C Extensions - Using the GNU Compiler Collection (GCC)
Description: Using the GNU Compiler Collection (GCC)

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## 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++.