GNU C provides several language features not found in ANSI standard C. (The option, ‘-pedantic’, directs GNU CC 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 GNU CC.
These extensions are available in C and Objective C. Most of the extensions are also available in C++. See Extensions to the C++ language for extensions that apply only to C++.
See the following documentation for more specific discussion regarding extensions to the C language family.