bits(3) - NetBSD Manual Pages

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BITS(3)                 NetBSD Library Functions Manual                BITS(3)


NAME
__BIT, __BITS, __SHIFTIN, __SHIFTOUT, __SHIFTOUT_MASK -- macros for pre- paring bitmasks and operating on bit fields
SYNOPSIS
#include <sys/param.h> #include <sys/cdefs.h> uint32_t __BIT(n); uint32_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 signifi- cant 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 signif- icant bit is bit 0. __SHIFTIN(), __SHIFTOUT(), and __SHIFTOUT_MASK() help read and write bit- fields 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 right- most 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 bit- field 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 imple- mentation. 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().
BUGS
__BIT() and __BITS() can only express 32-bit bitmasks. NetBSD 6.0 April 8, 2011 NetBSD 6.0
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