CLICK FAQ
=========

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*            The most up-to-date version of this FAQ is on line at            *
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*                    http://www.read.cs.ucla.edu/click/faq                    *
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SECTION 1: GENERAL QUESTIONS
----------------------------

Q. Is Click experimental software?

A. Yes.


Q. The Click Linux patch does not apply cleanly to my version of Linux.

A. Try a version of Linux for which we distribute a specific patch.  See
   the INSTALL file for a list of patches.


Q. How fast can Click route packets?

A. On a 700 MHz Pentium III, we could get 456,000 64-byte packets a second
   through a Click router with eight active DEC Tulip fast Ethernet cards.


Q. How do you do that?

A. Device driver improvements, including polling, and language-level
   optimizations.


Q. Can I get that many packets through a Click router?

A. You should be able to do that right now if you use DEC Tulip fast
   Ethernet cards or Intel E1000 gigabit Ethernet cards.  These are the
   main device drivers that we have changed to use our polling extensions
   to Linux.


Q. How does Click run inside a Linux kernel?

A. As a kernel thread.


Q. What are the Grid elements?

A. Grid is a set of protocols for routing in multi-hop ad-hoc
   wireless networks.  See http://pdos.lcs.mit.edu/grid for further
   details.


SECTION 2: QUESTIONS ABOUT POLLING
----------------------------------

Q. How can I change a device driver to use the Click polling extensions?

A. Benjie Chen, who designed the extensions, never wrote a document
   describing how to do this.  For now, you'll have to look at our
   additions to Linux's 'struct device', and the way they are used by our
   drivers.  You can always write us and ask for help:
   <click@pdos.lcs.mit.edu>.


Q. Can I use Click without updating device drivers (that is, without
   polling)?

A. Sure.  Just use FromDevice elements instead of PollDevice elements.
   Your performance will generally be worse than Linux; we haven't
   particularly optimized this.


SECTION 3: CREATING YOUR OWN ELEMENTS
-------------------------------------

Q. How can I add my own element class to Click?

A. There are two ways to add an element class to Click: in the main Click
   collection, or in a package. We recommend that you use packages for
   nontrivial collections of elements. It has several advantages -- for
   example, it will keep your code separate from the main Click code. Check
   out the sample package in 'etc/samplepackage'. However, if you just want
   to compile a single new element, it will be easier to add it to the main
   Click collection. This answer shows how.

   First, write your element class.

     Each element class should be written as two C++ source files, FILE.cc
     and FILE.hh. The easiest way to create an element this is to copy an
     existing element and change the C++ class's name. You must change at
     least the following function:

	const char *class_name() const;	// return your element's name

     Other common functions to override include:

	void push(int i, Packet *);	// process push request on input i
	Packet *pull(int i);		// process pull request on output i
	Packet *simple_action(Packet *); // for agnostic elements

	const char *port_count() const; // return port count code
	const char *processing() const; // return processing code
	int configure(Vector<String> &, ErrorHandler *);
					// process configuration string
	void add_handlers();		// set up element handlers
	int initialize(ErrorHandler *);	// initialize element
	void cleanup(CleanupStage);	// clean up element state

     All these functions are described in the Click programming manual,
     doc/click.texi.

     Make sure that your .cc file exports the element class with
     EXPORT_ELEMENT. For example, the nullelement.cc file ends with:

	EXPORT_ELEMENT(NullElement)

     EXPORT_ELEMENT takes a single argument, the name of the C++ class
     corresponding to your element. You can have multiple EXPORT_ELEMENT
     lines if your source file declares multiple element classes. If your
     element is meant only for the user-level driver, add this line near
     EXPORT_ELEMENT:

	ELEMENT_REQUIRES(userlevel)

     Or, if it is meant for the Linux kernel module:

	ELEMENT_REQUIRES(linuxmodule)

     ELEMENT_REQUIRES can also take element names and package names like
     'ip6':

	ELEMENT_REQUIRES(linuxmodule Storage ip6)

   Second, put your element in an 'elements/' directory.

     Choose the directory that seems most appropriate for your element.
     Often, this is 'elements/local', which is designed for locally-created
     elements. If you place your element in 'local', make sure you provide
     the '--enable-local' argument to 'configure'.

   Third, run 'make elemlist'.

     'make elemlist' checks the source files in the 'elements/'
     subdirectories for EXPORT_ELEMENT directives, and compiles a list of
     elements that Click should compile. After running 'make elemlist',
     check the 'userlevel/elements.conf' and 'linuxmodule/elements.conf'
     files to see if your .cc file made it into this list.

   Finally, run 'make install'!

     You are done.


Q. My element wasn't compiled! Click reports 'unknown element class'.

A. First, check whether Click's build process found your element, by
   running "grep ELEMENTNAME CLICKDIR/userlevel/elements.conf".  (If you
   are having trouble with the Linux kernel module, search
   CLICKDIR/linuxmodule/elements.conf instead.)  The elements.conf file is
   generated automatically, and contains information about every element
   Click plans to compile. If the grep command has no output, then Click
   didn't find your element. Check these things:

   * Do you have an EXPORT_ELEMENT statement?
   * Does your element require something with ELEMENT_REQUIRES that is not
     available?
   * Did you run 'make elemlist'?
   * Is the relevant elements/ directory enabled? (For instance, for
     elements/local, did you run './configure ... --enable-local'?)

   If the elements.conf file does list your element, then Click attempted
   to compile your element and everything should work.  If you still get
   'unknown element class', check these things:

   * Userlevel only: Are you running an installed version of Click? (For
     instance, are you typing "click FILE.click" instead of
     "CLICKDIR/userlevel/click FILE.click"?) If so, then you will not be
     able to use your new element until you run 'make install'.
   * Linuxmodule only: Did you unload the old version of the kernel module
     and load the new one that contains your element?  The easiest way to
     do this is with 'make install; click-install -u FILE.click'; the -u
     option unloads & reloads the kernel module.


Q. How can I make Click compile a C++ file that doesn't contain an element?

A. Add an ELEMENT_PROVIDES statement to your .cc file.  The Click build
   process searches for C and C++ files with 'ELEMENT_PROVIDES' as well as
   'EXPORT_ELEMENT'.  You'll have to come up with a one-word tag describing
   the functionality that your .cc file provides.  See the end of
   'elements/userlevel/fakepcap.cc' for an example; it looks like this:

	...
	CLICK_ENDDECLS
	ELEMENT_REQUIRES(userlevel|ns)
	ELEMENT_PROVIDES(FakePcap)

   Elements that use the 'FakePcap' functionality explicitly require it
   with ELEMENT_REQUIRES; see 'elements/userlevel/fromdump.cc' for an
   example.


Q. One of my elements must be linked against another library.  How can I
   make userlevel Click link with that library?

A. Add the required linker flags to an 'ELEMENT_LIBS' statement in your
   element's .cc file.  For example:

	...
	CLICK_ENDDECLS
	EXPORT_ELEMENT(MyElement)
	ELEMENT_LIBS((-L/usr/local/lib -lmy_library))

   Every element that needs a library should contain the relevant
   ELEMENT_LIBS.
