Definition of the Porting Layer for the X v11 Sample Server

   Copyright © 1994 X Consortium, Inc., 2004 X.org Foundation,
   Inc.

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                              Abstract

    The following document explains the structure of the X Window
      System display server and the interfaces among the larger
    pieces. It is intended as a reference for programmers who are
   implementing an X Display Server on their workstation hardware.
   It is included with the X Window System source tape, along with
   the document "Strategies for Porting the X v11 Sample Server."
       The order in which you should read these documents is:

      1. Read the first section of the "Strategies for Porting"
                 document (Overview of Porting Process).
        2. Skim over this document (the Definition document).
        3. Skim over the remainder of the Strategies document.
    4. Start planning and working, referring to the Strategies and
                          Definition documents.

        You may also want to look at the following documents:

             * "The X Window System" for an overview of X.
       * "Xlib - C Language X Interface" for a view of what the
                         client programmer sees.
      * "X Window System Protocol" for a terse description of the
           byte stream protocol between the client and server.

    To understand this document and the accompanying source code,
   you should know the C language. You should be familiar with 2D
     graphics and windowing concepts such as clipping, bitmaps,
   fonts, etc. You should have a general knowledge of the X Window
   System. To implement the server code on your hardware, you need
       to know a lot about your hardware, its graphic display
      device(s), and (possibly) its networking and multitasking
   facilities. This document depends a lot on the source code, so
            you should have a listing of the code handy.

   Some source in the distribution is directly compilable on your
   machine. Some of it will require modification. Other parts may
    have to be completely written from scratch. The distribution
    also includes source for a sample implementation of a display
        server which runs on a very wide variety of color and
      monochrome displays on Linux and *BSD which you will find
            useful for implementing any type of X server.

    Note to the 2004 edition: at this time this document must be
   considered incomplete. In particular, the new Render extension
    is still lacking good documentation, and has become vital to
    high performance X implementations. A new "fb" portable frame
      buffer graphics library (replacing "cfb") is used by most
      implementations to implement software rendering for most
   operations. Accelerating only a few of the old "core" graphics
    functions is now needed, as performance in software is "good
    enough" for most operations. Modern applications and desktop
   environments are now much more sensitive to good implementation
    of the Render extension than in most operations of the old X
   graphics model. The shadow frame buffer implementation is also
          very useful in many circumstances, and also needs
     documentation. We hope to rectify these shortcomings in our
   documentation in the future. Help would be greatly appreciated.
           _______________________________________________

   Table of Contents
   The X Window System
   Overview of the Server
   DIX Layer
   OS Layer
   DDX Layer
   Summary of Routines

The X Window System

   The X Window System, or simply "X," is a windowing system that
   provides high-performance, high-level, device-independent
   graphics.

   X is a windowing system designed for bitmapped graphic
   displays. The display can have a simple, monochrome display or
   it can have a color display with up to 32 bits per pixel with a
   special graphics processor doing the work. (In this document,
   monochrome means a black and white display with one bit per
   pixel. Even though the usual meaning of monochrome is more
   general, this special case is so common that we decided to
   reserve the word for this purpose.) In practice, monochrome
   displays are now almost unheard of, with 4 bit gray scale
   displays being the low end.

   X is designed for a networking environment where users can run
   applications on machines other than their own workstations.
   Sometimes, the connection is over an Ethernet network with a
   protocol such as TCP/IP; but, any "reliable" byte stream is
   allowable. A high-bandwidth byte stream is preferable; RS-232
   at 9600 baud would be slow without compression techniques.

   X by itself allows great freedom of design. For instance, it
   does not include any user interface standard. Its intent is to
   "provide mechanism, not policy." By making it general, it can
   be the foundation for a wide variety of interactive software.

   For a more detailed overview, see the document "The X Window
   System." For details on the byte stream protocol, see "X Window
   System protocol."
     __________________________________________________________

Overview of the Server

   The display server manages windows and simple graphics requests
   for the user on behalf of different client applications. The
   client applications can be running on any machine on the
   network. The server mainly does three things:

     * Responds to protocol requests from existing clients (mostly
       graphic and text drawing commands)
     * Sends device input (keystrokes and mouse actions) and other
       events to existing clients
     * Maintains client connections

   The server code is organized into four major pieces:

     * Device Independent (DIX) layer - code shared among all
       implementations
     * Operating System (OS) layer - code that is different for
       each operating system but is shared among all graphic
       devices for this operating system
     * Device Dependent (DDX) layer - code that is (potentially)
       different for each combination of operating system and
       graphic device
     * Extension Interface - a standard way to add features to the
       X server

   The "porting layer" consists of the OS and DDX layers; these
   are actually parallel and neither one is on top of the other.
   The DIX layer is intended to be portable without change to
   target systems and is not detailed here, although several
   routines in DIX that are called by DDX are documented.
   Extensions incorporate new functionality into the server; and
   require additional functionality over a simple DDX.

   The following sections outline the functions of the layers.
   Section 3 briefly tells what you need to know about the DIX
   layer. The OS layer is explained in Section 4. Section 5 gives
   the theory of operation and procedural interface for the DDX
   layer. Section 6 describes the functions which exist for the
   extension writer.
     __________________________________________________________

DIX Layer

   The DIX layer is the machine and device independent part of X.
   The source should be common to all operating systems and
   devices. The port process should not include changes to this
   part, therefore internal interfaces to DIX modules are not
   discussed, except for public interfaces to the DDX and the OS
   layers. The functions described in this section are available
   for extension writers to use.

   In the process of getting your server to work, if you think
   that DIX must be modified for purposes other than bug fixes,
   you may be doing something wrong. Keep looking for a more
   compatible solution. When the next release of the X server code
   is available, you should be able to just drop in the new DIX
   code and compile it. If you change DIX, you will have to
   remember what changes you made and will have to change the new
   sources before you can update to the new version.

   The heart of the DIX code is a loop called the dispatch loop.
   Each time the processor goes around the loop, it sends off
   accumulated input events from the input devices to the clients,
   and it processes requests from the clients. This loop is the
   most organized way for the server to process the asynchronous
   requests that it needs to process. Most of these operations are
   performed by OS and DDX routines that you must supply.
     __________________________________________________________

Server Resource System

   X resources are C structs inside the server. Client
   applications create and manipulate these objects according to
   the rules of the X byte stream protocol. Client applications
   refer to resources with resource IDs, which are 32-bit integers
   that are sent over the network. Within the server, of course,
   they are just C structs, and we refer to them by pointers.
     __________________________________________________________

Pre-Defined Resource Types

   The DDX layer has several kinds of resources:

     * Window
     * Pixmap
     * Screen
     * Device
     * Colormap
     * Font
     * Cursor
     * Graphics Contexts

   The type names of the more important server structs usually end
   in "Rec," such as "DeviceRec;" the pointer types usually end in
   "Ptr," such as "DevicePtr."

   The structs and important defined constants are declared in .h
   files that have names that suggest the name of the object. For
   instance, there are two .h files for windows, window.h and
   windowstr.h. window.h defines only what needs to be defined in
   order to use windows without peeking inside of them;
   windowstr.h defines the structs with all of their components in
   great detail for those who need it.

   Three kinds of fields are in these structs:

     * Attribute fields - struct fields that contain values like
       normal structs
     * Pointers to procedures, or structures of procedures, that
       operate on the object
     * A private field (or two) used by your DDX code to keep
       private data (probably a pointer to another data
       structure), or an array of private fields, which is sized
       as the server initializes.

   DIX calls through the struct's procedure pointers to do its
   tasks. These procedures are set either directly or indirectly
   by DDX procedures. Most of the procedures described in the
   remainder of this document are accessed through one of these
   structs. For example, the procedure to create a pixmap is
   attached to a ScreenRec and might be called by using the
   expression

     (* pScreen->CreatePixmap)(pScreen, width, height, depth).

   All procedure pointers must be set to some routine unless noted
   otherwise; a null pointer will have unfortunate consequences.

   Procedure routines will be indicated in the documentation by
   this convention:

     void pScreen->MyScreenRoutine(arg, arg, ...)

   as opposed to a free routine, not in a data structure:

     void MyFreeRoutine(arg, arg, ...)

   The attribute fields are mostly set by DIX; DDX should not
   modify them unless noted otherwise.
     __________________________________________________________

Creating Resources and Resource Types

   These functions should also be called from your
   extensionInitProc to allocate all of the various resource
   classes and types required for the extension. Each time the
   server resets, these types must be reallocated as the old
   allocations will have been discarded. Resource types are
   integer values starting at 1. Get a resource type by calling

    RESTYPE CreateNewResourceType(deleteFunc)


   deleteFunc will be called to destroy all resources with this
   type.

   Resource classes are masks starting at 1 << 31 which can be
   or'ed with any resource type to provide attributes for the
   type. To allocate a new class bit, call

    RESTYPE CreateNewResourceClass()


   There are two ways of looking up resources, by type or by
   class. Classes are non-exclusive subsets of the space of all
   resources, so you can lookup the union of multiple classes.
   (RC_ANY is the union of all classes).

   Note that the appropriate class bits must be or'ed into the
   value returned by CreateNewResourceType when calling resource
   lookup functions.

   If you need to create a ``private'' resource ID for internal
   use, you can call FakeClientID.

        XID FakeClientID(client)
            int client;


   This allocates from ID space reserved for the server.

   To associate a resource value with an ID, use AddResource.

        Bool AddResource(id, type, value)
            XID id;
            RESTYPE type;
            pointer value;


   The type should be the full type of the resource, including any
   class bits. If AddResource fails to allocate memory to store
   the resource, it will call the deleteFunc for the type, and
   then return False.

   To free a resource, use one of the following.

        void FreeResource(id, skipDeleteFuncType)
            XID id;
            RESTYPE skipDeleteFuncType;

        void FreeResourceByType(id, type, skipFree)
            XID id;
            RESTYPE type;
            Bool    skipFree;


   FreeResource frees all resources matching the given id,
   regardless of type; the type's deleteFunc will be called on
   each matching resource, except that skipDeleteFuncType can be
   set to a single type for which the deleteFunc should not be
   called (otherwise pass RT_NONE). FreeResourceByType frees a
   specific resource matching a given id and type; if skipFree is
   true, then the deleteFunc is not called.
     __________________________________________________________

Looking Up Resources

   To look up a resource, use one of the following.

        pointer LookupIDByType(id, rtype)
            XID id;
            RESTYPE rtype;

        pointer LookupIDByClass(id, classes)
            XID id;
            RESTYPE classes;


   LookupIDByType finds a resource with the given id and exact
   type. LookupIDByClass finds a resource with the given id whose
   type is included in any one of the specified classes.
     __________________________________________________________

Callback Manager

   To satisfy a growing number of requests for the introduction of
   ad hoc notification style hooks in the server, a generic
   callback manager was introduced in R6. A callback list object
   can be introduced for each new hook that is desired, and other
   modules in the server can register interest in the new callback
   list. The following functions support these operations.

   Before getting bogged down in the interface details, an typical
   usage example should establish the framework. Let's look at the
   ClientStateCallback in dix/dispatch.c. The purpose of this
   particular callback is to notify intereseted parties when a
   client's state (initial, running, gone) changes. The callback
   is "created" in this case by simply declaring a variable:

        CallbackListPtr ClientStateCallback;

   Whenever the client's state changes, the following code
   appears, which notifies all intereseted parties of the change:

        if (ClientStateCallback) CallCallbacks(&ClientStateCallback, (po
inter)client);

   Interested parties subscribe to the ClientStateCallback list by
   saying:

        AddCallback(&ClientStateCallback, func, data);

   When CallCallbacks is invoked on the list, func will be called
   thusly:

        (*func)(&ClientStateCallback, data, client)

   Now for the details.

        Bool CreateCallbackList(pcbl, cbfuncs)
            CallbackListPtr  *pcbl;
            CallbackFuncsPtr cbfuncs;


   CreateCallbackList creates a callback list. We envision that
   this function will be rarely used because the callback list is
   created automatically (if it doesn't already exist) when the
   first call to AddCallback is made on the list. The only reason
   to explicitly create the callback list with this function is if
   you want to override the implementation of some of the other
   operations on the list by passing your own cbfuncs. You also
   lose something by explicit creation: you introduce an order
   dependency during server startup because the list must be
   created before any modules subscribe to it. Returns TRUE if
   successful.

        Bool AddCallback(pcbl, callback, subscriber_data)
            CallbackListPtr *pcbl;
            CallbackProcPtr callback;
            pointer         subscriber_data;


   Adds the (callback, subscriber_data) pair to the given callback
   list. Creates the callback list if it doesn't exist. Returns
   TRUE if successful.

        Bool DeleteCallback(pcbl, callback, subscriber_data)
            CallbackListPtr *pcbl;
            CallbackProcPtr callback;
            pointer         subscriber_data;


   Removes the (callback, data) pair to the given callback list if
   present. Returns TRUE if (callback, data) was found.

        void CallCallbacks(pcbl, call_data)
            CallbackListPtr    *pcbl;
            pointer         call_data;


   For each callback currently registered on the given callback
   list, call it as follows:

        (*callback)(pcbl, subscriber_data, call_data);

        void DeleteCallbackList(pcbl)
            CallbackListPtr    *pcbl;


   Destroys the given callback list.
     __________________________________________________________

Extension Interfaces

   This function should be called from your extensionInitProc
   which should be called by InitExtensions.

        ExtensionEntry *AddExtension(name, NumEvents,NumErrors,
                MainProc, SwappedMainProc, CloseDownProc, MinorOpcodePro
c)

                char *name;  /*Null terminate string; case matters*/
                int NumEvents;
                int NumErrors;
                int (* MainProc)(ClientPtr);/*Called if client matches s
erver order*/
                int (* SwappedMainProc)(ClientPtr);/*Called if client di
ffers from server*/
                void (* CloseDownProc)(ExtensionEntry *);
                unsigned short (*MinorOpcodeProc)(ClientPtr);


   name is the name used by clients to refer to the extension.
   NumEvents is the number of event types used by the extension,
   NumErrors is the number of error codes needed by the extension.
   MainProc is called whenever a client accesses the major opcode
   assigned to the extension. SwappedMainProc is identical, except
   the client using the extension has reversed byte-sex.
   CloseDownProc is called at server reset time to deallocate any
   private storage used by the extension. MinorOpcodeProc is used
   by DIX to place the appropriate value into errors. The DIX
   routine StandardMinorOpcode can be used here which takes the
   minor opcode from the normal place in the request (i.e. just
   after the major opcode).
     __________________________________________________________

Macros and Other Helpers

   There are a number of macros in Xserver/include/dix.h which are
   useful to the extension writer. Ones of particular interest
   are: REQUEST, REQUEST_SIZE_MATCH, REQUEST_AT_LEAST_SIZE,
   REQUEST_FIXED_SIZE, LEGAL_NEW_RESOURCE, LOOKUP_DRAWABLE,
   VERIFY_GC, and VALIDATE_DRAWABLE_AND_GC. Useful byte swapping
   macros can be found in Xserver/include/misc.h: lswapl, lswaps,
   LengthRestB, LengthRestS, LengthRestL, SwapRestS, SwapRestL,
   swapl, swaps, cpswapl, and cpswaps.
     __________________________________________________________

OS Layer

   This part of the source consists of a few routines that you
   have to rewrite for each operating system. These OS functions
   maintain the client connections and schedule work to be done
   for clients. They also provide an interface to font files, font
   name to file name translation, and low level memory management.

     void OsInit()

   OsInit initializes your OS code, performing whatever tasks need
   to be done. Frequently there is not much to be done. The sample
   server implementation is in Xserver/os/osinit.c.
     __________________________________________________________

Scheduling and Request Delivery

   The main dispatch loop in DIX creates the illusion of
   multitasking between different windows, while the server is
   itself but a single process. The dispatch loop breaks up the
   work for each client into small digestible parts. Some parts
   are requests from a client, such as individual graphic
   commands. Some parts are events delivered to the client, such
   as keystrokes from the user. The processing of events and
   requests for different clients can be interleaved with one
   another so true multitasking is not needed in the server.

   You must supply some of the pieces for proper scheduling
   between clients.

        int WaitForSomething(pClientReady)
                int *pClientReady;

   WaitForSomething is the scheduler procedure you must write that
   will suspend your server process until something needs to be
   done. This call should make the server suspend until one or
   more of the following occurs:

     * There is an input event from the user or hardware (see
       SetInputCheck())
     * There are requests waiting from known clients, in which
       case you should return a count of clients stored in
       pClientReady
     * A new client tries to connect, in which case you should
       create the client and then continue waiting

   Before WaitForSomething() computes the masks to pass to select,
   poll or similar operating system interface, it needs to see if
   there is anything to do on the work queue; if so, it must call
   a DIX routine called ProcessWorkQueue.

        extern WorkQueuePtr     workQueue;

        if (workQueue)
                ProcessWorkQueue ();

   If WaitForSomething() decides it is about to do something that
   might block (in the sample server, before it calls select() or
   poll) it must call a DIX routine called BlockHandler().

        void BlockHandler(pTimeout, pReadmask)
                pointer pTimeout;
                pointer pReadmask;

   The types of the arguments are for agreement between the OS and
   DDX implementations, but the pTimeout is a pointer to the
   information determining how long the block is allowed to last,
   and the pReadmask is a pointer to the information describing
   the descriptors that will be waited on.

   In the sample server, pTimeout is a struct timeval **, and
   pReadmask is the address of the select() mask for reading.

   The DIX BlockHandler() iterates through the Screens, for each
   one calling its BlockHandler. A BlockHandler is declared thus:

        void xxxBlockHandler(nscreen, pbdata, pptv, pReadmask)
                int nscreen;
                pointer pbdata;
                struct timeval ** pptv;
                pointer pReadmask;

   The arguments are the index of the Screen, the blockData field
   of the Screen, and the arguments to the DIX BlockHandler().

   Immediately after WaitForSomething returns from the block, even
   if it didn't actually block, it must call the DIX routine
   WakeupHandler().

        void WakeupHandler(result, pReadmask)
                int result;
                pointer pReadmask;

   Once again, the types are not specified by DIX. The result is
   the success indicator for the thing that (may have) blocked,
   and the pReadmask is a mask of the descriptors that came
   active. In the sample server, result is the result from
   select() (or equivalent operating system function), and
   pReadmask is the address of the select() mask for reading.

   The DIX WakeupHandler() calls each Screen's WakeupHandler. A
   WakeupHandler is declared thus:

        void xxxWakeupHandler(nscreen, pbdata, err, pReadmask)
                int nscreen;
                pointer pbdata;
                unsigned long result;
                pointer pReadmask;

   The arguments are the index of the Screen, the blockData field
   of the Screen, and the arguments to the DIX WakeupHandler().

   In addition to the per-screen BlockHandlers, any module may
   register block and wakeup handlers (only together) using:

        Bool RegisterBlockAndWakeupHandlers (blockHandler, wakeupHandler
, blockData)
                BlockHandlerProcPtr    blockHandler;
                WakeupHandlerProcPtr   wakeupHandler;
                pointer blockData;

   A FALSE return code indicates that the registration failed for
   lack of memory. To remove a registered Block handler at other
   than server reset time (when they are all removed
   automatically), use:

        RemoveBlockAndWakeupHandlers (blockHandler, wakeupHandler, block
Data)
                BlockHandlerProcPtr   blockHandler;
                WakeupHandlerProcPtr  wakeupHandler;
                pointer blockData;

   All three arguments must match the values passed to
   RegisterBlockAndWakeupHandlers.

   These registered block handlers are called after the per-screen
   handlers:

        void (*BlockHandler) (blockData, pptv, pReadmask)
                pointer blockData;
                OSTimePtr pptv;
                pointer pReadmask;

   Sometimes block handlers need to adjust the time in a OSTimePtr
   structure, which on UNIX family systems is generally
   represented by a struct timeval consisting of seconds and
   microseconds in 32 bit values. As a convenience to reduce error
   prone struct timeval computations which require modulus
   arithmetic and correct overflow behavior in the face of
   millisecond wrapping throrugh 32 bits,

        void AdjustWaitForDelay(pointer /*waitTime*, unsigned long /* ne
wdelay */)


   has been provided.

   Any wakeup handlers registered with
   RegisterBlockAndWakeupHandlers will be called before the Screen
   handlers:

        void (*WakeupHandler) (blockData, err, pReadmask)
                pointer blockData;
                int err;
                pointer pReadmask;

   The WaitForSomething on the sample server also has a built in
   screen saver that darkens the screen if no input happens for a
   period of time. The sample server implementation is in
   Xserver/os/WaitFor.c.

   Note that WaitForSomething() may be called when you already
   have several outstanding things (events, requests, or new
   clients) queued up. For instance, your server may have just
   done a large graphics request, and it may have been a long time
   since WaitForSomething() was last called. If many clients have
   lots of requests queued up, DIX will only service some of them
   for a given client before going on to the next client (see
   isItTimeToYield, below). Therefore, WaitForSomething() will
   have to report that these same clients still have requests
   queued up the next time around.

   An implementation should return information on as many
   outstanding things as it can. For instance, if your
   implementation always checks for client data first and does not
   report any input events until there is no client data left,
   your mouse and keyboard might get locked out by an application
   that constantly barrages the server with graphics drawing
   requests. Therefore, as a general rule, input devices should
   always have priority over graphics devices.

