# Extensions

Because X can evolve by extensions to the core protocol, it is important that extensions not be perceived as second-class citizens. At some point, your favorite extensions may be adopted as additional parts of the X Standard.

Therefore, there should be little to distinguish the use of an extension from that of the core protocol. To avoid having to initialize extensions explicitly in application programs, it is also important that extensions perform lazy evaluations, automatically initializing themselves when called for the first time.

This appendix describes techniques for writing extensions to Xlib that will run at essentially the same performance as the core protocol requests.

It is expected that a given extension to X consists of multiple requests. Defining 10 new features as 10 separate extensions is a bad practice. Rather, they should be packaged into a single extension and should use minor opcodes to distinguish the requests.

The symbols and macros used for writing stubs to Xlib are listed in `<X11/Xlibint.h>`.

## Basic Protocol Support Routines

The basic protocol requests for extensions are and .

      Bool XQueryExtension
      Display *display
      char *name
      int *major_opcode_return
      int *first_event_return
      int *first_error_return

display

  Specifies the connection to the X server.

name

  Specifies the extension name.

major_opcode_return

  Returns the major opcode.

first_event_return

  Returns the first event code, if any.

first_error_return

  Returns the first error code, if any.

The function determines if the named extension is present. If the extension is not present, returns False; otherwise, it returns True. If the extension is present, returns the major opcode for the extension to major_opcode_return; otherwise, it returns zero. Any minor opcode and the request formats are specific to the extension. If the extension involves additional event types, returns the base event type code to first_event_return; otherwise, it returns zero. The format of the events is specific to the extension. If the extension involves additional error codes, returns the base error code to first_error_return; otherwise, it returns zero. The format of additional data in the errors is specific to the extension.

If the extension name is not in the Host Portable Character Encoding the result is implementation-dependent. Uppercase and lowercase matter; the strings ``thing'', ``Thing'', and ``thinG'' are all considered different names.

      char **XListExtensions
      Display *display
      int *nextensions_return

display

  Specifies the connection to the X server.

nextensions_return

  Returns the number of extensions listed.

The function returns a list of all extensions supported by the server. If the data returned by the server is in the Latin Portable Character Encoding, then the returned strings are in the Host Portable Character Encoding. Otherwise, the result is implementation-dependent.

      XFreeExtensionList
      char **list

list

  Specifies the list of extension names.

The function frees the memory allocated by .

## Hooking into Xlib

These functions allow you to hook into the library. They are not normally used by application programmers but are used by people who need to extend the core X protocol and the X library interface. The functions, which generate protocol requests for X, are typically called stubs.

In extensions, stubs first should check to see if they have initialized themselves on a connection. If they have not, they then should call to attempt to initialize themselves on the connection.

If the extension needs to be informed of GC/font allocation or deallocation or if the extension defines new event types, the functions described here allow the extension to be called when these events occur.

The XExtCodes structure returns the information from and is defined in `<X11/Xlib.h>`:

    typedef struct _XExtCodes {	/* public to extension, cannot be changed */
    	int extension;		/* extension number */
    	int major_opcode;	/* major op-code assigned by server */
    	int first_event;	/* first event number for the extension */
    	int first_error;	/* first error number for the extension */
    } XExtCodes;

      XExtCodes *XInitExtension
      Display *display
      char *name

display

  Specifies the connection to the X server.

name

  Specifies the extension name.

The function determines if the named extension exists. Then, it allocates storage for maintaining the information about the extension on the connection, chains this onto the extension list for the connection, and returns the information the stub implementor will need to access the extension. If the extension does not exist, returns NULL.

If the extension name is not in the Host Portable Character Encoding, the result is implementation-dependent. Uppercase and lowercase matter; the strings ``thing'', ``Thing'', and ``thinG'' are all considered different names.

The extension number in the XExtCodes structure is needed in the other calls that follow. This extension number is unique only to a single connection.

      XExtCodes *XAddExtension
      Display *display

display

  Specifies the connection to the X server.

For local Xlib extensions, the function allocates the XExtCodes structure, bumps the extension number count, and chains the extension onto the extension list. (This permits extensions to Xlib without requiring server extensions.)

### Hooks into the Library

These functions allow you to define procedures that are to be called when various circumstances occur. The procedures include the creation of a new GC for a connection, the copying of a GC, the freeing of a GC, the creating and freeing of fonts, the conversion of events defined by extensions to and from wire format, and the handling of errors.

All of these functions return the previous procedure defined for this extension.

      int XESetCloseDisplay
      Display *display
      int extension
      int (*proc)()

display

  Specifies the connection to the X server.

extension

  Specifies the extension number.

proc

  Specifies the procedure to call when the display is closed.

