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The &= operator is the bitwise AND assignment operator in C. It performs a bitwise AND operation between the binary representations of the left and right operands, and subsequently assigns the resulting value to the left operand. 
lvalue &= rvalue;
This compound assignment operator is syntactic sugar for the following equivalent expression, with the exception that the lvalue is evaluated only once: 
lvalue = lvalue & (rvalue);

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Mechanics and Constraints

  • Operand Types: Both operands must be of integral types (e.g., char, short, int, long, unsigned variants). The operator cannot be applied to floating-point types or pointers.
  • Evaluation Order: The rvalue expression is fully evaluated before the bitwise AND operation is performed.
  • Type Promotion: Standard integer promotion rules apply. If the operands are of different sizes or signedness, the compiler promotes them to a common type before executing the bitwise operation. The final result is then truncated or converted back to the type of the lvalue during assignment.
  • Bit-Level Logic: The operator compares each bit of the left operand to the corresponding bit of the right operand. The resulting bit is set to 1 if and only if both corresponding bits are 1. Otherwise, the resulting bit is 0.

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Truth Table

Left Operand BitRight Operand BitResulting Assigned Bit
000
010
100
111

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Execution Example

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

a &= b;
Step-by-step evaluation:
  1. Align the binary values: 0000 1100 (a) 0000 1010 (b)
  2. Apply bitwise AND logic per column: 0000 1000
  3. Assign the result back to a. The variable a now holds the decimal value 8.
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