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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++.  *Note Extensions to the C++ Language: C++
Extensions, for extensions that apply _only_ to C++.
   Some features that are in ISO C99 but not C89 or C++ are also, as
extensions, accepted by GCC in C89 mode and in C++.