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The |= operator is the bitwise inclusive OR assignment operator in C. It performs a bitwise OR operation between the binary representations of the left and right operands, and subsequently assigns the computed result to the left operand.

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Syntax and Equivalence

This compound assignment is semantically equivalent to the following expanded form, with the critical distinction that the lvalue is evaluated exactly once:
The single evaluation is significant when the left operand contains side effects, such as a post-increment operation (e.g., *ptr++ |= 0x0F;).

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Mechanics

The operator evaluates the operands bit by bit. For each corresponding bit position:
  • If either or both bits are 1, the resulting bit is 1.
  • If both bits are 0, the resulting bit is 0.

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Type Constraints

  • Integral Types Only: Both operands must be of integral types (e.g., char, short, int, long, unsigned variants, or _Bool).
  • Floating-Point Prohibition: Attempting to use |= with floating-point types (float, double) violates C language constraints and will trigger a compilation error.
  • Integer Promotions: Before the bitwise operation occurs, standard integer promotions are applied to the operands. If the operands have different types, standard arithmetic conversions are performed to bring them to a common type before the OR operation and subsequent assignment.
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