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A general description is here.
The non-API header files ‘R_ext/Callbacks.h’ and ‘R_ext/PrtUtil.h’ are no longer copied to the installed includes directory. The ‘R_ObjectTable’ type definitions formally in ‘R_ext/Callbacks.h’ are now available in the new header file ‘R_ext/ObjectTable.h’. This new header file should be used in packages instead of ‘R_ext/Callbacks.h’. To ease the transition, stub files ‘R_ext/Callbacks.h’ and ‘R_ext/PrtUtil.h’ have been added. These stub files contain ‘#warning’ directives to signal warnings that these files should not be used. The stub ‘R_ext/Callbacks.h’ also includes ‘R_ext/ObjectTable.h’. Packages including these headers will install successfully but with warnings for using these files.