(cpp) System Headers

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 2.8 System Headers
 ==================
 
 The header files declaring interfaces to the operating system and
 runtime libraries often cannot be written in strictly conforming C.
 Therefore, GCC gives code found in "system headers" special treatment.
 All warnings, other than those generated by `#warning' (
 Diagnostics), are suppressed while GCC is processing a system header.
 Macros defined in a system header are immune to a few warnings wherever
 they are expanded.  This immunity is granted on an ad-hoc basis, when
 we find that a warning generates lots of false positives because of
 code in macros defined in system headers.
 
    Normally, only the headers found in specific directories are
 considered system headers.  These directories are determined when GCC
 is compiled.  There are, however, two ways to make normal headers into
 system headers.
 
    The `-isystem' command line option adds its argument to the list of
 directories to search for headers, just like `-I'.  Any headers found
 in that directory will be considered system headers.
 
    All directories named by `-isystem' are searched _after_ all
 directories named by `-I', no matter what their order was on the
 command line.  If the same directory is named by both `-I' and
 `-isystem', the `-I' option is ignored.  GCC provides an informative
 message when this occurs if `-v' is used.
 
    There is also a directive, `#pragma GCC system_header', which tells
 GCC to consider the rest of the current include file a system header,
 no matter where it was found.  Code that comes before the `#pragma' in
 the file will not be affected.  `#pragma GCC system_header' has no
 effect in the primary source file.
 
    On very old systems, some of the pre-defined system header
 directories get even more special treatment.  GNU C++ considers code in
 headers found in those directories to be surrounded by an `extern "C"'
 block.  There is no way to request this behavior with a `#pragma', or
 from the command line.
 

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