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15.1 The Global targetm Variable

— Variable: struct gcc_target targetm

The target .c file must define the global targetm variable which contains pointers to functions and data relating to the target machine. The variable is declared in target.h; target-def.h defines the macro TARGET_INITIALIZER which is used to initialize the variable, and macros for the default initializers for elements of the structure. The .c file should override those macros for which the default definition is inappropriate. For example: 

          #include "target.h"
          #include "target-def.h"
          
          /* Initialize the GCC target structure.  */
          
          #undef TARGET_COMP_TYPE_ATTRIBUTES
          #define TARGET_COMP_TYPE_ATTRIBUTES machine_comp_type_attributes
          
          struct gcc_target targetm = TARGET_INITIALIZER;

Where a macro should be defined in the .c file in this manner to form part of the targetm structure, it is documented below as a “Target Hook” with a prototype. Many macros will change in future from being defined in the .h file to being part of the targetm structure.