Re: Problem only in release version!

On Mon, 10 Oct 2005 17:25:29 -0700, "Peter Oliphant"
<poliphant@xxxxxxxxxxxxxxxx> wrote:

>Just to affirm Jochen's point, the Debug version initializes most variables
>to zero. The Release version will not initialize anything 'naturally'
>(meaning without you writing code to do so), and hence many of the variable
>are often given a 'random' value upon loading ('random' in that they keep
>the contents of the memory addresses they happen to get assigned to).

The debug version doesn't initialize anything to zero that isn't
zero-initialized in the release version. The /RTCs documentation explains
why you're more likely to observe zero as the value of uninitialized
stack-based variables in debug builds that don't use /RTCs.

/RTC (Run-Time Error Checks)
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/vccore/html/vcrefrtcruntimechecks.asp

In addition, the debug heap manager sets memory to certain non-zero values
to help diagnose heap errors.

>Thus, your new errors are likely the result of some variable that works fine
>if initialized to zero, but don't work if not initialized to zero (or not in
>some range, non-negative, etc.). Since an application doesn't always get
>loaded at the same place and memory, and because even the same place in
>memory can have different values depending on when you load your program
>(that's why it's RAM not ROM...hehe), you can get random bugs and un-stable
>executions of the Release version (i.e., it might do different things on
>different runs depending on the 'random' nature of your variables) even if
>it is totally stable in Debug mode.

The OS (NT-based Windows anyway) only ever hands out memory pages
initialized to zero. In fact, there's an OS thread whose purpose in life is
to populate a zeroed page list:

Inside Memory Management, Part 2
http://www.windowsitpro.com/Articles/Print.cfm?ArticleID=3774
<q>
Pages on the standby page list move to the zeroed page list after a special
thread, called the zero-page thread, clears their content. The zero-page
thread executes in the background at priority 0. It runs only if no other
thread can run, and its job is to move pages from the free page list to the
zeroed page list as it clears their content.
....
The necessity of zeroing a page before assigning it to the working set of a
different process is a C2 security requirement.
</q>

--
Doug Harrison
VC++ MVP
.

Relevant Pages

  • Re: Problem only in release version!
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  • Re: Problem only in release version!
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    ... > Just to affirm Tomas's point, the Debug version initializes most variables ... > they keep the contents of the memory addresses they happen to get assigned ... > fine if initialized to zero, but don't work if not initialized to zero (or ... > variables) even if it is totally stable in Debug mode. ...
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