How to divide two 64-bit numbers in Linux Kernel?

Question

Some code that rounds up the division to demonstrate (C-syntax):

#define SINT64 long long int
#define SINT32 long int

SINT64 divRound(SINT64 dividend, SINT6         


        

Answer 1

I don't think (at least can't find a way to make) Chris' answer work in this case because do_div() actually changes the dividend in-place. Getting the absolute value implies a temporary variable whose value will change the way I require but can't be passed out of my __divdi3() override.

I don't see a way around the parameter-by-value signature of __divdi3() at this point except to mimic the technique used by do_div().

It might seem like I'm bending over backwards here and should just come up with an algorithm to do the 64-bit/32-bit division I actually need. The added complication here though is that I have a bunch of numerical code using the '/' operator and would need to go through that code and replace every '/' with my function calls.

I'm getting desperate enough to do just that though.

Thanks for any follow-up, Chad

Answer 2

Here's my really naive solution. Your mileage may vary.

Keep a sign bit, which is sign(dividend) ^ sign(divisor). (Or *, or /, if you're storing your sign as 1 and -1, as opposed to false and true. Basically, negative if either one is negative, positive if none or both are negative.)

Then, call the unsigned division function on the absolute values of both. Then tack the sign back onto the result.

P.S. That is actually how __divdi3 is implemented in libgcc2.c (from GCC 4.2.3, the version that's installed on my Ubuntu system). I just checked. :-)

Answer 3

ldiv ?

Edit: reread title, so you might want to ignore this. Or not, depending on if it has an appropriate non-library version.

Answer 4

This functionality is introduced in /linux/lib/div64.c as early as kernel v2.6.22.