Bit masking

Learn flag management with bit masking.

Bit masking

Bit masking uses bitwise AND (&) to select specific bits of a value. A mask is a number with 1s only at the bits of interest.

Example: flags & 0x02 tests the second bit of flags. A non-zero result means the bit is set; zero means it is clear. 0x02 is 00000010 in binary; only the second bit is 1. This pattern is the standard way to inspect hardware registers and permission flags. Each flag lives in its own bit, so one integer packs many states tightly.

Condition with bitwise AND

You can use the mask result in an if condition to branch on the bit state. Non-zero means true; zero means false.

Example: if (flags & mask) printf("on\n"); else printf("off\n");. Here, if flags & mask is non-zero, "on" is printed; if zero, "off". In C any non-zero value is true; so you can use the bitwise AND result directly as a condition. This structure appears constantly in hardware drivers and embedded systems; you use it to read a pin or check a flag.

Exercise Write a bit masking program

Declare unsigned int flags = 0x0F; and unsigned int mask = 0x02;. Check with if (flags & mask); if true print printf("açık\n");, else print printf("kapalı\n");.

Show example solution

#include <stdio.h>

int main(void) {
    unsigned int flags = 0x0F;
    unsigned int mask = 0x02;
    if (flags & mask) {
        printf("açık\n");
    } else {
        printf("kapalı\n");
    }
    return 0;
}

Test yourself

What does flags & 0x02 test?
- Whether the second bit is set
- Whether flags equals 2
- The total bit count
- The sign bit
Show answer

Whether the second bit is set — 0x02 has only the second bit as 1; AND tests that bit.

Complete the bitwise AND condition

if (flags & ____) printf("açık
");

Show answer

if (flags & mask) printf("açık
");

Match the masking parts

&

Bitwise AND
0x02

Second-bit mask
flags

Tested value
!= 0

Bit is set

Next lesson: Set and clear bits

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