Recently, Petr Machata created several trac tickets for requesting the use of attribute names with underscores for GCC. Since there is no link to the GCC's documentation in the tickets, I'd like to put a link to the documentation. In the GCC's manuals --> "Extensions to the C Language Family" --> "Declaring Attributes of Functions", "Specifying Attributes of Variables", and "Specifying Attributes of Types", there is description about attributes with preceding and following `__`: You may also specify attributes with `__' preceding and following each keyword. This allows you to use them in header files without being concerned about a possible macro of the same name. For example, you may use __noreturn__ instead of noreturn. Here are links to the gcc-2.9.5's documentation: http://gcc.gnu.org/onlinedocs/gcc-2.95.3/gcc_4.html#SEC84 http://gcc.gnu.org/onlinedocs/gcc-2.95.3/gcc_4.html#SEC90 http://gcc.gnu.org/onlinedocs/gcc-2.95.3/gcc_4.html#SEC91 I also checked manuals of gcc-3.0.4 and 4.0.4, and found the same description. Hope this helps, Michel