Is there a memcmp equivalent for comparing byte arrays in Mono?

Question

There is a well-known efficiency for comparing two byte-arrays in .Net by importing the memcmp function from msvcrt.dll, as described here.

Is there an equivalent library import in mono? Would it need to be different when running mono on linux or on windows? Or is there another fast byte array comparison technique that works well in mono? I'm looking for something better than just iterating over the arrays in c#.

Update

Based on Matt Patenaude's comment, I think this might work well:

#if __MonoCS__
    [DllImport("c", CallingConvention = CallingConvention.Cdecl)]
#else
    [DllImport("msvcrt.dll", CallingConvention = CallingConvention.Cdecl)]
#endif
    public static extern int memcmp(byte[] b1, byte[] b2, UIntPtr count);

But I have not yet tried it. I've never done p/invoke on mono before. I'm using the signature recommended on pinvoke.net. Is this going to be compatible?

Looking for a Mono-focused answer. Thanks.

I don't know much about Mono or .NET, but `memcmp` is a standard ISO C function, so most (all) UNIXes should support it already. When not on Windows, there might be a way in Mono to access the native libc? — Matt Patenaude, Dec 17 '12 at 23:34

Robert Rouhani · Accepted Answer · 2012-12-18T18:31:29.723

7

Based on your update, you shouldn't be using the __MonoCS__ preprocessor. It means you would have to recompile the library for Mono and .NET. The better way is to use dllmap functionality in Mono and only use the msvcrt.dll DllImport.

Instead define a "AssemblyName.dll.config" and use the dllmap tag to map msvcrt.dll to c when run on Mono.

Example:

<configuration>
    <dllmap dll="msvcrt.dll" target="libc.so.6" />
</configuration>

More detail on dllmap here: http://www.mono-project.com/Config_DllMap

EDIT

And if for some reason c doesn't work, libc.so should work.

edited Dec 18 '12 at 18:31

answered Dec 18 '12 at 04:27

Robert Rouhani

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Amen to "do not compile a separate version for Mono". – skolima Dec 18 '12 at 14:07
Ok, but is the memcmp signature still valid for mono? – Matt Johnson-Pint Dec 18 '12 at 18:07
Yeah, it should be. I'm not too sure about your `c` target though. Give me a minute to boot into my linux partition and try it out... – Robert Rouhani Dec 18 '12 at 18:14
Just tried it out. `c` doesn't work. For some reason, `libc.so` isn't executable, had to set the target to `libc.so.6`. Updated the answer with that. – Robert Rouhani Dec 18 '12 at 18:31

score 2 · Answer 2 · answered Dec 17 '12 at 23:39

2

You can use unsafe code blocks to access byte arrays almost as fast as native memcmp. Before you go down that road, make sure a straight for loop isn't fast enough for your purposes.

answered Dec 17 '12 at 23:39

Ben

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Now that I have a working answer, I will benchmark and see which makes more sense for my purposes. Thanks for the suggestion. – Matt Johnson-Pint Dec 18 '12 at 20:41

Is there a memcmp equivalent for comparing byte arrays in Mono?

2 Answers2