Lost at C: Variable Length Array

Variable Length Array, or VLA for short, is a somewhat controversial feature of C. As with all features of C, they can be used for good, or for bad.

So what is a VLA anyways?

Before we can understand VLA's, we need to understand arrays. Here is how you define an array in C:

int some_array[8];

This will create an identifier called some_array, which gives you a way to access the 8 new integers created on the stack:

int first = some_array[0];
int second = some_array[1]:

// or

int first_deref = *some_array;
int second_deref = *(some_array + 1);

In the first 2 lines, we use the subscript operator ([]), whereas in the next 2 lines, we use pointer arithmetic. These forms are equivalent, though you will often see *some_array used to dereference the first element of an array.

Also, you can initialize arrays like so:

int some_array[8] = {0, 1, 2, 3, 4, 5, 6, 7};

// or, if all elements are zero:

int some_array[8] = {0};

There are some things which C arrays cannot do, namely, once created, they cannot be resized. Also, these arrays are created on the stack, which is significantly smaller compared to the amount of storage available on the heap. If you use up too much stack memory, your program might crash, or worse, not crash, and start acting erratically.

You can dynamically allocate your arrays using malloc(), which will store your data on the heap instead of the stack. You can then resize your arrays using realloc(). This obviously works, but now you need to manage the memory of this array:

int *some_array = malloc(len * sizeof(int));
memset(some_array, 0, len);

// use array

free(some_array);

Surely there is a way for us to dynamically create arrays without having to manage their memory, right?

With C99 came the addition of Variable Length Arrays. As the name implies, the length of the array can vary (only when initially defining it though):

int some_len = 128;
int some_vla[some_int];

This will allocate 128 ints in the stack. Note that you cannot use an initializer with VLA's, you will have to use memset():

memset(some_vla, 0, some_len);

Take the following example:

int length = // read integer value from user
int vla[length];

This could become problematic if the user enters a large number, especially if this function is called recursively, or if you are allocating an array of large structs.

Also, since the stacksize of the function is unknown, there could be some performance degradation as a result.

If you really need to use VLA's, an alternative is to use the alloca() function. It is like malloc(), except it returns stack memory instead of heap memory:

int len = 8;
int *some_array = alloca(len * sizeof(int));
memset(some_array, 0, len);

You might ask, isn't this just a VLA with more steps? And yes, you would be right. But, this makes it so don't accidentally use VLA's (just make sure to enable the -Wvla flag).

VLA's where added in the C99 standard, and made optional in the C11 standard. Also, use of VLA's are strongly discouraged, and they even have their own -Wvla flag to warn of their use.

Knowing what VLA's are and how they are used is important, even if you don't use them everyday. If you do use them, just make sure to be aware of the possible drawbacks.