C Extensions

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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, 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++.

Menu

• 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.
• 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.
• 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.
• Other Builtins :: Other built-in functions.
• Target Builtins :: Built-in functions specific to particular targets.
• Pragmas :: Pragmas accepted by GCC.
• Unnamed Fields :: Unnamed struct/union fields within structs/unions.
• Thread-Local :: Per-thread variables.