the node is connected to), node ID equal to
	63 means "all nodes" (broadcast).
      </para>

      <para>
	The whole bus can thus be seen as a linear 64 bit address space by
	concatenating the node address (most significant bits) and and node
	address (least significant bits).  libraw1394 treats them separately in
	function arguments to save the application some fiddling with the bits.
	The node IDs are completely dynamic and determined during the bus reset.
      </para>

      <para>
	Unlike other buses there aren't many transactions or commands defined,
	higher level commands are defined in terms of addresses accessed instead
	of separate transaction types (comparable to memory mapped registers in
	hardware).  The 1394 transactions are:

	<itemizedlist>
	  <listitem>
	    <para>read (quadlets and blocks)</para>
	  </listitem>
	  <listitem>
	    <para>write (quadlets and blocks)</para>
	  </listitem>
	  <listitem>
	    <para>lock (some atomic modifications)</para>
	  </listitem>
	</itemizedlist>

	There is also the isochronous transaction (the above three are called
	asynchronous transactions), which is a broadcast stream with guaranteed
	bandwidth.  It doesn't contain any address but is distinguished by a 6
	bit channel number.
      </para>

      <para>
	The bus view is only logical, physically it consists of many
	point-to-point connections between nodes with every node forwarding data
	it receives to every other port which is capable of the speed the
	transaction is sent at (thus a S200 node in the path between two S400
	nodes would limit their communication speed to S200).  It forms a tree
	structure with all but one node having a parent and a number of
	children.  One node is the root node and has no parents.
      </para>
    </sect1>

    <sect1>
      <title>Bus Reset</title>
      
      <para>
	A bus reset occurs whenever the state of any node changes (including
	addition and removal of nodes).  At the beginning a root node is chosen,
	then the tree identification determines for every node which port is
	connected to a parent, child or nothing.  Then the SelfID phase begins.
	The root node sends a SelfID grant on its first port connected to a
	child.  If that is not a leaf node, it will itself forward the grant to
	its first child.  When a leaf node gets a grant, it will pick the lowest
	node ID not yet in use (starting with 0) and send out a SelfID packet
	with its node ID and more information, then acknowledge the SelfID grant
	to its parent, which will send a grant to its next child until it
	configured all its children, then pick a node ID itself, send SelfID
	packet and ack to parent.
      </para>

      <para>
	After bus reset the used node IDs are in a sequential range with no
	holes starting from 0 with the root node having the highest ID.  This
	also means that node IDs can change for many or all nodes with the
	insertion of a new node or moving the role of root to another node.  In
	libraw1394 all transactions are tagged automatically with a generation
	number which is increased in every bus reset and transactions with an
	obsolete generation will fail in order to avoid targetting the wrong
	node.  Nodes have to be identified in a different way than their
	volatile node IDs, namely by reading their globally unique ID (GUID)
	contained in the configuration ROM.
      </para>
    </sect1>

    <sect1>
      <title>Transactions</title>

      <para>
	The packets transmitted on the bus are acknowledged by the receiving end
	unless they are broadcast packets (broadcast writes and isochronous
	packets).  The acknowledge code contains an error code, which either
	signifies error, success or packet pending.  In the first two cases the
	transaction completes, in the last a response packet will follow at a
	later time from the targetted node to the source node (this is called a
	split transaction).  Only writes can succeed and complete in the ack
	code, reads and locks require a response.  Error and packet pending can
	happen for every transaction.
      </para>

      <para>
	The response packets contain a response code (rcode) which signifies
	success or type of error.  libraw1394 contains a function to convert
	ack/rcode pairs into errno codes which convey roughly the same meaning.
	This is done automatically for the synchronous read/write/lock wrapper
	functions, i.e. they return a negative value for failure and a standard
	error code can be found in the global variable <symbol>errno</symbol>.
      </para>

      <para>
	For read and write there are two different types, quadlet and block.
	The quadlet types have all their payload (exactly one quadlet) in the
	packet header, the block types have a variable length data block
	appended to the header.  Programs using libraw1394 don't have to care
	about that, quadlet transactions are automatically used when the data
	length is 4 bytes and block transactions otherwise.
      </para>

      <para>
	The lock transaction has several extended transaction codes defined
	which choose the atomic operation to perform, the most used being the
	compare-and-swap (code 0x2).  The transaction passes the data value and
	(depending on the operation) the arg value to the target node and
	returns the old value at the target address, but only when the
	transaction does not have an error.  All three values are of the same
	size, either one quadlet or one octlet.
      </para>

