Passing pointer from C# to C++

Question

I am trying to pass a 2D mask (all 0s, expect for a region of interest as 1s) from C# ( as short[]) to C++ (as unsigned short*), but I cannot get the right value in C++.

C#

[DllImport("StatsManager.dll", EntryPoint = "SetStatsMask")]
private static extern int SetStatsMask(IntPtr mask, int imgWidth, int imgHeight);

short[] mask;
mask = new short[8*8];
// some operation here making a ROI in mask all 1.  ex 0000111100000000 in 1D 
IntPtr maskPtr = Marshal.AllocHGlobal(2 * mask.Length);
Marshal.Copy(mask, 0, maskPtr, mask.Length);
SetStatsMask(maskPtr, width, height);

C++

long StatsManager::SetStatsMask(unsigned short *mask, long width, long height)
{
    //create memory to store the incoming mask
    //memcpy the mask to the new buffer 
    //pMask = realloc(pMask,width*height*sizeof(unsigned short));

    long ret = TRUE;

    if (NULL == _pMask)
    {
        _pMask = new unsigned short[width * height];
    }
    else
    {
        realloc(_pMask,width*height*sizeof(unsigned short));
    }

    memcpy(mask,_pMask,width*height*sizeof(unsigned short));

    SaveBuffer(_pMask,  width,  height);

    return ret;
}

But all I can see for mask in C++ using watch window is 52536 instead of 0000111100000000, so I am wondering where I messed up? Anyone can help? Thanks.

You'd be better off declaring the mask parameter in C# to accept a ushort[] -- the array will be marshaled automatically to unsigned short *. — cdhowie
– cdhowie, Commented Sep 8, 2014 at 22:12
Both the array you are using, and the IntPtr parameter should be changed to ushort[], then just pass your C# array directly. The P/Invoke layer will marshal it to a C-style array for you. You do not need to use any of the Marshal methods you are using. — cdhowie
– cdhowie, Commented Sep 8, 2014 at 22:15
Correct, nor AllocHGlobal(). Just pass in the array directly. SetStatsMask(mask, width, height);. (BTW this will not fix your problem, but frees you of the burden to get the array marshal code correct, including freeing your allocation. Let the P/Invoke layer do this for you!) — cdhowie
– cdhowie, Commented Sep 8, 2014 at 22:20

Anton Savin · Accepted Answer · 2014-09-08 22:17:31Z

2

I believe you misplaced the parameters of memcpy:

memcpy(mask,_pMask,width*height*sizeof(unsigned short));

As I understand you want to copy from mask to _pMask, so you should write:

memcpy(_pMask, mask, width*height*sizeof(unsigned short));

answered Sep 8, 2014 at 22:17

Anton Savin

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1 Comment

Ono Over a year ago

Ha, how can I miss that! Thank you, you are awesome!

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