   A list of indexes (client->index) for clients with data ready
   to be read or processed should be returned in pClientReady, and
   the count of indexes returned as the result value of the call.
   These are not clients that have full requests ready, but any
   clients who have any data ready to be read or processed. The
   DIX dispatcher will process requests from each client in turn
   by calling ReadRequestFromClient(), below.

   WaitForSomething() must create new clients as they are
   requested (by whatever mechanism at the transport level). A new
   client is created by calling the DIX routine:

        ClientPtr NextAvailableClient(ospriv)
                pointer ospriv;

   This routine returns NULL if a new client cannot be allocated
   (e.g. maximum number of clients reached). The ospriv argument
   will be stored into the OS private field (pClient->osPrivate),
   to store OS private information about the client. In the sample
   server, the osPrivate field contains the number of the socket
   for this client. See also "New Client Connections."
   NextAvailableClient() will call InsertFakeRequest(), so you
   must be prepared for this.

   If there are outstanding input events, you should make sure
   that the two SetInputCheck() locations are unequal. The DIX
   dispatcher will call your implementation of
   ProcessInputEvents() until the SetInputCheck() locations are
   equal.

   The sample server contains an implementation of
   WaitForSomething(). The following two routines indicate to
   WaitForSomething() what devices should be waited for. fd is an
   OS dependent type; in the sample server it is an open file
   descriptor.

        int AddEnabledDevice(fd)
                int fd;

        int RemoveEnabledDevice(fd)
                int fd;

   These two routines are usually called by DDX from the
   initialize cases of the Input Procedures that are stored in the
   DeviceRec (the routine passed to AddInputDevice()). The sample
   server implementation of AddEnabledDevice and
   RemoveEnabledDevice are in Xserver/os/connection.c.
     __________________________________________________________

Timer Facilities

   Similarly, the X server or an extension may need to wait for
   some timeout. Early X releases implemented this functionality
   using block and wakeup handlers, but this has been rewritten to
   use a general timer facilty, and the internal screen saver
   facilties reimplemented to use Timers. These functions are
   TimerInit, TimerForce, TimerSet, TimerCheck, TimerCancel, and
   TimerFree, as defined in Xserver/include/os.h. A callback
   function will be called when the timer fires, along with the
   current time, and a user provided argument.

        typedef struct _OsTimerRec *OsTimerPtr;

        typedef CARD32 (*OsTimerCallback)(
                OsTimerPtr /* timer */,
                CARD32 /* time */,
                pointer /* arg */);

         OsTimerPtr TimerSet( OsTimerPtr /* timer */,
                int /* flags */,
                CARD32 /* millis */,
                OsTimerCallback /* func */,
                pointer /* arg */);


   TimerSet returns a pointer to a timer structure and sets a
   timer to the specified time with the specified argument. The
   flags can be TimerAbsolute and TimerForceOld. The TimerSetOld
   flag controls whether if the timer is reset and the timer is
   pending, the whether the callback function will get called. The
   TimerAbsolute flag sets the callback time to an absolute time
   in the future rather than a time relative to when TimerSet is
   called. TimerFree should be called to free the memory allocated
   for the timer entry.

        void TimerInit(void)

        Bool TimerForce(OsTimerPtr /* pTimer */)

        void TimerCheck(void);

        void TimerCancel(OsTimerPtr /* pTimer */)

        void TimerFree(OSTimerPtr /* pTimer */)

   TimerInit frees any exisiting timer entries. TimerForce forces
   a call to the timer's callback function and returns true if the
   timer entry existed, else it returns false and does not call
   the callback function. TimerCancel will cancel the specified
   timer. TimerFree calls TimerCancel and frees the specified
   timer. Calling TimerCheck will force the server to see if any
   timer callbacks should be called.
     __________________________________________________________

New Client Connections

   The process whereby a new client-server connection starts up is
   very dependent upon what your byte stream mechanism. This
   section describes byte stream initiation using examples from
   the TCP/IP implementation on the sample server.

   The first thing that happens is a client initiates a connection
   with the server. How a client knows to do this depends upon
   your network facilities and the Xlib implementation. In a
   typical scenario, a user named Fred on his X workstation is
   logged onto a Cray supercomputer running a command shell in an
   X window. Fred can type shell commands and have the Cray
   respond as though the X server were a dumb terminal. Fred types
   in a command to run an X client application that was linked
   with Xlib. Xlib looks at the shell environment variable
   DISPLAY, which has the value "fredsbittube:0.0." The host name
   of Fred's workstation is "fredsbittube," and the 0s are for
   multiple screens and multiple X server processes. (Precisely
   what happens on your system depends upon how X and Xlib are
   implemented.)

   The client application calls a TCP routine on the Cray to open
   a TCP connection for X to communicate with the network node
   "fredsbittube." The TCP software on the Cray does this by
   looking up the TCP address of "fredsbittube" and sending an
   open request to TCP port 6000 on fredsbittube.

   All X servers on TCP listen for new clients on port 6000 by
   default; this is known as a "well-known port" in IP
   terminology.

   The server receives this request from its port 6000 and checks
   where it came from to see if it is on the server's list of
   "trustworthy" hosts to talk to. Then, it opens another port for
   communications with the client. This is the byte stream that
   all X communications will go over.

   Actually, it is a bit more complicated than that. Each X server
   process running on the host machine is called a "display." Each
   display can have more than one screen that it manages.
   "corporatehydra:3.2" represents screen 2 on display 3 on the
   multi-screened network node corporatehydra. The open request
   would be sent on well-known port number 6003.

   Once the byte stream is set up, what goes on does not depend
   very much upon whether or not it is TCP. The client sends an
   xConnClientPrefix struct (see Xproto.h) that has the version
   numbers for the version of Xlib it is running, some
   byte-ordering information, and two character strings used for
   authorization. If the server does not like the authorization
   strings or the version numbers do not match within the rules,
   or if anything else is wrong, it sends a failure response with
   a reason string.

   If the information never comes, or comes much too slowly, the
   connection should be broken off. You must implement the
   connection timeout. The sample server implements this by
   keeping a timestamp for each still-connecting client and, each
   time just before it attempts to accept new connections, it
   closes any connection that are too old. The connection timeout
   can be set from the command line.

   You must implement whatever authorization schemes you want to
   support. The sample server on the distribution tape supports a
   simple authorization scheme. The only interface seen by DIX is:

        char *
        ClientAuthorized(client, proto_n, auth_proto, string_n, auth_str
ing)
            ClientPtr client;
            unsigned int proto_n;
            char *auth_proto;
            unsigned int string_n;
            char *auth_string;

   DIX will only call this once per client, once it has read the
   full initial connection data from the client. If the connection
   should be accepted ClientAuthorized() should return NULL, and
   otherwise should return an error message string.

   Accepting new connections happens internally to
   WaitForSomething(). WaitForSomething() must call the DIX
   routine NextAvailableClient() to create a client object.
   Processing of the initial connection data will be handled by
   DIX. Your OS layer must be able to map from a client to
   whatever information your OS code needs to communicate on the
   given byte stream to the client. DIX uses this ClientPtr to
   refer to the client from now on. The sample server uses the
   osPrivate field in the ClientPtr to store the file descriptor
   for the socket, the input and output buffers, and authorization
   information.

   To initialize the methods you choose to allow clients to
   connect to your server, main() calls the routine

        void CreateWellKnownSockets()

   This routine is called only once, and not called when the
   server is reset. To recreate any sockets during server resets,
   the following routine is called from the main loop:

        void ResetWellKnownSockets()

   Sample implementations of both of these routines are found in
   Xserver/os/connection.c.

   For more details, see the section called "Connection Setup" in
   the X protocol specification.
     __________________________________________________________

Reading Data from Clients

   Requests from the client are read in as a byte stream by the OS
   layer. They may be in the form of several blocks of bytes
   delivered in sequence; requests may be broken up over block
   boundaries or there may be many requests per block. Each
   request carries with it length information. It is the
   responsibility of the following routine to break it up into
   request blocks.

        int ReadRequestFromClient(who)
                ClientPtr who;

   You must write the routine ReadRequestFromClient() to get one
   request from the byte stream belonging to client "who." You
   must swap the third and fourth bytes (the second 16-bit word)
   according to the byte-swap rules of the protocol to determine
   the length of the request. This length is measured in 32-bit
   words, not in bytes. Therefore, the theoretical maximum request
   is 256K. (However, the maximum length allowed is dependent upon
   the server's input buffer. This size is sent to the client upon
   connection. The maximum size is the constant MAX_REQUEST_SIZE
   in Xserver/include/os.h) The rest of the request you return is
   assumed NOT to be correctly swapped for internal use, because
   that is the responsibility of DIX.

   The 'who' argument is the ClientPtr returned from
   WaitForSomething. The return value indicating status should be
   set to the (positive) byte count if the read is successful, 0
   if the read was blocked, or a negative error code if an error
   happened.

   You must then store a pointer to the bytes of the request in
   the client request buffer field; who->requestBuffer. This can
   simply be a pointer into your buffer; DIX may modify it in
   place but will not otherwise cause damage. Of course, the
   request must be contiguous; you must shuffle it around in your
   buffers if not.

   The sample server implementation is in Xserver/os/io.c.
     __________________________________________________________

Inserting Data for Clients

   DIX can insert data into the client stream, and can cause a
   "replay" of the current request.

        Bool InsertFakeRequest(client, data, count)
            ClientPtr client;
            char *data;
            int count;

        int ResetCurrentRequest(client)
            ClientPtr client;

   InsertFakeRequest() must insert the specified number of bytes
   of data into the head of the input buffer for the client. This
   may be a complete request, or it might be a partial request.
   For example, NextAvailableCient() will insert a partial request
   in order to read the initial connection data sent by the
   client. The routine returns FALSE if memory could not be
   allocated. ResetCurrentRequest() should "back up" the input
   buffer so that the currently executing request will be
   reexecuted. DIX may have altered some values (e.g. the overall
   request length), so you must recheck to see if you still have a
   complete request. ResetCurrentRequest() should always cause a
   yield (isItTimeToYield).
     __________________________________________________________

Sending Events, Errors And Replies To Clients

        int WriteToClient(who, n, buf)
                ClientPtr who;
                int n;
                char *buf;

   WriteToClient should write n bytes starting at buf to the
   ClientPtr "who". It returns the number of bytes written, but
   for simplicity, the number returned must be either the same
   value as the number requested, or -1, signaling an error. The
   sample server implementation is in Xserver/os/io.c.

        void SendErrorToClient(client, majorCode, minorCode, resId, erro
rCode)
            ClientPtr client;
            unsigned int majorCode;
            unsigned int minorCode;
            XID resId;
            int errorCode;

   SendErrorToClient can be used to send errors back to clients,
   although in most cases your request function should simply
   return the error code, having set client->errorValue to the
   appropriate error value to return to the client, and DIX will
   call this function with the correct opcodes for you.

        void FlushAllOutput()

        void FlushIfCriticalOutputPending()

        void SetCriticalOutputPending()

   These three routines may be implemented to support buffered or
   delayed writes to clients, but at the very least, the stubs
   must exist. FlushAllOutput() unconditionally flushes all output
   to clients; FlushIfCriticalOutputPending() flushes output only
   if SetCriticalOutputPending() has be called since the last time
   output was flushed. The sample server implementation is in
   Xserver/os/io.c and actually ignores requests to flush output
   on a per-client basis if it knows that there are requests in
   that client's input queue.
     __________________________________________________________

Font Support

   In the sample server, fonts are encoded in disk files or
   fetched from the font server. For disk fonts, there is one file
   per font, with a file name like "fixed.pcf". Font server fonts
   are read over the network using the X Font Server Protocol. The
   disk directories containing disk fonts and the names of the
   font servers are listed together in the current "font path."

   In principle, you can put all your fonts in ROM or in RAM in
   your server. You can put them all in one library file on disk.
   You could generate them on the fly from stroke descriptions. By
   placing the appropriate code in the Font Library, you will
   automatically export fonts in that format both through the X
   server and the Font server.

   With the incorporation of font-server based fonts and the
   Speedo donation from Bitstream, the font interfaces have been
   moved into a separate library, now called the Font Library
   (../fonts/lib). These routines are shared between the X server
   and the Font server, so instead of this document specifying
   what you must implement, simply refer to the font library
   interface specification for the details. All of the interface
   code to the Font library is contained in dix/dixfonts.c
     __________________________________________________________

Memory Management

   Memory management is based on functions in the C runtime
   library. Xalloc(), Xrealloc(), and Xfree() work just like
   malloc(), realloc(), and free(), except that you can pass a
   null pointer to Xrealloc() to have it allocate anew or pass a
   null pointer to Xfree() and nothing will happen. The versions
   in the sample server also do some checking that is useful for
   debugging. Consult a C runtime library reference manual for
   more details.

   The macros ALLOCATE_LOCAL and DEALLOCATE_LOCAL are provided in
   Xserver/include/os.h. These are useful if your compiler
   supports alloca() (or some method of allocating memory from the
   stack); and are defined appropriately on systems which support
   it.

   Treat memory allocation carefully in your implementation.
   Memory leaks can be very hard to find and are frustrating to a
   user. An X server could be running for days or weeks without
   being reset, just like a regular terminal. If you leak a few
   dozen k per day, that will add up and will cause problems for
   users that leave their workstations on.
     __________________________________________________________

Client Scheduling

   The X server has the ability to schedule clients much like an
   operating system would, suspending and restarting them without
   regard for the state of their input buffers. This functionality
   allows the X server to suspend one client and continue
   processing requests from other clients while waiting for a
   long-term network activity (like loading a font) before
   continuing with the first client.

        Bool isItTimeToYield;

   isItTimeToYield is a global variable you can set if you want to
   tell DIX to end the client's "time slice" and start paying
   attention to the next client. After the current request is
   finished, DIX will move to the next client.

   In the sample server, ReadRequestFromClient() sets
   isItTimeToYield after 10 requests packets in a row are read
   from the same client.

   This scheduling algorithm can have a serious effect upon
   performance when two clients are drawing into their windows
   simultaneously. If it allows one client to run until its
   request queue is empty by ignoring isItTimeToYield, the
   client's queue may in fact never empty and other clients will
   be blocked out. On the other hand, if it switchs between
   different clients too quickly, performance may suffer due to
   too much switching between contexts. For example, if a graphics
   processor needs to be set up with drawing modes before drawing,
   and two different clients are drawing with different modes into
   two different windows, you may switch your graphics processor
   modes so often that performance is impacted.

   See the Strategies document for heuristics on setting
   isItTimeToYield.

   The following functions provide the ability to suspend request
   processing on a particular client, resuming it at some later
   time:

        int IgnoreClient (who)
                ClientPtr who;

        int AttendClient (who)
                ClientPtr who;

   Ignore client is responsible for pretending that the given
   client doesn't exist. WaitForSomething should not return this
   client as ready for reading and should not return if only this
   client is ready. AttendClient undoes whatever IgnoreClient did,
   setting it up for input again.

   Three functions support "process control" for X clients:

        Bool ClientSleep (client, function, closure)
                ClientPtr       client;
                Bool            (*function)();
                pointer         closure;


   This suspends the current client (the calling routine is
   responsible for making its way back to Dispatch()). No more X
   requests will be processed for this client until ClientWakeup
   is called.

        Bool ClientSignal (client)
                ClientPtr       client;


   This function causes a call to the (*function) parameter passed
   to ClientSleep to be queued on the work queue. This does not
   automatically "wakeup" the client, but the function called is
   free to do so by calling:

        ClientWakeup (client)
                ClientPtr       client;


   This re-enables X request processing for the specified client.
     __________________________________________________________

Other OS Functions

        void
        ErrorF(char *f, ...)

        void
        FatalError(char *f, ...)

        void
        Error(str)
            char *str;

   You should write these three routines to provide for diagnostic
   output from the dix and ddx layers, although implementing them
   to produce no output will not affect the correctness of your
   server. ErrorF() and FatalError() take a printf() type of
   format specification in the first argument and an
   implementation-dependent number of arguments following that.
   Normally, the formats passed to ErrorF() and FatalError()
   should be terminated with a newline. Error() provides an os
   interface for printing out the string passed as an argument
   followed by a meaningful explanation of the last system error.
   Normally the string does not contain a newline, and it is only
   called by the ddx layer. In the sample implementation, Error()
   uses the perror() function.

   After printing the message arguments, FatalError() must be
   implemented such that the server will call AbortDDX() to give
   the ddx layer a chance to reset the hardware, and then
   terminate the server; it must not return.

   The sample server implementation for these routines is in
   Xserver/os/util.c.
     __________________________________________________________

Idiom Support

   The DBE specification introduces the notion of idioms, which
   are groups of X requests which can be executed more efficiently
   when taken as a whole compared to being performed individually
   and sequentially. This following server internal support to
   allows DBE implementations, as well as other parts of the
   server, to do idiom processing.

        xReqPtr PeekNextRequest(xReqPtr req, ClientPtr client, Bool read
more)

   If req is NULL, the return value will be a pointer to the start
   of the complete request that follows the one currently being
   executed for the client. If req is not NULL, the function
   assumes that req is a pointer to a request in the client's
   request buffer, and the return value will be a pointer to the
   the start of the complete request that follows req. If the
   complete request is not available, the function returns NULL;
   pointers to partial requests will never be returned. If (and
   only if) readmore is TRUE, PeekNextRequest should try to read
   an additional request from the client if one is not already
   available in the client's request buffer. If PeekNextRequest
   reads more data into the request buffer, it should not move or
   change the existing data.

        void SkipRequests(xReqPtr req, ClientPtr client, int numskipped)

   The requests for the client up to and including the one
   specified by req will be skipped. numskipped must be the number
   of requests being skipped. Normal request processing will
   resume with the request that follows req. The caller must not
   have modified the contents of the request buffer in any way
   (e.g., by doing byte swapping in place).

   Additionally, two macros in os.h operate on the xReq pointer
   returned by PeekNextRequest:

        int ReqLen(xReqPtr req, ClientPtr client)

   The value of ReqLen is the request length in bytes of the given
   xReq.

        otherReqTypePtr CastxReq(xReq *req, otherReqTypePtr)

   The value of CastxReq is the conversion of the given request
   pointer to an otherReqTypePtr (which should be a pointer to a
   protocol structure type). Only those fields which come after
   the length field of otherReqType may be accessed via the
   returned pointer.

   Thus the first two fields of a request, reqType and data, can
   be accessed directly using the xReq * returned by
   PeekNextRequest. The next field, the length, can be accessed
   with ReqLen. Fields beyond that can be accessed with CastxReq.
   This complexity was necessary because of the reencoding of core
   protocol that can happen due to the BigRequests extension.
     __________________________________________________________

DDX Layer

   This section describes the interface between DIX and DDX. While
   there may be an OS-dependent driver interface between DDX and
   the physical device, that interface is left to the DDX
   implementor and is not specified here.

   The DDX layer does most of its work through procedures that are
   pointed to by different structs. As previously described, the
   behavior of these resources is largely determined by these
   procedure pointers. Most of these routines are for graphic
   display on the screen or support functions thereof. The rest
   are for user input from input devices.
     __________________________________________________________

Input

   In this document "input" refers to input from the user, such as
   mouse, keyboard, and bar code readers. X input devices are of
   several types: keyboard, pointing device, and many others. The
   core server has support for extension devices as described by
   the X Input Extension document; the interfaces used by that
   extension are described elsewhere. The core devices are
   actually implemented as two collections of devices, the mouse
   is a ButtonDevice, a ValuatorDevice and a PtrFeedbackDevice
   while the keyboard is a KeyDevice, a FocusDevice and a
   KbdFeedbackDevice. Each part implements a portion of the
   functionality of the device. This abstraction is hidden from
   view for core devices by DIX.

   You, the DDX programmer, are responsible for some of the
   routines in this section. Others are DIX routines that you
   should call to do the things you need to do in these DDX
   routines. Pay attention to which is which.
     __________________________________________________________

Input Device Data Structures

   DIX keeps a global directory of devices in a central data
   structure called InputInfo. For each device there is a device
   structure called a DeviceRec. DIX can locate any DeviceRec
   through InputInfo. In addition, it has a special pointer to
   identify the main pointing device and a special pointer to
   identify the main keyboard.

   The DeviceRec (Xserver/include/input.h) is a device-independent
   structure that contains the state of an input device. A
   DevicePtr is simply a pointer to a DeviceRec.

   An xEvent describes an event the server reports to a client.
   Defined in Xproto.h, it is a huge struct of union of structs
   that have fields for all kinds of events. All of the variants
   overlap, so that the struct is actually very small in memory.
     __________________________________________________________

Processing Events

   The main DDX input interface is the following routine:

        void ProcessInputEvents()

   You must write this routine to deliver input events from the
   user. DIX calls it when input is pending (see next section),
   and possibly even when it is not. You should write it to get
   events from each device and deliver the events to DIX. To
   deliver the events to DIX, DDX should call the following
   routine:

        void DevicePtr->processInputProc(pEvent, device, count)
                    xEventPtr events;
                    DeviceIntPtr device;
                    int count;

   This is the "input proc" for the device, a DIX procedure. DIX
   will fill in this procedure pointer to one of its own routines
   by the time ProcessInputEvents() is called the first time. Call
   this input proc routine as many times as needed to deliver as
   many events as should be delivered. DIX will buffer them up and
   send them out as needed. Count is set to the number of event
   records which make up one atomic device event and is always 1
   for the core devices (see the X Input Extension for
   descriptions of devices which may use count > 1).

