In C is _Nonnull universal, or is it some kind of compiler extension/c99 feature?

Question

const char *const _Nonnull GetString() {
    ...
}

Is that cross platform? Or is _Nonnull either a compiler extension or a c99 feature?

Answer 1

_Nonnull is a vendor extension

The _Nonnull token is not cross-platform, or standard. It's something that Apple used to use in its ObjectiveC compilers to trigger additional compile-time checking of the variable declared as _Nonnull. More modern versions of the language have upgraded this macro to a first-class keyword.

In this use, I believe the compiler will make sure that the function doesn't explicitly return a NULL-pointer value. (It's not perfect, though, as code can always misuse pointer-arithmetic to return NULL at runtime, something the compiler cannot detect)

I've never seen this used in standard C, only ever in old ObjectiveC.

A C99 equivalent for function parameters

For C99, the static keyword can be used to enforce non-NULL inputs to functions, although the syntax will make you wonder what's going on at first:

int processNonNullString(char myString[static 1])
{
    /* ... */
}

This use of static before the array size (1), tells the compiler that 'myString' is a character array of at least one element. Remember that arrays are pointers in C, so this restriction rules out the possibility of the pointer being null.