#### 6.3.6 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 `libsubdir/include`. Headers for non-C runtime libraries are also installed by GCC; these are not documented here. (FIXME: document them somewhere.)

Several of the headers GCC installs are in the `ginclude` directory. These headers, `iso646.h`, `stdarg.h`, `stdbool.h`, and `stddef.h`, are installed in `libsubdir/include`, unless the target Makefile fragment (see 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 `libsubdir/include`. `config.gcc` may set `extra_headers`; this specifies additional headers under `config` to be installed on some systems.

GCC installs its own version of `<float.h>`, from `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 `glimits.h`, together with `limitx.h` and `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 `libsubdir/include/syslimits.h`, which is copied from `gsyslimits.h` if it does not need fixing to work with GCC; if it needs fixing, `syslimits.h` is the fixed copy.

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