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(libc.info)ISO C

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ISO C
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   The GNU C library is compatible with the C standard adopted by the
American National Standards Institute (ANSI): `American National
Standard X3.159-1989--"ANSI C"' and later by the International
Standardization Organization (ISO): `ISO/IEC 9899:1990, "Programming
languages--C"'.  We here refer to the standard as ISO C since this is
the more general standard in respect of ratification.  The header files
and library facilities that make up the GNU library are a superset of
those specified by the ISO C standard.

   If you are concerned about strict adherence to the ISO C standard,
you should use the `-ansi' option when you compile your programs with
the GNU C compiler.  This tells the compiler to define _only_ ISO
standard features from the library header files, unless you explicitly
ask for additional features.  Note: Feature Test Macros, for
information on how to do this.

   Being able to restrict the library to include only ISO C features is
important because ISO C puts limitations on what names can be defined
by the library implementation, and the GNU extensions don't fit these
limitations.  Note: Reserved Names, for more information about these
restrictions.

   This manual does not attempt to give you complete details on the
differences between ISO C and older dialects.  It gives advice on how
to write programs to work portably under multiple C dialects, but does
not aim for completeness.