The function defines a procedure to be called whenever `XCloseDisplay` is called. It returns any previously defined procedure, usually NULL.

When `XCloseDisplay` is called, your procedure is called with these arguments:

      int (*proc)
      Display *display
      XExtCodes *codes

      int *XESetCreateGC
      Display *display
      int extension
      int (*proc)()

display

  Specifies the connection to the X server.

extension

  Specifies the extension number.

proc

  Specifies the procedure to call when a GC is closed.

The function defines a procedure to be called whenever a new GC is created. It returns any previously defined procedure, usually NULL.

When a GC is created, your procedure is called with these arguments:

      int (*proc)
      Display *display
      GC gc
      XExtCodes *codes

      int *XESetCopyGC
      Display *display
      int extension
      int (*proc)()

display

  Specifies the connection to the X server.

extension

  Specifies the extension number.

proc

  Specifies the procedure to call when GC components are copied.

The function defines a procedure to be called whenever a GC is copied. It returns any previously defined procedure, usually NULL.

When a GC is copied, your procedure is called with these arguments:

      int (*proc)
      Display *display
      GC gc
      XExtCodes *codes

      int *XESetFreeGC
      Display *display
      int extension
      int (*proc)()

display

  Specifies the connection to the X server.

extension

  Specifies the extension number.

proc

  Specifies the procedure to call when a GC is freed.

The function defines a procedure to be called whenever a GC is freed. It returns any previously defined procedure, usually NULL.

When a GC is freed, your procedure is called with these arguments:

      int (*proc)
      Display *display
      GC gc
      XExtCodes *codes

      int *XESetCreateFont
      Display *display
      int extension
      int (*proc)()

display

  Specifies the connection to the X server.

extension

  Specifies the extension number.

proc

  Specifies the procedure to call when a font is created.

The function defines a procedure to be called whenever and are called. It returns any previously defined procedure, usually NULL.

When or is called, your procedure is called with these arguments:

      int (*proc)
      Display *display
      XFontStruct *fs
      XExtCodes *codes

      int *XESetFreeFont
      Display *display
      int extension
      int (*proc)()

display

  Specifies the connection to the X server.

extension

  Specifies the extension number.

proc

  Specifies the procedure to call when a font is freed.

The function defines a procedure to be called whenever is called. It returns any previously defined procedure, usually NULL.

When is called, your procedure is called with these arguments:

      int (*proc)
      Display *display
      XFontStruct *fs
      XExtCodes *codes

The and functions allow you to define new events to the library. An XEvent structure always has a type code (type int) as the first component. This uniquely identifies what kind of event it is. The second component is always the serial number (type unsigned long) of the last request processed by the server. The third component is always a Boolean (type Bool) indicating whether the event came from a SendEvent protocol request. The fourth component is always a pointer to the display the event was read from. The fifth component is always a resource ID of one kind or another, usually a window, carefully selected to be useful to toolkit dispatchers. The fifth component should always exist, even if the event does not have a natural destination; if there is no value from the protocol to put in this component, initialize it to zero. There is an implementation limit such that your host event structure size cannot be bigger than the size of the XEvent union of structures. There also is no way to guarantee that more than 24 elements or 96 characters in the structure will be fully portable between machines.

      int *XESetWireToEvent
      Display *display
      int event_number
      Status (*proc)()

display

  Specifies the connection to the X server.

event_number

  Specifies the event code.

proc

  Specifies the procedure to call when converting an event.

The function defines a procedure to be called when an event needs to be converted from wire format (xEvent) to host format (XEvent). The event number defines which protocol event number to install a conversion procedure for. returns any previously defined procedure. You can replace a core event conversion function with one of your own, although this is not encouraged. It would, however, allow you to intercept a core event and modify it before being placed in the queue or otherwise examined. When Xlib needs to convert an event from wire format to host format, your procedure is called with these arguments:

      int (*proc)
      Display *display
      XEvent *re
      xEvent *event

Your procedure must return status to indicate if the conversion succeeded. The re argument is a pointer to where the host format event should be stored, and the event argument is the 32-byte wire event structure. In the XEvent structure you are creating, you must fill in the five required members of the event structure. You should fill in the type member with the type specified for the xEvent structure. You should copy all other members from the xEvent structure (wire format) to the XEvent structure (host format). Your conversion procedure should return True if the event should be placed in the queue or False if it should not be placed in the queue.

To initialize the serial number component of the event, call with the event and use the return value.

      unsigned long_XSetLastRequestRead
      Display *display
      xGenericReply *rep

display

  Specifies the connection to the X server.

rep

  Specifies the wire event structure.

The function computes and returns a complete serial number from the partial serial number in the event.

