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XS(3) User Contributed Perl Documentation XS(3)


Readonly::XS - Companion module for, to speed up read-only scalar variables.


This document describes version 1.05 of Readonly::XS, February 24, 2009.


  Install this module, but do not use it.


The Readonly module (q.v.) is an effective way to create non-modifiable variables. However, it's relatively slow.

The reason it's slow is that is implements the read-only-ness of variables via tied objects. This mechanism is inherently slow. Perl simply has to do a lot of work under the hood to make tied variables work.

This module corrects the speed problem, at least with respect to scalar variables. When Readonly::XS is installed, Readonly uses it to access the internals of scalar variables. Instead of creating a scalar variable object and tying it, Readonly simply flips the SvREADONLY bit in the scalar's FLAGS structure.

Readonly arrays and hashes are not sped up by this, since the SvREADONLY flag only works for scalars. Arrays and hashes always use the tie interface.

Why implement this as a separate module? Because not everyone can use XS. Not everyone has a C compiler. Also, installations with a statically-linked perl may not want to recompile their perl binary just for this module. Rather than render useless for
these people, the XS portion was put into a separate module.

Programs that you write do not need to know whether Readonly::XS is installed or not. They should just "use Readonly" and let Readonly worry about whether or not it can use XS. If the Readonly::XS is present, Readonly will be faster. If not, it won't. Either way, it will still work, and your code will not have to change.

Your program can check whether is using XS or not by examining the $Readonly::XSokay variable. It will be true if the XS module was found and is being used. Please do not change this variable.





Eric Roode,

Copyright (c) 2003-2009 by Eric J. Roode. All Rights Reserved. This module is free software; you can redistribute it and/or modify it under the same terms as Perl itself.

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2011-11-18 perl v5.40.0