NAME

stdc_leading_ones, stdc_leading_ones_uc, stdc_leading_ones_us, stdc_leading_ones_ui, stdc_leading_ones_ul, stdc_leading_ones_ull — count consecutive leading one bits

LIBRARY

Standard C Library (libc, -lc)

SYNOPSIS

#include <stdbit.h>

unsigned int
stdc_leading_ones(generic_value_type value);

unsigned int
stdc_leading_ones_uc(unsigned char value);

unsigned int
stdc_leading_ones_us(unsigned short value);

unsigned int
stdc_leading_ones_ui(unsigned int value);

unsigned int
stdc_leading_ones_ul(unsigned long value);

unsigned int
stdc_leading_ones_ull(unsigned long long value);

DESCRIPTION

The stdc_leading_ones() family of functions counts the number of consecutive one bits present in value starting at the most significant bit.

The stdc_leading_ones() 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_leading_ones() family always return the number of leading ones found in value. These functions cannot fail.

EXAMPLES

Example 1 Printing the number of leading zeros.

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

int
main(void)
{
	printf("0x%x 0x%x 0x%x 0x%x\n", stdc_leading_ones_uc(0xf6),
	    stdc_leading_ones_us(0x9009), stdc_leading_ones_ui(UINT32_MAX),
	    stdc_leading_ones_ull(0));
	return (0);
}

When compiled and run, this produces:

$ ./a.out
0x4 0x1 0x20 0x0

INTERFACE STABILITY

Committed

MT-LEVEL

Async-Signal-Safe