C Extensions

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

   GNU C provides several language features not found in ANSI standard
C.  (The `-pedantic' option directs GNU CC 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 GNU CC.

   These extensions are available in C and Objective C.  Most of them
are also available in C++.  See Extensions to the C++ Language: C++ Extensions, for extensions that apply *only* to 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.
Zero Length: Zero-length arrays.
Variable Length: Arrays whose length is computed at run time.
Macro Varargs: Macros with variable number of arguments.
Subscripting: Any array can be subscripted, even if not an lvalue.
Pointer Arith: Arithmetic on `void'-pointers and function pointers.
Initializers: Non-constant initializers.
Constructors: Constructor expressions give structures, unions or arrays as values.
Labeled Elements: Labeling elements of initializers.
Cast to Union: Casting to union type from any member of the union.
Case Ranges: `case 1 ... 9' and such.
Function Attributes: Declaring that functions have no side effects, or that they can never return.
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.

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