STDC_FIRST_TRAILING_ONE(3)                    FreeBSD Library Functions Manual

NAME

     stdc_first_trailing_one - find the first trailing one in an integer

SYNOPSIS

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

     unsigned int
     stdc_first_trailing_one_uc(unsigned char value);

     unsigned int
     stdc_first_trailing_one_us(unsigned short value);

     unsigned int
     stdc_first_trailing_one_ui(unsigned int value);

     unsigned int
     stdc_first_trailing_one_ul(unsigned long value);

     unsigned int
     stdc_first_trailing_one_ull(unsigned long long value);

     unsigned int
     stdc_first_trailing_one(value);

DESCRIPTION

     The stdc_first_trailing_one_type() family of functions returns the index
     of the least 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_trailing_one(value) type-generic macro picks the appropriate
     stdc_first_trailing_one_type() function based on the type of value.

     Functions stdc_first_trailing_one_ui(), stdc_first_trailing_one_ul(), and
     stdc_first_trailing_one_ull() are identical to 4.3BSD functions ffs(3),
     ffsl(3), and ffsll(3) respectively, except for operating on unsigned
     instead of signed values.

RETURN VALUES

     Returns the index of the least significant set bit in value.  The bits
     are numbered such that the least significant bit has number 1, and the
     most 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), one is
     returned.

SEE ALSO

     bit_ffs(3), ffs(3), ffsl(3), ffsll(3), stdbit(3), stdc_trailing_zeros(3),
     stdc_first_trailing_zero(3), stdc_first_leading_one(3)

STANDARDS

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

HISTORY

     The stdc_first_trailing_one_type() family of functions and the
     stdc_first_trailing_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_TRAILING_ONE(3)