Do C Compilers Accept Variable Length Arrays (VLAs), and If So, Why?

C Compiler's Acceptance of Variable Length Arrays (VLAs)

Although VLAs were standardized in C99, they are not part of the C standard. Nevertheless, compilers like g and clang accept VLA declarations with a warning under the -pedantic flag.

Compiler Acceptance

Compilers have the flexibility to include non-standard features to maintain compatibility with legacy code or to enhance programmer convenience. In this case, the compiler authors chose to allow VLA declarations to facilitate migration from C99 to C .

Standard Interpretation

The C standard explicitly states that arrays can only be declared with constant sizes. VLA declarations, where the size is not known at compile-time, violate this rule and are therefore deemed illegal in C .

Compiler Implementation

Despite the standard's prohibition, the compiler translates VLAs into code that appears to allocate memory on the stack. However, this behavior is not specified by the C standard and is solely an implementation detail of the compiler.

Conclusion

While VLAs offer flexibility in certain scenarios, it's crucial to understand that they are not part of the C standard. Compilers may accept them for compatibility reasons, but using them should be done with caution and should not be relied upon in standard-compliant code. Always consult the documentation of your compiler for specific non-standard features that are supported.

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