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stdc_leading_zeros

From cppreference.net
C
Numerics
Bit manipulation
Defined in header <stdbit.h>
unsigned int stdc_leading_zeros_uc ( unsigned char value ) [ [ unsequenced ] ] ;
(1) (since C23)
unsigned int stdc_leading_zeros_us ( unsigned short value ) [ [ unsequenced ] ] ;
(2) (since C23)
unsigned int stdc_leading_zeros_ui ( unsigned int value ) [ [ unsequenced ] ] ;
(3) (since C23)
unsigned int stdc_leading_zeros_ul ( unsigned long int value ) [ [ unsequenced ] ] ;
(4) (since C23)
unsigned int stdc_leading_zeros_ull ( unsigned long long int value ) [ [ unsequenced ] ] ;
(5) (since C23)
#define stdc_leading_zeros( value )

// exposed interface:

generic_return_type stdc_leading_zeros ( generic_value_type value ) [ [ unsequenced ] ] ;
(6) (since C23)
1-5) Returns the number of consecutive ​ 0 ​ bits in the value , starting from the most significant bit.
6) The type-generic function (marked by its generic_value_type argument) returns the appropriate value based on the type of the input value, so long as it is a:
  • standard unsigned integer type, excluding bool ;
  • extended unsigned integer type;
  • or, bit-precise unsigned integer type whose width matches a standard or extended integer type, excluding bool .
The generic_return_type shall be a suitable large unsigned integer type capable of representing the computed result.

Contents

Parameters

value - value of unsigned integer type

Return value

The number of consecutive ​ 0 ​ bits in the value , starting from the most significant bit.

Example

Run this code 
#include <limits.h>
#include <stdbit.h>
#include <stdint.h>
#include <stdio.h>
#define bits_num(value) (sizeof(value) * CHAR_BIT)
#define bin_impl(T, suffix) \
const char* bin_##suffix(T x) \
{ \
    static char buf[bits_num(x) * CHAR_BIT + 1]; \
    for (T i = 0, mask = ((T)1 << (bits_num(x) - 1)); mask; mask >>= 1) \
        buf[i++] = x & mask ? '1' : '0'; \
    buf[bits_num(x)] = '\0'; \
    return buf; \
}
bin_impl(uint8_t, u8)
bin_impl(uint16_t, u16)
bin_impl(uint32_t, u32)
bin_impl(uint64_t, u64)
#define bin(x) _Generic((x), \
    uint8_t: bin_u8, uint16_t: bin_u16, uint32_t: bin_u32, default: bin_u64)(x)
int main()
{
    puts("uint8_t:");
    for (uint8_t x = 0b11000000; ; x >>= 1)
    {
        printf("x = [%s], leading zeros: %d\n", bin(x), stdc_leading_zeros(x));
        if (!x)
            break;
    }
    puts("uint16_t:");
    for (uint16_t x = 0b11000000; ; x >>= 1)
    {
        printf("x = [%s], leading zeros: %d\n", bin(x), stdc_leading_zeros(x));
        if (!x)
            break;
    }
}

Output: 

uint8_t:
x = [11000000], leading zeros: 0
x = [01100000], leading zeros: 1
x = [00110000], leading zeros: 2
x = [00011000], leading zeros: 3
x = [00001100], leading zeros: 4
x = [00000110], leading zeros: 5
x = [00000011], leading zeros: 6
x = [00000001], leading zeros: 7
x = [00000000], leading zeros: 8
uint16_t:
x = [0000000011000000], leading zeros: 8
x = [0000000001100000], leading zeros: 9
x = [0000000000110000], leading zeros: 10
x = [0000000000011000], leading zeros: 11
x = [0000000000001100], leading zeros: 12
x = [0000000000000110], leading zeros: 13
x = [0000000000000011], leading zeros: 14
x = [0000000000000001], leading zeros: 15
x = [0000000000000000], leading zeros: 16

See also

(C23)
finds the first position of ​ 0 ​ bit, starting from the most significant bit
(type-generic function macro)
(C23)
counts the number of ​ 0 ​ bits in an unsigned integer
(type-generic function macro)
(C23)
counts the number of consecutive 1 bits, starting from the most significant bit
(type-generic function macro)
C++ documentation for countl_zero