One of my colleagues shared with me one of his application compatibility stories.
There was a program that would fail on some computers but not
others, and it wasn't clear why.
The problem was traced to
the size of an internal cache.
Now,
GlobalMemoryStatus
officially returns unsigned values,
but if the calling application
is not marked
/LARGEADDRESSAWARE,
then
GlobalMemoryStatus
reports a maximum
of
2GB − 1
bytes of memory,
for compatibility purposes.
You'd think that this would be enough to keep old
programs happy,
but apparently not.
This particular program wasn't content with the values
that it got from
GlobalMemoryStatus.
Instead, it took the
dwTotalPhys
and
added it to the dwTotalPageFile,
and treated the result as a signed value.
This means that on systems with more
than 2GB of memory, the addition will
produce a total of
0xFFFFFFFE,
which is
a negative value
when interpreted as a signed result,
which in turn causes the program to crash.
My colleague fixed the program by patching
out the instructions that added
dwTotalPhys
to
dwTotalPageFile,
and had the program operate solely on
dwTotalPhys,
which is probably
what it should have been operating on
in the first place.
You see, even though the field in
the
MEMORYSTATUS structure
is named
dwAvailPageFile,
it doesn't actually give you
the size of the page file.
The documentation of
dwAvailPageFile
says
The current size of the committed memory limit, in bytes. This is physical memory plus the size of the page file, minus a small overhead.
Yes, this is a case of bad naming. (You can come up with your own theories how we ended up with the bad name.)
By adding
dwTotalPhys and
dwTotalPageFile,
the code was double-counting the physical memory.
The conclusion my colleague drew from this exercise was that there are still programmers out there who are working hard to skip the documentation, come up with bad ideas, and implement them poorly.
I admire the program's dedication to getting everything wrong despite the operating system's efforts to save them from themselves.
