Question

In C99 there are variable-length arrays, and there can be static qualifiers (and type qualifiers) in parameter array declarators:

void f(int i, int *a);
void f(int i, int a[]);
void f(int i, int a[i]);
void f(int i, int a[*]);         // Only allowed in function prototypes.
void f(int i, int a[static i]);

Since array function parameters simply decay to pointers, is there any practical difference between the previous declarations, or is it a matter of style? When should any of them be used? In particular, what does the static qualifier imply? The standard does not render well clear the reason for each syntax.

Answer 1

As long as you are working with single-dimensional arrays only, the above declarations are all equivalent. The last one though

void f(int i, int a[static i])

has an extra effect. It is equivalent to the previous ones in terms of the parameter types, but also tells the compiler that it can rely on a parameter pointing to an array of at least i elements (which can be used in optimizations).

You are also forgetting another new declaration

void f(int i, int a[const])

This one actually does have an effect even in case of a single-dimensional array. It is equivalent to

void f(int i, int *const a)

although some might argue that const-qualifications on function parameters are useless. Before it was impossible to const-qualify the pointer the array parameter “decays” to when using the [] syntax for the declaration.

The * (as well as i) between the [] begins to matter only when it is used between the second (or greater) pair of [] in multi-dimensional array declaration. In essence, it is just like it has always been: array size in the parameter declaration always mattered only between the second or further pair of []. The * is used in prototype declarations for VLA parameters, when the size value is not named explicitly. For example, you can declare

void bar(int n, int m, int a[n][m]);

and the compiler will know that a is a VLA since the sizes are not constants. But if you prefer not to name parameters in prototypes, how are you going to tell the compiler that a is a VLA? That’s when * helps

void bar(int, int, int a[*][*]);