NAME

stdc_first_trailing_zero, stdc_first_trailing_zero_uc, stdc_first_trailing_zero_us, stdc_first_trailing_zero_ui, stdc_first_trailing_zero_ul, stdc_first_trailing_zero_ull — find index of least significant zero bit

LIBRARY

Standard C Library (libc, -lc)

SYNOPSIS

#include <stdbit.h>

unsigned int
stdc_first_trailing_zero(generic_value_type value);

unsigned int
stdc_first_trailing_zero_uc(unsigned char value);

unsigned int
stdc_first_trailing_zero_us(unsigned short value);

unsigned int
stdc_first_trailing_zero_ui(unsigned int value);

unsigned int
stdc_first_trailing_zero_ul(unsigned long value);

unsigned int
stdc_first_trailing_zero_ull(unsigned long long value);

DESCRIPTION

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

The stdc_first_trailing_zero() 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_first_trailing_zero() family always return the index of the first trailing zero bit in value plus one. Otherwise, if there are no zero bits in value, 0 will be returned. These functions cannot fail.

EXAMPLES

Example 1 Printing the index of the first trailing zero.

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

int
main(void)
{
	printf("0x%x 0x%x 0x%x 0x%x\n",
	    stdc_first_trailing_zero_uc(0xef),
	    stdc_first_trailing_zero_us(0x07ff),
	    stdc_first_trailing_zero_ui(UINT32_MAX),
	    stdc_first_trailing_zero_ull(0x7777777777777777));
	return (0);
}

When compiled and run, this produces:

$ ./a.out
0x5 0xc 0x0 0x4

INTERFACE STABILITY

Committed

MT-LEVEL

Async-Signal-Safe