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GNU C provides several language features not found in ISO standard C.(The-pedantic option directs GCC to print a warning message ifany of these features is used.) To test for the availability of thesefeatures 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 arealso available in C++. SeeExtensions to theC++ Language, for extensions that applyonly to C++.
Some features that are in ISO C99 but not C90 or C++ are also, asextensions, 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 onvoid-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 characterESC. | |
| •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 implementingoffsetof. | |
| •__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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