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

Variable Length Arrays (VLAs) in C and C

You inquire about the behavior and restrictions surrounding variable length arrays (VLAs) in C and C .

Variable Length Arrays in Local Scope

Your understanding is accurate. In C99, VLAs can be declared within local scopes, as demonstrated in your example:

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

Variable Length Arrays in Global Scope

However, VLAs are indeed forbidden in global scopes in C99. Specifically:

const int global_size = 100;
int global_array[global_size]; // forbidden in C99, allowed in C++

The reason for this restriction lies in the fact that, since const-qualified objects aren't considered integer constant expressions in C, the compiler cannot determine the size of the VLA at compile time. Hence, declaring such VLAs in global scope is prohibited.

Differences in Behavior

This behavior differs from that of arrays in global and local scopes because global arrays have their size determined at compile time, while the size of VLAs in local scope may be calculated dynamically based on the value of a variable during run-time.

Lack of VLA Support in C

Unlike C99, C does not allow for the use of VLAs. Instead, C provides features such as dynamic memory allocation and container classes like vectors and arrays, which cater to varying array sizes.

In conclusion, your understanding of the behavior and restrictions surrounding VLAs in C and C is correct. VLAs are allowed in local scopes but not in global scopes in C99, and are not supported in C . These restrictions stem from the need to determine the VLA's size at compile time, which is not always possible with global variables.

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