STDC_FIRST_LEADING_ONE(3)                     FreeBSD Library Functions Manual

NAME

     stdc_first_leading_one - find the first leading one in an integer

SYNOPSIS

     /* -lc */
     #include <stdbit.h>

     unsigned int
     stdc_first_leading_one_uc(unsigned char value);

     unsigned int
     stdc_first_leading_one_us(unsigned short value);

     unsigned int
     stdc_first_leading_one_ui(unsigned int value);

     unsigned int
     stdc_first_leading_one_ul(unsigned long value);

     unsigned int
     stdc_first_leading_one_ull(unsigned long long value);

     unsigned int
     stdc_first_leading_one(value);

DESCRIPTION

     The stdc_first_leading_one_type() family of functions returns the index
     of the most significant set bit in value, where value is of type unsigned
     char, unsigned short, unsigned int, unsigned long, or unsigned long long
     for type being uc, us, ui, ul, or ull respectively.  The
     stdc_first_leading_one(value) type-generic macro picks the appropriate
     stdc_first_leading_one_type() function based on the type of value.

RETURN VALUES

     Returns the index of the most significant set bit in value.  The bits are
     numbered such that the most significant bit has number 1, and the least
     significant bit has number w where w is the number of bits in the type of
     value.  If no bits are set in value (i.e. value is zero), zero is
     returned.

SEE ALSO

     fls(3), stdbit(3), stdc_leading_zeros(3), stdc_first_leading_zero(3),
     stdc_first_trailing_one(3)

STANDARDS

     The stdc_first_leading_one_type() family of functions and the
     stdc_first_leading_one() type-generic macro conform to ISO/IEC 9899:2024
     ("ISO C23").

HISTORY

     The stdc_first_leading_one_type() family of functions and the
     stdc_first_leading_one() type-generic macro were added in FreeBSD 15.1.

AUTHOR

     Robert Clausecker <fuz@FreeBSD.org>

FreeBSD 15.1-STABLE-HBSD       November 9, 2025      STDC_FIRST_LEADING_ONE(3)