The keyword __attribute__ allows you to specify special
attributes of variables or structure fields. This keyword is followed
by an attribute specification inside double parentheses. Some
attributes are currently defined generically for variables.
Other attributes are defined for variables on particular target
systems. Other attributes are available for functions
(see Declaring Attributes of Functions), labels (see Label Attributes),
enumerators (see Enumerator Attributes), and for types
(see Specifying Attributes of Types).
Other front ends might define more attributes
(see Extensions to the C++ Language).
See Attribute Syntax, for details of the exact syntax for using attributes.