Understanding the Bitwise OR Operator (|) in C: A Beginner’s Guide

The bitwise OR operator (|) in C is a crucial tool for manipulating binary data at the bit level. Unlike the logical OR operator (||), which works with Boolean values, the bitwise OR operates directly on the binary representations of integers. It is commonly used to set specific bits or combine binary values.

In this blog, we’ll explore the bitwise OR operator in detail, covering syntax, examples, edge cases, and practical applications.

1. What is the Bitwise OR Operator (|)?

The bitwise OR operator performs a bit-by-bit comparison between two numbers. For each corresponding pair of bits:

• If either bit is 1, the result is 1.
• If both bits are 0, the result is 0.

Syntax:

result = operand1 | operand2;

Bitwise OR Truth Table:

2. Basic Example

Let’s begin with a simple example to understand how the bitwise OR operator works with binary numbers.

#include <stdio.h>
 
int main() {
    int a = 5;  // Binary: 0101
    int b = 3;  // Binary: 0011
 
    int result = a | b;  // Binary: 0101 | 0011 = 0111 (Decimal 7)
 
    printf("Result of %d | %d = %d\n", a, b, result);
    return 0;
}

Output:

Result of 5 | 3 = 7

Explanation:

• Binary of 5: 0101
• Binary of 3: 0011
• Bitwise OR: 0111 (Decimal 7)

3. Practical Applications of Bitwise OR

1. Setting Specific Bits

The bitwise OR operator is often used to set specific bits in a number without affecting the other bits.

#include <stdio.h>
 
int main() {
    int number = 8;       // Binary: 1000
    int mask = 5;         // Binary: 0101
 
    int result = number | mask;  // Sets bits specified in the mask
 
    printf("Result after setting bits: %d\n", result);
    return 0;
}

Output:

Result after setting bits: 13

Explanation:

• 8: 1000
• 5: 0101
• Result: 1101 (Decimal 13)

2. Combining Flags

In many programming scenarios, especially in low-level programming or embedded systems, flags are combined using bitwise OR.

#include <stdio.h>
 
#define FLAG_READ  0x01  // Binary: 0001
#define FLAG_WRITE 0x02  // Binary: 0010
#define FLAG_EXEC  0x04  // Binary: 0100
 
int main() {
    int permissions = FLAG_READ | FLAG_WRITE;  // Combine read and write permissions
 
    printf("Combined permissions: %d\n", permissions);
    return 0;
}

Output:

Combined permissions: 3

Explanation:

• FLAG_READ: 0001
• FLAG_WRITE: 0010
• Result: 0011 (Decimal 3)

3. Setting a Specific Bit

To set a specific bit in a number, use the bitwise OR operator with a bit mask that has a 1 in the desired bit position.

#include <stdio.h>
 
int main() {
    int number = 4;  // Binary: 0100
    int bit_position = 1;  // Set the 1st bit (zero-indexed)
 
    number = number | (1 << bit_position);
 
    printf("Number after setting bit %d: %d\n", bit_position, number);
    return 0;
}

Output:

Number after setting bit 1: 6

Explanation:

• 4 in binary: 0100
• Mask created by 1 << 1: 0010
• 0100 | 0010: 0110 (Decimal 6)

4. Edge Cases and Considerations

1. Bitwise OR with Zero

Any number ORed with 0 remains unchanged:

#include <stdio.h>
 
int main() {
    int a = 5;
    printf("%d | 0 = %d\n", a, a | 0);
    return 0;
}

Output:

5 | 0 = 5

2. Bitwise OR with -1

Any number ORed with -1 (all bits set to 1) results in -1:

#include <stdio.h>
 
int main() {
    int a = 7;
    printf("%d | -1 = %d\n", a, a | -1);
    return 0;
}

Output:

7 | -1 = -1

3. Bitwise OR with Two Identical Numbers

ORing a number with itself results in the number:

#include <stdio.h>
 
int main() {
    int a = 12;
    printf("%d | %d = %d\n", a, a, a | a);
    return 0;
}

Output:

12 | 12 = 12

5. Operator Precedence and Associativity

The bitwise OR (|) operator has lower precedence than arithmetic and relational operators but higher precedence than the logical OR (||).

Precedence Example:

#include <stdio.h>
 
int main() {
    int a = 5, b = 3, c = 8;
 
    int result = a | b + c;  // Evaluated as a | (b + c)
 
    printf("Result: %d\n", result);
    return 0;
}

Output:

Result: 13

Explanation: The addition (b + c) is evaluated first, then bitwise OR with a.

Summary

• The bitwise OR operator (|) performs a binary OR operation on each pair of corresponding bits in two operands.
• It is widely used for setting specific bits, combining flags, and manipulating binary data.
• Understanding the behavior, precedence, and common use cases of the bitwise OR operator can help you write more efficient and powerful C programs, especially when working with low-level data or hardware interfaces.

Mastering bitwise operations unlocks the true potential of low-level programming and allows you to control binary data effectively!