Are Variable Length Arrays (VLAs) Allowed in Global Scope in C and C ?

Variable Length Arrays in C and C : Detailed Analysis

Variable length arrays (VLAs) offer dynamic memory allocation at compile time, allowing arrays to be resized at run time. While the concept is straightforward, it raises questions about their behavior and limitations.

Local vs. Global Scope in C

In C, VLAs can be declared within local scopes (e.g., functions). However, they are not permitted in global scopes. The 'const' modifier on a variable does not create a compile-time value in C99. Therefore, a VLA with a non-constant 'const' size (e.g., 'const int global_size = 100; int global_array[global_size];') is allowed in C99 but still considered a VLA.

Global Scope in C

In C , the 'const' modifier does create a compile-time value. This means that declaring a global array with a 'const' size (e.g., 'int global_array[global_size];') is not a VLA. VLAs are not allowed in global scope in C .

Rationale for Global Scope Prohibitions

VLAs in global scopes can create complexity and inconsistencies:

• Evaluation order: If the length expression refers to an object from a different compilation unit, the behavior would be unpredictable.
• Dynamic initialization in C : Global arrays with non-const lengths can lead to issues similar to dynamic initialization of file-scope objects, which can be problematic in multi-threaded environments.
• Limited use cases: The use cases for VLAs in global scope are limited, as they are typically intended for local or transient data structures.

Conclusion

The behavior described in the question is correct. VLAs are not allowed in global scopes in C because their evaluation and behavior can be ambiguous. C disallows VLAs in global scopes to avoid potential problems with evaluation order and dynamic initialization. While VLAs offer flexibility within local scopes, their use in global scopes should be approached with caution or avoided altogether.

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