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.Dd October 27, 2024
.Dt STDC_LEADING_ONES 3C
.Os
.Sh NAME
.Nm stdc_leading_ones ,
.Nm stdc_leading_ones_uc ,
.Nm stdc_leading_ones_us ,
.Nm stdc_leading_ones_ui ,
.Nm stdc_leading_ones_ul ,
.Nm stdc_leading_ones_ull
.Nd count consecutive leading one bits
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In stdbit.h
.Ft "unsigned int"
.Fo stdc_leading_ones
.Fa "generic_value_type value"
.Fc
.Ft "unsigned int"
.Fo stdc_leading_ones_uc
.Fa "unsigned char value"
.Fc
.Ft "unsigned int"
.Fo stdc_leading_ones_us
.Fa "unsigned short value"
.Fc
.Ft "unsigned int"
.Fo stdc_leading_ones_ui
.Fa "unsigned int value"
.Fc
.Ft "unsigned int"
.Fo stdc_leading_ones_ul
.Fa "unsigned long value"
.Fc
.Ft "unsigned int"
.Fo stdc_leading_ones_ull
.Fa "unsigned long long value"
.Fc
.Sh DESCRIPTION
The
.Fn stdc_leading_ones
family of functions counts the number of consecutive one bits present in
.Fa value
starting at the most significant bit.
.Pp
The
.Fn 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.
.Sh RETURN VALUES
The functions in the
.Fn stdc_leading_ones
family always return the number of leading ones found in
.Fa value .
These functions cannot fail.
.Sh EXAMPLES
.Sy Example 1
Printing the number of leading zeros.
.Bd -literal
#include <stdbit.h>
#include <stdio.h>
#include <limits.h>

int
main(void)
{
	printf("0x%x 0x%x 0x%x 0x%x\en", stdc_leading_ones_uc(0xf6),
	    stdc_leading_ones_us(0x9009), stdc_leading_ones_ui(UINT32_MAX),
	    stdc_leading_ones_ull(0));
	return (0);
}
.Ed
.Pp
When compiled and run, this produces:
.Bd -literal -offset indent
$ ./a.out
0x4 0x1 0x20 0x0
.Ed
.Sh INTERFACE STABILITY
.Sy Committed
.Sh MT-LEVEL
.Sy Async-Signal-Safe
.Sh SEE ALSO
.Xr stdc_bit_ceil 3C ,
.Xr stdc_bit_floor 3C ,
.Xr stdc_bit_width 3C ,
.Xr stdc_count_ones 3C ,
.Xr stdc_count_zeros 3C ,
.Xr stdc_first_leading_one 3C ,
.Xr stdc_first_leading_zero 3C ,
.Xr stdc_first_trailing_one 3C ,
.Xr stdc_first_trailing_zero 3C ,
.Xr stdc_has_single_bit 3C ,
.Xr stdc_leading_zeros 3C ,
.Xr stdc_trailing_ones 3C ,
.Xr stdc_trailing_zeros 3C ,
.Xr stdbit.h 3HEAD