Thread: I need a (64 bit * 64 bit) / 64 bit calculation function.

I'm currently using the Windows MulDiv function to do what I need but apparently, it only works for 32 bit systems.

I'm trying to convert the size points from the screen to any given printer.
It'll run on a 64 bit system.

Here's the code using MulDiv

Code:

long nPts = MulDiv(12, GetDeviceCaps(resource->dc, LOGPIXELSY), 72)

72 points = 1 inch.

This gives me the height of the screen's 12 points as the printer sees it.
This only works for 32 bit systems.

I need the same function, but for 64 bit computers.
From what I gather, the problem is 64bit * 64bit = 128bit which could produce an overflow.

I found a MulDiv64 library but can't make it compile. It compiles the files but if my code tries to call MulDiv64 it throws a compile error 'external symbol MulDiv64 unresolved'.

I'm compiling a php7 extension, written in c. I'm using VC 2015 and a php compile nmake.

If I could just make the MulDiv64 library work that might suffice.

Help would be greatly appreciated.

algorithm - Multiplication of very long integers - Stack Overflow

Thanks Salem. Yea I'm thinking I've started chasing a wild goose here.

I'm trying to print text using Windows GDI.
I need to specify the font size in points (ie Arial 12 point). The GDI prints it in 'logical units'.
So far, the font is too small.

Microsoft's documentation on their GDI CreateFont function suggests that we use:
nHeight = -MulDiv(PointSize, GetDeviceCaps(hDC, LOGPIXELSY), 72); The font width is determined by the default, given Y for the given font.
The explanation for MulDiv is "Multiplies two 32-bit values and then divides the 64-bit result by a third 32-bit value. "
Since the explanation for MulDiv indicates 32 bit values, I thought maybe the problem is that I'm on a 64 bit machine. I think I was wrong about that.

I just learned that font 'cell' height is used if the CreateFont function is passed a positive number for height.
If a negative number is passed, it translates it to 'absolute' value and returns the actual font height.
nHeight = -MulDiv(PointSize, GetDeviceCaps(hDC, LOGPIXELSY), 72);
My code didn't include the - so I'll add that and see if it works.