BITS(3)

NAME
__BIT, __BITS, __MASK, __SHIFTIN, __SHIFTOUT, __SHIFTOUT_MASK - macros for preparing bitmasks and operating on bit fields

SYNOPSIS
#include <sys/param.h>
#include <sys/cdefs.h>
uintmax_t
__BIT(n);
uintmax_t
__BITS(m, n);
uintmax_t
__MASK(n);
uintmax_t
__SHIFTIN(v, mask);
uintmax_t
__SHIFTOUT(v, mask);
uintmax_t
__SHIFTOUT_MASK(mask);

DESCRIPTION
These macros prepare bitmasks, extract bitfields from words, and insert bitfields into words. A “bitfield” is a span of consecutive bits defined by a bitmask, where 1s select the bits in the bitfield.

Use __BIT(), __BITS(), and __MASK() to define bitmasks:

__BIT(n)
    Return a bitmask with bit n set, where the least significant bit is bit 0.

__BITS(m, n)
    Return a bitmask with bits m through n, inclusive, set. It does not matter whether m > n or m <= n. The least significant bit is bit 0.

__MASK(n)
    Return a bitmask with the first n bits set. That is, bits 0 through n - 1, inclusive, set.

__SHIFTIN(), __SHIFTOUT(), and __SHIFTOUT_MASK() help read and write bitfields from words:

__SHIFTIN(v, mask)
    Left-shift bits v into the bitfield defined by mask, and return them. No side-effects.

__SHIFTOUT(v, mask)
    Extract and return the bitfield selected by mask from v, right-shifting the bits so that the rightmost selected bit is at bit 0. No side-effects.

__SHIFTOUT_MASK(mask)
    Right-shift the bits in mask so that the rightmost non-zero bit is at bit 0. This is useful for finding the greatest unsigned value that a bitfield can hold. No side-effects. Note that __SHIFTOUT_MASK(m) = __SHIFTOUT(m, m).

EXAMPLES
The following example demonstrates basic usage of the bits macros:

    uint32_t bits, mask, val;
    bits = __BITS(2, 3);			/* 00001100 */
    mask = __BIT(2) | __BIT(3);		/* 00001100 */
    val = __SHIFTIN(0x03, mask);		/* 00001100 */
    val = __SHIFTOUT(0xf, mask);		/* 00000011 */

SEE ALSO
bitops(3), cdefs(3)

HISTORY
The bits macros first appeared in atw(4), with different names and implementation. In their current form these macros appeared in NetBSD 4.0.

AUTHORS
The bits macros were written by David Young <dyoung@NetBSD.org>. Matt Thomas <matt@NetBSD.org> suggested important improvements to the implementation, and contributed the macro names SHIFTIN() and SHIFTOUT().