      <para>
	In the compare-and-swap case, the data value is written to the target
	address if the old value is identical to the arg value.  The old value
	is returned in any case and can be used to find out whether the swap
	succeeded by repeating the compare locally.  Isochronous resource
	allocation is done using compare-and-swap, as described below.  Since
	the old value is always returned, it more efficient to do the first
	attempt with the reset value of the target register as arg instead of
	reading it first.  Repeat with the returned old value as new arg value
	if it didn't succeed.
      </para>
    </sect1>

    <sect1>
      <title>Bus Management</title>

      <para>
	There are three basic bus service nodes defined in IEEE 1394 (higher
	level protocols may define more): cycle master, isochronous resource
	manager and bus manager.  These positions are contended for in and
	shortly after the bus reset and may all be taken by a single node.  A
	node does not have to support being any of those but if it is bus
	manager capable it also has to be iso manager capable, if it is iso
	manager capable it also has to be cycle master capable.
      </para>

      <para>
	The cycle master sends 8000 cycle start packets per second, which
	initiate an iso cycle.  Without that, no isochronous transmission is
	possible.  Only the root node is allowed to be cycle master, if it is
	not capable then no iso transmissions can occur (and the iso or bus
	manager have to select another node to become root and initiate a bus
	reset).
      </para>

      <para>
	The isochronous resource manager is the central point where channel and
	bandwidth allocations are stored.  A bit in the SelfID shows whether a
	node is iso manager capable or not, the iso manager capable node with
	the highest ID wins the position after a bus reset.  Apart from
	containing allocation registers, this one doesn't do much.  Only if
	there is no bus manager, it may determine a cycle master capable node to
	become root and initiate a bus reset.
      </para>

      <para>
	The bus manager has more responsibilities: power management (calculate
	power provision and consumption on the bus and turn on disabled nodes if
	enough power is available), bus optimization (calculate an effective gap
	count, optimize the topology by selecting a better positioned node for
	root) and some registers relevant to topology (topology map containing
	the SelfIDs of the last reset and a speed map, which is obsoleted in
	IEEE 1394a).  The bus manager capable nodes contend for the role by
	doing a lock transaction on the bus manager ID register in the iso
	manager, the first to successfully complete the transaction wins the
	role.
      </para>
    </sect1>

    <sect1>
      <title>Isochronous Transmissions</title>

      <para>
	Nodes can allocate a channel and bandwidth for isochronous transmissions
	at the iso manager to broadcast timing critical data (e.g. multimedia
	streams) on the bus.  However these transmissions are unreliable, there
	is no guarantee that every packet reaches the intended recipients (the
	software and hardware involved also take iso packets a bit more
	lightly).  After a cycle start packet, the isochronous cycle begins and
	every node can transmit iso packets, however only one packet per channel
	is allowed.  As soon as a gap of a certain length appears (i.e. no node
	sends anymore), the iso cycle ends and the rest of the time until the
	next cycle start is reserved for asynchronous packets.
      </para>

      <para>
	The channel register on the iso manager consists of 64 bits, each of
	which signifies one channel.  A channel can be allocated by any node by
	doing a compare-swap lock request with the new bitmask.  Likewise the
	bandwidth can be allocated by doing a lock request with the new value.
	The bandwidth register contains the remaining time available for every
	iso cycle.  Since you allocate time, the maximum data you are allowed to
	put into an iso packet depends on the speed you will send at.
      </para>

      <para>
	On every bus reset, the resource registers are resetted to their initial
	values (all channels free, all bandwidth minus some amount set aside for
	asynchronous communication available), this has to happen since the
	isochronous manager may have moved to another node.  Isochronous
	transmissions may continue with the old allocations for a certain
	(FIXME) amount of time.  During that time, the nodes have to reallocate
	their resources and no new allocations are allowed to occur.  Only after
	this period new allocations may be done, this avoids nodes losing their
	allocations over a bus reset.
      </para>

      <para>
	libraw1394 does not provide special functions for allocating iso
	resources nor does it clean up after programs when they exit.  Protocols
	exist that require the first node to use some resources to allocate it
	and then leave it for the last node using it to deallocate it.  This may
	be different nodes, so automatic behaviour would be very undesirable in
	these cases.
      </para>

  </chapter>

  <chapter id="general">
    <title>Data Structures and Program Flow</title>

    <sect1>
      <title>Overview</title>
      
      <para>
	The 1394 subsystem in Linux is divided into the classical three layers,
	like most other interface subsystems in Linux.  The subsystem consists
	of the core, which provides basic services like handling of the 1394
	protocol (converting the abstract transactions into packets and back),
	collecting information about bus and nodes and providing some services
	to the bus that are required to be available for standards conformant
	node (e.g. CSR registers).  Below that are the hardware drivers, which
	handle converting packets and bus events to and from hardware accesses.
      </para>