   For example, your ProcessInputEvents() routine might check the
   mouse and the keyboard. If the keyboard had several keystrokes
   queued up, it could just call the keyboard's processInputProc
   as many times as needed to flush its internal queue.

   event is an xEvent struct you pass to the input proc. When the
   input proc returns, it is finished with the event rec, and you
   can fill in new values and call the input proc again with it.

   You should deliver the events in the same order that they were
   generated.

   For keyboard and pointing devices the xEvent variant should be
   keyButtonPointer. Fill in the following fields in the xEvent
   record:

     * type - is one of the following: KeyPress, KeyRelease,
       ButtonPress, ButtonRelease, or MotionNotify
     * detail - for KeyPress or KeyRelease fields, this should be
       the key number (not the ASCII code); otherwise unused
     * time - is the time that the event happened (32-bits, in
       milliseconds, arbitrary origin)
     * rootX - is the x coordinate of cursor
     * rootY - is the y coordinate of cursor

   The rest of the fields are filled in by DIX.

   The time stamp is maintained by your code in the DDX layer, and
   it is your responsibility to stamp all events correctly.

   The x and y coordinates of the pointing device and the time
   must be filled in for all event types including keyboard
   events.

   The pointing device must report all button press and release
   events. In addition, it should report a MotionNotify event
   every time it gets called if the pointing device has moved
   since the last notify. Intermediate pointing device moves are
   stored in a special GetMotionEvents buffer, because most client
   programs are not interested in them.

   There are quite a collection of sample implementations of this
   routine, one for each supported device.
     __________________________________________________________

Telling DIX When Input is Pending

   In the server's dispatch loop, DIX checks to see if there is
   any device input pending whenever WaitForSomething() returns.
   If the check says that input is pending, DIX calls the DDX
   routine ProcessInputEvents().

   This check for pending input must be very quick; a procedure
   call is too slow. The code that does the check is a hardwired
   IF statement in DIX code that simply compares the values
   pointed to by two pointers. If the values are different, then
   it assumes that input is pending and ProcessInputEvents() is
   called by DIX.

   You must pass pointers to DIX to tell it what values to
   compare. The following procedure is used to set these pointers:

        void SetInputCheck(p1, p2)
                long *p1, *p2;

   You should call it sometime during initialization to indicate
   to DIX the correct locations to check. You should pay special
   attention to the size of what they actually point to, because
   the locations are assumed to be longs.

   These two pointers are initialized by DIX to point to arbitrary
   values that are different. In other words, if you forget to
   call this routine during initialization, the worst thing that
   will happen is that ProcessInputEvents will be called when
   there are no events to process.

   p1 and p2 might point at the head and tail of some shared
   memory queue. Another use would be to have one point at a
   constant 0, with the other pointing at some mask containing 1s
   for each input device that has something pending.

   The DDX layer of the sample server calls SetInputCheck() once
   when the server's private internal queue is initialized. It
   passes pointers to the queue's head and tail. See
   Xserver/mi/mieq.c.

        int TimeSinceLastInputEvent()

   DDX must time stamp all hardware input events. But DIX
   sometimes needs to know the time and the OS layer needs to know
   the time since the last hardware input event in order for the
   screen saver to work. TimeSinceLastInputEvent() returns the
   this time in milliseconds.
     __________________________________________________________

Controlling Input Devices

   You must write four routines to do various device-specific
   things with the keyboard and pointing device. They can have any
   name you wish because you pass the procedure pointers to DIX
   routines.

        int pInternalDevice->valuator->GetMotionProc(pdevice, coords, st
art, stop, pScreen)
                DeviceIntPtr pdevice;
                xTimecoord * coords;
                unsigned long start;
                unsigned long stop;
                ScreenPtr pScreen;

   You write this DDX routine to fill in coords with all the
   motion events that have times (32-bit count of milliseconds)
   between time start and time stop. It should return the number
   of motion events returned. If there is no motion events
   support, this routine should do nothing and return zero. The
   maximum number of coords to return is set in
   InitPointerDeviceStruct(), below.

   When the user drags the pointing device, the cursor position
   theoretically sweeps through an infinite number of points.
   Normally, a client that is concerned with points other than the
   starting and ending points will receive a pointer-move event
   only as often as the server generates them. (Move events do not
   queue up; each new one replaces the last in the queue.) A
   server, if desired, can implement a scheme to save these
   intermediate events in a motion buffer. A client application,
   like a paint program, may then request that these events be
   delivered to it through the GetMotionProc routine.

        void pInternalDevice->bell->BellProc(percent, pDevice, ctrl, unk
nown)
                int percent;
                DeviceIntPtr pDevice;
                pointer ctrl;
                int class;

   You need to write this routine to ring the bell on the
   keyboard. loud is a number from 0 to 100, with 100 being the
   loudest. Class is either BellFeedbackClass or KbdFeedbackClass
   (from XI.h).

        void pInternalDevice->somedevice->CtrlProc(device, ctrl)
                DevicePtr device;
                SomethingCtrl *ctrl;


   You write two versions of this procedure, one for the keyboard
   and one for the pointing device. DIX calls it to inform DDX
   when a client has requested changes in the current settings for
   the particular device. For a keyboard, this might be the repeat
   threshold and rate. For a pointing device, this might be a
   scaling factor (coarse or fine) for position reporting. See
   input.h for the ctrl structures.
     __________________________________________________________

Input Initialization

   Input initialization is a bit complicated. It all starts with
   InitInput(), a routine that you write to call AddInputDevice()
   twice (once for pointing device and once for keyboard.) You
   also want to call RegisterKeyboardDevice() and
   RegisterPointerDevice() on them.

   When you Add the devices, a routine you supply for each device
   gets called to initialize them. Your individual initialize
   routines must call InitKeyboardDeviceStruct() or
   InitPointerDeviceStruct(), depending upon which it is. In other
   words, you indicate twice that the keyboard is the keyboard and
   the pointer is the pointer.

        void InitInput(argc, argv)
            int argc;
            char **argv;

   InitInput is a DDX routine you must write to initialize the
   input subsystem in DDX. It must call AddInputDevice() for each
   device that might generate events. In addition, you must
   register the main keyboard and pointing devices by calling
   RegisterPointerDevice() and RegisterKeyboardDevice().

        DevicePtr AddInputDevice(deviceProc, autoStart)
                DeviceProc deviceProc;
                Bool autoStart;

   AddInputDevice is a DIX routine you call to create a device
   object. deviceProc is a DDX routine that is called by DIX to do
   various operations. AutoStart should be TRUE for devices that
   need to be turned on at initialization time with a special
   call, as opposed to waiting for some client application to turn
   them on. This routine returns NULL if sufficient memory cannot
   be allocated to install the device.

   Note also that except for the main keyboard and pointing
   device, an extension is needed to provide for a client
   interface to a device.

        void RegisterPointerDevice(device)
                DevicePtr device;

   RegisterPointerDevice is a DIX routine that your DDX code calls
   that makes that device the main pointing device. This routine
   is called once upon initialization and cannot be called again.

        void RegisterKeyboardDevice(device)
                DevicePtr device;

   RegisterKeyboardDevice makes the given device the main
   keyboard. This routine is called once upon initialization and
   cannot be called again.

   The following DIX procedures return the specified DevicePtr.
   They may or may not be useful to DDX implementors.

        DevicePtr LookupKeyboardDevice()

   LookupKeyboardDevice returns pointer for current main keyboard
   device.

        DevicePtr LookupPointerDevice()

   LookupPointerDevice returns pointer for current main pointing
   device.

   A DeviceProc (the kind passed to AddInputDevice()) in the
   following form:

        Bool pInternalDevice->DeviceProc(device, action);
                DeviceIntPtr device;
                int action;

   You must write a DeviceProc for each device. device points to
   the device record. action tells what action to take; it will be
   one of these defined constants (defined in input.h):

     * DEVICE_INIT - At DEVICE_INIT time, the device should
       initialize itself by calling InitPointerDeviceStruct(),
       InitKeyboardDeviceStruct(), or a similar routine (see
       below) and "opening" the device if necessary. If you return
       a non-zero (i.e., != Success) value from the DEVICE_INIT
       call, that device will be considered unavailable. If either
       the main keyboard or main pointing device cannot be
       initialized, the DIX code will refuse to continue booting
       up.
     * DEVICE_ON - If the DeviceProc is called with DEVICE_ON,
       then it is allowed to start putting events into the client
       stream by calling through the ProcessInputProc in the
       device.
     * DEVICE_OFF - If the DeviceProc is called with DEVICE_OFF,
       no further events from that device should be given to the
       DIX layer. The device will appear to be dead to the user.
     * DEVICE_CLOSE - At DEVICE_CLOSE (terminate or reset) time,
       the device should be totally closed down.

        void InitPointerDeviceStruct(device, map, mapLength,
                        GetMotionEvents, ControlProc, numMotionEvents)
                DevicePtr device;
                CARD8 *map;
                int mapLength;
                ValuatorMotionProcPtr ControlProc;
                PtrCtrlProcPtr GetMotionEvents;
                int numMotionEvents;

   InitPointerDeviceStruct is a DIX routine you call at
   DEVICE_INIT time to declare some operating routines and data
   structures for a pointing device. map and mapLength are as
   described in the X Window System protocol specification.
   ControlProc and GetMotionEvents are DDX routines, see above.

   numMotionEvents is for the motion-buffer-size for the
   GetMotionEvents request. A typical length for a motion buffer
   would be 100 events. A server that does not implement this
   capability should set numMotionEvents to zero.

        void InitKeyboardDeviceStruct(device, pKeySyms, pModifiers, Bell
, ControlProc)
                DevicePtr device;
                KeySymsPtr pKeySyms;
                CARD8 *pModifiers;
                BellProcPtr Bell;
                KbdCtrlProcPtr ControlProc;


   You call this DIX routine when a keyboard device is initialized
   and its device procedure is called with DEVICE_INIT. The
   formats of the keysyms and modifier maps are defined in
   Xserver/include/input.h. They describe the layout of keys on
   the keyboards, and the glyphs associated with them. ( See the
   next section for information on setting up the modifier map and
   the keysym map.) ControlProc and Bell are DDX routines, see
   above.
     __________________________________________________________

Keyboard Mapping and Keycodes

   When you send a keyboard event, you send a report that a given
   key has either been pressed or has been released. There must be
   a keycode for each key that identifies the key; the
   keycode-to-key mapping can be any mapping you desire, because
   you specify the mapping in a table you set up for DIX. However,
   you are restricted by the protocol specification to keycode
   values in the range 8 to 255 inclusive.

   The keycode mapping information that you set up consists of the
   following:

     * A minimum and maximum keycode number
     * An array of sets of keysyms for each key, that is of length
       maxkeycode - minkeycode + 1. Each element of this array is
       a list of codes for symbols that are on that key. There is
       no limit to the number of symbols that can be on a key.

   Once the map is set up, DIX keeps and maintains the client's
   changes to it.

   The X protocol defines standard names to indicate the symbol(s)
   printed on each keycap. (See X11/keysym.h)

   Legal modifier keys must generate both up and down transitions.
   When a client tries to change a modifier key (for instance, to
   make "A" the "Control" key), DIX calls the following routine,
   which should retuurn TRUE if the key can be used as a modifier
   on the given device:

        Bool LegalModifier(key, pDev)
            unsigned int key;
            DevicePtr pDev;
     __________________________________________________________

Screens

   Different computer graphics displays have different
   capabilities. Some are simple monochrome frame buffers that are
   just lying there in memory, waiting to be written into. Others
   are color displays with many bits per pixel using some color
   lookup table. Still others have high-speed graphic processors
   that prefer to do all of the work themselves, including
   maintaining their own high-level, graphic data structures.
     __________________________________________________________

Screen Hardware Requirements

   The only requirement on screens is that you be able to both
   read and write locations in the frame buffer. All screens must
   have a depth of 32 or less (unless you use an X extension to
   allow a greater depth). All screens must fit into one of the
   classes listed in the section in this document on Visuals and
   Depths.

   X uses the pixel as its fundamental unit of distance on the
   screen. Therefore, most programs will measure everything in
   pixels.

   The sample server assumes square pixels. Serious WYSIWYG (what
   you see is what you get) applications for publishing and
   drawing programs will adjust for different screen resolutions
   automatically. Considerable work is involved in compensating
   for non-square pixels (a bit in the DDX code for the sample
   server but quite a bit in the client applications).
     __________________________________________________________

Data Structures

   X supports multiple screens that are connected to the same
   server. Therefore, all the per-screen information is bundled
   into one data structure of attributes and procedures, which is
   the ScreenRec (see Xserver/include/scrnintstr.h). The procedure
   entry points in a ScreenRec operate on regions, colormaps,
   cursors, and fonts, because these resources can differ in
   format from one screen to another.

   Windows are areas on the screen that can be drawn into by
   graphic routines. "Pixmaps" are off-screen graphic areas that
   can be drawn into. They are both considered drawables and are
   described in the section on Drawables. All graphic operations
   work on drawables, and operations are available to copy patches
   from one drawable to another.

   The pixel image data in all drawables is in a format that is
   private to DDX. In fact, each instance of a drawable is
   associated with a given screen. Presumably, the pixel image
   data for pixmaps is chosen to be conveniently understood by the
   hardware. All screens in a single server must be able to handle
   all pixmaps depths declared in the connection setup
   information.

   Pixmap images are transferred to the server in one of two ways:
   XYPixmap or ZPimap. XYPixmaps are a series of bitmaps, one for
   each bit plane of the image, using the bitmap padding rules
   from the connection setup. ZPixmaps are a series of bits,
   nibbles, bytes or words, one for each pixel, using the format
   rules (padding and so on) for the appropriate depth.

   All screens in a given server must agree on a set of pixmap
   image formats (PixmapFormat) to support (depth, number of bits
   per pixel, etc.).

   There is no color interpretation of bits in the pixmap. Pixmaps
   do not contain pixel values. The interpretation is made only
   when the bits are transferred onto the screen.

   The screenInfo structure (in scrnintstr.h) is a global data
   structure that has a pointer to an array of ScreenRecs, one for
   each screen on the server. (These constitute the one and only
   description of each screen in the server.) Each screen has an
   identifying index (0, 1, 2, ...). In addition, the screenInfo
   struct contains global server-wide details, such as the bit-
   and byte- order in all bit images, and the list of pixmap image
   formats that are supported. The X protocol insists that these
   must be the same for all screens on the server.
     __________________________________________________________

Output Initialization

        InitOutput(pScreenInfo, argc, argv)
                ScreenInfo *pScreenInfo;
                int argc;
                char **argv;

   Upon initialization, your DDX routine InitOutput() is called by
   DIX. It is passed a pointer to screenInfo to initialize. It is
   also passed the argc and argv from main() for your server for
   the command-line arguments. These arguments may indicate what
   or how many screen device(s) to use or in what way to use them.
   For instance, your server command line may allow a "-D" flag
   followed by the name of the screen device to use.

   Your InitOutput() routine should initialize each screen you
   wish to use by calling AddScreen(), and then it should
   initialize the pixmap formats that you support by storing
   values directly into the screenInfo data structure. You should
   also set certain implementation-dependent numbers and
   procedures in your screenInfo, which determines the pixmap and
   scanline padding rules for all screens in the server.

        int AddScreen(scrInitProc, argc, argv)
                Bool (*scrInitProc)();
                int argc;
                char **argv;

   You should call AddScreen(), a DIX procedure, in InitOutput()
   once for each screen to add it to the screenInfo database. The
   first argument is an initialization procedure for the screen
   that you supply. The second and third are the argc and argv
   from main(). It returns the screen number of the screen
   installed, or -1 if there is either insufficient memory to add
   the screen, or (*scrInitProc) returned FALSE.

   The scrInitProc should be of the following form:

        Bool scrInitProc(iScreen, pScreen, argc, argv)
                int iScreen;
                ScreenPtr pScreen;
                int argc;
                char **argv;

   iScreen is the index for this screen; 0 for the first one
   initialized, 1 for the second, etc. pScreen is the pointer to
   the screen's new ScreenRec. argc and argv are as before. Your
   screen initialize procedure should return TRUE upon success or
   FALSE if the screen cannot be initialized (for instance, if the
   screen hardware does not exist on this machine).

   This procedure must determine what actual device it is supposed
   to initialize. If you have a different procedure for each
   screen, then it is no problem. If you have the same procedure
   for multiple screens, it may have trouble figuring out which
   screen to initialize each time around, especially if
   InitOutput() does not initialize all of the screens. It is
   probably easiest to have one procedure for each screen.

   The initialization procedure should fill in all the screen
   procedures for that screen (windowing functions, region
   functions, etc.) and certain screen attributes for that screen.
     __________________________________________________________

Region Routines in the ScreenRec

   A region is a dynamically allocated data structure that
   describes an irregularly shaped piece of real estate in XY
   pixel space. You can think of it as a set of pixels on the
   screen to be operated upon with set operations such as AND and
   OR.

   A region is frequently implemented as a list of rectangles or
   bitmaps that enclose the selected pixels. Region operators
   control the "clipping policy," or the operations that work on
   regions. (The sample server uses YX-banded rectangles. Unless
   you have something already implemented for your graphics
   system, you should keep that implementation.) The procedure
   pointers to the region operators are located in the ScreenRec
   data structure. The definition of a region can be found in the
   file Xserver/include/regionstr.h. The region code is found in
   Xserver/mi/miregion.c. DDX implementations using other region
   formats will need to supply different versions of the region
   operators.

   Since the list of rectangles is unbounded in size, part of the
   region data structure is usually a large, dynamically allocated
   chunk of memory. As your region operators calculate logical
   combinations of regions, these blocks may need to be
   reallocated by your region software. For instance, in the
   sample server, a RegionRec has some header information and a
   pointer to a dynamically allocated rectangle list.
   Periodically, the rectangle list needs to be expanded with
   Xrealloc(), whereupon the new pointer is remembered in the
   RegionRec.

   Most of the region operations come in two forms: a function
   pointer in the Screen structure, and a macro. The server can be
   compiled so that the macros make direct calls to the
   appropriate functions (instead of indirecting through a screen
   function pointer), or it can be compiled so that the macros are
   identical to the function pointer forms. Making direct calls is
   faster on many architectures.

        RegionPtr pScreen->RegionCreate( rect, size)
                BoxPtr rect;
                int size;

        macro: RegionPtr REGION_CREATE(pScreen, rect, size)


   RegionCreate creates a region that describes ONE rectangle. The
   caller can avoid unnecessary reallocation and copying by
   declaring the probable maximum number of rectangles that this
   region will need to describe itself. Your region routines,
   though, cannot fail just because the region grows beyond this
   size. The caller of this routine can pass almost anything as
   the size; the value is merely a good guess as to the maximum
   size until it is proven wrong by subsequent use. Your region
   procedures are then on their own in estimating how big the
   region will get. Your implementation might ignore size, if
   applicable.

        void pScreen->RegionInit (pRegion, rect, size)
                RegionPtr       pRegion;
                BoxPtr          rect;
                int             size;

        macro: REGION_INIT(pScreen, pRegion, rect, size)


   Given an existing raw region structure (such as an local
   variable), this routine fills in the appropriate fields to make
   this region as usable as one returned from RegionCreate. This
   avoids the additional dynamic memory allocation overhead for
   the region structure itself.

        Bool pScreen->RegionCopy(dstrgn, srcrgn)
                RegionPtr dstrgn, srcrgn;

        macro: Bool REGION_COPY(pScreen, dstrgn, srcrgn)


   RegionCopy copies the description of one region, srcrgn, to
   another already-created region, dstrgn; returning TRUE if the
   copy succeeded, and FALSE otherwise.

        void pScreen->RegionDestroy( pRegion)
                RegionPtr pRegion;

        macro: REGION_DESTROY(pScreen, pRegion)


   RegionDestroy destroys a region and frees all allocated memory.

        void pScreen->RegionUninit (pRegion)
                RegionPtr pRegion;

        macro: REGION_UNINIT(pScreen, pRegion)


   Frees everything except the region structure itself, useful
   when the region was originally passed to RegionInit instead of
   received from RegionCreate. When this call returns, pRegion
   must not be reused until it has been RegionInit'ed again.

        Bool pScreen->Intersect(newReg, reg1, reg2)
                RegionPtr newReg, reg1, reg2;

        macro: Bool REGION_INTERSECT(pScreen, newReg, reg1, reg2)

        Bool  pScreen->Union(newReg, reg1, reg2)
                RegionPtr newReg, reg1, reg2;

        macro: Bool REGION_UNION(pScreen, newReg, reg1, reg2)

        Bool  pScreen->Subtract(newReg, regMinuend, regSubtrahend)
                RegionPtr newReg, regMinuend, regSubtrahend;

        macro: Bool REGION_UNION(pScreen, newReg, regMinuend, regSubtrah
end)

        Bool pScreen->Inverse(newReg, pReg,  pBox)
                RegionPtr newReg, pReg;
                BoxPtr pBox;

        macro: Bool REGION_INVERSE(pScreen, newReg, pReg,  pBox)


   The above four calls all do basic logical operations on
   regions. They set the new region (which already exists) to
   describe the logical intersection, union, set difference, or
   inverse of the region(s) that were passed in. Your routines
   must be able to handle a situation where the newReg is the same
   region as one of the other region arguments.

