Using save Outside the Server Loop

Transcription

1 Using save Outside the Server Loop Adobe Developer Support Technical Note # February 1992 Adobe Systems Incorporated Adobe Developer Technologies 345 Park Avenue San Jose, CA PN LPS5129

2 Copyright 1992 by Adobe Systems Incorporated. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written consent of the publisher. Any software referred to herein is furnished under license and may only be used or copied in accordance with the terms of such license. PostScript is a registered trademark of Adobe Systems Incorporated. All instances of the name PostScript in the text are references to the PostScript language as defined by Adobe Systems Incorporated unless otherwise stated. The name PostScript also is used as a product trademark for Adobe Systems implementation of the PostScript language interpreter. Any references to a PostScript printer, a PostScript file, or a PostScript driver refer to printers, files, and driver programs (respectively) which are written in or support the PostScript language. The sentences in this book that use PostScript language as an adjective phrase are so constructed to reinforce that the name refers to the standard language definition as set forth by Adobe Systems Incorporated. PostScript, the PostScript logo, and the Adobe logo are trademarks of Adobe Systems Incorporated which may be registered in certain jurisdictions. Other brand or product names are the trademarks or registered trademarks of their respective holders. This publication and the information herein is furnished AS IS, is subject to change without notice, and should not be construed as a commitment by Adobe Systems Incorporated. Adobe Systems Incorporated assumes no responsibility or liability for any errors or inaccuracies, makes no warranty of any kind (express, implied or statutory) with respect to this publication, and expressly disclaims any and all warranties of merchantability, fitness for particular purposes and noninfringement of third party rights.

3 Contents Using save Outside the Server Loop 5 1 The Server Loop 5 save and restore Outside the Server Loop 5 iii

4 iv Contents (14 Feb 92)

5 Using save Outside the Server Loop 1 The Server Loop The PostScript interpreter encapsulates normal PostScript language programs in a server loop. The server loop protects one PostScript language program from the effects of another. It uses the save operator to keep a snapshot of the initial VM and interpreter state, and the restore operator to reimpose the initial state when the program finishes. It is possible to make a change to the state of the interpreter, so that the change is in effect for subsequent PostScript language programs. This is done by exiting the server loop and thereby escaping the encapsulation using save and restore. Changes to VM done outside the server loop persist for subsequent jobs until the interpreter is reset. 1.1 save and restore Outside the Server Loop A PostScript language program can also use the save and restore operators outside the server loop. However, it is important that they are balanced, that is, have a restore corresponding to each save. There is no need for any program to have unbalanced save operations. It is ineffective, because Level 2 interpreters re-balance unmatched save operations. That is, they perform an implicit restore for each unmatched save when re-entering the server loop. Changes made after the first unmatched save are undone. However, it is bad practice because Level 1 interpreters do not re-balance unmatched save operations. The result is that PostScript language programs that perform unmatched save operations outside the server loop might have errors on Level 2 interpreters, although they appeared to work correctly on Level 1 interpreters. The solution is to change those programs so that they either do not use save outside the server loop, or balance each save with a restore. This should not be difficult, because as stated before there is no reason why a program should rely on an unmatched save outside the server loop. 5

6 One reason might be wanting to manage memory outside the server loop. This is usually bad strategy programs that are prepared to manage memory are far better off managing it within the server loop. For applications that absolutely must manage memory outside the server loop, the undef operator can be useful. This note does not apply to the use of save and restore inside the server loop. That functions as it always has, and memory management, which works inside the server loop in Level 1 interpreters, should work identically in Level 2 interpreters. For more information about the server loop model, see section 3.7.7, Job Execution Environment, in the PostScript Language Reference Manual, Second Edition. 6 Using save Outside the Server Loop (14 Feb 92)