      Status *XESetEventToWire
      Display *display
      int event_number
      int (*proc)()

display

  Specifies the connection to the X server.

event_number

  Specifies the event code.

proc

  Specifies the procedure to call when converting an event.

The function defines a procedure to be called when an event needs to be converted from host format (XEvent) to wire format (xEvent) form. The event number defines which protocol event number to install a conversion procedure for. returns any previously defined procedure. It returns zero if the conversion fails or nonzero otherwise. You can replace a core event conversion function with one of your own, although this is not encouraged. It would, however, allow you to intercept a core event and modify it before being sent to another client. When Xlib needs to convert an event from host format to wire format, your procedure is called with these arguments:

      int (*proc)
      Display *display
      XEvent *re
      xEvent *event

The re argument is a pointer to the host format event, and the event argument is a pointer to where the 32-byte wire event structure should be stored. You should fill in the type with the type from the XEvent structure. All other members then should be copied from the host format to the xEvent structure.

      Bool *XESetWireToError
      Display *display
      int error_number
      Bool (*proc)()

display

  Specifies the connection to the X server.

error_number

  Specifies the error code.

proc

  Specifies the procedure to call when an error is received.

The function defines a procedure to be called when an extension error needs to be converted from wire format to host format. The error number defines which protocol error code to install the conversion procedure for. returns any previously defined procedure.

Use this function for extension errors that contain additional error values beyond those in a core X error, when multiple wire errors must be combined into a single Xlib error, or when it is necessary to intercept an X error before it is otherwise examined.

When Xlib needs to convert an error from wire format to host format, the procedure is called with these arguments:

      int (*proc)
      Display *display
      XErrorEvent *he
      xError *we

The he argument is a pointer to where the host format error should be stored. The structure pointed at by he is guaranteed to be as large as an XEvent structure and so can be cast to a type larger than an XErrorEvent to store additional values. If the error is to be completely ignored by Xlib (for example, several protocol error structures will be combined into one Xlib error), then the function should return False; otherwise, it should return True.

      int *XESetError
      Display *display
      int extension
      int (*proc)()

display

  Specifies the connection to the X server.

extension

  Specifies the extension number.

proc

  Specifies the procedure to call when an error is received.

Inside Xlib, there are times that you may want to suppress the calling of the external error handling when an error occurs. This allows status to be returned on a call at the cost of the call being synchronous (though most such functions are query operations, in any case, and are typically programmed to be synchronous).

When Xlib detects a protocol error in , it calls your procedure with these arguments:

      int (*proc)
      Display *display
      xError *err
      XExtCodes *codes
      int *ret_code

The err argument is a pointer to the 32-byte wire format error. The codes argument is a pointer to the extension codes structure. The ret_code argument is the return code you may want returned to.

If your procedure returns a zero value, the error is not suppressed, and the client's error handler is called. (For further information, see section 11.8.2.) If your procedure returns nonzero, the error is suppressed, and returns the value of ret_code.

      char *XESetErrorString
      Display *display
      int extension
      char *(*proc)()

display

  Specifies the connection to the X server.

extension

  Specifies the extension number.

proc

  Specifies the procedure to call to obtain an error string.

The function returns a string to the user for an error. allows you to define a procedure to be called that should return a pointer to the error message. The following is an example.

      int (*proc)
      Display *display
      int code
      XExtCodes *codes
      char *buffer
      int nbytes

Your procedure is called with the error code for every error detected. You should copy nbytes of a null-terminated string containing the error message into buffer.

      void *XESetPrintErrorValues
      Display *display
      int extension
      void (*proc)()

display

  Specifies the connection to the X server.

extension

  Specifies the extension number.

proc

  Specifies the procedure to call when an error is printed.

The function defines a procedure to be called when an extension error is printed, to print the error values. Use this function for extension errors that contain additional error values beyond those in a core X error. It returns any previously defined procedure.

When Xlib needs to print an error, the procedure is called with these arguments:

      void (*proc)
      Display *display
      XErrorEvent *ev
      void *fp

The structure pointed at by ev is guaranteed to be as large as an XEvent structure and so can be cast to a type larger than an XErrorEvent to obtain additional values set by using . The underlying type of the fp argument is system dependent; on a POSIX-compliant system, fp should be cast to type FILE*.

      int *XESetFlushGC
      Display *display
      int extension
      int *(*proc)()

display

  Specifies the connection to the X server.

extension

  Specifies the extension number.

proc

  Specifies the procedure to call when a GC is flushed.

The procedure set by the function has the same interface as the procedure set by the function, but is called when a GC cache needs to be updated in the server.

      int *XESetCopyGC
      Display *display
      int extension
      int *(*proc)()

display

  Specifies the connection to the X server.

extension

  Specifies the extension number.

proc

  Specifies the procedure to call when a buffer is flushed.

