NAME

stdc_first_leading_one, stdc_first_leading_one_uc, stdc_first_leading_one_us, stdc_first_leading_one_ui, stdc_first_leading_one_ul, stdc_first_leading_one_ull — find index of most significant one bit

LIBRARY

Standard C Library (libc, -lc)

SYNOPSIS

#include <stdbit.h>

unsigned int
stdc_first_leading_one(generic_value_type value);

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);

DESCRIPTION

The stdc_first_leading_one() family of functions returns the 1s-based index of the first one bit in value starting at the most significant bit. If there is no one bit in value then zero is returned.

The stdc_first_leading_one() 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.

The way that the index is constructed is not necessarily intuitive. The C standard counts the most significant index starting with the most significant bit as index value 0. Consider the 16-bit value 0x952b. Generally we would consider the value ‘b’ as bits 0 to 3 while the value ‘9’ as bits 12 to 15. Bit 15 is actually most significant index 0. Bit 14, most significant index 1. Bit 0, most significant index 15. This example, 0x952b, would return the value 1 (when using the generic or unsigned short form) as the function is defined to return this particular index plus one. Zero is reserved for when there is no leading zero bit at all.

Note that if a non-zero unsigned integer is promoted, it will always be filled with leading zeros which will cause the returned value to increase as the first one bit is further away from the most significant bit.

While this is similar in function to the fls(3C) functions which find the last set value and identify the same bits, the fls(3C) functions determine the index starting from the least significant bit, instead of the most significant bit.

RETURN VALUES

The functions in the stdc_first_leading_one() family always return the most significant index of the first leading one bit in value, plus one. Otherwise, if there are no one bits in value, 0 will be returned. These functions cannot fail.

EXAMPLES

Example 1 Printing the index of the first leading one (plus one).

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

int
main(void)
{
	printf("0x%x 0x%x 0x%x 0x%x\n",
	    stdc_first_leading_one_uc(0x7f),
	    stdc_first_leading_one_us(0x0000),
	    stdc_first_leading_one_ui(UINT32_MAX),
	    stdc_first_leading_one_ull(0x000fedcba9abcdef));
	return (0);
}

When compiled and run, this produces:

$ ./a.out
0x2 0x0 0x1 0xd

INTERFACE STABILITY

Committed

MT-LEVEL

Async-Signal-Safe