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(gcc-300.info)C Extensions

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Extensions to the C Language Family
***********************************

   GNU C provides several language features not found in ISO standard C.
(The `-pedantic' option directs GCC to print a warning message if any
of these features is used.)  To test for the availability of these
features in conditional compilation, check for a predefined macro
`__GNUC__', which is always defined under GCC.

   These extensions are available in C and Objective C.  Most of them
are also available in C++.  Note: Extensions to the C++ Language,
 for extensions that apply _only_ to C++.

   Some features that are in ISO C99 but not C89 or C++ are also, as
extensions, accepted by GCC in C89 mode and in C++.

Statement Exprs

Putting statements and declarations inside expressions.

Local Labels

Labels local to a statement-expression.

Labels as Values

Getting pointers to labels, and computed gotos.

Nested Functions

As in Algol and Pascal, lexical scoping of functions.

Constructing Calls

Dispatching a call to another function.

Naming Types

Giving a name to the type of some expression.

Typeof

`typeof': referring to the type of an expression.

Lvalues

Using `?:', `,' and casts in lvalues.

Conditionals

Omitting the middle operand of a `?:' expression.

Long Long

Double-word integers---`long long int'.

Complex

Data types for complex numbers.

Hex Floats

Hexadecimal floating-point constants.

Zero Length

Zero-length arrays.

Variable Length

Arrays whose length is computed at run time.

Variadic Macros

Macros with a variable number of arguments.

Escaped Newlines

Slightly looser rules for escaped newlines.

Multi-line Strings

String literals with embedded newlines.

Subscripting

Any array can be subscripted, even if not an lvalue.

Pointer Arith

Arithmetic on `void'-pointers and function pointers.

Initializers

Non-constant initializers.

Compound Literals

Compound literals give structures, unions

or arrays as values.

Designated Inits

Labeling elements of initializers.

Cast to Union

Casting to union type from any member of the union.

Case Ranges

`case 1 ... 9' and such.

Mixed Declarations

Mixing declarations and code.

Function Attributes

Declaring that functions have no side effects,

or that they can never return.

Attribute Syntax

Formal syntax for attributes.

Function Prototypes

Prototype declarations and old-style definitions.

C++ Comments

C++ comments are recognized.

Dollar Signs

Dollar sign is allowed in identifiers.

Character Escapes

`\e' stands for the character <ESC>.

Variable Attributes

Specifying attributes of variables.

Type Attributes

Specifying attributes of types.

Alignment

Inquiring about the alignment of a type or variable.

Inline

Defining inline functions (as fast as macros).

Extended Asm

Assembler instructions with C expressions as operands.

(With them you can define ``built-in'' functions.)

Asm Labels

Specifying the assembler name to use for a C symbol.

Explicit Reg Vars

Defining variables residing in specified registers.

Alternate Keywords

`__const__', `__asm__', etc., for header files.

Incomplete Enums

`enum foo;', with details to follow.

Function Names

Printable strings which are the name of the current

function.

Return Address

Getting the return or frame address of a function.

Other Builtins

Other built-in functions.