      <para>
	Above the core are the highlevel drivers, which use the services
	provided by the core to implement protocols for certain devices and act
	as drivers to these.  raw1394 is one such driver, however it is not
	specialized to handle one kind of device but is designed to accept
	commands from user space to do any transaction wanted (as far as
	possible from current core design).  Using raw1394, normal applications
	can access 1394 nodes on the bus and it is not neccessary to write
	kernel code just for that.
      </para>

      <para>
	raw1394 communicates to user space like most device drivers do, through
	device files in /dev.  It uses a defined protocol on that device, but
	applications don't have to and should not care about that.  All of this
	is taken care of by libraw1394, which provides a set of functions that
	convert to and from raw1394 protocol packets and are a lot easier to
	handle than that underlying protocol.
      </para>
    </sect1>

    <sect1>
      <title>Handles</title>

      <para>
	The handle presented to the application for using libraw1394 is the
	raw1394handle_t, an opaque data structure (which means you don't need to
	know its internals).  The handle (and with it a connection to the kernel
	side of raw1394) is obtained using
	<function>raw1394_new_handle()</function>.  Insufficient permissions to
	access the kernel driver will result in failure of this function, among
	other possibilities of failure.
      </para>

      <para>
	While initializing the handle, a certain order of function calls have to
	be obeyed or undefined results will occur.  This order reflects the
	various states of initialization to be done:
      </para>

      <para>
	<orderedlist>
	  <listitem>
	    <para><function>raw1394_new_handle()</function></para>
	  </listitem>
	  <listitem>
	    <para><function>raw1394_get_port_info()</function></para>
	  </listitem>
	  <listitem>
	    <para><function>raw1394_set_port()</function></para>
	  </listitem>
	</orderedlist>
      </para>
    </sect1>

    <sect1>
      <title>Ports</title>

      <para>
	A computer may have multiple 1394 buses connected by having multiple
	1394 chips.  Each of these is called a port, and the handle has to be
	connected to one port before it can be used for anything.  Even if no
	nodes are connected to the chip in question, it forms a complete bus
	(with just one node, itself).
      </para>

      <para>
	A list of available ports together with some information about it
	(name of the hardware, number of connected nodes) is available via
	<function>raw1394_get_port_info()</function>, which is to be called
	right getting a fresh handle.  The user should be presented with a
	choice of available ports if there is more than one.  It may be good
	practice to do that even if there is only one port, since that may
	result from a normally configured port just not being available,
	making it confusing to be dropped right into the application attached
	to a port without a choice and notion of anything going wrong.
      </para>

      <para>
	The choice of port is then reported using
	<function>raw1394_set_port()</function>.  If this function fails and
	<symbol>errno</symbol> is set to <symbol>ESTALE</symbol>, then
	something has changed about the ports (port was added or removed)
	between getting the port info and trying to set a port.  It is
	required that the current port list is fetched (presenting the user
	with the choice again) and setting the port is retried with the new
	data.
      </para>

      <para>
	After a successful <function>raw1394_set_port()</function>, the get and
	set port functions must not be used anymore on this handle.  Undefined
	results occur if you do so.  To make up for this, all the other
	functions are allowed now.
      </para>
    </sect1>

    <sect1>
      <title>The Event Loop</title>

      <para>
	All commands in libraw1394 are asynchronous, with some synchronous
	wrapper functions for some types of transactions.  This means that there
	are two streams of data, one going into raw1394 and one coming out.
	With this design you can send out multiple transactions without having
	to wait for the response before you can continue (sending out other
	transactions, for example).  The responses and other events (like bus
	resets and received isochronous packets) are queued, and you can get
	them with <function>raw1394_loop_iterate()</function>.
      </para>

      <para>
	This forms an event loop you may already know from similar systems like
	GUI toolkits.  <function>raw1394_loop_iterate()</function> gets one
	message from the event queue in raw1394, processes it with the
	configured callback functions and returns the value returned by the
	callback (so you can signal to the main loop from your callback; the
	standard callbacks all return 0).  It normally blocks when there are no
	events and always processes only one event.  If you are only receiving
	broadcast events like isochronous packets you thus have to set up a loop
	continuously calling the iterate function to get your callbacks called.
      </para>

