The keyword __attribute__ allows you to specify special
attributes of types. Some type attributes apply only to struct
and union types, while others can apply to any type defined
via a typedef declaration. Other attributes are defined for
functions (see Declaring Attributes of Functions), labels (see Label Attributes), enumerators (see Enumerator Attributes), and for
variables (see Specifying Attributes of Variables).
The __attribute__ keyword is followed by an attribute specification
inside double parentheses.
You may specify type attributes in an enum, struct or union type
declaration or definition by placing them immediately after the
struct, union or enum keyword. A less preferred
syntax is to place them just past the closing curly brace of the
definition.
You can also include type attributes in a typedef declaration.
See Attribute Syntax, for details of the exact syntax for using
attributes.