Why Do Some C Compilers Accept Variable Length Arrays (VLAs) Despite Standard Prohibition?

Compiler Acceptance of Variable Length Arrays (VLAs) in C

Despite the absence of Variable Length Arrays (VLAs) in the C standard, compilers like g and clang surprisingly accept VLA syntax. This has raised questions about the compiler's behavior and the implications of this non-standard feature.

Compiler Tolerance

The compiler's acceptance of VLAs stems from its inherent design. GCC, in particular, prioritizes compatibility with legacy C compilers, allowing it to recognize VLA syntax even though it's technically not part of the C standard.

Standard Position

The C grammar specifies that array sizes must be constant expressions. Since VLAs use variable expressions, they violate this rule and are therefore not considered part of the C standard.

Compiler Implementation

VLAs in C compilers are implemented as compiler extensions. The compiler handles them by allocating the array on the stack during execution, similar to a regular array. However, the standard does not define this behavior, so it's subject to variation across different compilers.

Consequences

Using VLAs in C introduces several implications:

• Compatibility issues: Code with VLAs may not compile or run on all C compilers that strictly adhere to the standard.
• Undefined behavior: The standard does not specify how VLAs are allocated, so their memory management behavior can be unpredictable.
• Reduced portability: Relying on compiler extensions makes the code less portable across different systems and compilers.

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