C Bit Manipulation, OR, XOR, Shift

Unsigned Bit Operations

Bit masks represent compact flags and fields. Perform shifts and masks on unsigned integer types so right shift and overflow behavior are predictable. The shift count must be less than the width of the promoted left operand, and shifting a negative signed value or into an unrepresentable signed result is unsafe.

What is Bit Manipulation?

Bit manipulation means directly operating on the individual bits of an integer using bitwise operators. It is used in systems programming, embedded systems, cryptography, compression, and performance-critical code where you need to pack multiple values into a single integer or toggle hardware flags.

Operator	Symbol	Description	Example (a=5, b=3)
AND	&	1 if both bits are 1	5 & 3 = 1 (0101 & 0011 = 0001)
OR	\|	1 if either bit is 1	5 \| 3 = 7 (0101 \| 0011 = 0111)
XOR	^	1 if bits differ	5 ^ 3 = 6 (0101 ^ 0011 = 0110)
NOT	~	Flips all bits	~5 = -6 (two's complement)
Left Shift	<<	Shift bits left (multiply by 2)	5 << 1 = 10
Right Shift	>>	Shift bits right (divide by 2)	5 >> 1 = 2

Bitwise Operators - All Examples

#include <stdio.h>

void printBinary(unsigned int n) {
    for (int i = 7; i >= 0; i--) {
        printf("%d", (n >> i) & 1);
    }
}

int main() {
    unsigned int a = 5;  // 0000 0101
    unsigned int b = 3;  // 0000 0011

    printf("a = %u (", a); printBinary(a); printf(")\n");
    printf("b = %u (", b); printBinary(b); printf(")\n\n");

    printf("a & b = %u  (AND)\n",  a & b);   // 1
    printf("a | b = %u  (OR)\n",   a | b);   // 7
    printf("a ^ b = %u  (XOR)\n",  a ^ b);   // 6
    printf("~a    = %d  (NOT)\n",  ~a);       // -6 (signed)
    printf("a << 1 = %u (LEFT SHIFT  = a*2)\n", a << 1);  // 10
    printf("a >> 1 = %u (RIGHT SHIFT = a/2)\n", a >> 1);  // 2

    return 0;
}

/*
a = 5 (00000101)
b = 3 (00000011)

a & b = 1  (AND)
a | b = 7  (OR)
a ^ b = 6  (XOR)
~a    = -6 (NOT)
a << 1 = 10 (LEFT SHIFT  = a*2)
a >> 1 = 2  (RIGHT SHIFT = a/2)
*/

Common Bit Tricks

These patterns appear constantly in real-world C code:

Set, Clear, Toggle, Check a Bit

#include <stdio.h>

int main() {
    unsigned int n = 0b00001010;  // 10 in binary

    int pos = 2;  // bit position (0 = rightmost)

    // SET bit at position pos (force it to 1)
    n = n | (1 << pos);
    printf("After SET   bit %d: %u\n", pos, n);  // 14 (00001110)

    // CLEAR bit at position pos (force it to 0)
    n = n & ~(1 << pos);
    printf("After CLEAR bit %d: %u\n", pos, n);  // 10 (00001010)

    // TOGGLE bit at position pos (flip it)
    n = n ^ (1 << pos);
    printf("After TOGGLE bit %d: %u\n", pos, n); // 14 (00001110)

    // CHECK if bit at position pos is set
    int isSet = (n >> pos) & 1;
    printf("Bit %d is %s\n", pos, isSet ? "SET" : "CLEAR");

    // Check if number is even or odd using bit 0
    int x = 17;
    printf("\n%d is %s\n", x, (x & 1) ? "odd" : "even");

    // Check if power of 2: n & (n-1) == 0
    int vals[] = {1, 2, 3, 4, 8, 12, 16};
    for (int i = 0; i < 7; i++) {
        int v = vals[i];
        printf("%2d is %s power of 2\n", v,
               (v > 0 && (v & (v-1)) == 0) ? "a" : "NOT a");
    }

    return 0;
}

Bit Fields in Structures

Bit fields let you pack multiple small values into a single integer within a struct. This is heavily used in embedded systems to map hardware registers.

Bit Fields - Packing Flags into a Struct

#include <stdio.h>

// Permissions struct using bit fields
// Each field uses only the specified number of bits
struct Permissions {
    unsigned int read    : 1;  // 1 bit
    unsigned int write   : 1;  // 1 bit
    unsigned int execute : 1;  // 1 bit
    unsigned int admin   : 1;  // 1 bit
};  // Total: 4 bits packed into one int

int main() {
    struct Permissions user = {1, 1, 0, 0};  // read+write, no exec, no admin

    printf("Read:    %d\n", user.read);     // 1
    printf("Write:   %d\n", user.write);    // 1
    printf("Execute: %d\n", user.execute);  // 0
    printf("Admin:   %d\n", user.admin);    // 0

    // Grant execute permission
    user.execute = 1;
    printf("\nAfter granting execute:\n");
    printf("Execute: %d\n", user.execute);  // 1

    printf("\nSize of Permissions struct: %zu bytes\n",
           sizeof(struct Permissions));  // 4 bytes (one int)

    return 0;
}

Practical: Swap Without Temp Variable

XOR Swap - No Temp Variable

#include <stdio.h>

int main() {
    int a = 15, b = 27;
    printf("Before: a=%d, b=%d\n", a, b);

    // XOR swap - works because a^b^b = a and a^a^b = b
    a = a ^ b;  // a = 15??7
    b = a ^ b;  // b = (15??7)??7 = 15
    a = a ^ b;  // a = (15??7)^15 = 27

    printf("After:  a=%d, b=%d\n", a, b);  // a=27, b=15

    // Note: only works when a and b are different variables
    // XOR swap with same variable: a ^= a -> a becomes 0!

    return 0;
}

Masks and Fields

Set bits with value | mask, clear with value & ~mask, toggle with value ^ mask, and test with (value & mask) != 0. Build masks in an unsigned type of known width, such as uint32_t, and use UINT32_C when a constant’s type matters.

For a field, mask before inserting and shift after extracting. Validate that the supplied field value fits its allotted bits instead of silently truncating important data. Endianness affects byte order in storage or protocols, while bit positions within an integer are defined by numeric value; do not confuse the two.

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