The `XESetBeforeFlush` function defines a procedure to be called when data is about to be sent to the server. When data is about to be sent, your procedure is called one or more times with these arguments:

      void (*proc)
      Display *display
      XExtCodes *codes
      char *data
      long len

The data argument specifies a portion of the outgoing data buffer, and its length in bytes is specified by the len argument. Your procedure must not alter the contents of the data and must not do additional protocol requests to the same display.

### Hooks onto Xlib Data Structures

Various Xlib data structures have provisions for extension procedures to chain extension supplied data onto a list. These structures are GC, Visual, Screen, ScreenFormat, Display, and XFontStruct. Because the list pointer is always the first member in the structure, a single set of procedures can be used to manipulate the data on these lists.

The following structure is used in the functions in this section and is defined in `<X11/Xlib.h>`

    typedef struct _XExtData {
    	int number;	/* number returned by XInitExtension */
    	struct _XExtData *next;	/* next item on list of data for structure */
    	int (*free_private)();	/* if defined,  called to free private */
    	XPointer private_data;	/* data private to this extension. */
    } XExtData;

When any of the data structures listed above are freed, the list is walked, and the structure's free procedure (if any) is called. If free is NULL, then the library frees both the data pointed to by the private_data member and the structure itself.

    union {	Display *display;
    	GC gc;
    	Visual *visual;
    	Screen *screen;
    	ScreenFormat *pixmap_format;
    	XFontStruct *font } XEDataObject;

      XExtData **XEHeadOfExtensionList
      XEDataObject object

object

  Specifies the object.

The function returns a pointer to the list of extension structures attached to the specified object. In concert with , allows an extension to attach arbitrary data to any of the structures of types contained in XEDataObject.

      XAddToExtensionList
      XExtData **structure
      XExtData *ext_data

structure

  Specifies the extension list.

ext_data

  Specifies the extension data structure to add.

The structure argument is a pointer to one of the data structures enumerated above. You must initialize ext_data->number with the extension number before calling this function.

      XExtData *XFindOnExtensionList
      struct_XExtData **structure
      int number

structure

  Specifies the extension list.

number

  Specifies the extension number from .

The function returns the first extension data structure for the extension numbered number. It is expected that an extension will add at most one extension data structure to any single data structure's extension data list. There is no way to find additional structures.

The macro, which allocates and returns a resource ID, is defined in `<X11/Xlib.h>`.

      XAllocID
      Display *display

display

  Specifies the connection to the X server.

This macro is a call through the Display structure to an internal resource ID allocator. It returns a resource ID that you can use when creating new resources.

The macro allocates and returns an array of resource ID.

      XAllocIDs
      Display *display
      XID *ids_return
      int count

display

  Specifies the connection to the X server.

ids_return

  Returns the resource IDs.

rep

  Specifies the number of resource IDs requested.

This macro is a call through the Display structure to an internal resource ID allocator. It returns resource IDs to the array supplied by the caller. To correctly handle automatic reuse of resource IDs, you must call when requesting multiple resource IDs. This call might generate protocol requests.

## GC Caching

GCs are cached by the library to allow merging of independent change requests to the same GC into single protocol requests. This is typically called a write-back cache. Any extension procedure whose behavior depends on the contents of a GC must flush the GC cache to make sure the server has up-to-date contents in its GC.

The macro checks the dirty bits in the library's GC structure and calls if any elements have changed. The macro is defined as follows:

      FlushGC
      Display *display
      GC gc

display

  Specifies the connection to the X server.

gc

  Specifies the GC.

Note that if you extend the GC to add additional resource ID components, you should ensure that the library stub sends the change request immediately. This is because a client can free a resource immediately after using it, so if you only stored the value in the cache without forcing a protocol request, the resource might be destroyed before being set into the GC. You can use the procedure to force the cache to be flushed. The procedure is defined as follows:

      _XFlushGCCache
      Display *display
      GC gc

display

  Specifies the connection to the X server.

gc

  Specifies the GC.

## Graphics Batching

If you extend X to add more poly graphics primitives, you may be able to take advantage of facilities in the library to allow back-to-back single calls to be transformed into poly requests. This may dramatically improve performance of programs that are not written using poly requests. A pointer to an xReq, called last_req in the display structure, is the last request being processed. By checking that the last request type, drawable, gc, and other options are the same as the new one and that there is enough space left in the buffer, you may be able to just extend the previous graphics request by extending the length field of the request and appending the data to the buffer. This can improve performance by five times or more in naive programs. For example, here is the source for the stub. (Writing extension stubs is discussed in the next section.)

