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csyntaxgrammardeclarationc89

ANSI C - direct-declarator grammar - Why does the C grammar allow syntactically legal, but sementically illegal declarations like int func()()?

The ANSI C grammar specifies:

declarator:
    pointer_opt direct-declarator

direct-declarator:
    identifier
    ( declarator )
    direct-declarator [ constant-expression_opt ]
    direct-declarator ( parameter-type-list )
    direct-declarator ( identifier-list_opt )

According to this grammar, it would be possible to derive

func()()

as a declarator, and 

int func()()

as a declaration, which is semantically illegal. Why does the C grammar allow such syntactically legal, but sementically illegal declarations?

Solution

  • Why does the C grammar allow syntactically legal, but semantically illegal declarations like int func()()?

    Your question basically answers itself: Quite simply, it's because it's a grammar's whole job to accept syntactically legal constructs. If something is syntactically legal, but semantically meaningless or illegal, it's not the grammar's job to reject it -- it gets rejected later, during semantic analysis.

    And if the question is, "Why wasn't the grammar written differently, so that semantically illegal constructs were also syntactically illegal (such that the grammar could reject them)?", the answer is that it's often a tradeoff whether to reject things during parsing or during semantic analysis. C's declaration syntax is pretty complicated, and there's an obvious desire to make the grammar which accepts it about as complicated as, but not significantly more complicated than, it has to be. Often, you can keep a grammar nicely simple by deferring certain checks to the semantic analysis phase.