Manual browser: bits(3)
BITS(3) | Library Functions Manual | BITS(3) |
NAME
__BIT, __BITS, __SHIFTIN, __SHIFTOUT, __SHIFTOUT_MASK — macros for preparing bitmasks and operating on bit fieldsSYNOPSIS
#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)
- 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 */
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 <dyoung@NetBSD.org>. <matt@NetBSD.org> suggested important improvements to the implementation, and contributed the macro names SHIFTIN() and SHIFTOUT().BUGS
__BIT() and __BITS() can only express 32-bit bitmasks.October 17, 2012 | NetBSD 7.0 |