   The subtract function removes the Subtrahend from the Minuend
   and puts the result in newReg.

   The inverse function returns a region that is the pBox minus
   the region passed in. (A true "inverse" would make a region
   that extends to infinity in all directions but has holes in the
   middle.) It is undefined for situations where the region
   extends beyond the box.

   Each routine must return the value TRUE for success.

        void pScreen->RegionReset(pRegion, pBox)
                RegionPtr pRegion;
                BoxPtr pBox;

        macro: REGION_RESET(pScreen, pRegion, pBox)


   RegionReset sets the region to describe one rectangle and
   reallocates it to a size of one rectangle, if applicable.

        void  pScreen->TranslateRegion(pRegion, x, y)
                RegionPtr pRegion;
                int x, y;

        macro: REGION_TRANSLATE(pScreen, pRegion, x, y)


   TranslateRegion simply moves a region +x in the x direction and
   +y in the y direction.

        int  pScreen->RectIn(pRegion, pBox)
                RegionPtr pRegion;
                BoxPtr pBox;

        macro: int RECT_IN_REGION(pScreen, pRegion, pBox)


   RectIn returns one of the defined constants rgnIN, rgnOUT, or
   rgnPART, depending upon whether the box is entirely inside the
   region, entirely outside of the region, or partly in and partly
   out of the region. These constants are defined in
   Xserver/include/region.h.

        Bool pScreen->PointInRegion(pRegion, x, y, pBox)
                RegionPtr pRegion;
                int x, y;
                BoxPtr pBox;

        macro: Bool POINT_IN_REGION(pScreen, pRegion, x, y, pBox)


   PointInRegion returns true if the point x, y is in the region.
   In addition, it fills the rectangle pBox with coordinates of a
   rectangle that is entirely inside of pRegion and encloses the
   point. In the mi implementation, it is the largest such
   rectangle. (Due to the sample server implementation, this comes
   cheaply.)

   This routine used by DIX when tracking the pointing device and
   deciding whether to report mouse events or change the cursor.
   For instance, DIX needs to change the cursor when it moves from
   one window to another. Due to overlapping windows, the shape to
   check may be irregular. A PointInRegion() call for every
   pointing device movement may be too expensive. The pBox is a
   kind of wake-up box; DIX need not call PointInRegion() again
   until the cursor wanders outside of the returned box.

        Bool pScreen->RegionNotEmpty(pRegion)
                RegionPtr pRegion;

        macro: Bool REGION_NOTEMPTY(pScreen, pRegion)


   RegionNotEmpty is a boolean function that returns true or false
   depending upon whether the region encloses any pixels.

        void pScreen->RegionEmpty(pRegion)
                RegionPtr pRegion;

        macro: REGION_EMPTY(pScreen, pRegion)


   RegionEmpty sets the region to be empty.

        BoxPtr pScreen->RegionExtents(pRegion)
                RegionPtr pRegion;

        macro: REGION_EXTENTS(pScreen, pRegion)


   RegionExtents returns a rectangle that is the smallest possible
   superset of the entire region. The caller will not modify this
   rectangle, so it can be the one in your region struct.

        Bool pScreen->RegionAppend (pDstRgn, pRegion)
                RegionPtr pDstRgn;
                RegionPtr pRegion;

        macro: Bool REGION_APPEND(pScreen, pDstRgn, pRegion)

        Bool pScreen->RegionValidate (pRegion, pOverlap)
                RegionPtr pRegion;
                Bool *pOverlap;

        macro: Bool REGION_VALIDATE(pScreen, pRegion, pOverlap)


   These functions provide an optimization for clip list
   generation and must be used in conjunction. The combined effect
   is to produce the union of a collection of regions, by using
   RegionAppend several times, and finally calling RegionValidate
   which takes the intermediate representation (which needn't be a
   valid region) and produces the desired union. pOverlap is set
   to TRUE if any of the original regions overlap; FALSE
   otherwise.

        RegionPtr pScreen->BitmapToRegion (pPixmap)
                PixmapPtr pPixmap;

        macro: RegionPtr BITMAP_TO_REGION(pScreen, pPixmap)


   Given a depth-1 pixmap, this routine must create a valid region
   which includes all the areas of the pixmap filled with 1's and
   excludes the areas filled with 0's. This routine returns NULL
   if out of memory.

        RegionPtr pScreen->RectsToRegion (nrects, pRects, ordering)
                int nrects;
                xRectangle *pRects;
                int ordering;

        macro: RegionPtr RECTS_TO_REGION(pScreen, nrects, pRects, orderi
ng)


   Given a client-supplied list of rectangles, produces a region
   which includes the union of all the rectangles. Ordering may be
   used as a hint which describes how the rectangles are sorted.
   As the hint is provided by a client, it must not be required to
   be correct, but the results when it is not correct are not
   defined (core dump is not an option here).

        void pScreen->SendGraphicsExpose(client,pRegion,drawable,major,m
inor)
                ClientPtr client;
                RegionPtr pRegion;
                XID drawable;
                int major;
                int minor;


   SendGraphicsExpose dispatches a list of GraphicsExposure events
   which span the region to the specified client. If the region is
   empty, or a NULL pointer, a NoExpose event is sent instead.
     __________________________________________________________

Cursor Routines for a Screen

   A cursor is the visual form tied to the pointing device. The
   default cursor is an "X" shape, but the cursor can have any
   shape. When a client creates a window, it declares what shape
   the cursor will be when it strays into that window on the
   screen.

   For each possible shape the cursor assumes, there is a
   CursorRec data structure. This data structure contains a
   pointer to a CursorBits data structure which contains a bitmap
   for the image of the cursor and a bitmap for a mask behind the
   cursor, in addition, the CursorRec data structure contains
   foreground and background colors for the cursor. The CursorBits
   data structure is shared among multiple CursorRec structures
   which use the same font and glyph to describe both source and
   mask. The cursor image is applied to the screen by applying the
   mask first, clearing 1 bits in its form to the background
   color, and then overwriting on the source image, in the
   foreground color. (One bits of the source image that fall on
   top of zero bits of the mask image are undefined.) This way, a
   cursor can have transparent parts, and opaque parts in two
   colors. X allows any cursor size, but some hardware cursor
   schemes allow a maximum of N pixels by M pixels. Therefore, you
   are allowed to transform the cursor to a smaller size, but be
   sure to include the hot-spot.

   CursorBits in Xserver/include/cursorstr.h is a
   device-independent structure containing a device-independent
   representation of the bits for the source and mask. (This is
   possible because the bitmap representation is the same for all
   screens.)

   When a cursor is created, it is "realized" for each screen. At
   realization time, each screen has the chance to convert the
   bits into some other representation that may be more convenient
   (for instance, putting the cursor into off-screen memory) and
   set up its device-private area in either the CursorRec data
   structure or CursorBits data structure as appropriate to
   possibly point to whatever data structures are needed. It is
   more memory-conservative to share realizations by using the
   CursorBits private field, but this makes the assumption that
   the realization is independent of the colors used (which is
   typically true). For instance, the following are the device
   private entries for a particular screen and cursor:

        pCursor->devPriv[pScreen->myNum]
        pCursor->bits->devPriv[pScreen->myNum]


   This is done because the change from one cursor shape to
   another must be fast and responsive; the cursor image should be
   able to flutter as fast as the user moves it across the screen.

   You must implement the following routines for your hardware:

        Bool pScreen->RealizeCursor( pScr, pCurs)
                ScreenPtr pScr;
                CursorPtr pCurs;

        Bool pScreen->UnrealizeCursor( pScr, pCurs)
                ScreenPtr pScr;
                CursorPtr pCurs;


   RealizeCursor and UnrealizeCursor should realize (allocate and
   calculate all data needed) and unrealize (free the dynamically
   allocated data) a given cursor when DIX needs them. They are
   called whenever a device-independent cursor is created or
   destroyed. The source and mask bits pointed to by fields in
   pCurs are undefined for bits beyond the right edge of the
   cursor. This is so because the bits are in Bitmap format, which
   may have pad bits on the right edge. You should inhibit
   UnrealizeCursor() if the cursor is currently in use; this
   happens when the system is reset.

        Bool pScreen->DisplayCursor( pScr, pCurs)
                ScreenPtr pScr;
                CursorPtr pCurs;


   DisplayCursor should change the cursor on the given screen to
   the one passed in. It is called by DIX when the user moves the
   pointing device into a different window with a different
   cursor. The hotspot in the cursor should be aligned with the
   current cursor position.

        void pScreen->RecolorCursor( pScr, pCurs, displayed)
                ScreenPtr pScr;
                CursorPtr pCurs;
                Bool displayed;

   RecolorCursor notifies DDX that the colors in pCurs have
   changed and indicates whether this is the cursor currently
   being displayed. If it is, the cursor hardware state may have
   to be updated. Whether displayed or not, state created at
   RealizeCursor time may have to be updated. A generic version,
   miRecolorCursor, may be used that does an unrealize, a realize,
   and possibly a display (in micursor.c); however this constrains
   UnrealizeCursor and RealizeCursor to always return TRUE as no
   error indication is returned here.

        void pScreen->ConstrainCursor( pScr, pBox)
                ScreenPtr pScr;
                BoxPtr pBox;


   ConstrainCursor should cause the cursor to restrict its motion
   to the rectangle pBox. DIX code is capable of enforcing this
   constraint by forcefully moving the cursor if it strays out of
   the rectangle, but ConstrainCursor offers a way to send a hint
   to the driver or hardware if such support is available. This
   can prevent the cursor from wandering out of the box, then
   jumping back, as DIX forces it back.

        void pScreen->PointerNonInterestBox( pScr, pBox)
                ScreenPtr pScr;
                BoxPtr pBox;


   PointerNonInterestBox is DIX's way of telling the pointing
   device code not to report motion events while the cursor is
   inside a given rectangle on the given screen. It is optional
   and, if not implemented, it should do nothing. This routine is
   called only when the client has declared that it is not
   interested in motion events in a given window. The rectangle
   you get may be a subset of that window. It saves DIX code the
   time required to discard uninteresting mouse motion events.
   This is only a hint, which may speed performance. Nothing in
   DIX currently calls PointerNonInterestBox.

        void pScreen->CursorLimits( pScr, pCurs, pHotBox, pTopLeftBox)
                ScreenPtr pScr;
                CursorPtr pCurs;
                BoxPtr pHotBox;
                BoxPtr pTopLeftBox;     /* return value */


   CursorLimits should calculate the box that the cursor hot spot
   is physically capable of moving within, as a function of the
   screen pScr, the device-independent cursor pCurs, and a box
   that DIX hypothetically would want the hot spot confined
   within, pHotBox. This routine is for informing DIX only; it
   alters no state within DDX.

        Bool pScreen->SetCursorPosition( pScr, newx, newy, generateEvent
)
                ScreenPtr pScr;
                int newx;
                int newy;
                Bool generateEvent;


   SetCursorPosition should artificially move the cursor as though
   the user had jerked the pointing device very quickly. This is
   called in response to the WarpPointer request from the client,
   and at other times. If generateEvent is True, the device should
   decide whether or not to call ProcessInputEvents() and then it
   must call DevicePtr->processInputProc. Its effects are, of
   course, limited in value for absolute pointing devices such as
   a tablet.

        void NewCurrentScreen(newScreen, x, y)
            ScreenPtr newScreen;
            int x,y;


   If your ddx provides some mechanism for the user to magically
   move the pointer between multiple screens, you need to inform
   DIX when this occurs. You should call NewCurrentScreen to
   accomplish this, specifying the new screen and the new x and y
   coordinates of the pointer on that screen.
     __________________________________________________________

Visuals, Depths and Pixmap Formats for Screens

   The "depth" of a image is the number of bits that are used per
   pixel to display it.

   The "bits per pixel" of a pixmap image that is sent over the
   client byte stream is a number that is either 4, 8, 16, 24 or
   32. It is the number of bits used per pixel in Z format. For
   instance, a pixmap image that has a depth of six is best sent
   in Z format as 8 bits per pixel.

   A "pixmap image format" or a "pixmap format" is a description
   of the format of a pixmap image as it is sent over the byte
   stream. For each depth available on a server, there is one and
   only one pixmap format. This pixmap image format gives the bits
   per pixel and the scanline padding unit. (For instance, are
   pixel rows padded to bytes, 16-bit words, or 32-bit words?)

   For each screen, you must decide upon what depth(s) it
   supports. You should only count the number of bits used for the
   actual image. Some displays store additional bits to indicate
   what window this pixel is in, how close this object is to a
   viewer, transparency, and other data; do not count these bits.

   A "display class" tells whether the display is monochrome or
   color, whether there is a lookup table, and how the lookup
   table works.

   A "visual" is a combination of depth, display class, and a
   description of how the pixel values result in a color on the
   screen. Each visual has a set of masks and offsets that are
   used to separate a pixel value into its red, green, and blue
   components and a count of the number of colormap entries. Some
   of these fields are only meaningful when the class dictates so.
   Each visual also has a screen ID telling which screen it is
   usable on. Note that the depth does not imply the number of
   map_entries; for instance, a display can have 8 bits per pixel
   but only 254 colormap entries for use by applications (the
   other two being reserved by hardware for the cursor).

   Each visual is identified by a 32-bit visual ID which the
   client uses to choose what visual is desired on a given window.
   Clients can be using more than one visual on the same screen at
   the same time.

   The class of a display describes how this translation takes
   place. There are three ways to do the translation.

     * Pseudo - The pixel value, as a whole, is looked up in a
       table of length map_entries to determine the color to
       display.
     * True - The pixel value is broken up into red, green, and
       blue fields, each of which are looked up in separate red,
       green, and blue lookup tables, each of length map_entries.
     * Gray - The pixel value is looked up in a table of length
       map_entries to determine a gray level to display.

   In addition, the lookup table can be static (resulting colors
   are fixed for each pixel value) or dynamic (lookup entries are
   under control of the client program). This leads to a total of
   six classes:

     * Static Gray - The pixel value (of however many bits)
       determines directly the level of gray that the pixel
       assumes.
     * Gray Scale - The pixel value is fed through a lookup table
       to arrive at the level of gray to display for the given
       pixel.
     * Static Color - The pixel value is fed through a fixed
       lookup table that yields the color to display for that
       pixel.
     * PseudoColor - The whole pixel value is fed through a
       programmable lookup table that has one color (including
       red, green, and blue intensities) for each possible pixel
       value, and that color is displayed.
     * True Color - Each pixel value consists of one or more bits
       that directly determine each primary color intensity after
       being fed through a fixed table.
     * Direct Color - Each pixel value consists of one or more
       bits for each primary color. Each primary color value is
       individually looked up in a table for that primary color,
       yielding an intensity for that primary color. For each
       pixel, the red value is looked up in the red table, the
       green value in the green table, and the blue value in the
       blue table.

   Here are some examples:

     * A simple monochrome 1 bit per pixel display is Static Gray.
     * A display that has 2 bits per pixel for a choice between
       the colors of black, white, green and violet is Static
       Color.
     * A display that has three bits per pixel, where each bit
       turns on or off one of the red, green or blue guns, is in
       the True Color class.
     * If you take the last example and scramble the
       correspondence between pixel values and colors it becomes a
       Static Color display.

   A display has 8 bits per pixel. The 8 bits select one entry out
   of 256 entries in a lookup table, each entry consisting of 24
   bits (8bits each for red, green, and blue). The display can
   show any 256 of 16 million colors on the screen at once. This
   is a pseudocolor display. The client application gets to fill
   the lookup table in this class of display.

   Imagine the same hardware from the last example. Your server
   software allows the user, on the command line that starts up
   the server program, to fill the lookup table to his liking once
   and for all. From then on, the server software would not change
   the lookup table until it exits. For instance, the default
   might be a lookup table with a reasonable sample of colors from
   throughout the color space. But the user could specify that the
   table be filled with 256 steps of gray scale because he knew
   ahead of time he would be manipulating a lot of black-and-white
   scanned photographs and not very many color things. Clients
   would be presented with this unchangeable lookup table.
   Although the hardware qualifies as a PseudoColor display, the
   facade presented to the X client is that this is a Static Color
   display.

   You have to decide what kind of display you have or want to
   pretend you have. When you initialize the screen(s), this class
   value must be set in the VisualRec data structure along with
   other display characteristics like the depth and other numbers.

   The allowable DepthRec's and VisualRec's are pointed to by
   fields in the ScreenRec. These are set up when InitOutput() is
   called; you should Xalloc() appropriate blocks or use static
   variables initialized to the correct values.
     __________________________________________________________

Colormaps for Screens

   A colormap is a device-independent mapping between pixel values
   and colors displayed on the screen.

   Different windows on the same screen can have different
   colormaps at the same time. At any given time, the most
   recently installed colormap(s) will be in use in the server so
   that its (their) windows' colors will be guaranteed to be
   correct. Other windows may be off-color. Although this may seem
   to be chaotic, in practice most clients use the default
   colormap for the screen.

   The default colormap for a screen is initialized when the
   screen is initialized. It always remains in existence and is
   not owned by any regular client. It is owned by client 0 (the
   server itself). Many clients will simply use this default
   colormap for their drawing. Depending upon the class of the
   screen, the entries in this colormap may be modifiable by
   client applications.
     __________________________________________________________

Colormap Routines

   You need to implement the following routines to handle the
   device-dependent aspects of color maps. You will end up placing
   pointers to these procedures in your ScreenRec data
   structure(s). The sample server implementations of many of
   these routines are in both cfbcmap.c and mfbcmap.c; since mfb
   does not do very much with color, the cfb versions are
   typically more useful prototypes.

        Bool pScreen->CreateColormap(pColormap)
                ColormapPtr pColormap;


   This routine is called by the DIX CreateColormap routine after
   it has allocated all the data for the new colormap and just
   before it returns to the dispatcher. It is the DDX layer's
   chance to initialize the colormap, particularly if it is a
   static map. See the following section for more details on
   initializing colormaps. The routine returns FALSE if creation
   failed, such as due to memory limitations. Notice that the
   colormap has a devPriv field from which you can hang any
   colormap specific storage you need. Since each colormap might
   need special information, we attached the field to the colormap
   and not the visual.

        void pScreen->DestroyColormap(pColormap)
                ColormapPtr pColormap;


   This routine is called by the DIX FreeColormap routine after it
   has uninstalled the colormap and notified all interested
   parties, and before it has freed any of the colormap storage.
   It is the DDX layer's chance to free any data it added to the
   colormap.

        void pScreen->InstallColormap(pColormap)
                ColormapPtr pColormap;


   InstallColormap should fill a lookup table on the screen with
   which the colormap is associated with the colors in pColormap.
   If there is only one hardware lookup table for the screen, then
   all colors on the screen may change simultaneously.

   In the more general case of multiple hardware lookup tables,
   this may cause some other colormap to be uninstalled, meaning
   that windows that subscribed to the colormap that was
   uninstalled may end up being off-color. See the note, below,
   about uninstalling maps.

        void pScreen->UninstallColormap(pColormap)
                ColormapPtr pColormap;


   UninstallColormap should remove pColormap from screen
   pColormap->pScreen. Some other map, such as the default map if
   possible, should be installed in place of pColormap if
   applicable. If pColormap is the default map, do nothing. If any
   client has requested ColormapNotify events, the DDX layer must
   notify the client. (The routine WalkTree() is be used to find
   such windows. The DIX routines TellNoMap(), TellNewMap() and
   TellGainedMap() are provided to be used as the procedure
   parameter to WalkTree. These procedures are in
   Xserver/dix/colormap.c.)

        int pScreen->ListInstalledColormaps(pScreen, pCmapList)
                ScreenPtr pScreen;
                XID *pCmapList;



   ListInstalledColormaps fills the pCMapList in with the resource
   ids of the installed maps and returns a count of installed
   maps. pCmapList will point to an array of size MaxInstalledMaps
   that was allocated by the caller.

        void pScreen->StoreColors (pmap, ndef, pdefs)
                ColormapPtr pmap;
                int ndef;
                xColorItem *pdefs;


   StoreColors changes some of the entries in the colormap pmap.
   The number of entries to change are ndef, and pdefs points to
   the information describing what to change. Note that partial
   changes of entries in the colormap are allowed. Only the colors
   indicated in the flags field of each xColorItem need to be
   changed. However, all three color fields will be sent with the
   proper value for the benefit of screens that may not be able to
   set part of a colormap value. If the screen is a static class,
   this routine does nothing. The structure of colormap entries is
   nontrivial; see colormapst.h and the definition of xColorItem
   in Xproto.h for more details.

        void pScreen->ResolveColor(pRed, pGreen, pBlue, pVisual)
                unsigned short *pRed, *pGreen, *pBlue;
                VisualPtr pVisual;



   Given a requested color, ResolveColor returns the nearest color
   that this hardware is capable of displaying on this visual. In
   other words, this rounds off each value, in place, to the
   number of bits per primary color that your screen can use.
   Remember that each screen has one of these routines. The level
   of roundoff should be what you would expect from the value you
   put in the bits_per_rgb field of the pVisual.

