Question

First, please forgive me if I say something silly, I am no IT guy but electronics engineer, but I have been assigned to something that needs more skills that I do.

I need to write and read physical sectors in a SD card. I have accomplished it in C++, but the main application is written in C#, so I thought that it was a good moment to write my first dll.

Here is the code working in c++ for writing a sector.

private: System::Void button4_Click(System::Object^  sender, System::EventArgs^  e) {
HANDLE hFile   = INVALID_HANDLE_VALUE;
    BOOL fSuccess  = FALSE;


    DWORD dwBytesWritten = 0; 

    unsigned char  chBuffer[SIZE]; 

    long SectorActual = 39;
    long PosicionInicio = SectorActual * 512;

        for (int i=0; i<SIZE; i++) // Garbage values to be written
        {
            chBuffer[i]= i % 16;
            if((i/16)%2==0)
            {
                chBuffer[i]= 15 - chBuffer[i];
            }
        }
        hFile = CreateFileA("\\\\.\\PhysicalDrive5",GENERIC_READ|GENERIC_WRITE,0,NULL,OPEN_EXISTING,FILE_ATTRIBUTE_NORMAL,NULL);

        if (hFile == INVALID_HANDLE_VALUE) 
        { 
            textBox1->Text += "Could not open file (error " + GetLastError() + ") \r\n";
            return; 
        }

        DWORD dwPtr = SetFilePointer( hFile, 
                                PosicionInicio, 
                                NULL, 
                                FILE_BEGIN ); 

        if (dwPtr == INVALID_SET_FILE_POINTER) // Test for failure
        { 
            textBox1->Text += "Error moving pointer (error " + GetLastError() + ") \r\n";
            return;     // Deal with failure 
        } // End of error handler 

        fSuccess = WriteFile(hFile, chBuffer, TAMANYO_SECTOR, &dwBytesWritten, NULL); 
        if (!fSuccess) 
        {
            textBox1->Text += "WriteFile failed\r\n";
        }
        else
        {
            textBox1->Text += "WriteFile wrote " + dwBytesWritten.ToString() + " bytes\r\n";
        }

        CloseHandle(hFile);
    }

Then I made the DLL. Function is this:

 int AccesoBajoNivel::EscribeBloqueFisico(char numeroDisco, unsigned char * buffer, long      BytesQueEscribir, long posicion_inicio)
{
    HANDLE hFile   = INVALID_HANDLE_VALUE;
            BOOL fSuccess  = FALSE;
    DWORD dwBytesWritten = 0; 

    char ArrayDeChars[] = "\\\\.\\PhysicalDrive5";
            ArrayDeChars[17] = numeroDisco;
    LPCSTR pathAlDisco = ArrayDeChars;

    hFile = CreateFileA(pathAlDisco,GENERIC_READ|GENERIC_WRITE,0,NULL,OPEN_EXISTING,FILE_ATTRIBUTE_NORMAL,NULL);

    if (hFile == INVALID_HANDLE_VALUE) 
    { 
        //printf("Could not open file (error %d)\n", GetLastError());
        CloseHandle(hFile);
        return -1; 
    }

    DWORD dwPtr = SetFilePointer( hFile, 
                            posicion_inicio, 
                            NULL, 
                            FILE_BEGIN ); 

    if (dwPtr == INVALID_SET_FILE_POINTER) // Test for failure
    { 
        CloseHandle(hFile);
        return -2;     // Deal with failure 
    } // End of error handler 

    fSuccess = WriteFile(hFile, buffer, BytesQueEscribir, &dwBytesWritten, NULL); 
    if (!fSuccess) 
    {
        CloseHandle(hFile);
        return -3;
    }
    else
    {
        CloseHandle(hFile);
        //return dwBytesWritten;
        return dwPtr;
    }

    CloseHandle(hFile);
}

Then I imported in the C# project:

[DllImport("AccesoBajoNivel.dll", CallingConvention = CallingConvention.Cdecl)]
static extern int EscribeBloqueFisico(char numeroDisco, byte[] buffer, long BytesQueEscribir, long posicion_inicio);

Then I call the function:

int ResultEscribir = EscribeBloqueFisico(numeroDiscoFisico, BufferEscritura, 512, SectorEscribir * 512);

Always writes to the first sector, no matter the value of “posicion_inicio” (Is the start offset, sorry about the spanish tags) So that’s why I changed the writing function in the api to return dwPtr (Result of SetFilePointer) instead of bytes written. This always seems to be zero, which is obviously not what I want. But can’t find why the first function works and the dll call don’t.

Maybe is something that should be seen at first glance, but as I said, I am electronics guy, not used to this kind of programming…

Other functions (As reading DISK_GEOMETRY or VOLUME_DISK_EXTENTS) work, which I use to determine which Physical Drive is a given logical unit. As I say, also, writing and reading work well, it’s just that I am always pointing to sector zero…

Thanks in advance. Also, this is the first time I post here. Readed many times (That is why I chosed to ask here) but if I made some mistake when posting the question, please point it out.

Answer 1

A long in C++ is 32-bits. Which makes it an int in C#. Replace long with int in the pinvoke declaration.

You should have gotten a PInvokeStackImbalance warning about that. Be sure the re-enable that warning if you turned it off. And it would have been easy to see that the value for *posicion_inicio* is wrong in the debugger. Be sure to enable unmanaged debugging, Project + Properties, Debug tab. Now you can set a breakpoint in your C++ code.