> ## Documentation Index > Fetch the complete documentation index at: https://docs.syntblaze.com/llms.txt > Use this file to discover all available pages before exploring further. # C Alignof The `_Alignof` operator is a unary operator introduced in C11 that evaluates to the alignment requirement of a specified type. It returns the number of bytes representing the boundary on which an object of the given type must be allocated in memory. ```c theme={"dark"} _Alignof(type-name) ``` ## Technical Specifications * **Return Type:** The operator evaluates to an integer constant expression of type `size_t` (defined in ``). * **Operand:** The operand must be a parenthesized type name. Unlike the `sizeof` operator, `_Alignof` cannot accept an expression as its operand. * **Evaluation:** The result is determined entirely at compile-time. ## Type-Specific Behavior * **Scalar Types:** Returns the architecture-specific alignment boundary for the type (e.g., `_Alignof(int)` typically returns `4`). * **Array Types:** When applied to an array type, `_Alignof` evaluates to the alignment requirement of the array's underlying element type, not the alignment of the entire array block. * **Struct and Union Types:** When applied to a `struct` or `union`, it evaluates to the strictest (largest) alignment requirement among all of its members. ## Constraints and Restrictions The `_Alignof` operator will result in a compilation error if applied to: 1. Function types. 2. Incomplete types (such as forward-declared structs without a defined body). 3. The `void` type. ## Standard Library Integration While `_Alignof` is the native keyword, the C standard library provides a convenience macro in the `` header: ```c theme={"dark"} #include // 'alignof' expands directly to '_Alignof' size_t alignment = alignof(int); ``` *Note: In the C23 standard, `alignof` becomes a native keyword, making the inclusion of `` obsolete for this purpose, though `_Alignof` remains valid for backward compatibility.* ## Syntax Visualization ```c theme={"dark"} #include struct MixedData { char a; // Typically requires 1-byte alignment double b; // Typically requires 8-byte alignment }; int main(void) { // Evaluates to 1 size_t char_align = _Alignof(char); // Evaluates to the alignment of 'int' (e.g., 4) size_t array_align = _Alignof(int[10]); // Evaluates to the strictest member alignment (e.g., 8 for 'double') size_t struct_align = _Alignof(struct MixedData); return 0; } ```

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