Why Do C Compilers Accept Variable Length Arrays Despite Them Being Non-Standard?

Variable Length Arrays in C : Compiler Acceptance Despite Non-Standard Status

In C , Variable Length Arrays (VLAs) are not part of the standard. However, compilers like g and clang surprisingly accept VLA declarations, albeit with warning messages about non-standard usage. This unexpected behavior raises questions about the compiler's motivation and the standard's stance on VLAs.

Compiler Acceptance

The reason behind the compiler's acceptance of VLAs despite their non-standard nature lies in the decision of compiler authors. C compilers, particularly GCC, prioritize compatibility with older C compilers, including support for features that were historically acceptable but do not conform to the C standard. This compatibility-first approach explains the compilers' reluctance to reject VLA declarations.

Standard Interpretation

The C standard explicitly forbids the use of VLAs in [dcl.array]. It states that array declarations must specify the size using a constant expression, which is not the case for VLAs. In other words, C does not natively support the allocation of arrays with sizes determined during runtime.

Compiler Implementation

Despite the standard's prohibition, compilers that accept VLAs do so by implementing them as extensions to the language. The exact implementation varies between compilers and platforms. However, examining the assembly code produced by the compiler reveals that VLA declarations are typically mapped onto stack allocated arrays, similar to how regular arrays are implemented.

Conclusion

Compilers' acceptance of VLAs in C is an example of compiler extensions that deviate from the language standard. While such extensions may be useful in specific situations, it is crucial to be aware of their limitations and to use them cautiously. Developers can rely on compiler warnings and documentation to determine the availability and behavior of VLA extensions in their target compiler environment.

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