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 section Extensions to the C++ Language, 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.
• Interrupt Functions: Compiling functions for interrupt calls
• 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.