illumos: manual page: stdc_first_leading

STDC_FIRST_LEADING_ONE(9F) Kernel Functions for Drivers

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

SYNOPSIS

#include <sys/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 ddi_fls(9F) functions which find
the last set value and identify the same bits, the ddi_fls(9F) functions
determine the index starting from the least significant bit, instead of the
most significant bit.

CONTEXT

These functions may be called from user, kernel, or interrupt context.

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.

INTERFACE STABILITY