c - What's the point of VLA anyway?

Question

I understand what variable length arrays are and how they are implemented. This question is about why they exist.

We know that VLAs are only allowed within function blocks (or prototypes) and that they basically cannot be anywhere but on the stack (assuming the normal implementation): C11, 6.7.6.2-2:

If an identifier is declared as having a variably modified type, it shall be an ordinary identifier (as defined in 6.2.3), have no linkage, and have either block scope or function prototype scope. If an identifier is declared to be an object with static or thread storage duration, it shall not have a variable length array type.

Let's take a small example:

void f(int n)
{
    int array[n];
    /* etc */
}

there are two cases that need to be taken care of:

• n <= 0: f has to guard against this, otherwise the behavior is undefined: C11, 6.7.6.2-5 (emphasis mine):

  If the size is an expression that is not an integer constant expression: if it occurs in a declaration at function prototype scope, it is treated as if it were replaced by *; otherwise, each time it is evaluated it shall have a value greater than zero. The size of each instance of a variable length array type does not change during its lifetime. Where a size expression is part of the operand of a sizeof operator and changing the value of the size expression would not affect the result of the operator, it is unspecified whether or not the size expression is evaluated.

• n > stack_space_left / element_size: There is no standard way of finding how much stack space is left (since there is no such thing as stack so long as the standard is concerned). So this test is impossible. Only sensible solution is to have a predefined maximum possible size for n, say N, to make sure stack overflow doesn't occur.

In other words, the programmer must make sure 0 < n <= N for some N of choice. However, the program should work for n == N anyway, so one might as well declare the array with constant size N rather than variable length n.

I am aware that VLAs were introduced to replace alloca (as also mentioned in this answer), but in effect they are the same thing (allocate variable size memory on stack).

So the question is why did alloca and consequently VLA exist and why weren't they deprecated? The only safe way to use VLAs seem to me to be with a bounded size in which case taking a normal array with the maximum size is always a viable solution.

Answer 1

For reasons that are not entirely clear to me, almost every time the topic of C99 VLA pops up in a discussion, people start talking predominantly about the possibility of declaring run-time-sized arrays as local objects (i.e. creating them "on the stack"). This is rather surprising and misleading, since this facet of VLA functionality - support for local array declarations - happens to be a rather auxiliary, secondary capability provided by VLA. It does not really play any significant role in what VLA can do. Most of the time, the matter of local VLA declarations and their accompanying potential pitfalls is forced into the foreground by VLA critics, who use it as a "straw man" intended to derail the discussion and bog it down among barely relevant details.

The essence of VLA support in C is, first and foremost, a revolutionary qualitative extension of the language's concept of type. It involves the introduction of such fundamentally new kind of types as variably modified types. Virtually every important implementation detail associated with VLA is actually attached to its type, not to the VLA object per se. It is the very introduction of variably modified types into the language that makes up the bulk of the proverbial VLA cake, while the ability to declare objects of such types in local memory is nothing more than a insignificant and fairly inconsequential icing on that cake.

Consider this: every time one declares something like this in one's code

/* Block scope */
int n = 10;
...
typedef int A[n];
...
n = 5; /* <- Does not affect `A` */

size-related characteristics of the variably modified type A (e.g. the value of n) are finalized at the exact moment when the control passes over the above typedef-declaration. Any changes in the value of n made further down the line (below this declaration of A) don't affect the size of A. Stop for a second and think about what it means. It means that the implementation is supposed to associate with A a hidden internal variable, which will store the size of the array type. This hidden internal variable is initialized from n at run time when the control passes over the declaration of A.

This gives the above typedef-declaration a rather interesting and unusual property, something we haven't seen before: this typedef-declaration generates executable code (!). Moreover, it doesn't just generate executable code, it generates critically important executable code. If we somehow forget to initialize the internal variable associated with such typedef-declaration, we'll end up with a "broken"/uninitialized typedef alias. The importance of that internal code is the reason why the language imposes some unusual restrictions on such variably modified declarations: the language prohibits passing control into their scope from outside of their scope

/* Block scope */
int n = 10;
goto skip; /* Error: invalid goto */

typedef int A[n];

skip:;

Note once again that the above code does not define any VLA arrays. It simply declares a seemingly innocent alias for a variably modified type. Yet, it is illegal to jump over such typedef-declaration. (We are already familiar with such jump-related restrictions in C++, albeit in other contexts).

