Big Requests Extension

X Consortium Standard

Bob Scheifler

   X Consortium

   X Version 11, Release 7.7

   Version 2.0

   Copyright © 1993, 1994 X Consortium

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     __________________________________________________________

   Table of Contents

   1. Overview
   2. Requests
   3. Events and Errors
   4. Encoding
   5. C language binding
   6. Acknowledgements

Chapter 1. Overview

   This extension enables the use of protocol requests that exceed
   262140 bytes in length.

   The core protocol restricts the maximum length of a protocol
   request to 262140 bytes, in that it uses a 16-bit length field
   specifying the number of 4-byte units in the request. This is a
   problem in the core protocol when joining large numbers of
   lines (PolyLine) or arcs (PolyArc), since these requests cannot
   be broken up into smaller requests without disturbing the
   rendering of the join points. It is also much more of a problem
   for protocol extensions, such as the PEX extension for 3D
   graphics and the XIE extension for imaging, that need to send
   long data lists in output commands.

   This extension defines a mechanism for extending the length
   field beyond 16 bits. If the normal 16-bit length field of the
   protocol request is zero, then an additional 32-bit field
   containing the actual length (in 4-byte units) is inserted into
   the request, immediately following the 16-bit length field.

   For example, a normal PolyLine encoding is:
   PolyLine
   1  65                   opcode
   1                       coordinate-mode
      0           Origin
      1           Previous
   2  3+n
   4  DRAWABLE             drawable
   4  GCONTEXT             gc
   4n LISTofPOINT          points

   An extended-length PolyLine encoding is:
   PolyLine
   1  65                   opcode
   1                       coordinate-mode
      0           Origin
      1           Previous
   2  0                    extended length flag
   4  4+n                  request length
   4  DRAWABLE             drawable
   4  GCONTEXT             gc
   4n LISTofPOINT          points

   Extended-length protocol encodings, once enabled, can be used
   on all protocol requests, including all extensions.

Chapter 2. Requests

   BigReqEnable

   =>

   maximum-request-length: CARD32

   This request enables extended-length protocol requests for the
   requesting client. It also returns the maximum length of a
   request, in 4-byte units, that can be used in extended-length
   protocol requests. This value will always be greater than the
   maximum-request-length returned in the connection setup
   information.

Chapter 3. Events and Errors

   No new events or errors are defined by this extension.

Chapter 4. Encoding

   Please refer to the X11 Protocol Encoding document as this
   document uses conventions established there.

   The name of this extension is "BIG-REQUESTS".
   BigReqEnable
   1 Card8 opcode
   1 0     bigreq opcode
   2 1     request length
   =>
   1  1      Reply
   1         unused
   2  CARD16 sequence number
   4  0      length
   4  CARD32 maximum-request-length
   20        unused

Chapter 5. C language binding

   It is desirable for core Xlib, and other extensions, to use
   this extension internally when necessary. It is also desirable
   to make the use of this extension as transparent as possible to
   the X client. For example, if enabling of the extension were
   delayed until the first time it was needed, an application that
   used XNextRequest to determine the sequence number of a request
   would no longer get the correct sequence number. As such,
   XOpenDisplay will determine if the extension is supported by
   the server and, if it is, enable extended-length encodings.

   The core Xlib functions XDrawLines, XDrawArcs, XFillPolygon,
   XChangeProperty, XSetClipRectangles, and XSetRegion are
   required to use extended-length encodings when necessary, if
   supported by the server. Use of extended-length encodings in
   other core Xlib functions (XDrawPoints, XDrawRectangles,
   XDrawSegments). XFillArcs, XFillRectangles, XPutImage is
   permitted but not required; an Xlib implementation may choose
   to split the data across multiple smaller requests instead.

   To permit clients to know what the maximum-request-length for
   extended-length encodings is, the following function is added
   to Xlib:

   long XExtendedMaxRequestSize(Display *display);

   Returns zero (0) if the specified display does not support this
   extension, otherwise returns the maximum-request-length (in
   4-byte units) supported by the server through the
   extended-length encoding.

Chapter 6. Acknowledgements

   Clive Feather (IXI) originated the extended-length encoding
   used in this extension proposal.
