BITS(3) Library Functions Manual BITS(3)
NAME
__BIT, __BITS, __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);
__SHIFTIN(v, mask);
__SHIFTOUT(v, mask);
__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() and __BITS() 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.
__SHIFTIN(), __SHIFTOUT(), and __SHIFTOUT_MASK() help read and write
bitfields from words:
__SHIFTIN(v, mask)
Leftshift bits v into the bitfield defined by
mask, and return them. No sideeffects.
__SHIFTOUT(v, mask)
Extract and return the bitfield selected by mask
from v, rightshifting the bits so that the
rightmost selected bit is at bit 0. No side
effects.
__SHIFTOUT_MASK(mask)
Rightshift the bits in mask so that the rightmost
nonzero bit is at bit 0. This is useful for
finding the greatest unsigned value that a
bitfield can hold. No sideeffects. 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 <dyoungATNetBSD.org>.
Matt Thomas <mattATNetBSD.org> suggested important improvements to the
implementation, and contributed the macro names SHIFTIN() and SHIFTOUT().
BUGS
__BIT() and __BITS() can only express 32bit bitmasks.
NetBSD 6.1.5 October 17, 2012 NetBSD 6.1.5