      <para>
	Often it is necessary to have multiple event loops and combine them,
	e.g. if your application uses a GUI toolkit which also has its own event
	loop.  In that case you can use <function>raw1394_get_fd()</function> to
	get the file descriptor used for this handle by libraw1394.  The fd can
	be used to for <function>select()</function> or
	<function>poll()</function> calls (testing for read availability)
	together with the other loop's fd, some event loops also allow to add
	other fds to their own set (GTK's event loop does).  If these trigger on
	the libraw1394 fd, you can call
	<function>raw1394_loop_iterate()</function> once and it is guaranteed
	that it will not block since at the very least one event waits.  After
	the first call you continue the main event loop.  If more events wait,
	the <function>select()</function>/<function>poll()</function> will
	immediately return again.
      </para>

      <para>
	You can also use the fd to set the <symbol>O_NONBLOCK</symbol> flag with
	<function>fcntl()</function>.  After that, the iterate function will not
	block anymore but fail with <symbol>errno</symbol> set to
	<symbol>EAGAIN</symbol> if no events wait.  These are the only legal
	uses for the fd returned by <function>raw1394_get_fd()</function>.
      </para>

      <para>
	There are some functions which provide a synchronous wrapper for
	transactions, note that these will call
	<function>raw1394_loop_iterate()</function> continuously until their
	transaction is completed, thus having implicit callback invocations
	during their execution.  The standard transaction functions have names
	of the form <function>raw1394_start_xxx</function>, the synchronous
	wrappers are called <function>raw1394_xxx</function>.
      </para>
    </sect1>

    <sect1>
      <title>Handlers</title>

      <para>
	There are a number of handlers which can be set using the appropriate
	function as described in the function reference and which libraw1394
	will call during a <function>raw1394_loop_iterate()</function>.  These
	are:

	<itemizedlist>
	  <listitem>
	    <para>tag handler (called for completed commands)</para>
	  </listitem>
	  <listitem>
	    <para>bus reset handler (called when a bus reset happens)</para>
	  </listitem>
	  <listitem>
	    <para>iso handler (called when an iso packet is received)</para>
	  </listitem>
	  <listitem>
	    <para>fcp handler (called when a FCP command or response is
	    received)</para>
	  </listitem>
	</itemizedlist>

	The bus reset handler is always called, the tag handler for every
	command that completes, the iso handler and fcp handler are only called
	when the application chooses to receive these packets.  Handlers return
	an integer value which is passed on by
	<function>raw1394_loop_iterate()</function> (only one handler is called
	per invocation), <constant>0</constant> is returned without a handler in
	place.
      </para>

      <para>
	The tag handler case is a bit special since the default handler is
	actually doing something.  Every command that you start can be given an
	unsigned long tag which is passed untouched to the tag handler when the
	event loop sees a completed command.  The default handler expects this
	value to be a pointer to a <structname>raw1394_reqhandle</structname>
	structure, which contains a data pointer and its own callback function
	pointer.  The callback gets the untouched data pointer and error code as
	arguments.  If you want to use tags that are not
	<structname>raw1394_reqhandle</structname> pointers you have to set up
	your own tag handler.
      </para>
    </sect1>

  </chapter>

  <chapter id="functions">
    <title>Function Reference</title>

    <refentry>
      <refmeta>
	<refentrytitle>raw1394_new_handle</refentrytitle>
	<manvolnum>3</manvolnum>
      </refmeta>

      <refnamediv>
	<refname>raw1394_net_handle</refname>
	<refpurpose>create new handle</refpurpose>
      </refnamediv>

      <refsynopsisdiv>
	<funcsynopsis>
	  <funcprototype>
	    <funcdef>raw1394handle_t <function>raw1394_new_handle</function></funcdef>
	    <void>
	  </funcprototype>
	</funcsynopsis>
      </refsynopsisdiv>

      <refsect1>
	<title>Description</title>

	<para>
	  Creates and returns a new handle.  It is not allowed to use the same
	  handle in multiple threads or forked processes.  It is allowed to
	  create and use multiple handles, however.  Use one handle per thread
	  which needs it in the multithreaded case.
	</para>
      </refsect1>

      <refsect1>
	<title>Return Value</title>

	<para>
	  Returns the created handle or <constant>NULL</constant> when
	  initialization fails.  In the latter case <varname>errno</varname>
	  either contains some OS specific error code or <constant>0</constant>
	  if the error is that libraw1394 and raw1394 don't support each other's
	  protocol versions.
	</para>
      </refsect1>
    </refentry>

    <refentry>
      <refmeta>
	<refentrytitle>raw1394_destroy_handle</refentrytitle>
	<manvolnum>3</manvolnum>
      </refmeta>

