PRB: DLL Function Returns Float or Double Value Incorrectly (86081)

MORE INFORMATION

The DLL and any application that calls the DLL each have separate floating-point accumulators. When an application calls a DLL function declared with the Pascal calling convention, the application allocates space on the stack to receive the returned data type. The DLL function pushes the value onto the stack for the application to use.

When an application calls a DLL function that uses the C calling convention, no stack space is allocated because the calling function cleans up the stack. Under the C calling convention, the DLL function returns a pointer (in DX:AX) to the floating-point accumulator, which contains the result. However, once the application regains control, the pointer is not valid.

The code examples below demonstrate returning a float value under the C and Pascal calling conventions:

C Calling Convention

// C calling convention - DLL
// Compile options required: /Asw /G2sw /Zp

HANDLE _far floatcalc(float fl1, float fl2);

HANDLE _far floatcalc(float fl1, float fl2)
{
   HANDLE hFloat;
   float _far *pFloat;

   hFloat = GlobalAlloc(GMEM_MOVEABLE, sizeof(float));
   pFloat = (float _far *)GlobalLock(hFloat);
   *pFloat = fl1 * fl2;
   GlobalUnlock(hFloat);
   return hFloat;
}

// C calling convention - Application
// Compile options required: /AS /G2sw /Zp

extern HANDLE _far floatcalc(float fl1, float  fl2);

void Calc(void)
{
   float _far *pFloat, fl;
   HANDLE lFloat;

   lFloat = floatcalc((float)3.0, (float)4.1);  // Call DLL function
   pFloat = (float _far *)GlobalLock(lFloat);
   fl = *pFloat;
   GlobalFree(lFloat);
}
				

Pascal Calling Convention

// Pascal calling convention - DLL
// Compile options required: /Asw /G2sw /Zp

float _far _pascal FloatCalc(float fl1, float fl2);

float _far _pascal FloatCalc(float fl1, float fl2)
{
return fl1 * fl2;
}

// Pascal calling convention - Application
// Compile options required: /G2sw /Zp

extern float _far _pascal FloatCalc(float fl1, float fl2);

void Calc(void)
{
   float temp;

   temp = FloatCalc((float)3.1, (float)4.2);  // Call DLL function
}