Who Moved My byte[] using DllImport?

Question

4.10/5 (3 votes)

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I wrote some testcase named LibTest.dll (act like strcpy) in VC++ 6.0

LibTest.h shown as below:

///////////////////////////////////////////////////////////////////////////////
#define DLLEXPORT extern "C" __declspec(dllexport)

DLLEXPORT void LibTestInit();
DLLEXPORT void LibTestStrCpy(char *in, char *out);
DLLEXPORT void LibTestCleanup();

LibTest.cpp shown as below:

///////////////////////////////////////////////////////////////////////////////
#include "LibTest.h"

#include <stdio.h>

namespace CCTEC 
{
    class Test 
    {
    public:
        Test() {}
        ~Test() {}

    public:
        void strcpy(char *in, char *out) 
        {
            while (*out++ = *in++);
        }
    };
};

static CCTEC::Test *m_Test = NULL;

DLLEXPORT void LibTestInit() 
{
    m_Test = new CCTEC::Test(); 
}

DLLEXPORT void LibTestCleanup() 
{
    if (m_Test) delete m_Test; m_Test = NULL; 
}

DLLEXPORT void LibTestStrCpy(char *in, char *out) 
{
    if (m_Test) m_Test->strcpy(in, out);
}</stdio.h>

Then using DllImport to call unmanaged functions (such as LibTestInit )
that are implemented in a DLL (LibTest.dll)

LibTest.cs (part of the source code) shown as below:

using System.Runtime.InteropServices;

public class LibTest
{
    #region DLL Import
    [DllImport("LibTest")]
    private static extern void LibTestInit();

    [DllImport("LibTest")]
    private static extern void LibTestCleanup();

    [DllImport("LibTest")]
    private static extern void LibTestStrCpy(byte[] inStr, byte[] outStr);
    #endregion

    public LibTest() 
    {
        LibTestInit();
    }

    public void Dispose() 
    {
        LibTestCleanup();
    }

    public void StrCpy(byte[] inStr, byte[] outStr) 
    {
        LibTestStrCpy(inStr, outStr);
    }
}

And Form1.cs (with a TextBox control to test the output) shown as below:

m_LibTest1 = new LibTest();
byte[] outStr1 = new byte[1024];
m_LibTest1.StrCpy(System.Text.Encoding.Default.GetBytes("Hello World"), outStr1);
m_LibTest1.Dispose();
m_LibTest1 = null;
textBox.Text += System.Text.Encoding.Default.GetString(outStr1);
MessageBox.Show(System.Text.Encoding.Default.GetString(outStr1));

m_LibTest2 = new LibTest();
byte[] outStr2 = new byte[1024];
m_LibTest2.StrCpy(System.Text.Encoding.Default.GetBytes("Hello Linux"), outStr2);
m_LibTest2.Dispose();
m_LibTest2 = null;
// FIXME: There is no Hello Linux shown in the textBox.Text
textBox.Text += System.Text.Encoding.Default.GetString(outStr2);
// XXX: But shown by MessageBox
MessageBox.Show(System.Text.Encoding.Default.GetString(outStr2));

Expected output: Hello WorldHello Linux
Actual output: Hello World

So Who Moved My "Hello Linux" byte[] ?
Please someone catched the thief who moved my byte[], thanks a lot!

Posted 31-Mar-11 19:53pm

Leslie Zhai

Updated 31-Mar-11 19:56pm

Wendelius

v2

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Wendelius 1-Apr-11 1:56am

Pre tags added

phil.o 27-Mar-15 16:16pm

Not me! I'm innocent, I swear.

4 solutions

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Debojyoti Majumder · Accepted Answer · 2011-03-31T20:16:00

Solution 2

You can use "MarshalAs" in your C# dllimport function.
I think it's a problem related to marshaling only.

Posted 31-Mar-11 20:16pm

Debojyoti Majumder

Comments

Sergey Alexandrovich Kryukov 1-Apr-11 2:29am

No! You could adivice not "calling" but application of attribute MarshalAs, but only one method has parameters.

--SA

Sergey Alexandrovich Kryukov · Accepted Answer · 2011-03-31T20:32:00

Solution 3

The problem looks trivial, but I would need some time to debug it. Right now, I see only one problem here: the type *char by default is marchalled as System.String. Seriosly! See standard marshalling.

—SA

Posted 31-Mar-11 20:32pm

Sergey Alexandrovich Kryukov

Comments

Olivier Levrey 1-Apr-11 4:54am

If OP wants to fill a string, the best option is StringBuilder. See my answer.

Sergey Alexandrovich Kryukov 1-Apr-11 5:07am

Thank you. Yes, I understand. I was just a quick hint.
Let me see your answer...
--SA

Olivier Levrey · Accepted Answer · 2011-03-31T22:52:00

From C# code:
- If you want to pass a string as input parameter, use string type.
- If you want to pass a string and modify its content, then use StringBuilder type.

C#

public class LibTest
{
    //and don't forget to tell how big is your buffer
    [DllImport("LibTest")]
    private static extern void LibTestStrCpy(
       //this in an input string. So pass it as a string
       string inStr,
       //this is an output string. So pass it as StringBuilder
       StringBuilder outStr,
       //this is the size of the output buffer (the capacity of the StringBuilder)
       int maxLength);

    public void Test()
    {
        string inputString = "testing...";
        //create a StringBuilder with enough storage
        StringBuilder str = new StringBuilder(100);
        //call the function
        LibTestStrCpy(inStr, str, str.Capacity);
        //gets the output string
        string outputString = str.ToString();
        ...
    }    
}

And modify your C++ code like that:

C++

class Test
{
...
    void strcpy(const char *in, char *out, int sizeOfOut)
    {
        for (int i = 0; i < sizeOfOut - 1 && *in != '\0'; i++)
            *out++ = *in++;
        *out = '\0';
    }
...
};

DLLEXPORT void LibTestStrCpy(const char *in, char *out, int sizeOfOut)
{
    if (m_Test)
        m_Test->strcpy(in, out, sizeOfOut);
}

Sergey Alexandrovich Kryukov · Accepted Answer · 2011-03-31T20:06:00

Solution 1

You're in real danger! Somebody is moving your bytes. However, it can be "what", not "who"… who knows how to call it? Changes are, SPC will need to capture it.

Please see: [WARNING!] Black Line of Death in windows phone 7.

Be extra careful,

—SA

Posted 31-Mar-11 20:06pm

Sergey Alexandrovich Kryukov

Updated 27-Mar-15 10:06am

v4

Who Moved My byte[] using DllImport?

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