    #include <X11/Xlibint.h>

    /* precompute the maximum size of batching request allowed */

    static int size = sizeof(xPolyPointReq) + EPERBATCH * sizeof(xPoint);

    XDrawPoint(dpy, d, gc, x, y)
        register Display *dpy;
        Drawable d;
        GC gc;
        int x, y; /* INT16 */
    {
        xPoint *point;
        LockDisplay(dpy);
        FlushGC(dpy, gc);
        {
        register xPolyPointReq *req = (xPolyPointReq *) dpy->last_req;
        /* if same as previous request, with same drawable, batch requests */
        if (
              (req->reqType == X_PolyPoint)
           && (req->drawable == d)
           && (req->gc == gc->gid)
           && (req->coordMode == CoordModeOrigin)
           && ((dpy->bufptr + sizeof (xPoint)) <= dpy->bufmax)
           && (((char *)dpy->bufptr - (char *)req) < size) ) {
             point = (xPoint *) dpy->bufptr;
             req->length += sizeof (xPoint) >> 2;
             dpy->bufptr += sizeof (xPoint);
             }

        else {
            GetReqExtra(PolyPoint, 4, req); /* 1 point = 4 bytes */
            req->drawable = d;
            req->gc = gc->gid;
            req->coordMode = CoordModeOrigin;
            point = (xPoint *) (req + 1);
            }
        point->x = x;
        point->y = y;
        }
        UnlockDisplay(dpy);
        SyncHandle();
    }

To keep clients from generating very long requests that may monopolize the server, there is a symbol defined in `<X11/Xlibint.h>` of EPERBATCH on the number of requests batched. Most of the performance benefit occurs in the first few merged requests. Note that is called before picking up the value of last_req, because it may modify this field.

## Writing Extension Stubs

All X requests always contain the length of the request, expressed as a 16-bit quantity of 32 bits. This means that a single request can be no more than 256K bytes in length. Some servers may not support single requests of such a length. The value of dpy->max_request_size contains the maximum length as defined by the server implementation. For further information, see X Window System Protocol.

### Requests, Replies, and Xproto.h

The `<X11/Xproto.h>` file contains three sets of definitions that are of interest to the stub implementor: request names, request structures, and reply structures.

You need to generate a file equivalent to `<X11/Xproto.h>` for your extension and need to include it in your stub procedure. Each stub procedure also must include `<X11/Xlibint.h>`.

The identifiers are deliberately chosen in such a way that, if the request is called X_DoSomething, then its request structure is xDoSomethingReq, and its reply is xDoSomethingReply. The GetReq family of macros, defined in `<X11/Xlibint.h>`, takes advantage of this naming scheme.

For each X request, there is a definition in `<X11/Xproto.h>` that looks similar to this:

    #define X_DoSomething   42

In your extension header file, this will be a minor opcode, instead of a major opcode.

### Request Format

Every request contains an 8-bit major opcode and a 16-bit length field expressed in units of 4 bytes. Every request consists of 4 bytes of header (containing the major opcode, the length field, and a data byte) followed by zero or more additional bytes of data. The length field defines the total length of the request, including the header. The length field in a request must equal the minimum length required to contain the request. If the specified length is smaller or larger than the required length, the server should generate a BadLength error. Unused bytes in a request are not required to be zero. Extensions should be designed in such a way that long protocol requests can be split up into smaller requests, if it is possible to exceed the maximum request size of the server. The protocol guarantees the maximum request size to be no smaller than 4096 units (16384 bytes).

Major opcodes 128 through 255 are reserved for extensions. Extensions are intended to contain multiple requests, so extension requests typically have an additional minor opcode encoded in the second data byte in the request header, but the placement and interpretation of this minor opcode as well as all other fields in extension requests are not defined by the core protocol. Every request is implicitly assigned a sequence number (starting with one) used in replies, errors, and events.

To help but not cure portability problems to certain machines, the B16 and B32 macros have been defined so that they can become bitfield specifications on some machines. For example, on a Cray, these should be used for all 16-bit and 32-bit quantities, as discussed below.

Most protocol requests have a corresponding structure typedef in `<X11/Xproto.h>`, which looks like:

    typedef struct _DoSomethingReq {
    	CARD8 reqType;		/* X_DoSomething */
    	CARD8 someDatum;	/* used differently in different requests */
    	CARD16 length B16;	/* total # of bytes in request, divided by 4 */
    	...
    	/* request-specific data */
    	...
    } xDoSomethingReq;

If a core protocol request has a single 32-bit argument, you need not declare a request structure in your extension header file. Instead, such requests use the xResourceReq structure in `<X11/Xproto.h>`. This structure is used for any request whose single argument is a Window, Pixmap, Drawable, GContext, Font, Cursor, Colormap, Atom, or VisualID.

    typedef struct _ResourceReq {
    	CARD8 reqType;	/* the request type, e.g. X_DoSomething */
    	BYTE pad;	/* not used */
    	CARD16 length B16;	/* 2 (= total # of bytes in request, divided by 4) */
    	CARD32 id B32;	/* the Window, Drawable, Font, GContext, etc. */
    } xResourceReq;

If convenient, you can do something similar in your extension header file.

