How to Check if a Bit is Set in Embedded C: Simple Guide

In embedded C, you check if a bit is set by using the bitwise AND operator & with a mask that has a 1 in the bit position you want to check. If the result is not zero, the bit is set. For example, if (value & (1 << bit_position)) checks if the bit at bit_position is set in value.

Syntax

To check if a bit is set, use the bitwise AND operator & with a mask created by shifting 1 to the bit position you want to check.

• value: The variable holding the bits.
• bit_position: The zero-based position of the bit to check (0 is the least significant bit).
• 1 << bit_position: Creates a mask with only the target bit set.
• value & (1 << bit_position): Checks if that bit is set.

c

if (value & (1 << bit_position)) {
    // bit is set
} else {
    // bit is not set
}

Example

This example shows how to check if the 3rd bit (bit position 2) is set in a byte variable.

c

#include <stdio.h>

int main() {
    unsigned char value = 0b00001100; // binary: bits 2 and 3 are set
    int bit_position = 2;

    if (value & (1 << bit_position)) {
        printf("Bit %d is set.\n", bit_position);
    } else {
        printf("Bit %d is not set.\n", bit_position);
    }

    return 0;
}

Output

Bit 2 is set.

Common Pitfalls

Common mistakes when checking bits include:

• Using = instead of & which assigns instead of checks.
• Not using parentheses around the shift operation, causing wrong precedence.
• Checking the wrong bit position (remember bit positions start at 0).
• Forgetting that the result of & is zero or non-zero, so always compare to zero or use it directly in conditions.

c

/* Wrong way: assignment instead of bitwise AND */
if (value = (1 << bit_position)) {
    // This sets value, does not check bit
}

/* Correct way: use bitwise AND with parentheses */
if (value & (1 << bit_position)) {
    // Bit is set
}

Quick Reference

Operation	Description	Example
Check bit set	Use bitwise AND with mask	if (value & (1 << bit_position))
Create mask	Shift 1 left by bit position	1 << bit_position
Bit positions	Start at 0 for least significant bit	bit 0 is rightmost bit
Result	Non-zero means bit is set	if (result != 0) or if (result)

Key Takeaways

Use bitwise AND with a shifted 1 to check if a bit is set.

Remember bit positions start at 0 from the right (least significant bit).

Always use parentheses around the shift operation for correct precedence.

Check if the result is non-zero to confirm the bit is set.

Avoid using assignment (=) instead of bitwise AND (&) when checking bits.