5. Extensions to the C Language Family

GNU C provides several language features not found in ISO 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 section 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++.

5.1 Statements and Declarations in Expressions		Putting statements and declarations inside expressions.
5.2 Locally Declared Labels		Labels local to a statement-expression.
5.3 Labels as Values		Getting pointers to labels, and computed gotos.
5.4 Nested Functions		As in Algol and Pascal, lexical scoping of functions.
5.5 Constructing Function Calls		Dispatching a call to another function.
5.6 Naming an Expression's Type		Giving a name to the type of some expression.
5.7 Referring to a Type with typeof		typeof: referring to the type of an expression.
5.8 Generalized Lvalues		Using `?:', `,' and casts in lvalues.
5.9 Conditionals with Omitted Operands		Omitting the middle operand of a `?:' expression.
5.10 Double-Word Integers		Double-word integers---long long int.
5.11 Complex Numbers		Data types for complex numbers.
5.12 Hex Floats		Hexadecimal floating-point constants.
5.13 Arrays of Length Zero		Zero-length arrays.
5.14 Arrays of Variable Length		Arrays whose length is computed at run time.
5.15 Macros with a Variable Number of Arguments.		Macros with a variable number of arguments.
5.16 Slightly Looser Rules for Escaped Newlines		Slightly looser rules for escaped newlines.
5.17 String Literals with Embedded Newlines		String literals with embedded newlines.
5.18 Non-Lvalue Arrays May Have Subscripts		Any array can be subscripted, even if not an lvalue.
5.19 Arithmetic on void- and Function-Pointers		Arithmetic on void-pointers and function pointers.
5.20 Non-Constant Initializers		Non-constant initializers.
5.21 Compound Literals		Compound literals give structures, unions or arrays as values.
5.22 Designated Initializers		Labeling elements of initializers.
5.24 Cast to a Union Type		Casting to union type from any member of the union.
5.23 Case Ranges		`case 1 ... 9' and such.
5.25 Mixed Declarations and Code		Mixing declarations and code.
5.26 Declaring Attributes of Functions		Declaring that functions have no side effects, or that they can never return.
5.27 Attribute Syntax		Formal syntax for attributes.
5.28 Prototypes and Old-Style Function Definitions		Prototype declarations and old-style definitions.
5.29 C++ Style Comments		C++ comments are recognized.
5.30 Dollar Signs in Identifier Names		Dollar sign is allowed in identifiers.
5.31 The Character ESC in Constants		`\e' stands for the character ESC.
5.33 Specifying Attributes of Variables		Specifying attributes of variables.
5.34 Specifying Attributes of Types		Specifying attributes of types.
5.32 Inquiring on Alignment of Types or Variables		Inquiring about the alignment of a type or variable.
5.35 An Inline Function is As Fast As a Macro		Defining inline functions (as fast as macros).
5.36 Assembler Instructions with C Expression Operands		Assembler instructions with C expressions as operands.

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

5.37 Controlling Names Used in Assembler Code		Specifying the assembler name to use for a C symbol.
5.38 Variables in Specified Registers		Defining variables residing in specified registers.
5.39 Alternate Keywords		__const__, __asm__, etc., for header files.
5.40 Incomplete enum Types		enum foo;, with details to follow.
5.41 Function Names as Strings		Printable strings which are the name of the current function.
5.42 Getting the Return or Frame Address of a Function		Getting the return or frame address of a function.
5.43 Other built-in functions provided by GNU CC		Other built-in functions.