Bitwise OR

The | operator in C is the bitwise inclusive OR operator. It performs a logical disjunction on each pair of corresponding bits of two integral operands. If either or both bits in the compared position are 1, the resulting bit is 1; if both bits are 0, the resulting bit is 0.

result = operand1 | operand2;

Truth Table

The operator evaluates operands at the binary level according to the following truth table: | Bit A | Bit B | A | B | | :---: | :---: | :---: | | 0 | 0 | 0 | | 0 | 1 | 1 | | 1 | 0 | 1 | | 1 | 1 | 1 |

Operand Constraints

The | operator strictly requires operands of integral types (e.g., char, short, int, long, unsigned variants, and _Bool). Attempting to use the | operator with floating-point types (float, double) or pointer types will result in a compilation error.

Integer Promotion and Type Conversion

Before the bitwise OR operation is executed, C applies standard integer promotions to the operands:

1. If the operands are of types smaller than int (such as char or short), they are promoted to int or unsigned int.
2. If the operands are of different types, the compiler applies the usual arithmetic conversions to establish a common type.
3. The operation is then performed, and the resulting value shares this common promoted type.

Evaluation Mechanics

When evaluating a | b, the compiler aligns the bit sequences of both operands and applies the OR logic vertically to each bit index.

unsigned char a = 10;  // Binary: 0000 1010
unsigned char b = 12;  // Binary: 0000 1100

unsigned char c = a | b; 
/* 
   Evaluation:
     0000 1010  (10)
   | 0000 1100  (12)

     0000 1110  (14)
*/

Compound Assignment

The bitwise OR operator can be combined with the assignment operator to form the compound assignment operator |=. This mutates the left operand by applying the bitwise OR operation between it and the right operand.

operand1 |= operand2; 
// Semantically equivalent to: operand1 = operand1 | operand2;

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