6 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++. See section Extensions to the C++ Language, for extensions that apply only to C++.

Some features that are in ISO C99 but not C90 or C++ are also, as extensions, accepted by GCC in C90 mode and in C++.

6.1 Statements and Declarations in Expressions		Putting statements and declarations inside expressions.
6.2 Locally Declared Labels		Labels local to a block.
6.3 Labels as Values		Getting pointers to labels, and computed gotos.
6.4 Nested Functions		As in Algol and Pascal, lexical scoping of functions.
6.5 Constructing Function Calls		Dispatching a call to another function.
6.6 Referring to a Type with `typeof`		`typeof`: referring to the type of an expression.
6.7 Conditionals with Omitted Operands		Omitting the middle operand of a ‘`?:`’ expression.
6.8 128-bit integers		128-bit integers—`__int128`.
6.9 Double-Word Integers		Double-word integers—`long long int`.
6.10 Complex Numbers		Data types for complex numbers.
6.11 Additional Floating Types
6.12 Half-Precision Floating Point
6.13 Decimal Floating Types
6.14 Hex Floats		Hexadecimal floating-point constants.
6.15 Fixed-Point Types
6.16 Named Address Spaces		Named address spaces.
6.17 Arrays of Length Zero		Zero-length arrays.
6.18 Structures With No Members		Structures with no members.
6.19 Arrays of Variable Length		Arrays whose length is computed at run time.
6.20 Macros with a Variable Number of Arguments.		Macros with a variable number of arguments.
6.21 Slightly Looser Rules for Escaped Newlines		Slightly looser rules for escaped newlines.
6.22 Non-Lvalue Arrays May Have Subscripts		Any array can be subscripted, even if not an lvalue.
6.23 Arithmetic on `void`- and Function-Pointers		Arithmetic on `void`-pointers and function pointers.
6.24 Non-Constant Initializers		Non-constant initializers.
6.25 Compound Literals		Compound literals give structures, unions or arrays as values.
6.26 Designated Initializers		Labeling elements of initializers.
6.27 Case Ranges		‘case 1 ... 9’ and such.
6.28 Cast to a Union Type		Casting to union type from any member of the union.
6.29 Mixed Declarations and Code		Mixing declarations and code.
6.30 Declaring Attributes of Functions		Declaring that functions have no side effects, or that they can never return.
6.31 Attribute Syntax		Formal syntax for attributes.
6.32 Prototypes and Old-Style Function Definitions		Prototype declarations and old-style definitions.
6.33 C++ Style Comments		C++ comments are recognized.
6.34 Dollar Signs in Identifier Names		Dollar sign is allowed in identifiers.
6.35 The Character <ESC> in Constants		‘`\e`’ stands for the character <ESC>.
6.36 Specifying Attributes of Variables		Specifying attributes of variables.
6.37 Specifying Attributes of Types		Specifying attributes of types.
6.38 Inquiring on Alignment of Types or Variables		Inquiring about the alignment of a type or variable.
6.39 An Inline Function is As Fast As a Macro		Defining inline functions (as fast as macros).
6.40 When is a Volatile Object Accessed?		What constitutes an access to a volatile object.
6.41 Assembler Instructions with C Expression Operands		Assembler instructions with C expressions as operands. (With them you can define “built-in” functions.)
6.42 Constraints for `asm` Operands		Constraints for asm operands
6.43 Controlling Names Used in Assembler Code		Specifying the assembler name to use for a C symbol.
6.44 Variables in Specified Registers		Defining variables residing in specified registers.
6.45 Alternate Keywords		`__const__`, `__asm__`, etc., for header files.
6.46 Incomplete `enum` Types		`enum foo;`, with details to follow.
6.47 Function Names as Strings		Printable strings which are the name of the current function.
6.48 Getting the Return or Frame Address of a Function		Getting the return or frame address of a function.
6.49 Using Vector Instructions through Built-in Functions		Using vector instructions through built-in functions.
6.50 Offsetof		Special syntax for implementing `offsetof`.
6.51 Legacy __sync Built-in Functions for Atomic Memory Access		Legacy built-in functions for atomic memory access.
6.52 Built-in functions for memory model aware atomic operations		Atomic built-in functions with memory model.
6.53 x86 specific memory model extensions for transactional memory		x86 memory models.
6.54 Object Size Checking Built-in Functions		Built-in functions for limited buffer overflow checking.
6.55 Other Built-in Functions Provided by GCC		Other built-in functions.
6.56 Built-in Functions Specific to Particular Target Machines		Built-in functions specific to particular targets.
6.57 Format Checks Specific to Particular Target Machines		Format checks specific to particular targets.
6.58 Pragmas Accepted by GCC		Pragmas accepted by GCC.
6.59 Unnamed struct/union fields within structs/unions
6.60 Thread-Local Storage		Per-thread variables.
6.61 Binary constants using the ‘`0b`’ prefix

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