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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++.