(gcc.info.gz) C Extensions

 
 5 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++.   Extensions to the C++ Language C++
 Extensions, 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 block.
* 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.
* Typeof              `typeof': referring to the type of an expression.
* Conditionals        Omitting the middle operand of a `?:' expression.
* Long Long           Double-word integers---`long long int'.
* Complex             Data types for complex numbers.
* Floating Types      Additional Floating Types.
* Decimal Float       Decimal Floating Types.
* Hex Floats          Hexadecimal floating-point constants.
* Fixed-Point         Fixed-Point Types.
* Zero Length         Zero-length arrays.
* Variable Length     Arrays whose length is computed at run time.
* Empty Structures    Structures with no members.
* Variadic Macros     Macros with a variable number of arguments.
* Escaped Newlines    Slightly looser rules for escaped 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.)
* Constraints         Constraints for asm operands
* 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.
* Vector Extensions   Using vector instructions through built-in functions.
* Offsetof            Special syntax for implementing `offsetof'.
* Atomic Builtins     Built-in functions for atomic memory access.
* Object Size Checking Built-in functions for limited buffer overflow
                         checking.
* Other Builtins      Other built-in functions.
* Target Builtins     Built-in functions specific to particular targets.
* Target Format Checks Format checks specific to particular targets.
* Pragmas             Pragmas accepted by GCC.
* Unnamed Fields      Unnamed struct/union fields within structs/unions.
* Thread-Local        Per-thread variables.
* Binary constants    Binary constants using the `0b' prefix.

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