A code-generating typedef, a typedef that requires run-time initialization is a significant departure from what typedef is in the "classic" language. (It also happens to pose a significant hurdle of the way of adoption of VLA in C++.)

When one declares an actual VLA object, in addition to allocating the actual array memory the compiler also creates one or more hidden internal variables, which hold the size(s) of the array in question. One has to understand that these hidden variables are associated not with the array itself, but rather with its variably modified type.

One important and remarkable consequence of this approach is as follows: the additional information about array size, associated with a VLA, is not built directly into the object representation of the VLA. It is actually stored besides the array, as "sidecar" data. This means that object representation of a (possibly multidimensional) VLA is fully compatible with object representation of an ordinary classic compile-time-sized array of the same dimensionality and the same sizes. For example

void foo(unsigned n, unsigned m, unsigned k, int a[n][m][k]) {}
void bar(int a[5][5][5]) {}

int main(void)
{
  unsigned n = 5;
  int vla_a[n][n][n];
  bar(a);

  int classic_a[5][6][7];
  foo(5, 6, 7, classic_a); 
}

Both function calls in the above code are perfectly valid and their behavior is fully defined by the language, despite the fact that we pass a VLA where a "classic" array is expected, and vice versa. Granted, the compiler cannot control the type compatibility in such calls (since at least one of the involved types is run-time-sized). However, if desired, the compiler (or the user) has everything necessary to perform the run-time check in debug version of code.

(Note: As usual, parameters of array type are always implicitly adjusted into parameters of pointer type. This applies to VLA parameter declarations exactly as it applies to "classic" array parameter declarations. This means that in the above example parameter a actually has type int (*)[m][k]. This type is unaffected by the value of n. I intentionally added a few extra dimensions to the array to maintain its dependence on run-time values.)

Compatibility between VLA and "classic" arrays as function parameters is also supported by the fact that the compiler does not have to accompany a variably modified parameter with any additional hidden information about its size. Instead, the language syntax forces the user to pass this extra information in the open. In the above example the user was forced to first include parameters n, m and k into function parameter list. Without declaring n, m and k first, the user would not have been able to declare a (see also the above note about n). These parameters, explicitly passed into the function by the user, will bring over the information about the actual sizes of a.

For another example, by taking advantage of VLA support we can write the following code

#include <stdio.h>
#include <stdlib.h>

void init(unsigned n, unsigned m, int a[n][m])
{
  for (unsigned i = 0; i < n; ++i)
    for (unsigned j = 0; j < m; ++j)
      a[i][j] = rand() % 100;
}

void display(unsigned n, unsigned m, int a[n][m])
{
  for (unsigned i = 0; i < n; ++i)
    for (unsigned j = 0; j < m; ++j)
      printf("%2d%s", a[i][j], j + 1 < m ? " " : "
");
  printf("
");
}

int main(void) 
{
  int a1[5][5] = { 42 }; 
  display(5, 5, a1);
  init(5, 5, a1);
  display(5, 5, a1);

  unsigned n = rand() % 10 + 5, m = rand() % 10 + 5;
  int (*a2)[n][m] = malloc(sizeof *a2);
  init(n, m, *a2);
  display(n, m, *a2);
  free(a2);
}

This code is intended to draw your attention to the following fact: this code makes heavy use of valuable properties of variably modified types. It is impossible to implement elegantly without VLA. This is the primary reason why these properties are desperately needed in C to replace the ugly hacks that were used in their place previously. Yet at the same time, not even a single VLA is created in local memory in the above program, meaning that this popular vector of VLA criticism is not applicable to this code at all.

Basically, the two last examples above is a concise illustration of what the point of VLA support is.