AccessViolationException in PInvoke function call

Question

I'm trying to write a wrapper for C library but I'm really struggling with this error.

I tried many approaches, here is one of them:

    [DllImport(DRIVER_FILENAME)]
    [return: MarshalAs(UnmanagedType.U4)]
    private static extern uint GetData(IntPtr handle,
        [MarshalAs(UnmanagedType.LPArray), In()] int[] buffer,
        [MarshalAs(UnmanagedType.U4)] uint size);

Here is function GetData from the library documentation:

LONG GetData( 
  IN HANDLE Handle,
  OUT PULONG Buffer, 
  IN ULONG Size
);

Function returns continuous data (about 16KB/s) in buffer which size is given in bytes. Buffer is int[16384]. My code looks like this:

public static uint GetLibData(IntPtr handle, int[] buffer, uint size)
    {
        size *= 4;            
        uint sizeRead = GetData(handle, buffer, size);

        sizeRead /= 4;

        return sizeRead;
    }

Problematic argument is buffer, I tried manage it previously in other ways, such as IntPtr bufferPtr and then allocating memory by Marshal.AllocHGlobal but I was getting the same error:

Attempted to read or write protected memory. This is often an indication that other memory is corrupt.

How to correctly invoke this function?

You didn't specify the CallingConvention, good odds it is Cdecl. — Hans Passant, Jan 18 '13 at 14:18
@HansPassant: thanks. I've just tried all possible coventions but none of them changed this error ("_Attempted to read or write protected memory. This is often an indication that other memory is corrupt._") — msu, Jan 18 '13 at 17:54
You'll need to start debugging the C code to narrow down why it crashed. If you don't have it then ask for help from the owner. — Hans Passant, Jan 18 '13 at 17:57

David Heffernan · Accepted Answer · 2013-01-18T23:36:46.020

The appropriate p/invoke declaration is

[DllImport(DRIVER_FILENAME)]
private static extern uint GetData(
    IntPtr handle,
    [Out] uint[] buffer,
    uint size
);

It is your responsibility to allocate the buffer before you call the function:

uint[] buffer = new uint[16384];
uint bufferSize = buffer.Length*Marshal.SizeOf(typeof(uint));
uint sizeRead = GetData(handle, buffer, bufferSize);
uint lenRead = sizeRead/Marshal.SizeOf(typeof(uint));

The only thing that's not 100% clear is the calling convention. I'd guess that this library uses cdecl which would mean your DllImport should be

[DllImport(DRIVER_FILENAME, CallingConvention=CallingConvention.Cdecl)]

score 0 · Answer 2 · answered Jan 18 '13 at 18:17

0

Try with the following PInvoke:

[DllImport(DRIVER_FILENAME)]
private static extern Int32 GetData
(
    [In] IntPtr handle,
    [Out] out IntPtr buffer,
    [In] UInt32 size
);

answered Jan 18 '13 at 18:17

Tommaso Belluzzo

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The second parameter is `PULONG`. That's a point to `uint` passed by value. So, one fewer levels of indirection than in your answer. – David Heffernan Jan 18 '13 at 20:50

AccessViolationException in PInvoke function call

2 Answers2