prev up next   top/contents search

comp.lang.c FAQ list · Question 20.7

Q: How can I manipulate individual bits?

A: Bit manipulation is straightforward in C, and commonly done. To extract (test) a bit, use the bitwise AND (&) operator, along with a bit mask representing the bit(s) you're interested in:

	value & 0x04
To set a bit, use the bitwise OR (| or |=) operator:
	value |= 0x04
To clear a bit, use the bitwise complement (~) and the AND (& or &=) operators:
	value &= ~0x04
(The preceding three examples all manipulate the third-least significant, or 2**2, bit, expressed as the constant bitmask 0x04.)

To manipulate an arbitrary bit, use the shift-left operator (<<) to generate the mask you need:

	value & (1 << bitnumber)
	value |= (1 << bitnumber)
	value &= ~(1 << bitnumber)
Alternatively, you may wish to precompute an array of masks:
	unsigned int masks[] =
		{0x01, 0x02, 0x04, 0x08, 0x10, 0x20, 0x40, 0x80};

	value & masks[bitnumber]
	value |= masks[bitnumber]
	value &= ~masks[bitnumber]

To avoid surprises involving the sign bit, it is often a good idea to use unsigned integral types in code which manipulates bits and bytes.

See also questions 9.2 and 20.8.

References: K&R1 Sec. 2.9 pp. 44-45
K&R2 Sec. 2.9 pp. 48-49
ISO Sec., Sec. 6.3.7, Sec. 6.3.10, Sec. 6.3.12
H&S Sec. 7.5.5 p. 197, Sec. 7.6.3 pp. 205-6, Sec. 7.6.6 p. 210

prev up next   contents search
about this FAQ list   about eskimo   search   feedback   copyright

Hosted by Eskimo North