Why does GCC's __attribute__((...)) syntax use double parentheses?

Question

The documentation for GCC's __attribute__((...)) syntax indicates that attributes must be surrounded by double parentheses, but does not give a rationale for this design decision.

What practical reason would have caused GCC's designers to require this? Does it have anything to do with the preprocessor's handling of double parentheses?

Answer 1

To make it easier to eliminate it for different compiler. If you have portable code, you have to remove them for other compilers, so you do

#ifndef __GNUC__ #define __attribute__(x) #endif 

The problem is that attributes have various number of arguments and you can combine multiple attributes in one __attribute__ declaration, but C only introduced variadic macros in C99. With double parenthesis the above definition does not need variadic macros.

Answer 2

probably the idea is that you can declare a simple macro that helps to ignore all this in any other C and C++ compiler. If you wouldn't have the second pair of parenthesis that macro would be necessarily one with .... So for compilers that don't support that you would be screwed.

Edit: With this syntax it can simply look like

#ifdef __GNUC__ # define attribute(X) __attribute__(X) #else # define attribute(X) #endif 

and then you would use attribute for your function declarations, e.g.