C allows you to manipulate individual bits within a variable. One way is to use bitwise operations. The following operations (not valid for floats or doubles) are available.
Examples
AND
Bitwise AND to mask off some bits:
c= n & 0177;
sets to zero all but the lowest 7 bits.
Note the use of an octal constant here. The convention is:
You can make a constant long by appending L eg 0X1234L
OR
The bitwise OR can be used to force bits to be set to 1.
x= x | 0XFF
forces the lowest 8 bits to 1.
Exclusive OR
Exclusive OR is true if either, but not both, of its two expressions is true. In other words, it produces an output of 1 if and only if precisely one of its two inputs its 1.
The exclusive OR operator is commonly used to change certain bits:
x= x ^ 03;
means that the lowest pair of bits of x are changed.
Shift
The left-shift is straightforward:
x << 2
moves the bit pattern x two places to the left, pulling in zeroes in the lowest (rightmost) bit positions. The original top two bits are lost from the left end.
In contrast, the right-shift needs considerable care.
x >> 2
moves the bit pattern two places to the right. If x is an unsigned quantity, zeroes feed in at the top.
If it is signed, the result depends on the machine: some will fill with zeroes, others with the sign bit.
One's complement
The one's complement operator (~) turns 0's to 1's and vice versa. C also allows you to work directly with bit fields.
Example
Bit fields definitions:
unsigned flag1 : 1; unsigned flag2 : 1;
The fields are defined to be n bits long, with : n. We can now write:
flag1= 1; flag2= 0; if (flag2 == 1)etc.
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