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csyntaxgrammaratomicc11

Does the C11 keyword '_Atomic' count as type qualifier or specifier if followed by a whitespace and a left parenthesis?

Reading the N1570 draft of the C11 standard, it says on p. 121 about the _Atomic keyword:

If the _Atomic keyword is immediately followed by a left parenthesis, it is interpreted as a type specifier (with a type name), not as a type qualifier.

Now I am wondering, what constitutes as 'immediate' in this context?

I find the wording quite ambiguous: Are the two following lines of code guaranteed by the standard to always be identical?

Unambiguous:

static _Atomic(type) var;

Ambiguous:

static _Atomic (type) var;

Does the insertion of a whitespace destroy the immediacy of the left parenthesis?

While in the first case, the keyword is always a type specifier, in the second case I am not sure whether it is a type specifier or a type qualifier and whether that is a question of interpretation or firmly defined by the standard. I am also referring to cases where 'var' is a pointer.

Solution

  • _Atomic as a type specifier or type qualifier is shown in the grammar in clauses 6.7.2.4 and 6.7.3, respectively. The grammar is expressed in tokens (the terminal symbols of the grammar are the tokens defined by the C specification), and the grammar is analyzed in translation phase 7 (clause 5.1.1.2):

    White-space characters separating tokens are no longer significant. Each preprocessing token is converted into a token. The resulting tokens are syntactically and semantically analyzed and translated as a translation unit.

    Thus, white space is irrelevant.