Unbalanced parenthesis using __attribute__ in g++

Question

Today I tried clang on a project I have developed some time ago. I was suprised when it encountered a compilation error, since I had compiled my project successfully using g++.

This short snippet reproduces the line where the error was encountered:

int main() {
    __attribute__((aligned(16)) char arr[5];
}

Which produces this error:

test.cpp:2:32: error: expected ')'
    __attribute__((aligned(16)) char arr[5];
                               ^
                               )

As you can see, there is an umbalanced parenthesis. There are three '(', and two ')'. This clearly looks like it should actually produce a compilation error.

Is this a valid usage of this keyword? I can't seem to find anything on the documentation that indicates it is.

I'm using g++ 4.5.2 and clang 2.8.

Note that this error is detected when using gcc instead of g++.

Answer 1

This would be a compiler bug. The compiler sees __attribute__ followed by two opening parentheses, some other tokens and then two closing parentheses, which is probably the “definition” of what an __attribute__ should look like, e.g.

<attribute> ::= __attribute__ '((' something '))'

My guess is that the tokens in between are being interpreted as aligned(16 and by some miracle it still works.