Set and clear bits

Learn to set and clear specific bits.

Setting and clearing bits

To turn a bit on use bitwise OR (|=); to turn it off use bitwise AND with NOT (&= ~); to toggle use XOR (^=).

Example: flags |= 0x01; sets the first bit. flags &= ~0x01; clears it; ~0x01 makes all bits 1 except the first. flags ^= 0x01; flips the first bit. These three operations are the core of managing bit flags. Each bit is independent; changing one does not affect the others. So one integer can pack many flags tightly.

Managing multiple bits

To manage several bits at once you combine masks. With bitwise OR you can set multiple bits in one operation.

Example: flags |= 0x01; flags |= 0x04; sets the first and third bits. After these, flags is 5 (binary 00000101). Then flags &= ~0x01; clears the first bit; flags stays 4 (binary 00000100). Each operation touches only the relevant bit; the rest are preserved. This is a common pattern in permission systems and state machines.

Exercise Set and clear bits

Declare unsigned int flags = 0x00;. Set bits with flags |= 0x01; and flags |= 0x04;. Print with printf("%u\n", flags);. Clear a bit with flags &= ~0x01;. Print with printf("%u\n", flags);.

Show example solution

#include <stdio.h>

int main(void) {
    unsigned int flags = 0x00;
    flags |= 0x01;
    flags |= 0x04;
    printf("%u\n", flags);
    flags &= ~0x01;
    printf("%u\n", flags);
    return 0;
}

Test yourself

Match the bit operations

|=

Set bit
&= ~

Clear bit
^=

Toggle bit
&

Test bit
What does flags &= ~0x01 do?
- Makes the value 1
- Resets all bits
- Sets the first bit
- Clears the first bit
Show answer

Clears the first bit — &= ~0x01 zeroes the first bit; the rest are preserved.
Complete the bit clear expression
```
flags &= ~____;
```
Show answer
```
flags &= ~0x01;
```

Next lesson: Bit shifting

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