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).
#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 */
}
}
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