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## 15 Target Description Macros and Functions

In addition to the file machine.md, a machine description includes a C header file conventionally given the name machine.h and a C source file named machine.c. The header file defines numerous macros that convey the information about the target machine that does not fit into the scheme of the .md file. The file tm.h should be a link to machine.h. The header file config.h includes tm.h and most compiler source files include config.h. The source file defines a variable `targetm`, which is a structure containing pointers to functions and data relating to the target machine. machine.c should also contain their definitions, if they are not defined elsewhere in GCC, and other functions called through the macros defined in the .h file.