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: | 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. |
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| • Conditionals: | Omitting the middle operand of a ‘?:’ expression. | |
| • __int128: | 128-bit integers—__int128. |
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| • Long Long: | Double-word integers—long long int. |
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| • 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. |
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| • 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. | |
| • 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: | Inquiring about the alignment of a 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. |
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| • Incomplete Enums: |
enum foo;, with details to follow. |
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| • 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. |
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| • __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. | |
| • Pointer Bounds Checker builtins: | Built-in functions for Pointer Bounds Checker. | |
| • Cilk Plus Builtins: | Built-in functions for the Cilk Plus language extension. | |
| • 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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