fls64(9) centos man page | unix.com

FLS64(9)						     Basic C Library Functions							  FLS64(9)

NAME
       fls64 - find last set bit in a 64-bit word

SYNOPSIS
       int fls64(__u64 x);

ARGUMENTS
       x
	   the word to search

DESCRIPTION
       This is defined in a similar way as the libc and compiler builtin ffsll, but returns the position of the most significant set bit.

       fls64(value) returns 0 if value is 0 or the position of the last set bit if value is nonzero. The last (most significant) bit is at
       position 64.

COPYRIGHT
Kernel Hackers Manual 3.10					     June 2014								  FLS64(9)