lib wrapped into dll imported function access memory error

Question

I have a lib file need to use in c# I have wrapped it into dll but when I call one of the function it always show me the error of attempted to read or write protected memory. the function in lib header file is as the following LONG WINAPI mdRandREx( LONG LONG LONG LPVOID LPVOID LONG ); In c++ I wrapped it as the following long MmdRandREx(long pathlong netnolong stnolong dev[]short *buflong bufsize) { buf=(short *)malloc(bufsize); return mdRandREx(pathnetnostnodevbufbufsize); } in c# I import it [DllImport(@dllname CallingConvention = CallingConvention.Cdecl)] static extern long MmdRandREx(long path long netno long stno long[] dev out short buf long bufsize); and call it short buf; long retval=MmdRandREx(path1 stanoPLC dev1out buf bufsize); I need to use this function to read PLC input and another function to write PLC access a PCI card. can any one help me on this? thanks in advance.

Vulpes · Answer

Well one problem I can see is that the long type in C/C++ is a 4 byte integer whereas the long type in C# is an 8 byte integer. So you want to replace long with int on the C# side. The short type is 2 bytes in C/C++/C# so you have the right type there. However as it's a pointer to a buffer I'd have thought that it would be expecting an array of shorts rather than a scalar short (albeit as an 'out' parameter). I'm also a bit worried about the dev parameter. If you pass that as an int array how will the unmanaged function know what its length is? One other possible problem is the calling convention. I see the unmanaged function is using WINAPI (typically StdCall on Windows) whereas you're using Cdecl. For consistency I'd change to Winapi. So taking all that into account and hoping for the best with the int[] I'd try: [DllImport(@dllname CallingConvention = CallingConvention.Winapi)] static extern int MmdRandREx(int path int netno int stno int[] dev [In Out] short[] buf int bufsize); and call with: short[] buf = new short[bufsize/2]; // assuming bufsize is in bytes not elements int retval = MmdRandREx(path 1 stanoPLC dev1 buf bufsize);

Sunny Wang · Answer

thank you very much  problem solved it is the length of the data in c++ different to c#.