## 19 Host Configuration

Most details about the machine and system on which the compiler is actually running are detected by the `configure` script. Some things are impossible for `configure` to detect; these are described in two ways, either by macros defined in a file named `xm-machine.h` or by hook functions in the file specified by the out_host_hook_obj variable in `config.gcc`. (The intention is that very few hosts will need a header file but nearly every fully supported host will need to override some hooks.)

If you need to define only a few macros, and they have simple definitions, consider using the `xm_defines` variable in your `config.gcc` entry instead of creating a host configuration header. See System Config.