In both of these structures, the reqType field identifies the type of the request (for example, X_MapWindow or X_CreatePixmap). The length field tells how long the request is in units of 4-byte longwords. This length includes both the request structure itself and any variable-length data, such as strings or lists, that follow the request structure. Request structures come in different sizes, but all requests are padded to be multiples of four bytes long.

A few protocol requests take no arguments at all. Instead, they use the xReq structure in `<X11/Xproto.h>`, which contains only a reqType and a length (and a pad byte).

If the protocol request requires a reply, then `<X11/Xproto.h>` also contains a reply structure typedef:

    typedef struct _DoSomethingReply {
    	BYTE type;	/* always X_Reply */
    	BYTE someDatum;	/* used differently in different requests */
    	CARD16 sequenceNumber B16;	/* # of requests sent so far */
    	CARD32 length B32;	/* # of additional bytes, divided by 4 */
    	...
    	/* request-specific data */
    	...
    } xDoSomethingReply;

Most of these reply structures are 32 bytes long. If there are not that many reply values, then they contain a sufficient number of pad fields to bring them up to 32 bytes. The length field is the total number of bytes in the request minus 32, divided by 4. This length will be nonzero only if:

- The reply structure is followed by variable-length data, such as a list or string.

- The reply structure is longer than 32 bytes.

Only GetWindowAttributesl, QueryFont, QueryKeymap, and GetKeyboardControl have reply structures longer than 32 bytes in the core protocol.

A few protocol requests return replies that contain no data. `<X11/Xproto.h>` does not define reply structures for these. Instead, they use the xGenericReply structure, which contains only a type, length, and sequence number (and sufficient padding to make it 32 bytes long).

### Starting to Write a Stub Procedure

An Xlib stub procedure should start like this:

    #include "<X11/Xlibint.h>

    XDoSomething (arguments, ... )
    /* argument declarations */
    {

    register XDoSomethingReq *req;
    ...

If the protocol request has a reply, then the variable declarations should include the reply structure for the request. The following is an example:

    xDoSomethingReply rep;

### Locking Data Structures

To lock the display structure for systems that want to support multithreaded access to a single display connection, each stub will need to lock its critical section. Generally, this section is the point from just before the appropriate GetReq call until all arguments to the call have been stored into the buffer. The precise instructions needed for this locking depend upon the machine architecture. Two calls, which are generally implemented as macros, have been provided.

      LockDisplay
      Display *display

      UnlockDisplay
      Display *display

display

  Specifies the connection to the X server.

### Sending the Protocol Request and Arguments

After the variable declarations, a stub procedure should call one of four macros defined in `<X11/Xlibint.h>`: `GetReq`, `GetReqExtra`, `GetResReq`, or `GetEmptyReq`. All of these macros take, as their first argument, the name of the protocol request as declared in `<X11/Xproto.h>` except with X_ removed. Each one declares a Display structure pointer, called dpy, and a pointer to a request structure, called req, which is of the appropriate type. The macro then appends the request structure to the output buffer, fills in its type and length field, and sets req to point to it.

If the protocol request has no arguments (for instance, X_GrabServer), then use `GetEmptyReq`.

    GetEmptyReq (DoSomething, req);

If the protocol request has a single 32-bit argument (such as a Pixmap, Window, Drawable, Atom, and so on), then use `GetResReq`. The second argument to the macro is the 32-bit object. X_MapWindow is a good example.

    GetResReq (DoSomething, rid, req);

The rid argument is the Pixmap, Window, or other resource ID.

If the protocol request takes any other argument list, then call `GetReq`. After the `GetReq`, you need to set all the other fields in the request structure, usually from arguments to the stub procedure.

    GetReq (DoSomething, req);
    /* fill in arguments here */
    req->arg1 = arg1;
    req->arg2 = arg2;
    ...

A few stub procedures (such as and ) return a resource ID to the caller but pass a resource ID as an argument to the protocol request. Such procedures use the macro to allocate a resource ID from the range of IDs that were assigned to this client when it opened the connection.

    rid = req->rid = XAllocID();
    ...
    return (rid);

Finally, some stub procedures transmit a fixed amount of variable-length data after the request. Typically, these procedures (such as and ) are special cases of more general functions like and . These procedures use `GetReqExtra`, which is the same as `GetReq` except that it takes an additional argument (the number of extra bytes to allocate in the output buffer after the request structure). This number should always be a multiple of four.

