Double return value?

[Farabi]

How to retrieve a qword floating point return value?

double cvGetCaptureProperty( CvCapture* capture, int property_id );

[BogdanOntanu]

It will probably be returned in eax:edx but you will have to check to be sure.

corrected: floating point values will be returned on the FPU stack.

[MichaelW]

Floating-point return values are normally returned on the FPU stack in ST. This might seem like a strange way to return a value, but actually it's very well thought out and flexible. Because the value is stored in the FPU as a REAL10 it retains the maximum possible precision, and the caller can convert the value and store it to memory (as well as perform the essential step of removing it from the FPU stack) as any type that the FPU supports. I don't have OpenCV, so this example uses the CRT sqrt function instead:

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    include \masm32\include\masm32rt.inc
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    .data
      two_r8  REAL8 2.0
      rtn_r8  REAL8 0.0
      rtn_dq  dq 0
      rtn_dd  dd 0
    .code
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start:
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    invoke crt_sqrt, two_r8
    fstp rtn_r8
    invoke crt_printf, chr$("%.15f",10), rtn_r8

    invoke crt_sqrt, two_r8
    fistp rtn_dq
    invoke crt_printf, chr$("%I64d",10), rtn_dq

    invoke crt_sqrt, two_r8
    fistp rtn_dd
    invoke crt_printf, chr$("%d",10,10), rtn_dd

    inkey "Press any key to exit..."
    exit

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end start


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In my quick search, all of the examples that I encountered stored the return value as a 32-bit signed integer. For example:

frame_size.height = (int) cvGetCaptureProperty( . . .

[Farabi]

I tried poping the FPU st0 but the result is 0. Also I load the [eax] value as a qword and it return zero, eax value is 3.

[MichaelW]

Try posting the code that you are using to do the call, get the return value, and display the return value.

[Farabi]

      invoke cvGetCaptureProperty,ftest,CV_CAP_PROP_FRAME_WIDTH
   ;   push eax
   ;   fild dword ptr[esp]
      fstp FPS
   ;   pop eax
invoke FloatToStr2,FPS,addr buff
      invoke lstrlen,addr buff
      invoke TextOut,main_screen.DC,0,45,addr buff,eax