visual c libraries: I was wondering if anyone knew of any changes in the CRT that would cause
malloc behavior to change between code built with VC6/VC7? We discovered a
segfault bug in production code built with VC6 that was not crashing
frequently at all. We fixed the bug, but in the meantime, our builds were in
the process of being switched to VC7. In the VC7 built code, the same crash
occurs much more frequently. This alarmed us, so we are investigating why VC7
behavior would differ so much from VC6.

The nature of the bug is such that many small buffers (~16 bytes) are being
malloc'd but the bug causes the code to read past the end of the buffer by 8
bytes. Straightforward, and this crashes whenever the address to the 17th
byte lives on the next memory page which happens to not be allocated.

The issue is in VC6 built code, this bug rarely occurs, almost as if the
memory allocation never allocates that close to an invalid page, or never
completely
fills up an entire page. When the code is built in VC7, the crash occurs
very, very frequently, as if the 16 bytes are being allocated at the end of
the
page prior to an invalid one.

My question is if anything regarding malloc or the heap changed between
VC6/VC7 that could be behind this. Do any of the compiler optimization flags
affect malloc's ability to alloc up to the last available byte in a page?

We have discovered the source of the bug in remote code and have fixed it,

[quoted text, click to view]

I don't know the details, but I do recall comments that it was changed
significantly, so that may well account for the differences you see.

[quoted text, click to view]

Quickly!

VC8 is out now you know ;)

Dave
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