NAME

stdc_first_trailing_one, stdc_first_trailing_one_uc, stdc_first_trailing_one_us, stdc_first_trailing_one_ui, stdc_first_trailing_one_ul, stdc_first_trailing_one_ull — find index of least significant one bit

LIBRARY

Standard C Library (libc, -lc)

SYNOPSIS

#include <stdbit.h>

unsigned int
stdc_first_trailing_one(generic_value_type value);

unsigned int
stdc_first_trailing_one_uc(unsigned char value);

unsigned int
stdc_first_trailing_one_us(unsigned short value);

unsigned int
stdc_first_trailing_one_ui(unsigned int value);

unsigned int
stdc_first_trailing_one_ul(unsigned long value);

unsigned int
stdc_first_trailing_one_ull(unsigned long long value);

DESCRIPTION

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

These functions are sometimes called “find first set” and signed equivalents exist with ffs(3C).

The stdc_first_trailing_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.

RETURN VALUES

The functions in the stdc_first_trailing_one() family always return the index of the first trailing 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 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_one_uc(0x2b),
	    stdc_first_trailing_one_us(0x8000),
	    stdc_first_trailing_one_ui(UINT32_MAX),
	    stdc_first_trailing_one_ull(0xff60000000000000));
	return (0);
}

When compiled and run, this produces:

$ ./a.out
0x1 0x10 0x1 0x36

INTERFACE STABILITY

Committed

MT-LEVEL

Async-Signal-Safe