C Academy · Lesson

Bit Masks and Flags

Set, clear, toggle bits.

Lesson 3 of 413 steps

Bit Masks and Flags is a free C Academy lesson on CoddyKit. This is lesson 3 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the C Academy learning path, and your progress syncs across the web and the CoddyKit app. The C Academy course includes 4 lessons in total.

Bits as Flags

A single integer can store many on/off settings, one per bit. Each bit is a flag.

This is compact and fast, common in operating systems and embedded code.

#include <stdio.h>

int main(void) {
    unsigned flags = 0;
    printf("Initial flags = %u\n", flags);
    return 0;
}

Defining Masks

A mask is a value with specific bits set. Define each flag as a power of two so it occupies its own bit.

#include <stdio.h>

#define READ  (1u << 0)
#define WRITE (1u << 1)
#define EXEC  (1u << 2)

int main(void) {
    printf("READ=%u WRITE=%u EXEC=%u\n", READ, WRITE, EXEC);
    return 0;
}

Setting a Bit

To turn a flag on, OR the value with the mask: flags |= MASK.

Bits already set stay set.

#include <stdio.h>
#define READ (1u << 0)
#define WRITE (1u << 1)

int main(void) {
    unsigned flags = 0;
    flags |= READ;
    flags |= WRITE;
    printf("flags = %u\n", flags);
    return 0;
}

Clearing a Bit

To turn a flag off, AND with the inverted mask: flags &= ~MASK.

The inverted mask has 0 in the target bit and 1 everywhere else.

#include <stdio.h>
#define READ (1u << 0)
#define WRITE (1u << 1)

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

Toggling a Bit

To flip a flag, XOR with the mask: flags ^= MASK.

If it was on it turns off, and vice versa.

#include <stdio.h>
#define EXEC (1u << 2)

int main(void) {
    unsigned flags = 0;
    flags ^= EXEC;
    printf("after toggle: %u\n", flags);
    flags ^= EXEC;
    printf("after toggle: %u\n", flags);
    return 0;
}

Testing a Bit

To check whether a flag is on, AND with the mask and test the result: flags & MASK is nonzero when set.

#include <stdio.h>
#define READ (1u << 0)
#define WRITE (1u << 1)

int main(void) {
    unsigned flags = READ;
    if (flags & READ)  printf("READ is on\n");
    if (flags & WRITE) printf("WRITE is on\n");
    else printf("WRITE is off\n");
    return 0;
}

Setting Multiple Flags

Combine masks with OR to set several flags at once.

#include <stdio.h>
#define READ (1u << 0)
#define WRITE (1u << 1)
#define EXEC (1u << 2)

int main(void) {
    unsigned flags = READ | WRITE | EXEC;
    printf("all set: %u\n", flags);
    return 0;
}

Checking Several at Once

To test that all of a group are set, AND with the combined mask and compare to the mask itself.

#include <stdio.h>
#define READ (1u << 0)
#define WRITE (1u << 1)

int main(void) {
    unsigned flags = READ | WRITE;
    unsigned need = READ | WRITE;
    if ((flags & need) == need) printf("both present\n");
    return 0;
}

Printing Active Flags

Loop over your masks to report which flags are currently on.

#include <stdio.h>
#define READ (1u << 0)
#define WRITE (1u << 1)
#define EXEC (1u << 2)

int main(void) {
    unsigned flags = READ | EXEC;
    if (flags & READ)  printf("R");
    if (flags & WRITE) printf("W");
    if (flags & EXEC)  printf("X");
    printf("\n");
    return 0;
}

Counting Set Bits

To know how many flags are on, count the set bits by repeatedly clearing the lowest set bit.

#include <stdio.h>

int main(void) {
    unsigned v = 0b10110;
    int count = 0;
    while (v) {
        v &= (v - 1);
        count++;
    }
    printf("set bits = %d\n", count);
    return 0;
}

Enums for Flags

Using an enum keeps flag definitions organized and readable.

#include <stdio.h>

enum { OPT_A = 1, OPT_B = 2, OPT_C = 4 };

int main(void) {
    unsigned opts = OPT_A | OPT_C;
    printf("opts = %u\n", opts);
    if (opts & OPT_C) printf("C enabled\n");
    return 0;
}

Quick Check

Test your knowledge of masks and flags.

Answer it in the interactive lesson

Recap

You learned to manage bit flags:

Set with flags |= MASK.
Clear with flags &= ~MASK.
Toggle with flags ^= MASK.
Test with flags & MASK.
Define masks as powers of two, often with macros or enums.

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Frequently Asked Questions

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Yes — the full text of “Bit Masks and Flags” is free to read here on the web. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the C Academy course, upgrade to CoddyKit PRO. The C Academy course includes 4 lessons in total.

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Set, clear, toggle bits. You practise C Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

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No prior experience is required. C Academy on CoddyKit is structured for beginners through advanced learners, so you can start here or from the beginning and move at your own pace. This is lesson 3 of 4.

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