What are the Rules and Limitations of Variable Length Arrays (VLA) in C and C ?

Variable Length Arrays (VLA) in C and C : Clarifying Their Behavior

In the realm of programming, certain concepts may leave you with lingering questions. One such topic is the usage of Variable Length Arrays (VLA) in C and C . Let's delve into some key points to clarify their behavior.

Local Scope Declaration of VLA

As you mentioned, C99 introduced the ability to declare VLA within local scopes. This is indeed true. By specifying the size of the array dynamically at runtime, you can create VLA like so:

int main(int argc, char **argv) {
    int size = 100;
    int array[size];
    return 0;
}

Restriction on Global VLA Declaration

However, as you also observed, VLA are prohibited in global scope in C99. This is where your reasoning comes into play. In C99, the const modifier does not guarantee a compile-time value. Therefore, global_array in your example remains a VLA, violating the prohibition.

In C , however, const does enforce compile-time evaluation. As a result, global_size becomes a compile-time constant, and global_array is no longer a VLA.

Why the Restriction on Global VLA?

The ban on VLA in global scope stems from practical considerations. Imagine a global VLA whose size is determined by an expression that references an object in a different compilation unit. Determining the evaluation order becomes challenging, and such intricate dependencies could lead to unpredictable behavior.

Differences in Behavior

The behavior of VLA and arrays in global and local scopes is indeed different. Global arrays have a fixed size that is determined at compile-time. In contrast, VLA in local scopes can have their size adjusted dynamically based on input or runtime conditions.

In summary, VLA can be declared in local scopes in C99 and later, while they are not allowed in global scopes for consistency and clarity reasons. The behavior of arrays in global and local scopes differs due to their fixed or variable size characteristics.

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