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## 16 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.