1*d790b16bSRobert Clausecker.\" 2*d790b16bSRobert Clausecker.\" Copyright (c) 2025 Robert Clausecker <fuz@FreeBSD.org> 3*d790b16bSRobert Clausecker.\" 4*d790b16bSRobert Clausecker.\" SPDX-License-Identifier: BSD-2-Clause 5*d790b16bSRobert Clausecker.\" 6*d790b16bSRobert Clausecker.Dd November 9, 2025 7*d790b16bSRobert Clausecker.Dt STDC_FIRST_LEADING_ONE 3 8*d790b16bSRobert Clausecker.Os 9*d790b16bSRobert Clausecker.Sh NAME 10*d790b16bSRobert Clausecker.Nm stdc_first_leading_one 11*d790b16bSRobert Clausecker.Nd find the first leading one in an integer 12*d790b16bSRobert Clausecker.Sh SYNOPSIS 13*d790b16bSRobert Clausecker.Lb libc 14*d790b16bSRobert Clausecker.In stdbit.h 15*d790b16bSRobert Clausecker.Ft unsigned int 16*d790b16bSRobert Clausecker.Fn stdc_first_leading_one_uc "unsigned char value" 17*d790b16bSRobert Clausecker.Ft unsigned int 18*d790b16bSRobert Clausecker.Fn stdc_first_leading_one_us "unsigned short value" 19*d790b16bSRobert Clausecker.Ft unsigned int 20*d790b16bSRobert Clausecker.Fn stdc_first_leading_one_ui "unsigned int value" 21*d790b16bSRobert Clausecker.Ft unsigned int 22*d790b16bSRobert Clausecker.Fn stdc_first_leading_one_ul "unsigned long value" 23*d790b16bSRobert Clausecker.Ft unsigned int 24*d790b16bSRobert Clausecker.Fn stdc_first_leading_one_ull "unsigned long long value" 25*d790b16bSRobert Clausecker.Ft unsigned int 26*d790b16bSRobert Clausecker.Fn stdc_first_leading_one "value" 27*d790b16bSRobert Clausecker.Sh DESCRIPTION 28*d790b16bSRobert ClauseckerThe 29*d790b16bSRobert Clausecker.Nm stdc_first_leading_one_ Ns Em type Ns () 30*d790b16bSRobert Clauseckerfamily of functions returns the index of the most significant set bit in 31*d790b16bSRobert Clausecker.Fa value , 32*d790b16bSRobert Clauseckerwhere 33*d790b16bSRobert Clausecker.Fa value 34*d790b16bSRobert Clauseckeris of type 35*d790b16bSRobert Clausecker.Va unsigned char , 36*d790b16bSRobert Clausecker.Va unsigned short , 37*d790b16bSRobert Clausecker.Va unsigned int , 38*d790b16bSRobert Clausecker.Va unsigned long , 39*d790b16bSRobert Clauseckeror 40*d790b16bSRobert Clausecker.Va unsigned long long 41*d790b16bSRobert Clauseckerfor 42*d790b16bSRobert Clausecker.Em type 43*d790b16bSRobert Clauseckerbeing 44*d790b16bSRobert Clausecker.Sy uc , 45*d790b16bSRobert Clausecker.Sy us , 46*d790b16bSRobert Clausecker.Sy ui , 47*d790b16bSRobert Clausecker.Sy ul , 48*d790b16bSRobert Clauseckeror 49*d790b16bSRobert Clausecker.Sy ull 50*d790b16bSRobert Clauseckerrespectively. 51*d790b16bSRobert ClauseckerThe 52*d790b16bSRobert Clausecker.Fn stdc_first_leading_one "value" 53*d790b16bSRobert Clauseckertype-generic macro picks the appropriate 54*d790b16bSRobert Clausecker.Nm stdc_first_leading_one_ Ns Em type Ns () 55*d790b16bSRobert Clauseckerfunction based on the type of 56*d790b16bSRobert Clausecker.Fa value . 57*d790b16bSRobert Clausecker.Sh RETURN VALUES 58*d790b16bSRobert ClauseckerReturns the index of the most significant set bit in 59*d790b16bSRobert Clausecker.Fa value . 60*d790b16bSRobert ClauseckerThe bits are numbered such that the most significant bit has number\~1, 61*d790b16bSRobert Clauseckerand the least significant bit has number 62*d790b16bSRobert Clausecker.Ms w 63*d790b16bSRobert Clauseckerwhere 64*d790b16bSRobert Clausecker.Ms w 65*d790b16bSRobert Clauseckeris the number of bits in the type of 66*d790b16bSRobert Clausecker.Fa value . 67*d790b16bSRobert ClauseckerIf no bits are set in 68*d790b16bSRobert Clausecker.Fa value 69*d790b16bSRobert Clausecker(i.\^e.\& 70*d790b16bSRobert Clausecker.Fa value 71*d790b16bSRobert Clauseckeris zero), zero is returned. 72*d790b16bSRobert Clausecker.Sh SEE ALSO 73*d790b16bSRobert Clausecker.Xr fls 3 , 74*d790b16bSRobert Clausecker.Xr stdbit 3 , 75*d790b16bSRobert Clausecker.Xr stdc_leading_zeros 3 , 76*d790b16bSRobert Clausecker.Xr stdc_first_leading_zero 3 , 77*d790b16bSRobert Clausecker.Xr stdc_first_trailing_one 3 78*d790b16bSRobert Clausecker.Sh STANDARDS 79*d790b16bSRobert ClauseckerThe 80*d790b16bSRobert Clausecker.Nm stdc_first_leading_one_ Ns Em type Ns () 81*d790b16bSRobert Clauseckerfamily of functions and the 82*d790b16bSRobert Clausecker.Fn stdc_first_leading_one 83*d790b16bSRobert Clauseckertype-generic macro conform to 84*d790b16bSRobert Clausecker.St -isoC-2023 . 85*d790b16bSRobert Clausecker.Sh HISTORY 86*d790b16bSRobert ClauseckerThe 87*d790b16bSRobert Clausecker.Nm stdc_first_leading_one_ Ns Em type Ns () 88*d790b16bSRobert Clauseckerfamily of functions and the 89*d790b16bSRobert Clausecker.Fn stdc_first_leading_one 90*d790b16bSRobert Clauseckertype-generic macro were added in 91*d790b16bSRobert Clausecker.Fx 15.1. 92*d790b16bSRobert Clausecker.Sh AUTHOR 93*d790b16bSRobert Clausecker.An Robert Clausecker Aq Mt fuz@FreeBSD.org 94