Variable Length Arrays (VLAs) in C

A Variable Length Array is an array whose size is not fixed at compile-time, but instead is decided at runtime.

Stored on the stack (automatic storage). Their size is determined when execution reaches the declaration.
They must be declared inside a function (block scope) or in a function prototype scope, not globally.
VLAs were introduced in the C99 standard, some later standards (like C11) made them optional, so not all compilers may support them.

C++

#include <stdio.h>

// function to initialize array
void initialize(int *arr, int size)
{
    for (int i = 0; i < size; i++)
    {
        arr[i] = i + 1;
    }
}

// function to print an array
void printArray(int size)
{
    // variable length array
    int arr[size];
    initialize(arr, size);

    for (int i = 0; i < size; i++)
    {
        printf("%d ", arr[i]);
    }
}

int main()
{
    int n;
    printf("Enter the Size: ");
    scanf("%d", &n);
    printArray(n);

    return 0;
}

Output

Enter the Size: 5
1 2 3 4 5

Unlike fixed-size arrays, VLAs cannot be initialized using {0} in standard C.
This works only for arrays with compile-time constants size, e.g, int arr[5] ={0};

C++

#include <stdio.h>

int main()
{
    int n = 5;
    int arr[n] = {0};

    for (int i = 0; i < n; i++)
        printf("%d ", arr[i]);

    printf("\n");
    return 0;
}

Note : However, many modern compilers like GCC and Clang allow this as an extension, so they let it compile without error.

Advantages of VLAs

Makes code more flexible than fixed-size arrays.
No need to use dynamic memory functions (malloc/free) for temporary arrays.
Syntax is simple and similar to normal arrays.

Disadvantages of VLAs

VLAs are stored on the stack, which has limited memory, very large VLAs may cause stack overflow.
Cannot be used as global or static arrays.
Not guaranteed to be supported on all compilers (especially in modern standards like C11).

Variable Length Arrays (VLAs) in C

Advantages of VLAs

Disadvantages of VLAs

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