Calling a Delphi DLL from C# produces unexpected results

Question

I have a Delphi DLL that I did not write, but need to call from a C# ASP.NET 3.5 app. Here is the function definition I got from the developers:

function Cr         


        

Answer 1

I was messing around the other day trying to learn about calling conventions and I wrote some methods to convert between various ones. Here is one for StdCall->FastCall.

typedef struct
{
    USHORT ParameterOneOffset;  // The offset of the first parameter in dwords starting at one
    USHORT ParameterTwoOffset;  // The offset of the second parmaeter in dwords starting at one
} FastCallParameterInfo;



    __declspec( naked,dllexport ) void __stdcall InvokeFast()
{
    FastCallParameterInfo paramInfo;
    int functionAddress;
    int retAddress;
    int paramOne, paramTwo;
    __asm
    {
        // Pop the return address and parameter info.  Store in memory.
        pop retAddress;
        pop paramInfo;
        pop functionAddress;

        // Check if any parameters should be stored in edx                          
        movzx ecx, paramInfo.ParameterOneOffset;     
        cmp ecx,0;
        je NoRegister;  

        // Calculate the offset for parameter one.
        movzx ecx, paramInfo.ParameterOneOffset;    // Move the parameter one offset to ecx
        dec ecx;                                    // Decrement by 1
        mov eax, 4;                                 // Put 4 in eax
        mul ecx;                                    // Multiple offset by 4

        // Copy the value from the stack on to the register.
        mov ecx, esp;                               // Move the stack pointer to ecx
        add ecx, eax;                               // Subtract the offset.
        mov eax, ecx;                               // Store in eax for later.
        mov ecx, [ecx];                             // Derefernce the value
        mov paramOne, ecx;                          // Store the value in memory.

        // Fix up stack
        add esp,4;                                  // Decrement the stack pointer
        movzx edx, paramInfo.ParameterOneOffset;    // Move the parameter one offset to edx
        dec edx;                                    // Decrement by 1
        cmp edx,0;                                  // Compare offset with zero
        je ParamOneNoShift;                         // If first parameter then no shift.

    ParamOneShiftLoop:
        mov ecx, eax;
        sub ecx, 4;
        mov ecx, [ecx]
        mov [eax], ecx;                             // Copy value over
        sub eax, 4;                                 // Go to next 
        dec edx;                                    // decrement edx
        jnz ParamOneShiftLoop;                      // Loop
    ParamOneNoShift:
        // Check if any parameters should be stored in edx                          
        movzx ecx, paramInfo.ParameterTwoOffset;     
        cmp ecx,0;
        je NoRegister;  

        movzx ecx, paramInfo.ParameterTwoOffset;    // Move the parameter two offset to ecx
        sub ecx, 2;                                 // Increment the offset by two.  One extra for since we already shifted for ecx
        mov eax, 4;                                 // Put 4 in eax
        mul ecx;                                    // Multiple by 4

        // Copy the value from the stack on to the register.
        mov ecx, esp;                               // Move the stack pointer to ecx
        add ecx, eax;                               // Subtract the offset.
        mov eax, ecx;                               // Store in eax for later.
        mov ecx, [ecx];                             // Derefernce the value
        mov paramTwo, ecx;                          // Store the value in memory.           

        // Fix up stack
        add esp,4;                                  // Decrement the stack pointer
        movzx edx, paramInfo.ParameterTwoOffset;    // Move the parameter two offset to ecx
        dec edx;                                    // Decrement by 1
        cmp edx,0;                                  // Compare offset with zero
        je NoRegister;                              // If first parameter then no shift.
    ParamTwoShiftLoop:
        mov ecx, eax;
        sub ecx, 4;
        mov ecx, [ecx]
        mov [eax], ecx;                             // Copy value over
        sub eax, 4;                                 // Go to next 
        dec edx;                                    // decrement edx
        jnz ParamTwoShiftLoop;                      // Loop


    NoRegister:
        mov ecx, paramOne;                          // Copy value from memory to ecx register
        mov edx, paramTwo;                          // 
        push retAddress;
        jmp functionAddress;
    }
}

}