   Each value is an unsigned value ranging from 0 to 65535. The
   bits least likely to be used are the lowest ones.

   For example, if you had a pseudocolor display with any number
   of bits per pixel that had a lookup table supplying 6 bits for
   each color gun (a total of 256K different colors), you would
   round off each value to 6 bits. Please don't simply truncate
   these values to the upper 6 bits, scale the result so that the
   maximum value seen by the client will be 65535 for each
   primary. This makes color values more portable between
   different depth displays (a 6-bit truncated white will not look
   white on an 8-bit display).
     __________________________________________________________

Initializing a Colormap

   When a client requests a new colormap and when the server
   creates the default colormap, the procedure CreateColormap in
   the DIX layer is invoked. That procedure allocates memory for
   the colormap and related storage such as the lists of which
   client owns which pixels. It then sets a bit, BeingCreated, in
   the flags field of the ColormapRec and calls the DDX layer's
   CreateColormap routine. This is your chance to initialize the
   colormap. If the colormap is static, which you can tell by
   looking at the class field, you will want to fill in each color
   cell to match the hardwares notion of the color for that pixel.
   If the colormap is the default for the screen, which you can
   tell by looking at the IsDefault bit in the flags field, you
   should allocate BlackPixel and WhitePixel to match the values
   you set in the pScreen structure. (Of course, you picked those
   values to begin with.)

   You can also wait and use AllocColor() to allocate blackPixel
   and whitePixel after the default colormap has been created. If
   the default colormap is static and you initialized it in
   pScreen->CreateColormap, then use can use AllocColor afterwards
   to choose pixel values with the closest rgb values to those
   desired for blackPixel and whitePixel. If the default colormap
   is dynamic and uninitialized, then the rgb values you request
   will be obeyed, and AllocColor will again choose pixel values
   for you. These pixel values can then be stored into the screen.

   There are two ways to fill in the colormap. The simplest way is
   to use the DIX function AllocColor.

int AllocColor (pmap, pred, pgreen, pblue, pPix, client)
    ColormapPtr         pmap;
    unsigned short      *pred, *pgreen, *pblue;
    Pixel               *pPix;
    int                 client;


   This takes three pointers to 16 bit color values and a pointer
   to a suggested pixel value. The pixel value is either an index
   into one colormap or a combination of three indices depending
   on the type of pmap. If your colormap starts out empty, and you
   don't deliberately pick the same value twice, you will always
   get your suggested pixel. The truly nervous could check that
   the value returned in *pPix is the one AllocColor was called
   with. If you don't care which pixel is used, or would like them
   sequentially allocated from entry 0, set *pPix to 0. This will
   find the first free pixel and use that.

   AllocColor will take care of all the bookkeeping and will call
   StoreColors to get the colormap rgb values initialized. The
   hardware colormap will be changed whenever this colormap is
   installed.

   If for some reason AllocColor doesn't do what you want, you can
   do your own bookkeeping and call StoreColors yourself. This is
   much more difficult and shouldn't be necessary for most
   devices.
     __________________________________________________________

Fonts for Screens

   A font is a set of bitmaps that depict the symbols in a
   character set. Each font is for only one typeface in a given
   size, in other words, just one bitmap for each character.
   Parallel fonts may be available in a variety of sizes and
   variations, including "bold" and "italic." X supports fonts for
   8-bit and 16-bit character codes (for oriental languages that
   have more than 256 characters in the font). Glyphs are bitmaps
   for individual characters.

   The source comes with some useful font files in an ASCII,
   plain-text format that should be comprehensible on a wide
   variety of operating systems. The text format, referred to as
   BDF, is a slight extension of the current Adobe 2.1 Bitmap
   Distribution Format (Adobe Systems, Inc.).

   A short paper in PostScript format is included with the sample
   server that defines BDF. It includes helpful pictures, which is
   why it is done in PostScript and is not included in this
   document.

   Your implementation should include some sort of font compiler
   to read these files and generate binary files that are directly
   usable by your server implementation. The sample server comes
   with the source for a font compiler.

   It is important the font properties contained in the BDF files
   are preserved across any font compilation. In particular,
   copyright information cannot be casually tossed aside without
   legal ramifications. Other properties will be important to some
   sophisticated applications.

   All clients get font information from the server. Therefore,
   your server can support any fonts it wants to. It should
   probably support at least the fonts supplied with the X11 tape.
   In principle, you can convert fonts from other sources or dream
   up your own fonts for use on your server.
     __________________________________________________________

Portable Compiled Format

   A font compiler is supplied with the sample server. It has
   compile-time switches to convert the BDF files into a portable
   binary form, called Portable Compiled Format or PCF. This
   allows for an arbitrary data format inside the file, and by
   describing the details of the format in the header of the file,
   any PCF file can be read by any PCF reading client. By
   selecting the format which matches the required internal format
   for your renderer, the PCF reader can avoid reformatting the
   data each time it is read in. The font compiler should be quite
   portable.

   The fonts included with the tape are stored in fonts/bdf. The
   font compiler is found in fonts/tools/bdftopcf.
     __________________________________________________________

Font Realization

   Each screen configured into the server has an opportunity at
   font-load time to "realize" a font into some internal format if
   necessary. This happens every time the font is loaded into
   memory.

   A font (FontRec in Xserver/include/dixfontstr.h) is a
   device-independent structure containing a device-independent
   representation of the font. When a font is created, it is
   "realized" for each screen. At this point, the screen has the
   chance to convert the font into some other format. The DDX
   layer can also put information in the devPrivate storage.

        Bool pScreen->RealizeFont(pScr, pFont)
                ScreenPtr pScr;
                FontPtr pFont;

        Bool pScreen->UnrealizeFont(pScr, pFont)
                ScreenPtr pScr;
                FontPtr pFont;


   RealizeFont and UnrealizeFont should calculate and allocate
   these extra data structures and dispose of them when no longer
   needed. These are called in response to OpenFont and CloseFont
   requests from the client. The sample server implementation is
   in mfbfont.c (which does very little).
     __________________________________________________________

Other Screen Routines

   You must supply several other screen-specific routines for your
   X server implementation. Some of these are described in other
   sections:

     * GetImage() is described in the Drawing Primitives section.
     * GetSpans() is described in the Pixblit routine section.
     * Several window and pixmap manipulation procedures are
       described in the Window section under Drawables.
     * The CreateGC() routine is described under Graphics
       Contexts.

        void pScreen->QueryBestSize(kind, pWidth, pHeight)
                int kind;
                unsigned short *pWidth, *pHeight;
                ScreenPtr pScreen;


   QueryBestSize() returns the best sizes for cursors, tiles, and
   stipples in response to client requests. kind is one of the
   defined constants CursorShape, TileShape, or StippleShape
   (defined in X.h). For CursorShape, return the maximum width and
   height for cursors that you can handle. For TileShape and
   StippleShape, start with the suggested values in pWidth and
   pHeight and modify them in place to be optimal values that are
   greater than or equal to the suggested values. The sample
   server implementation is in Xserver/mfb/mfbmisc.c.

        pScreen->SourceValidate(pDrawable, x, y, width, height)
                DrawablePtr pDrawable;
                int x, y, width, height;


   SourceValidate should be called by CopyArea/CopyPlane
   primitives when the source drawable is not the same as the
   destination, and the SourceValidate function pointer in the
   screen is non-null. If you know that you will never need
   SourceValidate, you can avoid this check. Currently,
   SourceValidate is used by the mi software cursor code to remove
   the cursor from the screen when the source rectangle overlaps
   the cursor position. x,y,width,height describe the source
   rectangle (source relative, that is) for the copy operation.

        Bool pScreen->SaveScreen(pScreen, on)
                ScreenPtr pScreen;
                int on;


   SaveScreen() is used for Screen Saver support (see
   WaitForSomething()). pScreen is the screen to save.

        Bool pScreen->CloseScreen(pScreen)
            ScreenPtr pScreen;


   When the server is reset, it calls this routine for each
   screen.

        Bool pScreen->CreateScreenResources(pScreen)
            ScreenPtr pScreen;


   If this routine is not NULL, it will be called once per screen
   per server initialization/reset after all modules have had a
   chance to register their devPrivates on all structures that
   support them (see the section on devPrivates below). If you
   need to create any resources that have dynamic devPrivates as
   part of your screen initialization, you should do so in this
   function instead of in the screen init function passed to
   AddScreen to guarantee that the resources have a complete set
   of devPrivates. This routine returns TRUE if successful.
     __________________________________________________________

Drawables

   A drawable is a descriptor of a surface that graphics are drawn
   into, either a window on the screen or a pixmap in memory.

   Each drawable has a type, class, ScreenPtr for the screen it is
   associated with, depth, position, size, and serial number. The
   type is one of the defined constants DRAWABLE_PIXMAP,
   DRAWABLE_WINDOW and UNDRAWABLE_WINDOW. (An undrawable window is
   used for window class InputOnly.) The serial number is
   guaranteed to be unique across drawables, and is used in
   determining the validity of the clipping information in a GC.
   The screen selects the set of procedures used to manipulate and
   draw into the drawable. Position is used (currently) only by
   windows; pixmaps must set these fields to 0,0 as this reduces
   the amount of conditional code executed throughout the mi code.
   Size indicates the actual client-specified size of the
   drawable. There are, in fact, no other fields that a window
   drawable and pixmap drawable have in common besides those
   mentioned here.

   Both PixmapRecs and WindowRecs are structs that start with a
   drawable and continue on with more fields. Pixmaps have
   devPrivate pointers which usually point to the pixmap data but
   could conceivably be used for anything that DDX wants. Both
   windows and pixmaps have an array of devPrivates unions, one
   entry of which will probably be used for DDX specific data.
   Entries in this array are allocated using
   Allocate{Window|Pixmap}PrivateIndex() (see Wrappers and
   devPrivates below). This is done because different graphics
   hardware has different requirements for management; if the
   graphics is always handled by a processor with an independent
   address space, there is no point having a pointer to the bit
   image itself.

   The definition of a drawable and a pixmap can be found in the
   file Xserver/include/pixmapstr.h. The definition of a window
   can be found in the file Xserver/include/windowstr.h.
     __________________________________________________________

Pixmaps

   A pixmap is a three-dimensional array of bits stored somewhere
   offscreen, rather than in the visible portion of the screen's
   display frame buffer. It can be used as a source or destination
   in graphics operations. There is no implied interpretation of
   the pixel values in a pixmap, because it has no associated
   visual or colormap. There is only a depth that indicates the
   number of significant bits per pixel. Also, there is no implied
   physical size for each pixel; all graphic units are in numbers
   of pixels. Therefore, a pixmap alone does not constitute a
   complete image; it represents only a rectangular array of pixel
   values.

   Note that the pixmap data structure is reference-counted.

   The server implementation is free to put the pixmap data
   anywhere it sees fit, according to its graphics hardware setup.
   Many implementations will simply have the data dynamically
   allocated in the server's address space. More sophisticated
   implementations may put the data in undisplayed framebuffer
   storage.

   In addition to dynamic devPrivates (see the section on
   devPrivates below), the pixmap data structure has two fields
   that are private to the device. Although you can use them for
   anything you want, they have intended purposes. devKind is
   intended to be a device specific indication of the pixmap
   location (host memory, off-screen, etc.). In the sample server,
   since all pixmaps are in memory, devKind stores the width of
   the pixmap in bitmap scanline units. devPrivate is probably a
   pointer to the bits in the pixmap.

   A bitmap is a pixmap that is one bit deep.

        PixmapPtr pScreen->CreatePixmap(pScreen, width, height, depth)
                ScreenPtr pScreen;
                int width, height, depth;


   This ScreenRec procedure must create a pixmap of the size
   requested. It must allocate a PixmapRec and fill in all of the
   fields. The reference count field must be set to 1. If width or
   height are zero, no space should be allocated for the pixmap
   data, and if the implementation is using the devPrivate field
   as a pointer to the pixmap data, it should be set to NULL. If
   successful, it returns a pointer to the new pixmap; if not, it
   returns NULL. See Xserver/mfb/mfbpixmap.c for the sample server
   implementation.

        Bool pScreen->DestroyPixmap(pPixmap)
                PixmapPtr pPixmap;


   This ScreenRec procedure must "destroy" a pixmap. It should
   decrement the reference count and, if zero, it must deallocate
   the PixmapRec and all attached devPrivate blocks. If
   successful, it returns TRUE. See Xserver/mfb/mfbpixmap.c for
   the sample server implementation.

        Bool
        pScreen->ModifyPixmapHeader(pPixmap, width, height, depth, bitsP
erPixel, devKind, pPixData)
                PixmapPtr   pPixmap;
                int         width;
                int         height;
                int         depth;
                int         bitsPerPixel;
                int         devKind;
                pointer     pPixData;


   This routine takes a pixmap header (the PixmapRec plus all the
   dynamic devPrivates) and initializes the fields of the
   PixmapRec to the parameters of the same name. pPixmap must have
   been created via pScreen->CreatePixmap with a zero width or
   height to avoid allocating space for the pixmap data. pPixData
   is assumed to be the pixmap data; it will be stored in an
   implementation-dependent place (usually
   pPixmap->devPrivate.ptr). This routine returns TRUE if
   successful. See Xserver/mi/miscrinit.c for the sample server
   implementation.

        PixmapPtr
        GetScratchPixmapHeader(pScreen, width, height, depth, bitsPerPix
el, devKind, pPixData)
                ScreenPtr   pScreen;
                int         width;
                int         height;
                int         depth;
                int         bitsPerPixel;
                int         devKind;
                pointer     pPixData;

        void FreeScratchPixmapHeader(pPixmap)
                PixmapPtr pPixmap;


   DDX should use these two DIX routines when it has a buffer of
   raw image data that it wants to manipulate as a pixmap
   temporarily, usually so that some other part of the server can
   be leveraged to perform some operation on the data. The data
   should be passed in pPixData, and will be stored in an
   implementation-dependent place (usually
   pPixmap->devPrivate.ptr). The other fields go into the
   corresponding PixmapRec fields. If successful,
   GetScratchPixmapHeader returns a valid PixmapPtr which can be
   used anywhere the server expects a pixmap, else it returns
   NULL. The pixmap should be released when no longer needed
   (usually within the same function that allocated it) with
   FreeScratchPixmapHeader.
     __________________________________________________________

Windows

   A window is a visible, or potentially visible, rectangle on the
   screen. DIX windowing functions maintain an internal n-ary tree
   data structure, which represents the current relationships of
   the mapped windows. Windows that are contained in another
   window are children of that window and are clipped to the
   boundaries of the parent. The root window in the tree is the
   window for the entire screen. Sibling windows constitute a
   doubly-linked list; the parent window has a pointer to the head
   and tail of this list. Each child also has a pointer to its
   parent.

   The border of a window is drawn by a DDX procedure when DIX
   requests that it be drawn. The contents of the window is drawn
   by the client through requests to the server.

   Window painting is orchestrated through an expose event system.
   When a region is exposed, DIX generates an expose event,
   telling the client to repaint the window and passing the region
   that is the minimal area needed to be repainted.

   As a favor to clients, the server may retain the output to the
   hidden parts of windows in off-screen memory; this is called
   "backing store". When a part of such a window becomes exposed,
   it can quickly move pixels into place instead of triggering an
   expose event and waiting for a client on the other end of the
   network to respond. Even if the network response is
   insignificant, the time to intelligently paint a section of a
   window is usually more than the time to just copy
   already-painted sections. At best, the repainting involves
   blanking out the area to a background color, which will take
   about the same amount of time. In this way, backing store can
   dramatically increase the performance of window moves.

   On the other hand, backing store can be quite complex, because
   all graphics drawn to hidden areas must be intercepted and
   redirected to the off-screen window sections. Not only can this
   be complicated for the server programmer, but it can also
   impact window painting performance. The backing store
   implementation can choose, at any time, to forget pieces of
   backing that are written into, relying instead upon expose
   events to repaint for simplicity.

   In X, the decision to use the backing-store scheme is made by
   you, the server implementor. X provides hooks for implementing
   backing store, therefore the decision to use this strategy can
   be made on the fly. For example, you may use backing store only
   for certain windows that the user requests or you may use
   backing store until memory runs out, at which time you start
   dropping pieces of backing as needed to make more room.

   When a window operation is requested by the client, such as a
   window being created or moved, a new state is computed. During
   this transition, DIX informs DDX what rectangles in what
   windows are about to become obscured and what rectangles in
   what windows have become exposed. This provides a hook for the
   implementation of backing store. If DDX is unable to restore
   exposed regions, DIX generates expose events to the client. It
   is then the client's responsibility to paint the window parts
   that were exposed but not restored.

   If a window is resized, pixels sometimes need to be moved,
   depending upon the application. The client can request
   "Gravity" so that certain blocks of the window are moved as a
   result of a resize. For instance, if the window has controls or
   other items that always hang on the edge of the window, and
   that edge is moved as a result of the resize, then those pixels
   should be moved to avoid having the client repaint it. If the
   client needs to repaint it anyway, such an operation takes
   time, so it is desirable for the server to approximate the
   appearance of the window as best it can while waiting for the
   client to do it perfectly. Gravity is used for that, also.

   The window has several fields used in drawing operations:

     * clipList - This region, in conjunction with the client clip
       region in the gc, is used to clip output. clipList has the
       window's children subtracted from it, in addition to pieces
       of sibling windows that overlap this window. To get the
       list with the children included (subwindow-mode is
       IncludeInferiors), the routine NotClippedByChildren(pWin)
       returns the unclipped region.
     * borderClip is the region used by CopyWindow and includes
       the area of the window, its children, and the border, but
       with the overlapping areas of sibling children removed.

   Most of the other fields are for DIX use only.
     __________________________________________________________

Window Procedures in the ScreenRec

   You should implement all of the following procedures and store
   pointers to them in the screen record.

   The device-independent portion of the server "owns" the window
   tree. However, clever hardware might want to know the
   relationship of mapped windows. There are pointers to
   procedures in the ScreenRec data structure that are called to
   give the hardware a chance to update its internal state. These
   are helpers and hints to DDX only; they do not change the
   window tree, which is only changed by DIX.

        Bool pScreen->CreateWindow(pWin)
                WindowPtr pWin;


   This routine is a hook for when DIX creates a window. It should
   fill in the "Window Procedures in the WindowRec" below and also
   allocate the devPrivate block for it.

   See Xserver/mfb/mfbwindow.c for the sample server
   implementation.

        Bool pScreen->DestroyWindow(pWin);
                WindowPtr pWin;


   This routine is a hook for when DIX destroys a window. It
   should deallocate the devPrivate block for it and any other
   blocks that need to be freed, besides doing other cleanup
   actions.

   See Xserver/mfb/mfbwindow.c for the sample server
   implementation.

        Bool pScreen->PositionWindow(pWin, x, y);
                WindowPtr pWin;
                int x, y;


   This routine is a hook for when DIX moves or resizes a window.
   It should do whatever private operations need to be done when a
   window is moved or resized. For instance, if DDX keeps a pixmap
   tile used for drawing the background or border, and it keeps
   the tile rotated such that it is longword aligned to longword
   locations in the frame buffer, then you should rotate your
   tiles here. The actual graphics involved in moving the pixels
   on the screen and drawing the border are handled by
   CopyWindow(), below.

   See Xserver/mfb/mfbwindow.c for the sample server
   implementation.

        Bool pScreen->RealizeWindow(pWin);
                WindowPtr pWin;

        Bool  pScreen->UnrealizeWindow(pWin);
                WindowPtr pWin;


   These routines are hooks for when DIX maps (makes visible) and
   unmaps (makes invisible) a window. It should do whatever
   private operations need to be done when these happen, such as
   allocating or deallocating structures that are only needed for
   visible windows. RealizeWindow does NOT draw the window border,
   background or contents; UnrealizeWindow does NOT erase the
   window or generate exposure events for underlying windows; this
   is taken care of by DIX. DIX does, however, call
   PaintWindowBackground() and PaintWindowBorder() to perform some
   of these.

