NAME

stdc_bit_floor, stdc_bit_floor_uc, stdc_bit_floor_us, stdc_bit_floor_ui, stdc_bit_floor_ul, stdc_bit_floor_ull — find largest power of 2 smaller than value

LIBRARY

Standard C Library (libc, -lc)

SYNOPSIS

#include <stdbit.h>

generic_value_type
stdc_bit_floor(generic_value_type value);

unsigned char
stdc_bit_floor_uc(unsigned char value);

unsigned short
stdc_bit_floor_us(unsigned short value);

unsigned int
stdc_bit_floor_ui(unsigned int value);

unsigned long
stdc_bit_floor_ul(unsigned long value);

unsigned long long
stdc_bit_floor_ull(unsigned long long value);

DESCRIPTION

The stdc_bit_floor() family of functions determine the largest power of 2 that is not greater than value. If value is 0, then the function returns 0.

The 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.

RETURN VALUES

The functions in the stdc_bit_floor() family returns the largest power of 2 that is not greater than value. These functions cannot fail.

EXAMPLES

Example 1 Printing the bit floor of an integer.

#include <stdbit.h>
#include <stdio.h>
#include <limits.h>

int
main(void)
{
	printf("0x%x 0x%x 0x%x 0x%llx\n",
	    stdc_bit_floor_uc(0x2b),
	    stdc_bit_floor_us(0x1000),
	    stdc_bit_floor_ui(UINT32_MAX),
	    stdc_bit_floor_ull(0));
	return (0);
}

When compiled and run, this produces:

$ ./a.out
0x20 0x1000 0x80000000 0x0

INTERFACE STABILITY

Committed

MT-LEVEL

Async-Signal-Safe