Can You Declare an Array with a Variable Size in C ?

Variable Array Size Declaration in C

In C , the size of an array is typically expected to be a constant integer value. However, the question arises whether it's possible to declare an array with a non-constant variable as its size.

As highlighted by Bjarne Stroustrup in The C Programming Language, "array bound, must be a constant expression." As a result, the code below is considered incorrect:

<code class="cpp">int ArraySize = 5;
int MyArray[ArraySize]; // incorrect</code>

However, surprisingly, on some systems like GCC v4.4.0, the code compiles successfully:

<code class="cpp">void f(int i) {
    int v2[i];
}

int main() {
    int i = 3;
    int v1[i];
    f(5);
}</code>

This unexpected behavior is due to a GCC extension to the standard. By default, GCC allows the usage of a non-constant variable as an array size. However, this may not be a feature supported across all compilers or platforms.

If portability is a requirement, it's highly recommended to use the -pedantic option with GCC to enable warnings for this extension or use -std=c 98 to raise an error. By adhering to the standard, code will be more reliable and cross-platform compatible.

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