        Bool pScreen->ChangeWindowAttributes(pWin, vmask)
                WindowPtr pWin;
                unsigned long vmask;


   ChangeWindowAttributes is called whenever DIX changes window
   attributes, such as the size, front-to-back ordering, title, or
   anything of lesser severity that affects the window itself. The
   sample server implements this routine. It computes accelerators
   for quickly putting up background and border tiles. (See
   description of the set of routines stored in the WindowRec.)

        int pScreen->ValidateTree(pParent,  pChild, kind)
                WindowPtr pParent, pChild;
                VTKind kind;


   ValidateTree calculates the clipping region for the parent
   window and all of its children. This routine must be provided.
   The sample server has a machine-independent version in
   Xserver/mi/mivaltree.c. This is a very difficult routine to
   replace.

        void pScreen->PostValidateTree(pParent,  pChild, kind)
                WindowPtr pParent, pChild;
                VTKind kind;


   If this routine is not NULL, DIX calls it shortly after calling
   ValidateTree, passing it the same arguments. This is useful for
   managing multi-layered framebuffers. The sample server sets
   this to NULL.

        void pScreen->WindowExposures(pWin, pRegion, pBSRegion)
                WindowPtr pWin;
                RegionPtr pRegion;
                RegionPtr pBSRegion;


   The WindowExposures() routine paints the border and generates
   exposure events for the window. pRegion is an unoccluded region
   of the window, and pBSRegion is an occluded region that has
   backing store. Since exposure events include a rectangle
   describing what was exposed, this routine may have to send back
   a series of exposure events, one for each rectangle of the
   region. The count field in the expose event is a hint to the
   client as to the number of regions that are after this one.
   This routine must be provided. The sample server has a
   machine-independent version in Xserver/mi/miexpose.c.

        void pScreen->ClipNotify (pWin, dx, dy)
                WindowPtr pWin;
                int dx, dy;


   Whenever the cliplist for a window is changed, this function is
   called to perform whatever hardware manipulations might be
   necessary. When called, the clip list and border clip regions
   in the window are set to the new values. dx,dy are the distance
   that the window has been moved (if at all).
     __________________________________________________________

Window Painting Procedures

   In addition to the procedures listed above, there are four
   routines which manipulate the actual window image directly. In
   the sample server, mi implementations will work for most
   purposes and mfb/cfb routines speed up situations, such as
   solid backgrounds/borders or tiles that are 8, 16 or 32 pixels
   square.

   These three routines are used for systems that implement a
   backing-store scheme for it to know when to stash away areas of
   pixels and to restore or reposition them.

        void pScreen->ClearToBackground(pWin, x, y, w, h, generateExposu
res);
                WindowPtr pWin;
                int x, y, w, h;
                Bool generateExposures;


   This routine is called on a window in response to a
   ClearToBackground request from the client. This request has two
   different but related functions, depending upon
   generateExposures.

   If generateExposures is true, the client is declaring that the
   given rectangle on the window is incorrectly painted and needs
   to be repainted. The sample server implementation calculates
   the exposure region and hands it to the DIX procedure
   HandleExposures(), which calls the WindowExposures() routine,
   below, for the window and all of its child windows.

   If generateExposures is false, the client is trying to simply
   erase part of the window to the background fill style.
   ClearToBackground should write the background color or tile to
   the rectangle in question (probably using
   PaintWindowBackground). If w or h is zero, it clears all the
   way to the right or lower edge of the window.

   The sample server implementation is in Xserver/mi/miwindow.c.

        void pScreen->PaintWindowBackground(pWin, region, kind)
                WindowPtr pWin;
                RegionPtr region;
                int kind;       /* must be PW_BACKGROUND */

        void pScreen->PaintWindowBorder(pWin, region, kind)
                WindowPtr pWin;
                RegionPtr region;
                int kind;       /* must be PW_BORDER */


   These two routines are for painting pieces of the window
   background or border. They both actually paint the area
   designated by region. The kind parameter is a defined constant
   that is always PW_BACKGROUND or PW_BORDER, as shown. Therefore,
   you can use the same routine for both. The defined constant
   tells the routine whether to use the window's border fill style
   or its background fill style to paint the given region. Both
   fill styles consist of a union which holds a tile pointer and a
   pixel value, along with a separate variable which indicates
   which entry is valid. For PW_BORDER, borderIsPixel != 0
   indicates that the border PixUnion contains a pixel value, else
   a tile. For PW_BACKGROUND there are four values, contained in
   backgroundState; None, ParentRelative, BackgroundPixmap and
   BackgroundPixel. None indicates that the region should be left
   unfilled, while ParentRelative indicates that the background of
   the parent is inherited (see the Protocol document for the
   exact semantics).

        void pScreen->CopyWindow(pWin, oldpt, oldRegion);
                WindowPtr pWin;
                DDXPointRec oldpt;
                RegionPtr oldRegion;


   CopyWindow is called when a window is moved, and graphically
   moves to pixels of a window on the screen. It should not change
   any other state within DDX (see PositionWindow(), above).

   oldpt is the old location of the upper-left corner. oldRegion
   is the old region it is coming from. The new location and new
   region is stored in the WindowRec. oldRegion might modified in
   place by this routine (the sample implementation does this).

   CopyArea could be used, except that this operation has more
   complications. First of all, you do not want to copy a
   rectangle onto a rectangle. The original window may be obscured
   by other windows, and the new window location may be similarly
   obscured. Second, some hardware supports multiple windows with
   multiple depths, and your routine needs to take care of that.

   The pixels in oldRegion (with reference point oldpt) are copied
   to the window's new region (pWin->borderClip). pWin->borderClip
   is gotten directly from the window, rather than passing it as a
   parameter.

   The sample server implementation is in Xserver/mfb/mfbwindow.c.
     __________________________________________________________

Screen Operations for Backing Store

   Each ScreenRec has six functions which provide the backing
   store interface. For screens not supporting backing store,
   these pointers may be nul. Servers that implement some backing
   store scheme must fill in the procedure pointers for the
   procedures below, and must maintain the backStorage field in
   each window struct. The sample implementation is in
   mi/mibstore.c.

        void pScreen->SaveDoomedAreas(pWin, pRegion, dx, dy)
                WindowPtr pWin;
                RegionPtr pRegion;
                int dx, dy;


   This routine saves the newly obscured region, pRegion, in
   backing store. dx, dy indicate how far the window is being
   moved, useful as the obscured region is relative to the window
   as it will appear in the new location, rather then relative to
   the bits as the are on the screen when the function is invoked.

        RegionPtr pScreen->RestoreAreas(pWin, pRegion)
                WindowPtr pWin;
                RegionPtr pRegion;


   This looks at the exposed region of the window, pRegion, and
   tries to restore to the screen the parts that have been saved.
   It removes the restored parts from the backing storage (because
   they are now on the screen) and subtracts the areas from the
   exposed region. The returned region is the area of the window
   which should have expose events generated for and can be either
   a new region, pWin->exposed, or NULL. The region left in
   pRegion is set to the area of the window which should be
   painted with the window background.

        RegionPtr pScreen->TranslateBackingStore(pWin, dx, dy, oldClip,
oldx, oldy)
                WindowPtr pWin;
                int dx, dy;
                RegionPtr oldClip;
                int oldx, oldy;


   This is called when the window is moved or resized so that the
   backing store can be translated if necessary. oldClip is the
   old cliplist for the window, which is used to save doomed areas
   if the window is moved underneath its parent as a result of
   bitgravity. The returned region represents occluded areas of
   the window for which the backing store contents are invalid.

        void pScreen->ExposeCopy(pSrc, pDst, pGC, prgnExposed, srcx, src
y, dstx, dsty, plane)
                WindowPtr pSrc;
                DrawablePtr pDst;
                GCPtr pGC;
                RegionPtr prgnExposed;
                int srcx;
                int srcy;
                int dstx;
                int dsty;
                unsigned long plane;


   Copies a region from the backing store of pSrc to pDs.

        RegionPtr pScreen->ClearBackingStore(pWindow, x, y, w, h, genera
teExposures)
                WindowPtr pWindow;
                int x;
                int y;
                int w;
                int h;
                Bool generateExposures;


   Clear the given area of the backing pixmap with the background
   of the window. If generateExposures is TRUE, generate exposure
   events for the area. Note that if the area has any part outside
   the saved portions of the window, we do not allow the count in
   the expose events to be 0, since there will be more expose
   events to come.

        void pScreen->DrawGuarantee(pWindow, pGC, guarantee)
                WindowPtr pWindow;
                GCPtr pGC;
                int guarantee;


   This informs the backing store layer that you are about to
   validate a gc with a window, and that subsequent output to the
   window is (or is not) guaranteed to be already clipped to the
   visible regions of the window.
     __________________________________________________________

Screen Operations for Multi-Layered Framebuffers

   The following screen functions are useful if you have a
   framebuffer with multiple sets of independent bit planes, e.g.
   overlays or underlays in addition to the "main" planes. If you
   have a simple single-layer framebuffer, you should probably use
   the mi versions of these routines in mi/miwindow.c. This can be
   easily accomplished by calling miScreenInit.

    void pScreen->MarkWindow(pWin)
        WindowPtr pWin;


   This formerly dix function MarkWindow has moved to ddx and is
   accessed via this screen function. This function should store
   something, usually a pointer to a device-dependent structure,
   in pWin->valdata so that ValidateTree has the information it
   needs to validate the window.

    Bool pScreen->MarkOverlappedWindows(parent, firstChild, ppLayerWin)
        WindowPtr parent;
        WindowPtr firstChild;
        WindowPtr * ppLayerWin;


   This formerly dix function MarkWindow has moved to ddx and is
   accessed via this screen function. In the process, it has grown
   another parameter: ppLayerWin, which is filled in with a
   pointer to the window at which save under marking and
   ValidateTree should begin. In the single-layered framebuffer
   case, pLayerWin == pWin.

    Bool pScreen->ChangeSaveUnder(pLayerWin, firstChild)
        WindowPtr pLayerWin;
        WindowPtr firstChild;


   The dix functions ChangeSaveUnder and CheckSaveUnder have moved
   to ddx and are accessed via this screen function. pLayerWin
   should be the window returned in the ppLayerWin parameter of
   MarkOverlappedWindows. The function may turn on backing store
   for windows that might be covered, and may partially turn off
   backing store for windows. It returns TRUE if
   PostChangeSaveUnder needs to be called to finish turning off
   backing store.

    void pScreen->PostChangeSaveUnder(pLayerWin, firstChild)
        WindowPtr pLayerWin;
        WindowPtr firstChild;


   The dix function DoChangeSaveUnder has moved to ddx and is
   accessed via this screen function. This function completes the
   job of turning off backing store that was started by
   ChangeSaveUnder.

    void pScreen->MoveWindow(pWin, x, y, pSib, kind)
        WindowPtr pWin;
        int x;
        int y;
        WindowPtr pSib;
        VTKind kind;


   The formerly dix function MoveWindow has moved to ddx and is
   accessed via this screen function. The new position of the
   window is given by x,y. kind is VTMove if the window is only
   moving, or VTOther if the border is also changing.

    void pScreen->ResizeWindow(pWin, x, y, w, h, pSib)
        WindowPtr pWin;
        int x;
        int y;
        unsigned int w;
        unsigned int h;
        WindowPtr pSib;


   The formerly dix function SlideAndSizeWindow has moved to ddx
   and is accessed via this screen function. The new position is
   given by x,y. The new size is given by w,h.

    WindowPtr pScreen->GetLayerWindow(pWin)
        WindowPtr pWin


   This is a new function which returns a child of the layer
   parent of pWin.

    void pScreen->HandleExposures(pWin)
        WindowPtr pWin;


   The formerly dix function HandleExposures has moved to ddx and
   is accessed via this screen function. This function is called
   after ValidateTree and uses the information contained in
   valdata to send exposures to windows.

    void pScreen->ReparentWindow(pWin, pPriorParent)
        WindowPtr pWin;
        WindowPtr pPriorParent;


   This function will be called when a window is reparented. At
   the time of the call, pWin will already be spliced into its new
   position in the window tree, and pPriorParent is its previous
   parent. This function can be NULL.

    void pScreen->SetShape(pWin)
        WindowPtr pWin;


   The formerly dix function SetShape has moved to ddx and is
   accessed via this screen function. The window's new shape will
   have already been stored in the window when this function is
   called.

    void pScreen->ChangeBorderWidth(pWin, width)
        WindowPtr pWin;
        unsigned int width;


   The formerly dix function ChangeBorderWidth has moved to ddx
   and is accessed via this screen function. The new border width
   is given by width.

    void pScreen->MarkUnrealizedWindow(pChild, pWin, fromConfigure)
        WindowPtr pChild;
        WindowPtr pWin;
        Bool fromConfigure;


   This function is called for windows that are being unrealized
   as part of an UnrealizeTree. pChild is the window being
   unrealized, pWin is an ancestor, and the fromConfigure value is
   simply propogated from UnrealizeTree.
     __________________________________________________________

Graphics Contexts and Validation

   This graphics context (GC) contains state variables such as
   foreground and background pixel value (color), the current line
   style and width, the current tile or stipple for pattern
   generation, the current font for text generation, and other
   similar attributes.

   In many graphics systems, the equivalent of the graphics
   context and the drawable are combined as one entity. The main
   distinction between the two kinds of status is that a drawable
   describes a writing surface and the writings that may have
   already been done on it, whereas a graphics context describes
   the drawing process. A drawable is like a chalkboard. A GC is
   like a piece of chalk.

   Unlike many similar systems, there is no "current pen
   location." Every graphic operation is accompanied by the
   coordinates where it is to happen.

   The GC also includes two vectors of procedure pointers, the
   first operate on the GC itself and are called GC funcs. The
   second, called GC ops, contains the functions that carry out
   the fundamental graphic operations such as drawing lines,
   polygons, arcs, text, and copying bitmaps. The DDX graphic
   software can, if it wants to be smart, change these two vectors
   of procedure pointers to take advantage of hardware/firmware in
   the server machine, which can do a better job under certain
   circumstances. To reduce the amount of memory consumed by each
   GC, it is wise to create a few "boilerplate" GC ops vectors
   which can be shared by every GC which matches the constraints
   for that set. Also, it is usually reasonable to have every GC
   created by a particular module to share a common set of GC
   funcs. Samples of this sort of sharing can be seen in
   cfb/cfbgc.c and mfb/mfbgc.c.

   The DDX software is notified any time the client (or DIX) uses
   a changed GC. For instance, if the hardware has special support
   for drawing fixed-width fonts, DDX can intercept changes to the
   current font in a GC just before drawing is done. It can plug
   into either a fixed-width procedure that makes the hardware
   draw characters, or a variable-width procedure that carefully
   lays out glyphs by hand in software, depending upon the new
   font that is selected.

   A definition of these structures can be found in the file
   Xserver/include/gcstruct.h.

   Also included in each GC is an array of devPrivates which
   portions of the DDX can use for any reason. Entries in this
   array are allocated with AllocateGCPrivateIndex() (see Wrappers
   and Privates below).

   The DIX routines available for manipulating GCs are CreateGC,
   ChangeGC, CopyGC, SetClipRects, SetDashes, and FreeGC.


        GCPtr CreateGC(pDrawable, mask, pval, pStatus)
            DrawablePtr pDrawable;
            BITS32 mask;
            XID *pval;
            int *pStatus;

        int ChangeGC(pGC, mask, pval)
            GCPtr pGC;
            BITS32 mask;
            XID *pval;

        int CopyGC(pgcSrc, pgcDst, mask)
            GCPtr pgcSrc;
            GCPtr pgcDst;
            BITS32 mask;

        int SetClipRects(pGC, xOrigin, yOrigin, nrects, prects, ordering
)
            GCPtr pGC;
            int xOrigin, yOrigin;
            int nrects;
            xRectangle *prects;
            int ordering;

        SetDashes(pGC, offset, ndash, pdash)
            GCPtr pGC;
            unsigned offset;
            unsigned ndash;
            unsigned char *pdash;

        int FreeGC(pGC, gid)
            GCPtr pGC;
            GContext gid;


   As a convenience, each Screen structure contains an array of
   GCs that are preallocated, one at each depth the screen
   supports. These are particularly useful in the mi code. Two DIX
   routines must be used to get these GCs:

        GCPtr GetScratchGC(depth, pScreen)
            int depth;
            ScreenPtr pScreen;

        FreeScratchGC(pGC)
            GCPtr pGC;


   Always use these two routines, don't try to extract the scratch
   GC yourself -- someone else might be using it, so a new one
   must be created on the fly.

   If you need a GC for a very long time, say until the server is
   restarted, you should not take one from the pool used by
   GetScratchGC, but should get your own using CreateGC or
   CreateScratchGC. This leaves the ones in the pool free for
   routines that only need it for a little while and don't want to
   pay a heavy cost to get it.

        GCPtr CreateScratchGC(pScreen, depth)
            ScreenPtr pScreen;
            int depth;


   NULL is returned if the GC cannot be created. The GC returned
   can be freed with FreeScratchGC.
     __________________________________________________________

Details of Operation

   At screen initialization, a screen must supply a GC creation
   procedure. At GC creation, the screen must fill in GC funcs and
   GC ops vectors (Xserver/include/gcstruct.h). For any particular
   GC, the func vector must remain constant, while the op vector
   may vary. This invariant is to ensure that Wrappers work
   correctly.

   When a client request is processed that results in a change to
   the GC, the device-independent state of the GC is updated. This
   includes a record of the state that changed. Then the ChangeGC
   GC func is called. This is useful for graphics subsystems that
   are able to process state changes in parallel with the server
   CPU. DDX may opt not to take any action at GC-modify time. This
   is more efficient if multiple GC-modify requests occur between
   draws using a given GC.

   Validation occurs at the first draw operation that specifies
   the GC after that GC was modified. DIX calls then the
   ValidateGC GC func. DDX should then update its internal state.
   DDX internal state may be stored as one or more of the
   following: 1) device private block on the GC; 2) hardware
   state; 3) changes to the GC ops.

   The GC contains a serial number, which is loaded with a number
   fetched from the window that was drawn into the last time the
   GC was used. The serial number in the drawable is changed when
   the drawable's clipList or absCorner changes. Thus, by
   comparing the GC serial number with the drawable serial number,
   DIX can force a validate if the drawable has been changed since
   the last time it was used with this GC.

   In addition, the drawable serial number is always guaranteed to
   have the most significant bit set to 0. Thus, the DDX layer can
   set the most significant bit of the serial number to 1 in a GC
   to force a validate the next time the GC is used. DIX also uses
   this technique to indicate that a change has been made to the
   GC by way of a SetGC, a SetDashes or a SetClip request.
     __________________________________________________________

GC Handling Routines

   The ScreenRec data structure has a pointer for CreateGC().

        Bool pScreen->CreateGC(pGC)
                GCPtr pGC;

   This routine must fill in the fields of a dynamically allocated
   GC that is passed in. It does NOT allocate the GC record itself
   or fill in the defaults; DIX does that.

   This must fill in both the GC funcs and ops; none of the
   drawing functions will be called before the GC has been
   validated, but the others (dealing with allocating of clip
   regions, changing and destroying the GC, etc.) might be.

   The GC funcs vector contains pointers to 7 routines and a
   devPrivate field:

        pGC->funcs->ChangeGC(pGC, changes)
                GCPtr pGC;
                unsigned long changes;


   This GC func is called immediately after a field in the GC is
   changed. changes is a bit mask indicating the changed fields of
   the GC in this request.

   The ChangeGC routine is useful if you have a system where
   state-changes to the GC can be swallowed immediately by your
   graphics system, and a validate is not necessary.

        pGC->funcs->ValidateGC(pGC, changes, pDraw)
                GCPtr pGC;
                unsigned long changes;
                DrawablePtr pDraw;


   ValidateGC is called by DIX just before the GC will be used
   when one of many possible changes to the GC or the graphics
   system has happened. It can modify a devPrivates field of the
   GC or its contents, change the op vector, or change hardware
   according to the values in the GC. It may not change the
   device-independent portion of the GC itself.

   In almost all cases, your ValidateGC() procedure should take
   the regions that drawing needs to be clipped to and combine
   them into a composite clip region, which you keep a pointer to
   in the private part of the GC. In this way, your drawing
   primitive routines (and whatever is below them) can easily
   determine what to clip and where. You should combine the
   regions clientClip (the region that the client desires to clip
   output to) and the region returned by NotClippedByChildren(),
   in DIX. An example is in Xserver/mfb/mfbgc.c.

   Some kinds of extension software may cause this routine to be
   called more than originally intended; you should not rely on
   algorithms that will break under such circumstances.

   See the Strategies document for more information on creatively
   using this routine.

        pGC->funcs->CopyGC(pGCSrc, mask, pGCDst)
                GCPtr pGCSrc;
                unsigned long mask;
                GCPtr pGCDst;


   This routine is called by DIX when a GC is being copied to
   another GC. This is for situations where dynamically allocated
   chunks of memory are hanging off a GC devPrivates field which
   need to be transferred to the destination GC.

        pGC->funcs->DestroyGC(pGC)
                GCPtr pGC;


   This routine is called before the GC is destroyed for the
   entity interested in this GC to clean up after itself. This
   routine is responsible for freeing any auxiliary storage
   allocated.
     __________________________________________________________

GC Clip Region Routines

   The GC clientClip field requires three procedures to manage it.
   These procedures are in the GC funcs vector. The underlying
   principle is that dix knows nothing about the internals of the
   clipping information, (except when it has come from the
   client), and so calls ddX whenever it needs to copy, set, or
   destroy such information. It could have been possible for dix
   not to allow ddX to touch the field in the GC, and require it
   to keep its own copy in devPriv, but since clip masks can be
   very large, this seems like a bad idea. Thus, the server allows
   ddX to do whatever it wants to the clientClip field of the GC,
   but requires it to do all manipulation itself.

        void pGC->funcs->ChangeClip(pGC, type, pValue, nrects)
                GCPtr pGC;
                int type;
                char *pValue;
                int nrects;


   This routine is called whenever the client changes the client
   clip region. The pGC points to the GC involved, the type tells
   what form the region has been sent in. If type is CT_NONE, then
   there is no client clip. If type is CT_UNSORTED, CT_YBANDED or
   CT_YXBANDED, then pValue pointer to a list of rectangles,
   nrects long. If type is CT_REGION, then pValue pointer to a
   RegionRec from the mi region code. If type is CT_PIXMAP pValue
   is a pointer to a pixmap. (The defines for CT_NONE, etc. are in
   Xserver/include/gc.h.) This routine is responsible for
   incrementing any necessary reference counts (e.g. for a pixmap
   clip mask) for the new clipmask and freeing anything that used
   to be in the GC's clipMask field. The lists of rectangles
   passed in can be freed with Xfree(), the regions can be
   destroyed with the RegionDestroy field in the screen, and
   pixmaps can be destroyed by calling the screen's DestroyPixmap
   function. DIX and MI code expect what they pass in to this to
   be freed or otherwise inaccessible, and will never look inside
   what's been put in the GC. This is a good place to be wary of
   storage leaks.

