Variable Length Arrays (VLAs) in C

In C, variable length arrays (VLAs) are also known as runtime-sized or variable-sized arrays. The size of such arrays is defined at run-time.

Variably modified types include variable-length arrays and pointers to variable-length arrays. Variably changed types must be declared at either block scope or function prototype scope.

Variable length arrays are a feature where we can allocate an auto array (on stack) of variable size. It can be used in a typedef statement. C supports variable-sized arrays from the C99 standard. For example, the below program compiles and runs fine in C.

Syntax of VLAs in C

void fun(int size)
{
     int arr[size];
     // code
}

Note: In C99 or C11 standards, there is a feature called flexible array members, which works the same as the above. 

Example of Variable Length Arrays

The below example shows the implementation of variable length arrays in C program

// C program to demonstrate variable length array
#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]);
    }
}

// driver code
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 

Explanation: The above program illustrate how to create a variable size array in a function in C program. This size is passed as parameter and the variable array is created on the stack memory.