(gcc.info.gz) C Extensions

 
 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++.   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.
* Hex Floats          Hexadecimal floating-point constants.
* 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.
* Return Address      Getting the return or frame address of a function.
* Other Builtins      Other built-in functions.
* Deprecated Features Things might disappear from g++.

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