   In the sample server, this routine transforms either the bitmap
   or the rectangle list into a region, so that future routines
   will have a more predictable starting point to work from. (The
   validate routine must take this client clip region and merge it
   with other regions to arrive at a composite clip region before
   any drawing is done.)

        void pGC->funcs->DestroyClip(pGC)
                GCPtr pGC;


   This routine is called whenever the client clip region must be
   destroyed. The pGC points to the GC involved. This call should
   set the clipType field of the GC to CT_NONE. In the sample
   server, the pointer to the client clip region is set to NULL by
   this routine after destroying the region, so that other
   software (including ChangeClip() above) will recognize that
   there is no client clip region.

        void pGC->funcs->CopyClip(pgcDst, pgcSrc)
                GCPtr pgcDst, pgcSrc;


   This routine makes a copy of the clipMask and clipType from
   pgcSrc into pgcDst. It is responsible for destroying any
   previous clipMask in pgcDst. The clip mask in the source can be
   the same as the clip mask in the dst (clients do the strangest
   things), so care must be taken when destroying things. This
   call is required because dix does not know how to copy the clip
   mask from pgcSrc.
     __________________________________________________________

Drawing Primitives

   The X protocol (rules for the byte stream that goes between
   client and server) does all graphics using primitive
   operations, which are called Drawing Primitives. These include
   line drawing, area filling, arcs, and text drawing. Your
   implementation must supply 16 routines to perform these on your
   hardware. (The number 16 is arbitrary.)

   More specifically, 16 procedure pointers are in each GC op
   vector. At any given time, ALL of them MUST point to a valid
   procedure that attempts to do the operation assigned, although
   the procedure pointers may change and may point to different
   procedures to carry out the same operation. A simple server
   will leave them all pointing to the same 16 routines, while a
   more optimized implementation will switch each from one
   procedure to another, depending upon what is most optimal for
   the current GC and drawable.

   The sample server contains a considerable chunk of code called
   the mi (machine independent) routines, which serve as drawing
   primitive routines. Many server implementations will be able to
   use these as-is, because they work for arbitrary depths. They
   make no assumptions about the formats of pixmaps and frame
   buffers, since they call a set of routines known as the
   "Pixblit Routines" (see next section). They do assume that the
   way to draw is through these low-level routines that apply
   pixel values rows at a time. If your hardware or firmware gives
   more performance when things are done differently, you will
   want to take this fact into account and rewrite some or all of
   the drawing primitives to fit your needs.
     __________________________________________________________

GC Components

   This section describes the fields in the GC that affect each
   drawing primitive. The only primitive that is not affected is
   GetImage, which does not use a GC because its destination is a
   protocol-style bit image. Since each drawing primitive mirrors
   exactly the X protocol request of the same name, you should
   refer to the X protocol specification document for more
   details.

   ALL of these routines MUST CLIP to the appropriate regions in
   the drawable. Since there are many regions to clip to
   simultaneously, your ValidateGC routine should combine these
   into a unified clip region to which your drawing routines can
   quickly refer. This is exactly what the cfb and mfb routines
   supplied with the sample server do. The mi implementation
   passes responsibility for clipping while drawing down to the
   Pixblit routines.

   Also, all of them must adhere to the current plane mask. The
   plane mask has one bit for every bit plane in the drawable;
   only planes with 1 bits in the mask are affected by any drawing
   operation.

   All functions except for ImageText calls must obey the alu
   function. This is usually Copy, but could be any of the
   allowable 16 raster-ops.

   All of the functions, except for CopyArea, might use the
   current foreground and background pixel values. Each pixel
   value is 32 bits. These correspond to foreground and background
   colors, but you have to run them through the colormap to find
   out what color the pixel values represent. Do not worry about
   the color, just apply the pixel value.

   The routines that draw lines (PolyLine, PolySegment, PolyRect,
   and PolyArc) use the line width, line style, cap style, and
   join style. Line width is in pixels. The line style specifies
   whether it is solid or dashed, and what kind of dash. The cap
   style specifies whether Rounded, Butt, etc. The join style
   specifies whether joins between joined lines are Miter, Round
   or Beveled. When lines cross as part of the same polyline, they
   are assumed to be drawn once. (See the X protocol specification
   for more details.)

   Zero-width lines are NOT meant to be really zero width; this is
   the client's way of telling you that you can optimize line
   drawing with little regard to the end caps and joins. They are
   called "thin" lines and are meant to be one pixel wide. These
   are frequently done in hardware or in a streamlined assembly
   language routine.

   Lines with widths greater than zero, though, must all be drawn
   with the same algorithm, because client software assumes that
   every jag on every line at an angle will come at the same
   place. Two lines that should have one pixel in the space
   between them (because of their distance apart and their widths)
   should have such a one-pixel line of space between them if
   drawn, regardless of angle.

   The solid area fill routines (FillPolygon, PolyFillRect,
   PolyFillArc) all use the fill rule, which specifies subtle
   interpretations of what points are inside and what are outside
   of a given polygon. The PolyFillArc routine also uses the arc
   mode, which specifies whether to fill pie segments or
   single-edge slices of an ellipse.

   The line drawing, area fill, and PolyText routines must all
   apply the correct "fill style." This can be either a solid
   foreground color, a transparent stipple, an opaque stipple, or
   a tile. Stipples are bitmaps where the 1 bits represent that
   the foreground color is written, and 0 bits represent that
   either the pixel is left alone (transparent) or that the
   background color is written (opaque). A tile is a pixmap of the
   full depth of the GC that is applied in its full glory to all
   areas. The stipple and tile patterns can be any rectangular
   size, although some implementations will be faster for certain
   sizes such as 8x8 or 32x32. The mi implementation passes this
   responsibility down to the Pixblit routines.

   See the X protocol document for full details. The description
   of the CreateGC request has a very good, detailed description
   of these attributes.
     __________________________________________________________

The Primitives

   The Drawing Primitives are as follows:

        RegionPtr pGC->ops->CopyArea(src, dst, pGC, srcx, srcy, w, h, ds
tx, dsty)
                DrawablePtr dst, src;
                GCPtr pGC;
                int srcx, srcy, w, h, dstx, dsty;


   CopyArea copies a rectangle of pixels from one drawable to
   another of the same depth. To effect scrolling, this must be
   able to copy from any drawable to itself, overlapped. No
   squeezing or stretching is done because the source and
   destination are the same size. However, everything is still
   clipped to the clip regions of the destination drawable.

   If pGC->graphicsExposures is True, any portions of the
   destination which were not valid in the source (either occluded
   by covering windows, or outside the bounds of the drawable)
   should be collected together and returned as a region (if this
   resultant region is empty, NULL can be returned instead).
   Furthermore, the invalid bits of the source are not copied to
   the destination and (when the destination is a window) are
   filled with the background tile. The sample routine
   miHandleExposures generates the appropriate return value and
   fills the invalid area using pScreen->PaintWindowBackground.

   For instance, imagine a window that is partially obscured by
   other windows in front of it. As text is scrolled on your
   window, the pixels that are scrolled out from under obscuring
   windows will not be available on the screen to copy to the
   right places, and so an exposure event must be sent for the
   client to correctly repaint them. Of course, if you implement
   some sort of backing store, you could do this without resorting
   to exposure events.

   An example implementation is mfbCopyArea() in
   Xserver/mfb/mfbbitblt.c.

        RegionPtr pGC->ops->CopyPlane(src, dst, pGC, srcx, srcy, w, h, d
stx, dsty, plane)
                DrawablePtr dst, src;
                GCPtr pGC;
                int srcx, srcy, w, h, dstx, dsty;
                unsigned long plane;


   CopyPlane must copy one plane of a rectangle from the source
   drawable onto the destination drawable. Because this routine
   only copies one bit out of each pixel, it can copy between
   drawables of different depths. This is the only way of copying
   between drawables of different depths, except for copying
   bitmaps to pixmaps and applying foreground and background
   colors to it. All other conditions of CopyArea apply to
   CopyPlane too.

   An example implementation is mfbCopyPlane() in
   Xserver/mfb/mfbbitblt.c.

        void pGC->ops->PolyPoint(dst, pGC, mode, n, pPoint)
                DrawablePtr dst;
                GCPtr pGC;
                int mode;
                int n;
                DDXPointPtr pPoint;


   PolyPoint draws a set of one-pixel dots (foreground color) at
   the locations given in the array. mode is one of the defined
   constants Origin (absolute coordinates) or Previous (each
   coordinate is relative to the last). Note that this does not
   use the background color or any tiles or stipples.

   Example implementations are mfbPolyPoint() in
   Xserver/mfb/mfbpolypnt.c and miPolyPoint in
   Xserver/mi/mipolypnt.c.

        void pGC->ops->Polylines(dst, pGC, mode, n, pPoint)
                DrawablePtr dst;
                GCPtr pGC;
                int mode;
                int n;
                DDXPointPtr pPoint;


   Similar to PolyPoint, Polylines draws lines between the
   locations given in the array. Zero-width lines are NOT meant to
   be really zero width; this is the client's way of telling you
   that you can maximally optimize line drawing with little regard
   to the end caps and joins. mode is one of the defined constants
   Previous or Origin, depending upon whether the points are each
   relative to the last or are absolute.

   Example implementations are miWideLine() and miWideDash() in
   mi/miwideline.c and miZeroLine() in mi/mizerline.c.

        void pGC->ops->PolySegment(dst, pGC, n, pPoint)
                DrawablePtr dst;
                GCPtr pGC;
                int n;
                xSegment *pSegments;


   PolySegments draws unconnected lines between pairs of points in
   the array; the array must be of even size; no interconnecting
   lines are drawn.

   An example implementation is miPolySegment() in mipolyseg.c.

        void pGC->ops->PolyRectangle(dst, pGC, n, pRect)
                DrawablePtr dst;
                GCPtr pGC;
                int n;
                xRectangle *pRect;


   PolyRectangle draws outlines of rectangles for each rectangle
   in the array.

   An example implementation is miPolyRectangle() in
   Xserver/mi/mipolyrect.c.

        void pGC->ops->PolyArc(dst, pGC, n, pArc)
                DrawablePtr dst;
                GCPtr pGC;
                int n;
                xArc*pArc;


   PolyArc draws connected conic arcs according to the
   descriptions in the array. See the protocol specification for
   more details.

   Example implementations are miZeroPolyArc in
   Xserver/mi/mizerarc. and miPolyArc() in Xserver/mi/miarc.c.

        void pGC->ops->FillPolygon(dst, pGC, shape, mode, count, pPoint)
                DrawablePtr dst;
                GCPtr pGC;
                int shape;
                int mode;
                int count;
                DDXPointPtr pPoint;


   FillPolygon fills a polygon specified by the points in the
   array with the appropriate fill style. If necessary, an extra
   border line is assumed between the starting and ending lines.
   The shape can be used as a hint to optimize filling; it
   indicates whether it is convex (all interior angles less than
   180), nonconvex (some interior angles greater than 180 but
   border does not cross itself), or complex (border crosses
   itself). You can choose appropriate algorithms or hardware
   based upon mode. mode is one of the defined constants Previous
   or Origin, depending upon whether the points are each relative
   to the last or are absolute.

   An example implementation is miFillPolygon() in
   Xserver/mi/mipoly.c.

        void pGC->ops->PolyFillRect(dst, pGC, n, pRect)
                DrawablePtr dst;
                GCPtr pGC;
                int n;
                xRectangle *pRect;


   PolyFillRect fills multiple rectangles.

   Example implementations are mfbPolyFillRect() in
   Xserver/mfb/mfbfillrct.c and miPolyFillRect() in
   Xserver/mi/mifillrct.c.

        void pGC->ops->PolyFillArc(dst, pGC, n, pArc)
                DrawablePtr dst;
                GCPtr pGC;
                int n;
                xArc *pArc;


   PolyFillArc fills a shape for each arc in the list that is
   bounded by the arc and one or two line segments with the
   current fill style.

   An example implementation is miPolyFillArc() in
   Xserver/mi/mifillarc.c.

        void pGC->ops->PutImage(dst, pGC, depth, x, y, w, h, leftPad, fo
rmat, pBinImage)
                DrawablePtr dst;
                GCPtr pGC;
                int x, y, w, h;
                int format;
                char *pBinImage;


   PutImage copies a pixmap image into the drawable. The pixmap
   image must be in X protocol format (either Bitmap, XYPixmap, or
   ZPixmap), and format tells the format. (See the X protocol
   specification for details on these formats). You must be able
   to accept all three formats, because the client gets to decide
   which format to send. Either the drawable and the pixmap image
   have the same depth, or the source pixmap image must be a
   Bitmap. If a Bitmap, the foreground and background colors will
   be applied to the destination.

   An example implementation is miPutImage() in
   Xserver/mfb/mibitblt.c.

        void pScreen->GetImage(src, x, y, w, h, format, planeMask, pBinI
mage)
                 DrawablePtr src;
                 int x, y, w, h;
                 unsigned int format;
                 unsigned long planeMask;
                 char *pBinImage;


   GetImage copies the bits from the source drawable into the
   destination pointer. The bits are written into the buffer
   according to the server-defined pixmap padding rules. pBinImage
   is guaranteed to be big enough to hold all the bits that must
   be written.

   This routine does not correspond exactly to the X protocol
   GetImage request, since DIX has to break the reply up into
   buffers of a size requested by the transport layer. If format
   is ZPixmap, the bits are written in the ZFormat for the depth
   of the drawable; if there is a 0 bit in the planeMask for a
   particular plane, all pixels must have the bit in that plane
   equal to 0. If format is XYPixmap, planemask is guaranteed to
   have a single bit set; the bits should be written in Bitmap
   format, which is the format for a single plane of an XYPixmap.

   An example implementation is miGetImage() in
   Xserver/mi/mibitblt.c.

        void pGC->ops->ImageText8(pDraw, pGC, x, y, count, chars)
                DrawablePtr pDraw;
                GCPtr pGC;
                int x, y;
                int count;
                char *chars;


   ImageText8 draws text. The text is drawn in the foreground
   color; the background color fills the remainder of the
   character rectangles. The coordinates specify the baseline and
   start of the text.

   An example implementation is miImageText8() in
   Xserver/mi/mipolytext.c.

        int pGC->ops->PolyText8(pDraw, pGC, x, y, count, chars)
                DrawablePtr pDraw;
                GCPtr pGC;
                int x, y;
                int count;
                char *chars;


   PolyText8 works like ImageText8, except it draws with the
   current fill style for special effects such as shaded text. See
   the X protocol specification for more details.

   An example implementation is miPolyText8() in
   Xserver/mi/mipolytext.c.

        int pGC->ops->PolyText16(pDraw, pGC, x, y, count, chars)
                DrawablePtr pDraw;
                GCPtr pGC;
                int x, y;
                int count;
                unsigned short *chars;

        void pGC->ops->ImageText16(pDraw, pGC, x, y, count, chars)
                DrawablePtr pDraw;
                GCPtr pGC;
                int x, y;
                int count;
                unsigned short *chars;


   These two routines are the same as the "8" versions, except
   that they are for 16-bit character codes (useful for oriental
   writing systems).

   The primary difference is in the way the character information
   is looked up. The 8-bit and the 16-bit versions obviously have
   different kinds of character values to look up; the main goal
   of the lookup is to provide a pointer to the CharInfo structs
   for the characters to draw and to pass these pointers to the
   Glyph routines. Given a CharInfo struct, lower-level software
   can draw the glyph desired with little concern for other
   characteristics of the font.

   16-bit character fonts have a row-and-column scheme, where the
   2bytes of the character code constitute the row and column in a
   square matrix of CharInfo structs. Each font has row and column
   minimum and maximum values; the CharInfo structures form a
   two-dimensional matrix.

   Example implementations are miPolyText16() and miImageText16()
   in Xserver/mi/mipolytext.c.

   See the X protocol specification for more details on these
   graphic operations.

   There is a hook in the GC ops, called LineHelper, that used to
   be used in the sample implementation by the code for wide
   lines. It no longer servers any purpose in the sample servers,
   but still exists, #ifdef'ed by NEED_LINEHELPER, in case someone
   needs it.
     __________________________________________________________

Pixblit Procedures

   The Drawing Primitive functions must be defined for your
   server. One possible way to do this is to use the mi routines
   from the sample server. If you choose to use the mi routines
   (even part of them!) you must implement these Pixblit routines.
   These routines read and write pixel values and deal directly
   with the image data.

   The Pixblit routines for the sample server are part of the
   "mfb" routines (for Monochrome Frame Buffer), and "cfb"
   routines (for Color Frame Buffer). As with the mi routines, the
   mfb and cfb routines are portable but are not as portable as
   the mi routines.

   The mfb routines only work for monochrome frame buffers, the
   simplest type of display. Furthermore, they only work for
   screens that organize their bits in rows of pixels on the
   screen. (See the Strategies document for more details on
   porting mfb.) The cfb routines work for packed-pixel displays
   from 2 to 32 bits in depth, although they have a bit of code
   which has been tuned to run on 8-bit (1 pixel per byte)
   displays.

   In other words, if you have a "normal" frame buffer type
   display, you can probably use either the mfb or cfb code, and
   the mi code. If you have a stranger hardware, you will have to
   supply your own Pixblit routines, but you can use the mi
   routines on top of them. If you have better ways of doing some
   of the Drawing Primitive functions, then you may want to supply
   some of your own Drawing Primitive routines. (Even people who
   write their own Drawing Primitives save at least some of the mi
   code for certain special cases that their hardware or library
   or fancy algorithm does not handle.)

   The client, DIX, and the machine-independent routines do not
   carry the final responsibility of clipping. They all depend
   upon the Pixblit routines to do their clipping for them. The
   rule is, if you touch the frame buffer, you clip.

   (The higher level routines may decide to clip at a high level,
   but this is only for increased performance and cannot
   substitute for bottom-level clipping. For instance, the mi
   routines, DIX, or the client may decide to check all character
   strings to be drawn and chop off all characters that would not
   be displayed. If so, it must retain the character on the edge
   that is partly displayed so that the Pixblit routines can clip
   off precisely at the right place.)

   To make this easier, all of the reasons to clip can be combined
   into one region in your ValidateGC procedure. You take this
   composite clip region with you into the Pixblit routines. (The
   sample server does this.)

   Also, FillSpans() has to apply tile and stipple patterns. The
   patterns are all aligned to the window origin so that when two
   people write patches that are contiguous, they will merge
   nicely. (Really, they are aligned to the patOrg point in the
   GC. This defaults to (0, 0) but can be set by the client to
   anything.)

   However, the mi routines can translate (relocate) the points
   from window-relative to screen-relative if desired. If you set
   the miTranslate field in the GC (set it in the CreateGC or
   ValidateGC routine), then the mi output routines will translate
   all coordinates. If it is false, then the coordinates will be
   passed window-relative. Screens with no hardware translation
   will probably set miTranslate to TRUE, so that geometry (e.g.
   polygons, rectangles) can be translated, rather than having the
   resulting list of scanlines translated; this is good because
   the list vertices in a drawing request will generally be much
   smaller than the list of scanlines it produces. Similarly,
   hardware that does translation can set miTranslate to FALSE,
   and avoid the extra addition per vertex, which can be (but is
   not always) important for getting the highest possible
   performance. (Contrast the behavior of GetSpans, which is not
   expected to be called as often, and so has different
   constraints.) The miTranslate field is settable in each GC, if
   , for example, you are mixing several kinds of destinations
   (offscreen pixmaps, main memory pixmaps, backing store, and
   windows), all of which have different requirements, on one
   screen.

   As with other drawing routines, there are fields in the GC to
   direct higher code to the correct routine to execute for each
   function. In this way, you can optimize for special cases, for
   example, drawing solids versus drawing stipples.

   The Pixblit routines are broken up into three sets. The Span
   routines simply fill in rows of pixels. The Glyph routines fill
   in character glyphs. The PushPixels routine is a three-input
   bitblt for more sophisticated image creation.

   It turns out that the Glyph and PushPixels routines actually
   have a machine-independent implementation that depends upon the
   Span routines. If you are really pressed for time, you can use
   these versions, although they are quite slow.
     __________________________________________________________

Span Routines

   For these routines, all graphic operations have been reduced to
   "spans." A span is a horizontal row of pixels. If you can
   design these routines which write into and read from rows of
   pixels at a time, you can use the mi routines.

