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The != (inequality) operator is a binary equality operator in C that evaluates whether two operands are not equal in value. It yields an integer result of 1 (representing true) if the operands differ, and 0 (representing false) if they evaluate to the same value. 
operand1 != operand2

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Return Type and Evaluation

Unlike languages with a dedicated boolean primitive built into the core syntax, the != operator in C strictly evaluates to an int.
  • Precedence: In the C standard (C11 Section 6.5.9), != is strictly classified as an equality operator, which places it at a lower precedence level than relational operators (<, <=, >, >=), arithmetic operators, bitwise NOT (~), and bitwise shift operators (<<, >>). It has higher precedence than bitwise AND (&), XOR (^), OR (|), and logical operators (&&, ||).
  • Associativity: Left-to-right.

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Type Conversions and Coercion

When operand1 and operand2 are of different types, the C compiler applies implicit type promotion rules before performing the comparison:
  1. Usual Arithmetic Conversions: If both operands are arithmetic types (integer or floating-point), they are promoted to a common type. For example, if comparing an int and a double, the int is implicitly promoted to a double before the inequality check occurs.
  2. Pointer Comparison: The != operator can compare two pointers. This is strictly valid only if:
    • Both pointers point to compatible types.
    • One operand is a pointer to an object or incomplete type and the other is a pointer to void.
    • One operand is a pointer and the other is a null pointer constant (e.g., NULL or 0).
  3. Structs and Unions: The != operator cannot be used directly on struct or union types. The compiler will throw an error as C does not support implicit deep-comparison of composite data types.

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Floating-Point Mechanics

When utilizing != with IEEE 754 floating-point numbers (float, double), specific hardware-level rules apply:
  • Negative Zero: 0.0 != -0.0 evaluates to 0 (they are considered equal).
  • NaN (Not a Number): If either operand is NaN, the != operator always evaluates to 1. Consequently, NaN != NaN evaluates to 1.
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