How to use Logarithmic (log) function in the kernel code?

[walking]

I am writing a modul which need be integrated to the kernel.

In the module, I need the value of log (x). Usually in the applications,
I just need include math.h header file. However, I don't know how to
do it in the kernel. It said:

unresolved symbol log


How to use log function under the kernel?

Thanks a lot!

[lakerdonald]

The Kernel does not provide support for floating point operations, and log tends to be a floating point operation. (Very rarely is the result of a logarithm an integer). For this reason, the only way that you'd be able to implement a logarithm that I know of would be to write your own version, bearing in mind that you'd only be dealing with a granularity of one (that is, it could only work on integers, and fp is not/never has been/never will be supported by the Linux Kernel).

[walking]

The Kernel does not provide support for floating point operations, and log tends to be a floating point operation. (Very rarely is the result of a logarithm an integer). For this reason, the only way that you'd be able to implement a logarithm that I know of would be to write your own version, bearing in mind that you'd only be dealing with a granularity of one (that is, it could only work on integers, and fp is not/never has been/never will be supported by the Linux Kernel).

Thanks a lot for your reply!

[Dolda2000]

Indeed, it is as lakerdonald says that kernel mode code cannot use floating point code (since the kernel doesn't save the FPU state, that would disrupt user-space). However, it's more than well possible to implement a logarithmic algorithm with finer granularity than integral numbers without resorting to floating point support by using fixed point fractional numbers.

[lakerdonald]

True. I suppose you could also implement a convoluted floating point type, and then write a bunch of drop-in replacement functions that calculate integers and then shift accordingly. It would be woefully inefficient, so I'd recommend Dolda's method.