### Variable Length Arguments

Some protocol requests take additional variable-length data that follow the xDoSomethingReq structure. The format of this data varies from request to request. Some requests require a sequence of 8-bit bytes, others a sequence of 16-bit or 32-bit entities, and still others a sequence of structures.

It is necessary to add the length of any variable-length data to the length field of the request structure. That length field is in units of 32-bit longwords. If the data is a string or other sequence of 8-bit bytes, then you must round the length up and shift it before adding:

    req->length += (nbytes+3)>>2;

To transmit variable-length data, use the macros. If the data fits into the output buffer, then this macro copies it to the buffer. If it does not fit, however, the macro calls `_XSend`, which transmits first the contents of the buffer and then your data. The macros take three arguments: the display, a pointer to the beginning of the data, and the number of bytes to be sent.

      Data
       display
      (char *

, `Data16`, and `Data32` are macros that may use their last argument more than once, so that argument should be a variable rather than an expression such as ``nitems*sizeof(item)''. You should do that kind of computation in a separate statement before calling them. Use the appropriate macro when sending byte, short, or long data.

If the protocol request requires a reply, then call the procedure `_XSend` instead of the macro. `_XSend` takes the same arguments, but because it sends your data immediately instead of copying it into the output buffer (which would later be flushed anyway by the following call on ), it is faster.

### Replies

If the protocol request has a reply, then call after you have finished dealing with all the fixed-length and variable-length arguments. flushes the output buffer and waits for an xReply packet to arrive. If any events arrive in the meantime, places them in the queue for later use.

      Status _XReply
      Display *display
      xReply *rep
      int extra
      Bool discard

display

  Specifies the connection to the X server.

rep

  Specifies the reply structure.

extra

  Specifies the number of 32-bit words expected after the replay.

discard

  Specifies if any data beyond that specified in the extra argument should be discarded.

The function waits for a reply packet and copies its contents into the specified rep. handles error and event packets that occur before the reply is received. takes four arguments:

- A Display * structure

- A pointer to a reply structure (which must be cast to an xReply *)

- The number of additional 32-bit words (beyond `sizeof( xReply`) = 32 bytes) in the reply structure

- A Boolean that indicates whether is to discard any additional bytes beyond those it was told to read

Because most reply structures are 32 bytes long, the third argument is usually 0. The only core protocol exceptions are the replies to GetWindowAttributesl, QueryFont, QueryKeymap, and GetKeyboardControl, which have longer replies.

The last argument should be False if the reply structure is followed by additional variable-length data (such as a list or string). It should be True if there is not any variable-length data. This last argument is provided for upward-compatibility reasons to allow a client to communicate properly with a hypothetical later version of the server that sends more data than the client expected. For example, some later version of GetWindowAttributesl might use a larger, but compatible, xGetWindowAttributesReply that contains additional attribute data at the end. returns True if it received a reply successfully or False if it received any sort of error.

For a request with a reply that is not followed by variable-length data, you write something like:

    _XReply(display, (xReply *)&rep, 0, True);
    *ret1 = rep.ret1;
    *ret2 = rep.ret2;
    *ret3 = rep.ret3;
    ...
    UnlockDisplay(dpy);
    SyncHandle();
    return (rep.ret4);
    }

If there is variable-length data after the reply, change the True to False, and use the appropriate function to read the variable-length data.

      _XRead
      Display *display
      char *data_return
      long nbytes

display

  Specifies the connection to the X server.

data_return

  Specifies the buffer.

nbytes

  Specifies the number of bytes required.

The function reads the specified number of bytes into data_return.

      _XRead16
      Display *display
      short *data_return
      long nbytes

display

  Specifies the connection to the X server.

data_return

  Specifies the buffer.

nbytes

  Specifies the number of bytes required.

The function reads the specified number of bytes, unpacking them as 16-bit quantities, into the specified array as shorts.

      _XRead32
      Display *display
      long *data_return
      long nbytes

display

  Specifies the connection to the X server.

data_return

  Specifies the buffer.

nbytes

  Specifies the number of bytes required.

The function reads the specified number of bytes, unpacking them as 32-bit quantities, into the specified array as longs.

      _XRead16Pad
      Display *display
      short *data_return
      long nbytes

display

  Specifies the connection to the X server.

data_return

  Specifies the buffer.

nbytes

  Specifies the number of bytes required.

The function reads the specified number of bytes, unpacking them as 16-bit quantities, into the specified array as shorts. If the number of bytes is not a multiple of four, reads and discards up to two additional pad bytes.

      _XReadPad
      Display *display
      char *data_return
      long nbytes

display

  Specifies the connection to the X server.

data_return

  Specifies the buffer.

nbytes

  Specifies the number of bytes required.

