How to use `static` with an array of array in C

Question

Given,

int last(int arr[3][6]) {
    return arr[2][5];
}

I want to make it clear that arr can't be null with the static keyword.
The problem is that the following code won't work.

int last(int arr[static 3][static 6]) {
    return arr[2][5];
}

Using gcc13 -std=c2x I get:

error: static or type qualifiers in non-parameter array declarator

Why can I do arr[static 3][6] and not arr[static 3][static 6] ?

Answer 1

In a parameter declared as an array of arrays, the C standard defines static only in the first dimension, and that is the only dimension you need it in.

C 2018 6.7.6.2 1 says:

… The optional type qualifiers and the keyword static shall appear only in a declaration of a function parameter with an array type, and then only in the outermost array type derivation.

static is useful because the outermost dimension otherwise would be lost. The parameter declaration int a[3][6] is automatically adjusted to int (*a)[6], which is a pointer to an array of 6 int, but there is no information about how many arrays of 6 int are at that location.

int a[static 3][6] is also adjusted to int (*a)[6] but with the additional information that there should be at least 3 arrays of 6 int at that location.

There is no need for static on the second dimension because the information about the second dimension is not lost. int [6] is a complete type; it is an array of 6 int. If we have a pointer to one array of 6 int, there are 6 int there. If we have a pointer to three arrays of 6 int, there are 3 arrays of 6 int there (18 int total).