Bitwise Operators
C provides six operators for bit manipulation; these may only be applied to integral operands, that is, char, short, int, and long, whether signed or unsigned.
& bitwise AND
| bitwise inclusive OR
^ bitwise exclusive OR
<< left shift
>> right shift
~ one’s complement (unary)
The bitwise AND operator & is often used to mask off some set of bits; for example,
n = n & 0177;
sets to zero all but the low-order 7 bits of n.
The bitwise OR operator I is used to turn bits on:
x = x | SET_ON;
sets to one in x the bits that are set to one in SET_ON.
The bitwise exclusive OR operator ^ sets a one in each bit position where its
operands have different bits, and zero where they are the same.
One must distinguish the bitwise operators & and I from the logical operators && and ||, which imply left-to-right evaluation of a truth value. For example, if x is 1 and y is 2, then x & y is zero while x && y is one. (참조: https://msdn.microsoft.com/ko-kr/library/z68fx2f1.aspx)
The shift operators < < and > > perform left and right shifts of their left operand by the number of bit positions given by the right operand, which must be positive. Thus x < < 2 shifts the value of x left by two positions, filling vacated bits with zero; this is equivalent to multiplication by 4. Right shifting an unsigned quantity always fills vacated bits with zero. Right shifting a signed quantity will fill with sign bits (“arithmetic shift”) on some machines and with 0-bits (“logical shift”) on others.
The unary operator ~ yields the one’s complement of an integer; that is, it
converts each 1-bit into a 0-bit and vice versa. For example,
x = x & ~077
sets the last six bits of x to zero. Note that x & ~077 is independent of word length, and is thus preferable to, for example, x & 0177700, which assumes that x is a 16-bit quantity. The portable form involves no extra cost, since ~077 is a constant expression that can be evaluated at compile time.
As an illustration of some of the bit operators, consider the function getbits (x, p, n) that returns the (right adjusted) n-bit field of x that begins at position p. We assume that bit position 0 is at the right end and that n and p are sensible positive values. For example, getbits(x, 4, 3) returns the three bits in bit positions 4, 3 and 2, right adjusted.
/* getbits: get n bits from position p */
unsigned getbits(unsigned x, int p, int n)
{
return (x >> (p + 1 - n)) & ~(~0 << n);
}
The expression x > > ( p+ 1-n ) moves the desired field to the right end of the word. ~0 is all 1-bits; shifting it left n bit positions with ~0 < < n places zeros in
the rightmost n bits; complementing that with ~ makes a mask with ones in the
rightmost n bits.
[The C Programming Language p.48-49]
