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The first MsgBox in the following script shows an invisible character for x, length 1. The second MsgBox shows the correct ! character surrounded by [ and ].Removing the VarSetCapacity line cures the problem.

This is caused by the fact that the program internally maintains the length of each variable's contents. But when you alter the contents by passing the address to a function, the program has no way of knowing what happened and thus the length is now inaccurate, which in turn causes misbehavior.

I'm not sure if this can be easily fixed, but I'll look into it more. Thanks.

I forgot to mention that you can work around this in most cases by passing the variable as a "str" argument and omitting the ampersand prefix. By passing it as a string, the program knows that the length might have changed and so updates it after the call:

DllCall("RtlFillMemory", Str, x, UInt, 1, UChar, 33)

Thanks, Chris! "Str" cures the problem.

I wonder, if you store the length of the strings, why do you need the terminating \0 ? Without it binary data could be handled more easily.

Since all the C library functions require terminated strings, it simplifies a lot of things to have it that way. A lot of code would have to be rewritten (and some new code added) to be able to omit the terminators.

The length-tracking is done because it improves performance by avoiding CPU-consuming calls to strlen() [which calculates the length of a string].

Update: In v1.0.44.03, VarSetCapacity() has been improved to interpret a capacity of -1 as "update this variable's internally-stored length". This is useful in cases where a variable has been altered indirectly, such as by passing its address via DllCall().