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.Dd October 27, 2024
.Dt STDC_BIT_FLOOR 3C
.Os
.Sh NAME
.Nm stdc_bit_floor ,
.Nm stdc_bit_floor_uc ,
.Nm stdc_bit_floor_us ,
.Nm stdc_bit_floor_ui ,
.Nm stdc_bit_floor_ul ,
.Nm stdc_bit_floor_ull
.Nd find largest power of 2 smaller than value
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In stdbit.h
.Ft generic_value_type
.Fo stdc_bit_floor
.Fa "generic_value_type value"
.Fc
.Ft "unsigned char"
.Fo stdc_bit_floor_uc
.Fa "unsigned char value"
.Fc
.Ft "unsigned short"
.Fo stdc_bit_floor_us
.Fa "unsigned short value"
.Fc
.Ft "unsigned int"
.Fo stdc_bit_floor_ui
.Fa "unsigned int value"
.Fc
.Ft "unsigned long"
.Fo stdc_bit_floor_ul
.Fa "unsigned long value"
.Fc
.Ft "unsigned long long"
.Fo stdc_bit_floor_ull
.Fa "unsigned long long value"
.Fc
.Sh DESCRIPTION
The
.Fn stdc_bit_floor
family of functions determine the largest power of 2 that is not greater
than
.Fa value .
If
.Fa value
is 0, then the function returns 0.
.Pp
The
.Fn stdc_bit_floor
function is generic and will operate on all 8, 16, 32, and 64-bit
unsigned integers; however, it is only available in C23.
The other functions all operate on a specific integer type, but
otherwise behave the same and are available regardless of the C language
version.
.Sh RETURN VALUES
The functions in the
.Fn stdc_bit_floor
family returns the largest power of 2 that is not greater than
.Fa value .
These functions cannot fail.
.Sh EXAMPLES
.Sy Example 1
Printing the bit floor of an integer.
.Bd -literal
#include <stdbit.h>
#include <stdio.h>
#include <limits.h>

int
main(void)
{
	printf("0x%x 0x%x 0x%x 0x%llx\en",
	    stdc_bit_floor_uc(0x2b),
	    stdc_bit_floor_us(0x1000),
	    stdc_bit_floor_ui(UINT32_MAX),
	    stdc_bit_floor_ull(0));
	return (0);
}
.Ed
.Pp
When compiled and run, this produces:
.Bd -literal -offset indent
$ ./a.out
0x20 0x1000 0x80000000 0x0
.Ed
.Sh INTERFACE STABILITY
.Sy Committed
.Sh MT-LEVEL
.Sy Async-Signal-Safe
.Sh SEE ALSO
.Xr stdc_bit_ceil 3C ,
.Xr stdc_bit_width 3C ,
.Xr stdc_count_ones 3C ,
.Xr stdc_count_zero 3C ,
.Xr stdc_first_leading_one 3C ,
.Xr stdc_first_leading_zero 3C ,
.Xr stdc_first_trailing_one 3C ,
.Xr stdc_first_trailing_zero 3C ,
.Xr stdc_has_single_bit 3C ,
.Xr stdc_leading_ones 3C ,
.Xr stdc_leading_zeros 3C ,
.Xr stdc_trailing_ones 3C ,
.Xr stdc_trailing_zeros 3C ,
.Xr stdbit.h 3HEAD