Is your 64bit program really solid ?

Posted by – August 16, 2009

If you ask Windows for memory it will generally start with low addresses first. There is nothing wrong with that. For a 64bit program however, this means that under normal circumstances unless you actually use up 4Gb of memory, your pointers will still fit into a 32bit value. So if you store a pointer into a too small variable (say a long) you will not notice, because the upper half of the 64bit pointer value isn’t even used. So your program may look quite solid and usable right now but if you feed it something like a 4Gb file to load it will start crashing. This may sound like an unlikely case but it will become much more common even for programs that are not meant to deal with large amounts of data, simply because as disk space, memory and computer speeds grow, so do the amounts of data that users deal with.

Anyway, I recently found this link: http://msdn.microsoft.com/en-us/library/bb613473.aspx

Notice at the very bottom the mention of the AllocationPreference registry setting. If you develop 64bit applications (especially if you are porting code from 32bit to 64bit) then this key will help you a great deal. What it does is it instructs Windows to give you high addresses first. The result are pointers that don’t fit into 32bit right from the start. So these errors that usually go unnoticed will blow up right in your face and you can fix them.

The link doesn’t explicitly state the OS this works on but i tested both on XP 64bit and Vista 64bit and it works fine on both. You have to restart the computer after setting the registry value for the change to take effect. You can then verify with a simple program if the high addresses are returned (make sure you compile it with the 64bit compiler ;) ):

For i = 1 To 10
  a$ + RSet(Hex(AllocateMemory(1000), #PB_Integer), 16, "0") + Chr(13)
Next i
MessageRequester("Pointer values", a$)

In this mode you can easily distinguish a valid pointer from a truncated one by looking at it as hex like above. Since the virtual address space is limited to 8TB on current x64 Windows versions, a pointer allocated in this mode will look something like 000007FFFXXXXXXX because the addresses cannot go any higher. If a pointer got cut somewhere it will either look like 00000000XXXXXXXX or FFFFFFFFXXXXXXXX depending on wether the cut 32bit value was positive or negative (the sign-extension of the negative numbers will create all the F’s).

The PureBasic package itself is a good example of how easily such bugs go unnoticed. We payed a lot of attention to this when porting the libraries and also the programs like IDE, debugger and compiler to 64bit with PB 4.30. There was a lot of testing by the users after that too and PB 4.31 x64 is quite stable. However, if you turn on this mode the 4.31 IDE and debugger will not even start. Even the compiler won’t compile anything. So if you want to try this with your own programs you obviously cannot use PB 4.31 for it. The new 4.40 version was tested with this mode and these bugs are now fixed. Since it is still in beta, you may run into the odd crash though. If this happens please let us know.

Keep in mind that this setting is systemwide. So other x64 programs could well crash on you too if they have such unnoticed bugs.

Have fun fixing a ton of bug you never knew existed :P

1 Comment on Is your 64bit program really solid ?

  1. luis says:

    Great find! Thank you for sharing it.

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