cpp.info: System Headers

Go backward to Wrapper Headers
Go up to Header Files
Go to the top op cpp

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' (*note
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.