Purpose of static keyword in array parameter of function

Question

While browsing some source code I came across a function like this:

void someFunction(char someArray[static 100])
{
    // do something cool here
}

With some experimentation it appears other qualifiers may appear there too:

void someFunction(char someArray[const])
{
    // do something cool here
}

It appears that qualifiers are only allowed inside the [ ] when the array is declared as a parameter of a function. What do these do? Why is it different for function parameters?

Answer 1

The first declaration tells the compiler that someArray is at least 100 Elements long. This can be used for optimizations. For example it also means that someArray is never NULL.

The second declaration simply declars someArray (not someArray's elements!) as const, i.e. you can not write someArray=someOtherArray

It's different for function parameters because it really on makes sense for function parameters. If you declare an array, then you've actually told the compiler how many elements there are. And an array variable with unspecified size (int x[]), has a different meaning than an array parameter (the variable must be initialized and yields a sizeof() of it's raw size)

UPDATE: I'm a bit on a loss to find the clause in C99 which defines this. There's a clause 6.7.5.3-21 which says:

[...]
void f(double a[restrict static 3][5]);
(Note that the last declaration also specifies that the argument corresponding to a in any call to f must be a non-null pointer to the first of at least three arrays of 5 doubles, which the others do not.)

and that's it. Anybody there with C99-fu?

Apparently yes, thank you @Gilles (from the comments):

In N1124 and N1256, the defining clause is 6.7.5.3-7: "If the keyword static also appears within the [ and ] of the array type derivation, then for each call to the function, the value of the corresponding actual argument shall provide access to the first element of an array with at least as many elements as specified by the size expression."

UPDATE: http://publib.boulder.ibm.com/infocenter/zos/v1r9/index.jsp?topic=/com.ibm.zos.r9.cbclx01/static_array_index.htm

Explicitly recommends "static" to indicate non-null array parameters.