Big Requests Extension

X Consortium Standard

Bob Scheifler

X Consortium

X Version 11, Release 7.7

Version 2.0

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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.

