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++.

• Statements and Declarations in Expressions
• Locally Declared Labels
• Labels as Values
• Nested Functions
• Nonlocal Gotos
• Constructing Function Calls
• Referring to a Type with typeof
• Conditionals with Omitted Operands
• 128-bit Integers
• Double-Word Integers
• Complex Numbers
• Additional Floating Types
• Half-Precision Floating Point
• Decimal Floating Types
• Hex Floats
• Fixed-Point Types
• Named Address Spaces
• Arrays of Length Zero
• Structures with No Members
• Arrays of Variable Length
• Macros with a Variable Number of Arguments.
• Slightly Looser Rules for Escaped Newlines
• Non-Lvalue Arrays May Have Subscripts
• Arithmetic on void- and Function-Pointers
• Pointer Arguments in Variadic Functions
• Pointers to Arrays with Qualifiers Work as Expected
• Non-Constant Initializers
• Compound Literals
• Designated Initializers
• Case Ranges
• Cast to a Union Type
• Mixed Declarations, Labels and Code
• Declaring Attributes of Functions
• Specifying Attributes of Variables
• Specifying Attributes of Types
• Label Attributes
• Enumerator Attributes
• Statement Attributes
• Attribute Syntax
• Prototypes and Old-Style Function Definitions
• C++ Style Comments
• Dollar Signs in Identifier Names
• The Character ESC in Constants
• Determining the Alignment of Functions, Types or Variables
• An Inline Function is As Fast As a Macro
• Const and Volatile Functions
• When is a Volatile Object Accessed?
• How to Use Inline Assembly Language in C Code
• Alternate Keywords
• Incomplete enum Types
• Function Names as Strings
• Getting the Return or Frame Address of a Function
• Stack scrubbing internal interfaces
• Using Vector Instructions through Built-in Functions
• Support for offsetof
• Legacy __sync Built-in Functions for Atomic Memory Access
• Built-in Functions for Memory Model Aware Atomic Operations
• Built-in Functions to Perform Arithmetic with Overflow Checking
• x86-Specific Memory Model Extensions for Transactional Memory
• Object Size Checking
• Other Built-in Functions Provided by GCC
• Built-in Functions Specific to Particular Target Machines
• Format Checks Specific to Particular Target Machines
• Pragmas Accepted by GCC
• Unnamed Structure and Union Fields
• Thread-Local Storage
• Binary Constants using the ‘0b’ Prefix