Re: C bug of the day

[Kilgallen@SpamCop.net (Larry Kilgallen)]

In article <d6652001.0306160158.8f93aab@posting.google.com>, kanze@gabi-soft.fr writes:
 > brangdon@cix.co.uk (Dave Harris) wrote in message
 > news:<memo.20030613191305.38347A@brangdon.madasafish.com>...
 >> jimmaureenrogers@att.net (James Rogers) wrote (abridged):

 >>  > why would you ever *want* uninitialized variables?
 >
 >> Mainly for speed. We would want to be able to say, eg,
 >>      std::vector<int> vec( 1000000, unitialised );
 >
 >> to avoid writing a million zeros. Which I suspect makes it a
 >> non-trivial language change. What is the type of "initialised"?
 >
 > Hmmm.  Either you write a million zeros, or you write a million words
 > with junk.  Either way, the actual execution time will not change on
 > typical machines.

Certainly an operating system can map arbitrary existing memory pages
into a particular chunk of virtual address space.  The only reason to
expect them to be zeroed or not based on operating system interests
would be confidentiality of the data formerly on those pages of memory.

VMS has "demand zero" pages that provide zeroed pages only on the first
access to the page.  I don't know if typical compilers are smart enough
to use that feature on arrays explicitly initialized to zero.

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