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 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++.

• 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:		Nested function in GNU C.
• Nonlocal Gotos:		Nonlocal gotos.
• Constructing Calls:		Dispatching a call to another function.
• Typeof:		typeof: referring to the type of an expression.
• Conditionals:		Omitting the middle operand of a ‘?:’ expression.
• __int128:		128-bit integers—__int128.
• Long Long:		Double-word integers—long long int.
• Complex:		Data types for complex numbers.
• Floating Types:		Additional Floating Types.
• Half-Precision:		Half-Precision Floating Point.
• Decimal Float:		Decimal Floating Types.
• Hex Floats:		Hexadecimal floating-point constants.
• Fixed-Point:		Fixed-Point Types.
• Named Address Spaces:		Named address spaces.
• Zero Length:		Zero-length arrays.
• Empty Structures:		Structures with no members.
• Variable Length:		Arrays whose length is computed at run time.
• 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.
• Variadic Pointer Args:		Pointer arguments to variadic functions.
• Pointers to Arrays:		Pointers to arrays with qualifiers work as expected.
• Initializers:		Non-constant initializers.
• Compound Literals:		Compound literals give structures, unions or arrays as values.
• Designated Inits:		Labeling elements of initializers.
• Case Ranges:		‘case 1 ... 9’ and such.
• Cast to Union:		Casting to union type from any member of the union.
• Mixed Declarations:		Mixing declarations and code.
• Function Attributes:		Declaring that functions have no side effects, or that they can never return.
• Variable Attributes:		Specifying attributes of variables.
• Type Attributes:		Specifying attributes of types.
• Label Attributes:		Specifying attributes on labels.
• Enumerator Attributes:		Specifying attributes on enumerators.
• Statement Attributes:		Specifying attributes on statements.
• 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.
• Alignment:		Determining the alignment of a function, type or variable.
• Inline:		Defining inline functions (as fast as macros).
• Volatiles:		What constitutes an access to a volatile object.
• Using Assembly Language with C:		Instructions and extensions for interfacing C with assembler.
• 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.
• Offsetof:		Special syntax for implementing offsetof.
• __sync Builtins:		Legacy built-in functions for atomic memory access.
• __atomic Builtins:		Atomic built-in functions with memory model.
• Integer Overflow Builtins:		Built-in functions to perform arithmetics and arithmetic overflow checking.
• x86 specific memory model extensions for transactional memory:		x86 memory models.
• Object Size Checking:		Built-in functions for limited buffer overflow checking.
• Other Builtins:		Other built-in functions.
• Target Builtins:		Built-in functions specific to particular targets.
• Target Format Checks:		Format checks specific to particular targets.
• Pragmas:		Pragmas accepted by GCC.
• Unnamed Fields:		Unnamed struct/union fields within structs/unions.
• Thread-Local:		Per-thread variables.
• Binary constants:		Binary constants using the ‘0b’ prefix.

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