..
  Copyright 1988-2022 Free Software Foundation, Inc.
  This is part of the GCC manual.
  For copying conditions, see the copyright.rst file.

.. _headers:

Headers Installed by GCC
^^^^^^^^^^^^^^^^^^^^^^^^

In general, GCC expects the system C library to provide most of the
headers to be used with it.  However, GCC will fix those headers if
necessary to make them work with GCC, and will install some headers
required of freestanding implementations.  These headers are installed
in :samp:`{libsubdir}/include`.  Headers for non-C runtime
libraries are also installed by GCC; these are not documented here.

.. todo:: document them somewhere

Several of the headers GCC installs are in the :samp:`ginclude`
directory.  These headers, :samp:`iso646.h`,
:samp:`stdarg.h`, :samp:`stdbool.h`, and :samp:`stddef.h`,
are installed in :samp:`{libsubdir}/include`,
unless the target Makefile fragment (see :ref:`target-fragment`)
overrides this by setting ``USER_H``.

In addition to these headers and those generated by fixing system
headers to work with GCC, some other headers may also be installed in
:samp:`{libsubdir}/include`.  :samp:`config.gcc` may set
``extra_headers`` ; this specifies additional headers under
:samp:`config` to be installed on some systems.

GCC installs its own version of ``<float.h>``, from :samp:`ginclude/float.h`.
This is done to cope with command-line options that change the
representation of floating point numbers.

GCC also installs its own version of ``<limits.h>`` ; this is generated
from :samp:`glimits.h`, together with :samp:`limitx.h` and
:samp:`limity.h` if the system also has its own version of
``<limits.h>``.  (GCC provides its own header because it is
required of ISO C freestanding implementations, but needs to include
the system header from its own header as well because other standards
such as POSIX specify additional values to be defined in
``<limits.h>``.)  The system's ``<limits.h>`` header is used via
:samp:`{libsubdir}/include/syslimits.h`, which is copied from
:samp:`gsyslimits.h` if it does not need fixing to work with GCC; if it
needs fixing, :samp:`syslimits.h` is the fixed copy.

GCC can also install ``<tgmath.h>``.  It will do this when
:samp:`config.gcc` sets ``use_gcc_tgmath`` to ``yes``.