In GNU C, you can use function attributes to declare certain things
about functions called in your program which help the compiler
optimize calls and check your code more carefully. For example, you
can use attributes to declare that a function never returns
(noreturn), returns a value depending only on its arguments
(pure), or has printf-style arguments (format).
You can also use attributes to control memory placement, code generation options or call/return conventions within the function being annotated. Many of these attributes are target-specific. For example, many targets support attributes for defining interrupt handler functions, which typically must follow special register usage and return conventions.
Function attributes are introduced by the __attribute__ keyword
on a declaration, followed by an attribute specification inside double
parentheses. You can specify multiple attributes in a declaration by
separating them by commas within the double parentheses or by
immediately following an attribute declaration with another attribute
declaration. See Attribute Syntax, for the exact rules on attribute
syntax and placement. Compatible attribute specifications on distinct
declarations of the same function are merged. An attribute specification
that is not compatible with attributes already applied to a declaration
of the same function is ignored with a warning.
GCC also supports attributes on variable declarations (see Variable Attributes), labels (see Label Attributes), enumerators (see Enumerator Attributes), statements (see Statement Attributes), and types (see Type Attributes).
There is some overlap between the purposes of attributes and pragmas
(see Pragmas Accepted by GCC). It has been
found convenient to use __attribute__ to achieve a natural
attachment of attributes to their corresponding declarations, whereas
#pragma is of use for compatibility with other compilers
or constructs that do not naturally form part of the grammar.
In addition to the attributes documented here, GCC plugins may provide their own attributes.