The function reads the specified number of bytes into data_return. If the number of bytes is not a multiple of four, reads and discards up to three additional pad bytes.

Each protocol request is a little different. For further information, see the Xlib sources for examples.

### Synchronous Calling

Each procedure should have a call, just before returning to the user, to a macro called SyncHandle. If synchronous mode is enabled (see `XSynchronize`), the request is sent immediately. The library, however, waits until any error the procedure could generate at the server has been handled.

### Allocating and Deallocating Memory

To support the possible reentry of these procedures, you must observe several conventions when allocating and deallocating memory, most often done when returning data to the user from the window system of a size the caller could not know in advance (for example, a list of fonts or a list of extensions). The standard C library functions on many systems are not protected against signals or other multithreaded uses. The following analogies to standard I/O library functions have been defined:

These should be used in place of any calls you would make to the normal C library functions.

If you need a single scratch buffer inside a critical section (for example, to pack and unpack data to and from the wire protocol), the general memory allocators may be too expensive to use (particularly in output functions, which are performance critical). The following function returns a scratch buffer for use within a critical section:

      char *_XAllocScratch
      Display *display
      unsignedlong nbytes

display

  Specifies the connection to the X server.

nbytes

  Specifies the number of bytes required.

This storage must only be used inside of a critical section of your stub. The returned pointer cannot be assumed valid after any call that might permit another thread to execute inside Xlib. For example, the pointer cannot be assumed valid after any use of the `GetReq` or families of macros, after any use of , or after any use of the `_XSend` or families of functions.

The following function returns a scratch buffer for use across critical sections:

      char *_XAllocTemp
      Display *display
      unsignedlong nbytes

display

  Specifies the connection to the X server.

nbytes

  Specifies the number of bytes required.

This storage can be used across calls that might permit another thread to execute inside Xlib. The storage must be explicitly returned to Xlib. The following function returns the storage:

      void _XFreeTemp
      Display *display
      char *buf
      unsignedlong nbytes

display

  Specifies the connection to the X server.

buf

  Specifies the buffer to return.

nbytes

  Specifies the size of the buffer.

You must pass back the same pointer and size that were returned by .

### Portability Considerations

Many machine architectures, including many of the more recent RISC architectures, do not correctly access data at unaligned locations; their compilers pad out structures to preserve this characteristic. Many other machines capable of unaligned references pad inside of structures as well to preserve alignment, because accessing aligned data is usually much faster. Because the library and the server use structures to access data at arbitrary points in a byte stream, all data in request and reply packets must be naturally aligned; that is, 16-bit data starts on 16-bit boundaries in the request and 32-bit data on 32-bit boundaries. All requests must be a multiple of 32 bits in length to preserve the natural alignment in the data stream. You must pad structures out to 32-bit boundaries. Pad information does not have to be zeroed unless you want to preserve such fields for future use in your protocol requests. Floating point varies radically between machines and should be avoided completely if at all possible.

This code may run on machines with 16-bit ints. So, if any integer argument, variable, or return value either can take only nonnegative values or is declared as a CARD16 in the protocol, be sure to declare it as unsigned int and not as int. (This, of course, does not apply to Booleans or enumerations.)

Similarly, if any integer argument or return value is declared CARD32 in the protocol, declare it as an unsigned long and not as int or long. This also goes for any internal variables that may take on values larger than the maximum 16-bit unsigned int.

The library currently assumes that a char is 8 bits, a short is 16 bits, an int is 16 or 32 bits, and a long is 32 bits. The `PackData` macro is a half-hearted attempt to deal with the possibility of 32 bit shorts. However, much more work is needed to make this work properly.

### Deriving the Correct Extension Opcode

The remaining problem a writer of an extension stub procedure faces that the core protocol does not face is to map from the call to the proper major and minor opcodes. While there are a number of strategies, the simplest and fastest is outlined below.

- Declare an array of pointers, _NFILE long (this is normally found in `<stdio.h>` and is the number of file descriptors supported on the system) of type XExtCodes. Make sure these are all initialized to NULL.

- When your stub is entered, your initialization test is just to use the display pointer passed in to access the file descriptor and an index into the array. If the entry is NULL, then this is the first time you are entering the procedure for this display. Call your initialization procedure and pass to it the display pointer.

- Once in your initialization procedure, call ; if it succeeds, store the pointer returned into this array. Make sure to establish a close display handler to allow you to zero the entry. Do whatever other initialization your extension requires. (For example, install event handlers and so on.) Your initialization procedure would normally return a pointer to the XExtCodes structure for this extension, which is what would normally be found in your array of pointers.

- After returning from your initialization procedure, the stub can now continue normally, because it has its major opcode safely in its hand in the XExtCodes structure.