Null-terminated Unicode strings in java.io on Windows

[Alan Bateman]

Roman Kennke wrote:
> Hi,
>
> I'm trying to understand a piece of code in java.io . Let me try to
> explain:
>
> When you look into WinNTFileSystem.c in the method
> getBooleanAttributes(), you see that the file object is converted to a
> WCHAR* using fileToNTPath(). In io_util.c, fileToNTPath(), the filename
> string is extracted from the File object, and passed to pathToNTPath().
>
> This is where it gets interesting. The pathToNTPath() function first
> converts the string into a jchar* using the macro WITH_UNICODE_STRING.
> This macro uses GetStringChars() to do this conversion. Now this is
> where I'm lost. Java strings are not null-terminated, and neither are
> the jchar* returned by GetStringChars() (which is in itself a long
> discussed problem in the JNI spec, but that's another story). But back
> in pathToNTPath() this jchar* is treated just like a null-terminated
> string, for example, we call wcslen() to determine its length, which
> relies on the string beeing null-terminated. Now I assume that this
> works somehow, and I only see the following options:
> 1. There's something in this picture that I don't see. Maybe the string
> ends up null-terminated somehow?
> 2. Maybe this works by accident because Hotspot terminates strings with
> a null internally?
> 3. Or this is a serious bug, that for some reason doesn't bomb all the
> time. After all, it _does_ bomb in the JamaicaVM, where I'm trying to
> port the code to...
>
> Any ideas? I'd be happy to get an explanation for this problem.
>
> Cheers, Roman
>   
The GetStringChars implementation in HotSpot always returns a copy that 
is length+1 and zero terminated. There is a long-standing bug to clarify 
the JNI specification on this topic. I believe it should say that the 
returned array of Unicode characters is not required to be zero 
terminated and that one should use GetStringLength to determine the 
length. Steve Bohne (cc'ed) has done the recent maintenance on the JNI 
spec and may wish to comment. In any case, I did a quick cscope and 
aside from java.io, it only appears to impact a small number of places.

-Alan.