Bitwise Operators

Bitwise operators can be used to perform bit level operation on variables.

Below is a list of all six bitwise operators supported in C:

Symbol | Operator | —— | —————————––| & | bitwise AND | | | bitwise inclusive OR | ^ | bitwise exclusive OR (XOR) | ~ | bitwise not (one’s complement) | << | logical left shift | >> | logical right shift |

Following program illustrates the use of all bitwise operators:

#include <stdio.h>

int main(void)
{
   unsigned int a = 29;    /* 29 = 0001 1101 */  
   unsigned int b = 48;    /* 48 = 0011 0000 */
   int c = 0;           

   c = a & b;              /* 32 = 0001 0000 */ 
   printf("%d & %d = %d\n", a, b, c );

   c = a | b;              /* 61 = 0011 1101 */
   printf("%d | %d = %d\n", a, b, c );

   c = a ^ b;              /* 45 = 0010 1101 */
   printf("%d ^ %d = %d\n", a, b, c );

   c = ~a;                 /* -30 = 1110 0010 */
   printf("~%d = %d\n", a, c );

   c = a << 2;             /* 116 = 0111 0100 */
   printf("%d << 2 = %d\n", a, c );

   c = a >> 2;             /* 7 = 0000 0111 */
   printf("%d >> 2 = %d\n", a, c );

   return 0;
}

Bitwise operations with signed types should be avoided because the sign bit of such a bit representation has a particular meaning. Particular restrictions apply to the shift operators:

• Left shifting a 1 bit into the signed bit is erroneous and leads to undefined behavior.
• Right shifting a negative value (with sign bit 1) is implementation defined and therefore not portable.
• If the value of the right operand of a shift operator is negative or is greater than or equal to the width of the promoted left operand, the behavior is undefined.

Masking:

Masking refers to the process of extracting the desired bits from (or transforming the desired bits in) a variable by using logical bitwise operations. The operand (a constant or variable) that is used to perform masking is called a mask.

Masking is used in many different ways:

• To decide the bit pattern of an integer variable.
• To copy a portion of a given bit pattern to a new variable, while the remainder of the new variable is filled with 0s (using bitwise AND)
• To copy a portion of a given bit pattern to a new variable, while the remainder of the new variable is filled with 1s (using bitwise OR).
• To copy a portion of a given bit pattern to a new variable, while the remainder of the original bit pattern is inverted within the new variable (using bitwise exclusive OR).

The following function uses a mask to display the bit pattern of a variable:

#include <limits.h>
void bit_pattern(int u)
{
    int i, x, word;
    unsigned mask = 1;
    word = CHAR_BIT * sizeof(int);
    mask = mask << (word - 1);    /* shift 1 to the leftmost position */
    for(i = 1; i <= word; i++)
    {
        x = (u & mask) ? 1 : 0;  /* identify the bit */
        printf("%d", x);         /* print bit value */
        mask >>= 1;              /* shift mask to the right by 1 bit */
    }
}