   Each routine takes a destination drawable to draw into, a GC to
   use while drawing, the number of spans to do, and two pointers
   to arrays that indicate the list of starting points and the
   list of widths of spans.

        void pGC->ops->FillSpans(dst, pGC, nSpans, pPoints, pWidths, sor
ted)
                DrawablePtr dst;
                GCPtr pGC;
                int nSpans;
                DDXPointPtr pPoints;
                int *pWidths;
                int sorted;


   FillSpans should fill horizontal rows of pixels with the
   appropriate patterns, stipples, etc., based on the values in
   the GC. The starting points are in the array at pPoints; the
   widths are in pWidths. If sorted is true, the scan lines are in
   increasing y order, in which case you may be able to make
   assumptions and optimizations.

   GC components: alu, clipOrg, clientClip, and fillStyle.

   GC mode-dependent components: fgPixel (for fillStyle Solid);
   tile, patOrg (for fillStyle Tile); stipple, patOrg, fgPixel
   (for fillStyle Stipple); and stipple, patOrg, fgPixel and
   bgPixel (for fillStyle OpaqueStipple).

        void pGC->ops->SetSpans(pDrawable, pGC, pSrc, ppt, pWidths, nSpa
ns, sorted)
                DrawablePtr pDrawable;
                GCPtr pGC;
                char *pSrc;
                DDXPointPtr pPoints;
                int *pWidths;
                int nSpans;
                int sorted;


   For each span, this routine should copy pWidths bits from pSrc
   to pDrawable at pPoints using the raster-op from the GC. If
   sorted is true, the scan lines are in increasing y order. The
   pixels in pSrc are padded according to the screen's padding
   rules. These can be used to support interesting extension
   libraries, for example, shaded primitives. It does not use the
   tile and stipple.

   GC components: alu, clipOrg, and clientClip

   The above functions are expected to handle all modifiers in the
   current GC. Therefore, it is expedient to have different
   routines to quickly handle common special cases and reload the
   procedure pointers at validate time, as with the other output
   functions.

        void pScreen->GetSpans(pDrawable, wMax, pPoints, pWidths, nSpans
)
                DrawablePtr pDrawable;
                int wMax;
                DDXPointPtr pPoints;
                int *pWidths;
                int nSpans;
                char *pDst;


   For each span, GetSpans gets bits from the drawable starting at
   pPoints and continuing for pWidths bits. Each scanline returned
   will be server-scanline padded. The routine can return NULL if
   memory cannot be allocated to hold the result.

   GetSpans never translates -- for a window, the coordinates are
   already screen-relative. Consider the case of hardware that
   doesn't do translation: the mi code that calls ddX will
   translate each shape (rectangle, polygon,. etc.) before
   scan-converting it, which requires many fewer additions that
   having GetSpans translate each span does. Conversely, consider
   hardware that does translate: it can set its translation point
   to (0, 0) and get each span, and the only penalty is the small
   number of additions required to translate each shape being
   scan-converted by the calling code. Contrast the behavior of
   FillSpans and SetSpans (discussed above under miTranslate),
   which are expected to be used more often.

   Thus, the penalty to hardware that does hardware translation is
   negligible, and code that wants to call GetSpans() is greatly
   simplified, both for extensions and the machine-independent
   core implementation.
     __________________________________________________________

Glyph Routines

   The Glyph routines draw individual character glyphs for text
   drawing requests.

   You have a choice in implementing these routines. You can use
   the mi versions; they depend ultimately upon the span routines.
   Although text drawing will work, it will be very slow.

        void pGC->ops->PolyGlyphBlt(pDrawable, pGC, x, y, nglyph, ppci,
pglyphBase)
                DrawablePtr pDrawable;
                GCPtr pGC;
                int x , y;
                unsigned int nglyph;
                CharInfoRec **ppci;             /* array of character in
fo */
                pointer unused;                 /* unused since R5 */


   GC components: alu, clipOrg, clientClip, font, and fillStyle.

   GC mode-dependent components: fgPixel (for fillStyle Solid);
   tile, patOrg (for fillStyle Tile); stipple, patOrg, fgPixel
   (for fillStyle Stipple); and stipple, patOrg, fgPixel and
   bgPixel (for fillStyle OpaqueStipple).

        void pGC->ops->ImageGlyphBlt(pDrawable, pGC, x, y, nglyph, ppci,
 pglyphBase)
                DrawablePtr pDrawable;
                GCPtr pGC;
                int x , y;
                unsigned int nglyph;
                CharInfoRec **ppci;     /* array of character info */
                pointer unused;         /* unused since R5 */


   GC components: clipOrg, clientClip, font, fgPixel, bgPixel

   These routines must copy the glyphs defined by the bitmaps in
   pglyphBase and the font metrics in ppci to the DrawablePtr,
   pDrawable. The poly routine follows all fill, stipple, and tile
   rules. The image routine simply blasts the glyph onto the
   glyph's rectangle, in foreground and background colors.

   More precisely, the Image routine fills the character rectangle
   with the background color, and then the glyph is applied in the
   foreground color. The glyph can extend outside of the character
   rectangle. ImageGlyph() is used for terminal emulators and
   informal text purposes such as button labels.

   The exact specification for the Poly routine is that the glyph
   is painted with the current fill style. The character rectangle
   is irrelevant for this operation. PolyText, at a higher level,
   includes facilities for font changes within strings and such;
   it is to be used for WYSIWYG word processing and similar
   systems.

   Both of these routines must clip themselves to the overall
   clipping region.

   Example implementations in mi are miPolyGlyphBlt() and
   miImageGlyphBlt() in Xserver/mi/miglblt.c.
     __________________________________________________________

PushPixels routine

   The PushPixels routine writes the current fill style onto the
   drawable in a certain shape defined by a bitmap. PushPixels is
   equivalent to using a second stipple. You can thing of it as
   pushing the fillStyle through a stencil. PushPixels is not used
   by any of the mi rendering code, but is used by the mi software
   cursor code.

     Suppose the stencil is: 00111100 and the stipple is:
     10101010 PushPixels result: 00101000

   You have a choice in implementing this routine. You can use the
   mi version which depends ultimately upon FillSpans(). Although
   it will work, it will be slow.

        void pGC->ops->PushPixels(pGC, pBitMap, pDrawable, dx, dy, xOrg,
 yOrg)
                GCPtr pGC;
                PixmapPtr pBitMap;
                DrawablePtr pDrawable;
                int dx, dy, xOrg, yOrg;


   GC components: alu, clipOrg, clientClip, and fillStyle.

   GC mode-dependent components: fgPixel (for fillStyle Solid);
   tile, patOrg (for fillStyle Tile); stipple, patOrg, fgPixel
   (for fillStyle Stipple); and stipple, patOrg, fgPixel and
   bgPixel (for fillStyle OpaqueStipple).

   PushPixels applys the foreground color, tile, or stipple from
   the pGC through a stencil onto pDrawable. pBitMap points to a
   stencil (of which we use an area dx wide by dy high), which is
   oriented over the drawable at xOrg, yOrg. Where there is a 1
   bit in the bitmap, the destination is set according to the
   current fill style. Where there is a 0 bit in the bitmap, the
   destination is left the way it is.

   This routine must clip to the overall clipping region.

   An Example implementation is miPushPixels() in
   Xserver/mi/mipushpxl.c.
     __________________________________________________________

Shutdown Procedures

        void AbortDDX()
        void ddxGiveUp()

   Some hardware may require special work to be done before the
   server exits so that it is not left in an intermediate state.
   As explained in the OS layer, FatalError() will call AbortDDX()
   just before terminating the server. In addition, ddxGiveUp()
   will be called just before terminating the server on a "clean"
   death. What AbortDDX() and ddxGiveUP do is left unspecified,
   only that stubs must exist in the ddx layer. It is up to local
   implementors as to what they should accomplish before
   termination.
     __________________________________________________________

Command Line Procedures

        int ddxProcessArgument(argc, argv, i)
            int argc;
            char *argv[];
            int i;

        void
        ddxUseMsg()


   You should write these routines to deal with device-dependent
   command line arguments. The routine ddxProcessArgument() is
   called with the command line, and the current index into argv;
   you should return zero if the argument is not a
   device-dependent one, and otherwise return a count of the
   number of elements of argv that are part of this one argument.
   For a typical option (e.g., "-realtime"), you should return the
   value one. This routine gets called before checks are made
   against device-independent arguments, so it is possible to peek
   at all arguments or to override device-independent argument
   processing. You can document the device-dependent arguments in
   ddxUseMsg(), which will be called from UseMsg() after printing
   out the device-independent arguments.
     __________________________________________________________

Wrappers and devPrivates

   Two new extensibility concepts have been developed for release
   4, Wrappers and devPrivates. These replace the R3 GCInterest
   queues, which were not a general enough mechanism for many
   extensions and only provided hooks into a single data
   structure.
     __________________________________________________________

devPrivates

   devPrivates are arrays of values attached to various data
   structures (Screens, GCs, Windows, and Pixmaps currently).
   These arrays are sized dynamically at server startup (and
   reset) time as various modules allocate portions of them. They
   can be used for any purpose; each array entry is actually a
   union, DevUnion, of common useful types (pointer, long and
   unsigned long). devPrivates must be allocated on startup and
   whenever the server resets. To make this easier, the global
   variable "serverGeneration" is incremented each time
   devPrivates should be allocated, but before the initialization
   process begins, typical usage would be:

static int privateGeneration = 0;

        if (privateGeneration != serverGeneration)
        {
                allocate devPrivates here.

                privateGeneration = serverGeneration;
        }
     __________________________________________________________

Screen devPrivates

   An index into every screen devPrivates array is allocated with

        int AllocateScreenPrivateIndex()

   This call can occur at any time, each existing devPrivates
   array is resized to accommodate the new entry. This routine
   returns -1 indicating an allocation failure. Otherwise, the
   return value can be used to index the array of devPrivates on
   any screen:

        private = (PrivatePointer) pScreen->devPrivates[screenPrivateInd
ex].ptr;

   The pointer in each screen is not initialized by
   AllocateScreenPrivateIndex().
     __________________________________________________________

Window devPrivates

   An index into every window devPrivates array is allocated with

        int AllocateWindowPrivateIndex ()

   AllocateWindowPrivateIndex() never returns an error. This call
   must be associated with a call which causes a chunk of memory
   to be automatically allocated and attached to the devPrivate
   entry on every screen which the module will need to use the
   index:

        Bool AllocateWindowPrivate (pScreen, index, amount)
                ScreenPtr pScreen;
                int index;
                unsigned amount;

   If this space is not always needed for every object, use 0 as
   the amount. In this case, the pointer field of the entry in the
   devPrivates array is initialized to NULL. This call exists so
   that DIX may preallocate all of the space required for an
   object with one call; this reduces memory fragmentation
   considerably. AllocateWindowPrivate returns FALSE on allocation
   failure. Both of these calls must occur before any window
   structures are allocated; the server is careful to avoid window
   creation until all modules are initialized, but do not call
   this after initialization. A typical allocation sequence for
   WindowPrivates would be:

    privateInitialize (pScreen)
        ScreenPtr pScreen;
    {
        if (privateGeneration != serverGeneration)
        {
            windowPrivateIndex = AllocateWindowPrivateIndex();
            privateGeneration = serverGeneration;
        }

        return (AllocateWindowPrivate(pScreen, windowPrivateIndex,
                                      sizeof(windowPrivateStructure)));
    }
     __________________________________________________________

GC and Pixmap devPrivates

   The calls for GCs and Pixmaps mirror the Window calls exactly;
   they have the same requirements and limitations:

        int AllocateGCPrivateIndex ()

        Bool AllocateGCPrivate (pScreen, index, amount)
                ScreenPtr pScreen;
                int index;
                unsigned amount;

        int AllocatePixmapPrivateIndex ()

        Bool AllocatePixmapPrivate (pScreen, index, amount)
                ScreenPtr pScreen;
                int index;
                unsigned amount;
     __________________________________________________________

Wrappers

   Wrappers are not a body of code, nor an interface spec. They
   are, instead, a technique for hooking a new module into an
   existing calling sequence. There are limitations on other
   portions of the server implementation which make using wrappers
   possible; limits on when specific fields of data structures may
   be modified. They are intended as a replacement for GCInterest
   queues, which were not general enough to support existing
   modules; in particular software cursors and backing store both
   needed more control over the activity. The general mechanism
   for using wrappers is:

privateWrapperFunction (object, ...)
        ObjectPtr       object;
{
        pre-wrapped-function-stuff ...

        object->functionVector = (void *) object->devPrivates[privateInd
ex].ptr;
        (*object->functionVector) (object, ...);
        /*
         * this next line is occasionally required by the rules governin
g
         * wrapper functions.  Always using it will not cause problems.
         * Not using it when necessary can cause severe troubles.
         */
        object->devPrivates[privateIndex].ptr = (pointer) object->functi
onVector;
        object->functionVector = privateWrapperFunction;

        post-wrapped-function-stuff ...
}

privateInitialize (object)
        ObjectPtr       object;
{
        object->devPrivates[privateIndex].ptr = (pointer) object->functi
onVector;
        object->functionVector = privateWrapperFunction;
}

   Thus the privateWrapperFunction provides hooks for performing
   work both before and after the wrapped function has been
   called; the process of resetting the functionVector is called
   "unwrapping" while the process of fetching the wrapped function
   and replacing it with the wrapping function is called
   "wrapping". It should be clear that GCInterest queues could be
   emulated using wrappers. In general, any function vectors
   contained in objects can be wrapped, but only vectors in GCs
   and Screens have been tested.

   Wrapping screen functions is quite easy; each vector is
   individually wrapped. Screen functions are not supposed to
   change after initialization, so rewrapping is technically not
   necessary, but causes no problems.

   Wrapping GC functions is a bit more complicated. GC's have two
   tables of function vectors, one hanging from gc->ops and the
   other from gc->funcs, which should be initially wrapped from a
   CreateGC wrapper. Wrappers should modify only table pointers,
   not the contents of the tables, as they may be shared by more
   than one GC (and, in the case of funcs, are probably shared by
   all gcs). Your func wrappers may change the GC funcs or ops
   pointers, and op wrappers may change the GC op pointers but not
   the funcs.

   Thus, the rule for GC wrappings is: wrap the funcs from
   CreateGC and, in each func wrapper, unwrap the ops and funcs,
   call down, and re-wrap. In each op wrapper, unwrap the ops,
   call down, and rewrap afterwards. Note that in re-wrapping you
   must save out the pointer you're replacing again. This way the
   chain will be maintained when wrappers adjust the funcs/ops
   tables they use.
     __________________________________________________________

Work Queue

   To queue work for execution when all clients are in a stable
   state (i.e. just before calling select() in WaitForSomething),
   call:

        Bool QueueWorkProc(function,client,closure)
                Bool            (*function)();
                ClientPtr       client;
                pointer         closure;

   When the server is about to suspend itself, the given function
   will be executed:

        (*function) (client, closure)

   Neither client nor closure are actually used inside the work
   queue routines.
     __________________________________________________________

Summary of Routines

   This is a summary of the routines discussed in this document.
   The procedure names are in alphabetical order. The Struct is
   the structure it is attached to; if blank, this procedure is
   not attached to a struct and must be named as shown. The sample
   server provides implementations in the following categories.
   Notice that many of the graphics routines have both mi and mfb
   implementations.

     * dix portable to all systems; do not attempt to rewrite
       (Xserver/dix)
     * os routine provided in Xserver/os or Xserver/include/os.h
     * ddx frame buffer dependent (examples in
       Xserver/mfb,Xserver/cfb)
     * mi routine provided in Xserver/mi
     * hd hardware dependent (examples in many Xserver/hw
       directories)
     * none not implemented in sample implementation

   Table 1. Server Routines (Page 1)
            Procedure           Port Struct
   ALLOCATE_LOCAL               os

   AbortDDX                     hd

   AddCallback                  dix

   AddEnabledDevice             os

   AddInputDevice               dix

   AddScreen                    dix

   AdjustWaitForDelay           os

   Bell                         hd

   Device
   ChangeClip                   mi

   GC func
   ChangeGC

   GC func
   ChangeWindowAttributes       ddx

   Screen
   ClearToBackground            ddx

   Window
   ClientAuthorized             os

   ClientSignal                 dix

   ClientSleep                  dix

   ClientWakeup                 dix

   ClipNotify                   ddx

   Screen
   CloseScreen                  hd

   ConstrainCursor              hd

   Screen
   CopyArea                     mi

   GC op
   CopyGCDest                   ddx

   GC func
   CopyGCSource                 none

   GC func
   CopyPlane                    mi

   GC op
   CopyWindow                   ddx

   Window
   CreateGC                     ddx

   Screen
   CreateCallbackList           dix

   CreatePixmap                 ddx

   Screen
   CreateScreenResources        ddx

   Screen
   CreateWellKnowSockets        os

   CreateWindow                 ddx

   Screen
   CursorLimits                 hd

   Screen
   DEALLOCATE_LOCAL             os

   DeleteCallback               dix

   DeleteCallbackList           dix

   DestroyClip                  ddx

   GC func
   DestroyGC                    ddx

   GC func
   DestroyPixmap                ddx

   Screen
   DestroyWindow                ddx

   Screen
   DisplayCursor                hd

   Screen
   Error                        os

   ErrorF                       os

   FatalError                   os

   FillPolygon                  mi

   GC op
   FillSpans                    ddx

   GC op
   FlushAllOutput               os

   FlushIfCriticalOutputPending os

   FreeScratchPixmapHeader      dix

   GetImage                     mi

   Screen
   GetMotionEvents              hd

   Device
   GetScratchPixmapHeader       dix

   GetSpans                     ddx

   Screen
   GetStaticColormap            ddx

   Screen

   Table 2. Server Routines (Page 2)
          Procedure         Port Struct
   ImageGlyphBlt            mi

   GC op
   ImageText16              mi

   GC op
   ImageText8               mi

   GC op
   InitInput                hd

   InitKeyboardDeviceStruct dix

   InitOutput               hd

   InitPointerDeviceStruct  dix

   InsertFakeRequest        os

   InstallColormap          ddx

   Screen
   Intersect                mi

   Screen
   Inverse                  mi

   Screen
   LegalModifier            hd

   LineHelper               mi

   GC op
   ListInstalledColormaps   ddx

   Screen
   LookupKeyboardDevice     dix

   LookupPointerDevice      dix

   ModifyPixmapheader       mi

   Screen
   NextAvailableClient      dix

   OsInit                   os

   PaintWindowBackground    mi

   Window
   PaintWindowBorder        mi

   Window
   PointerNonInterestBox    hd

   Screen
   PointInRegion            mi

   Screen
   PolyArc                  mi

   GC op
   PolyFillArc              mi

   GC op
   PolyFillRect             mi

   GC op
   PolyGlyphBlt             mi

   GC op
   Polylines                mi

   GC op
   PolyPoint                mi

   GC op
   PolyRectangle            mi

   GC op
   PolySegment              mi

   GC op
   PolyText16               mi

   GC op
   PolyText8                mi

   GC op
   PositionWindow           ddx

   Screen
   ProcessInputEvents       hd

   PushPixels               mi

   GC op
   PutImage                 mi

   GC op
   QueryBestSize            hd

   Screen
   ReadRequestFromClient    os

   RealizeCursor            hd

   Screen
   RealizeFont              ddx

   Screen
   RealizeWindow            ddx

   Screen
   RecolorCursor            hd

   Screen
   RectIn                   mi

   Screen
   RegionCopy               mi

   Screen
   RegionCreate             mi

   Screen
   RegionDestroy            mi

   Screen
   RegionEmpty              mi

   Screen
   RegionExtents            mi

   Screen
   RegionNotEmpty           mi

   Screen
   RegionReset              mi

   Screen
   ResolveColor             ddx

   Screen

   Table 3. Server Routines (Page 3)
          Procedure         Port Struct
   RegisterKeyboardDevice   dix

   RegisterPointerDevice    dix

   RemoveEnabledDevice      os

   ResetCurrentRequest      os

   RestoreAreas             none

   BackingStore
   SaveDoomedAreas          none

   BackingStore
   SaveScreen               ddx

   Screen
   SetCriticalOutputPending os

   SetCursorPosition        hd

   Screen
   SetInputCheck            dix

   SetSpans                 ddx

   GC op
   StoreColors              ddx

   Screen
   Subtract                 mi

   Screen
   TimerCancel              os

   TimerCheck               os

   TimerForce               os

   TimerFree                os

   TimerInit                os

   TimerSet                 os

   TimeSinceLastInputEvent  hd

   TranslateBackingStore    none

   BackingStore
   TranslateRegion          mi

   Screen
   UninstallColormap        ddx

   Screen
   Union                    mi

   Screen
   UnrealizeCursor          hd

   Screen
   UnrealizeFont            ddx

   Screen
   UnrealizeWindow          ddx

   Screen
   ValidateGC               ddx

   GC func
   ValidateTree             mi

   Screen
   WaitForSomething         os

   WindowExposures          mi

   Window
   WriteToClient            os

   Xalloc                   os

   Xfree                    os

   Xrealloc                 os
