Programming Tutorials

Bitwise Logical Operators in C

By: Grenfel in C Tutorials on 2007-10-03  

Three bitwise logical operators are used to manipulate individual bits in an integer data type, as shown in Table below. These operators have names similar to the TRUE/FALSE logical operators, but their operations differ.

The bitwise logical operators.

These are all binary operators, setting bits in the result to 1 or 0 depending on the bits in the operands. They operate as follows:

• Bitwise AND sets a bit in the result to 1 only if the corresponding bits in both operands are 1; otherwise, the bit is set to 0. The AND operator is used to turn off, or clear, one or more bits in a value.
• Bitwise inclusive OR sets a bit in the result to 0 only if the corresponding bits in both operands are 0; otherwise, the bit is set to 1. The OR operator is used to turn on, or set, one or more bits in a value.
• Bitwise exclusive OR sets a bit in the result to 1 if the corresponding bits in the operands are different (if one is 1 and the other is 0); otherwise, the bit is set to 0.

The following are examples of how these operators work:

You just read that bitwise AND and bitwise inclusive OR can be used to clear or set, respectively, specified bits in an integer value. Here's what that means. Suppose you have a type char variable, and you want to ensure that the bits in positions 0 and 4 are cleared (that is, equal to 0) and that the other bits stay at their original values. If you AND the variable with a second value that has the binary value 11101110, you'll obtain the desired result. Here's how this works:

In each position where the second value has a 1, the result will have the same value, 0 or 1, as was present in that position in the original variable:

0 & 1 == 0
1 & 1 == 1

In each position where the second value has a 0, the result will have a 0 regardless of the value that was present in that position in the original variable:

0 & 0 == 0
1 & 0 == 0

Setting bits with OR works in a similar way. In each position where the second value has a 1, the result will have a 1, and in each position where the second value has a 0, the result will be unchanged:

0 | 1 == 1
1 | 1 == 1
0 | 0 == 0
1 | 0 == 1