by Michael S. Kaplan, published on 2006/02/09 03:01 -05:00, original URI: http://blogs.msdn.com/b/michkap/archive/2006/02/09/528297.aspx
I'm not sure how many of you remember when I posted Hungarian is even more complicated than I thought and More on the fabled EqualString.
Not because I don't have stats or anything, but because there is no way to gauge how many of you are new readers and how many of you really have nothing better to do that read what I am posting here. :-)
Anyway, I am going to talk about RtlEqualUnicodeString and RtlCompareUnicodeString, the functions in ntdll.dll that do binary comparisons that can be cae insensitive, again.
I found out something really interesting about them the other day.
Now it is obvious how RtlEqualUnicodeString might be used -- I mean, if you have two strings and you need to know in a binary sense whether they are equal (possibly ignoring case) then it can be very handy. Because no matter how un-natural the comparison seems to humans, the fact is that lots of Windows loves it.
Of course the actual usage of RtlCompareUnicodeString is a bit less clear -- I mean, the order has no meaning to humans. So a function that uses it to order two strings seems like a ripe source for incorrect usage.
Don't worry, it turns out that nobody is using that order inappropriately.
In just about every case, the return value of the function is tested to see whether it was equal to or not equal to zero.
Yes, that is right -- almost everyone who uses it is essentially duplicating the functionality of RtlEqualUnicodeString.
When you get down to it, one has to wonder how much more expensive is operation A than operation B:
A - compare two strings, one WCHAR at a time, return the difference if there is one as soon as you find it, then compare that number to zero to see if there is in fact a difference.
B - compare two strings, one WCHAR at a time, return TRUE or FALSE as soon as you know whether they are in fact equal.
Remembering for a moment that a difference that makes no difference, makes no difference -- do you think it makes a significant difference?
Hopefully not. Though it worries me that no one seems to be doing anything beyond what RtlEqualUnicodeString would provide. So why take a hit at all?
I resisted the temptation to just go and fix all of the occurrences (it is a 100% safe change but even so, I hate when people do it to code I own).
I also resisted the temptation to send out a bunch of mail to all of the owners to tell them to change their code (I hate when people do that to me, too).
Now that I read this post again, it occurs to me that this will probably not actually be very interesting to people. It just seemed weird to me.
Though if you own one of those calls to RtlCompareUnicodeString, then feel free to change it; at worst it will just be more self-documenting as to the intention, and at best (if the code is called many times in a tight loop) it could even help performance!
This post brought to you by "P" (U+0050, a.k.a. LATIN CAPITAL LETTER P)
# Siva on 9 Feb 2006 3:45 AM:
# Nick Lamb on 9 Feb 2006 7:04 AM:
# Michael S. Kaplan on 9 Feb 2006 8:48 AM:
# Dean Harding on 9 Feb 2006 5:29 PM:
# Christian Kaiser on 10 Feb 2006 2:32 AM:
# Michael S. Kaplan on 10 Feb 2006 3:16 AM:
# Serge Wautier on 11 Feb 2006 8:07 AM:
Yuhong Bao on 12 Mar 2009 1:00 AM:
"Ouch... for a while, I figured Rtl stands for Right-to-left! "
Nope, it stands for "Run-Time Library". Do you know about the NT Native APIs in NTDLL.DLL? Rtl* APIs were utility functions that in user mode did not call directly to kernel mode. Nt* and Zw* APIs were in user mode stubs to do a system call to kernel mode which implements the API. In kernel mode, the Win32 API do not exist, and so the NT Native APIs are always used, and in kernel mode there is a difference between Zw* and Nt*. These APIs are mostly undocumented, the third-party that created Interix (a replacement for the POSIX subsystem) had to obtain NT source code to do so.
2006/04/29 'Which comes first?' vs. 'Are they equal?'
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