How to call memcmp() on two parts of byte[] (with offset)?

Question

I want to compare parts of byte[] efficiently - so I understand memcmp() should be used.

I know I can using PInvoke to call memcmp() - Comparing two byte arrays in .NET

But, I want to compare only parts of the byte[] - using offset, and there is no memcmp() with offset since it uses pointers.

int CompareBuffers(byte[] buffer1, int offset1, byte[] buffer2, int offset2, int count)
{
  // Somehow call memcmp(&buffer1+offset1, &buffer2+offset2, count)
}

Should I use C++/CLI to do that?

Should I use PInvoke with IntPtr? How?

Thank you.

Why do you think `memcmp` would be more efficient than the C# equivalent? — jalf, Jun 08 '10 at 20:00
What is the .NET equivalent to memcmp()? I don't want to write it myself since it would either be inefficient or buggy. — brickner, Jun 08 '10 at 20:07
read this thread about http://www.codeproject.com/Messages/863856/Re-memcmp-in-Csharp.aspx — lsalamon, Jun 08 '10 at 20:17
@Isalamon, I don't see where in the thread there is something about an offset? — brickner, Jun 08 '10 at 20:41

score 14 · Answer 1 · answered Sep 15 '10 at 13:42

14

[DllImport("msvcrt.dll")]
private static extern unsafe int memcmp(byte* b1, byte* b2, int count);

public static unsafe int CompareBuffers(byte[] buffer1, int offset1, byte[] buffer2, int offset2, int count)
{
    fixed (byte* b1 = buffer1, b2 = buffer2)
    {
        return memcmp(b1 + offset1, b2 + offset2, count);
    }
}

You might also want to add some parameter validation.

answered Sep 15 '10 at 13:42

Cory

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1

Need to add CallingConvention = CallingConvention.Cdecl. The default is WinApi or stdcall – zhaorufei Sep 10 '13 at 06:49
`count` is `size_t` as an unsigned integral type – Andrei Krasutski Dec 18 '19 at 14:55

score 5 · Accepted Answer · answered Jun 09 '10 at 02:48

5

C++/CLI will definitely be the cleanest, but this hardly justifies adding C++/CLI to your project if you aren't already using it.

How about Marshal.UnsafeAddrOfPinnedArrayElement(array, offset)?

answered Jun 09 '10 at 02:48

Ben Voigt

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score 3 · Answer 3 · answered Jun 08 '10 at 20:36

3

No matter what you do, you should check that the offset/count values are valid for the given byte arrays. After you do that, I don't see how just doing a for loop in C# is going to be any slower than P/Invoking a Win32 method. Seems like there would be a lot of overhead in the P/Invoke that it would not be worthwhile.

Also, there is always unsafe C#.

As with all performance questions, you should do your own performance testing. But it sounds to me like you are trying to optimize for performance prematurely.

answered Jun 08 '10 at 20:36

Bryan

2,775
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1

Optimized `memcmp` uses SIMD instructions to compare 16 bytes per clock cycle (when the buffers are properly aligned) and also provides cache hinting. One you write in C# won't do that. For small lengths, the p/invoke overhead is too much. But larger buffer sizes really win from calling optimized native code. – Ben Voigt Dec 05 '14 at 16:23

score 2 · Answer 4 · answered Jun 08 '10 at 20:32

2

No need to P/Invoke in C++/CLI. Use pin_ptr<> to pin the array. That gets you a byte*, just add the offset.

answered Jun 08 '10 at 20:32

Hans Passant

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I know. So you suggest using C++/CLI to call memcpy() and not PInvoke? – brickner Jun 08 '10 at 20:38
1

PInvoke, like so many references I saw but without offset capabilities. Or maybe just some unsafe code? – brickner Jun 08 '10 at 20:51
I'll let you think about this for a while. – Hans Passant Jun 08 '10 at 20:57
Thanks, I'm just not sure what will work and be fastest. If they are equivalent, I prefer C#, but I'm not sure how to write it so it would work best. – brickner Jun 08 '10 at 20:59

score 0 · Answer 5 · answered Jul 18 '17 at 12:15

0

There is one more way

SequenceEqual from System.Linq

 byte[] ByteArray1 = null;
 byte[] ByteArray2 = null;

 ByteArray1 = MyFunct1();
 ByteArray2 = MyFunct2();

 if (ByteArray1.SequenceEqual<byte>(ByteArray2))
 {
    MessageBox.Show("Same");
 }
 else
 {
   MessageBox.Show("Not Equal");
 }

answered Jul 18 '17 at 12:15

4

This solution is very slow. – SubqueryCrunch Aug 10 '17 at 07:00
See this answer for a way more performant approach which works on .NET 6+: https://stackoverflow.com/a/48599119/227779 – Per Lundberg Mar 07 '23 at 19:12

How to call memcmp() on two parts of byte[] (with offset)?

5 Answers5