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Bit masking

  • C course
  • Intermediate
  • Lesson 100/116
  • 5 min

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.

Practice 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");.

Starter code

main.c
#include <stdio.h>

// Define flags and mask variables
// Check with bitwise AND
// Print "açık" or "kapalı"

int main(void) {
    return 0;
}

Sample solution

main.c
#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;
}

Expected output

açık

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 What does flags & 0x02 test?

  • Whether the second bit is set Correct answer
  • Whether flags equals 2
  • The total bit count
  • The sign bit

Explanation: 0x02 has only the second bit as 1; AND tests that bit.

2 Complete the bitwise AND condition

Question 
if (flags & ____) printf("açık
");
Answer 
if (flags & mask) printf("açık
");

Explanation: flags & mask tests the masked bit.

3 Match the masking parts

Term Meaning
& Bitwise AND
0x02 Second-bit mask
flags Tested value
!= 0 Bit is set

All lessons in Intermediate

1 Advanced pointers

  1. Pointer to pointer
  2. Array of pointers
  3. Function pointers
  4. Function pointer parameter
  5. void pointer
  6. Summary - Advanced pointers

2 Memory management

  1. Dynamic array
  2. Resize with realloc
  3. Memory leaks
  4. Dangling pointers
  5. Allocation patterns
  6. Summary - Memory management

3 Project: Helper library

  1. Module design
  2. String helpers
  3. Number helpers
  4. Array helpers
  5. Putting it together

4 Advanced string handling

  1. Dynamic strings
  2. String concatenation
  3. String search
  4. String conversion
  5. String safety
  6. Summary - Advanced strings

5 Function pointers

  1. Function pointer definition
  2. Calling function pointers
  3. qsort
  4. Callback patterns
  5. Function pointer array
  6. Summary - Function pointers

6 Advanced structs

  1. Structs and pointers
  2. Dynamic struct
  3. Struct and function
  4. Nested structs
  5. Array of structs
  6. Summary - Advanced structs

7 Unions and enums

  1. union
  2. enum
  3. Struct and union
  4. typedef enum
  5. Bit fields
  6. Summary - Unions and enums

8 Project: Quiz app

  1. Quiz model
  2. Adding questions
  3. Running the quiz
  4. Score calculation
  5. Putting it together

9 File I/O

  1. FILE and fopen
  2. fprintf and fscanf
  3. fgets and fputs
  4. fclose
  5. fread and fwrite
  6. Summary - File I/O

10 Error handling

  1. errno
  2. perror
  3. strerror
  4. assert
  5. Error return values
  6. Summary - Error handling

11 Multi-file projects

  1. Headers and sources
  2. extern
  3. static
  4. Makefile basics
  5. Multiple modules
  6. Summary - Multi-file

12 Project: Library app

  1. Model design
  2. Library module
  3. Checking out
  4. Search and filter
  5. Putting it together

13 Dynamic data structures

  1. Linked list
  2. Adding to list
  3. Traversing list
  4. Deleting from list
  5. Stack and queue
  6. Summary - Dynamic data

14 Variable arguments

  1. stdarg.h
  2. va_start and va_end
  3. printf-like function
  4. vsnprintf
  5. Variadic patterns
  6. Summary - Variable arguments

15 Standard library

  1. stdlib.h
  2. string.h
  3. math.h
  4. time.h
  5. ctype.h
  6. Summary - Standard library

16 Command-line arguments

  1. argc and argv
  2. Argument processing
  3. getopt
  4. Argument validation
  5. Exit codes
  6. Summary - Command line

17 Debugging

  1. gdb basics
  2. Breakpoints
  3. Watch and print
  4. valgrind
  5. printf debugging
  6. Summary - Debugging

18 Advanced bitwise

  1. Bit masking
  2. Set and clear bits
  3. Bit shifting
  4. Bit hacks
  5. Pack and unpack
  6. Summary - Advanced bitwise

19 Advanced typedef

  1. typedef and struct
  2. typedef and function pointer
  3. typedef and array
  4. Opaque types
  5. typedef and enum
  6. Summary - Advanced typedef

20 Intermediate final

  1. Project design
  2. Data model
  3. Functional logic
  4. File I/O
  5. Putting it together