      <refnamediv>
	<refname>raw1394_destroy_handle</refname>
	<refpurpose>deallocate handle</refpurpose>
      </refnamediv>

      <refsynopsisdiv>
	<funcsynopsis>
	  <funcprototype>
	    <funcdef>void <function>raw1394_destroy_handle</function></funcdef>
	    <paramdef>raw1394handle_t <parameter>handle</parameter></paramdef>
	  </funcprototype>
	</funcsynopsis>
      </refsynopsisdiv>

      <refsect1>
	<title>Arguments</title>

	<variablelist>
	  <varlistentry>
	    <term><parameter>handle</parameter></term>
	    <listitem>
	      <para>handle to be deallocated</para>
	    </listitem>
	  </varlistentry>
	</variablelist>
      </refsect1>

      <refsect1>
	<title>Description</title>

	<para>
	  Closes connection with raw1394 on this handle and deallocates
	  everything associated with it.  It is safe to pass
	  <constant>NULL</constant> as handle, nothing is done in this case.
	</para>
      </refsect1>
    </refentry>

    <refentry>
      <refmeta>
	<refentrytitle>raw1394_get_fd</refentrytitle>
	<manvolnum>3</manvolnum>
      </refmeta>

      <refnamediv>
	<refname>raw1394_get_fd</refname>
	<refpurpose>get the communication file descriptor</refpurpose>
      </refnamediv>

      <refsynopsisdiv>
	<funcsynopsis>
	  <funcprototype>
	    <funcdef>int <function>raw1394_get_fd</function></funcdef>
	    <paramdef>raw1394handle_t <parameter>handle</parameter></paramdef>
	  </funcprototype>
	</funcsynopsis>
      </refsynopsisdiv>

      <refsect1>
	<title>Arguments</title>

	<variablelist>
	  <varlistentry>
	    <term><parameter>handle</parameter></term>
	    <listitem>
	      <para>handle of which the fd is to be returned from</para>
	    </listitem>
	  </varlistentry>
	</variablelist>
      </refsect1>

      <refsect1>
	<title>Description</title>

	<para>
	  Returns the fd used for communication with the raw1394 kernel module.
	  This can be used for
	  <function>select()</function>/<function>poll()</function> calls if you
	  wait on other fds or can be integrated into another event loop
	  (e.g. from a GUI application framework).  It can also be used to
	  set/remove the <constant>O_NONBLOCK</constant> flag using
	  <function>fcntl()</function> to modify the block behaviour in
	  <function>raw1394_loop_iterate()</function>.  It must not be used for
	  anything else.
	</para>
      </refsect1>

      <refsect1>
	<title>Return Value</title>

	<para>
	  The fd of the communication stream.  Invalid fds may be returned
	  before a port was set using <function>raw1394_set_port()</function>.
	</para>
      </refsect1>
    </refentry>

    <refentry>
      <refmeta>
	<refentrytitle>raw1394_(get|set)_userdata</refentrytitle>
	<manvolnum>3</manvolnum>
      </refmeta>

      <refnamediv>
	<refname>raw1394_get_userdata</refname>
	<refname>raw1394_set_userdata</refname>
	<refpurpose>associate user data with a handle</refpurpose>
      </refnamediv>

      <refsynopsisdiv>
	<funcsynopsis>
	  <funcprototype>
	    <funcdef>void *<function>raw1394_get_userdata</function></funcdef>
	    <paramdef>raw1394handle_t <parameter>handle</parameter></paramdef>
	  </funcprototype>
	  <funcprototype>
	    <funcdef>void <function>raw1394_set_userdata</function></funcdef>
	    <paramdef>raw1394handle_t <parameter>handle</parameter></paramdef>
	  </funcprototype>
	</funcsynopsis>
      </refsynopsisdiv>

      <refsect1>
	<title>Arguments</title>

	<variablelist>
	  <varlistentry>
	    <term><parameter>handle</parameter></term>
	    <listitem>
	      <para>handle associated with the user data</para>
	    </listitem>
	  </varlistentry>
	</variablelist>
      </refsect1>

      <refsect1>
	<title>Description</title>

	<para>
	  Allows to associate one void pointer with a handle.  libraw1394 does
	  not care about the data, it just stores it in the handle allowing it
	  to be retrieved at any time.  This can be useful when multiple handles
	  are used, so that callbacks can identify the handle.
	</para>
      </refsect1>

      <refsect1>
	<title>Return Value</title>

	<para>
	  <function>raw1394_get_userdata()</function> returns the void pointer
	  associated with the handle.
	</para>
      </refsect1>
    </refentry>
  </chapter>

</book>