Extensions to the C Language Family
GNU C provides several language features not found in ISO standard C.
-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 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++.
- 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: referring to the type of an expression.
- Lvalues: Using
, and casts in lvalues.
- Conditionals: Omitting the middle operand of a
- 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:
__asm__, etc., for header files.
- Incomplete Enums:
enum foo;, with details to follow.
- Function Names: Printable strings which are the name of the current
- 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.