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A general description is here.
Task callback support declared in ‘R_ext/Callbacks.h’ has been enabled again. It was disabled for R 4.5.0.
The non-API header file ‘R_ext/PrtUtil.h’ is no longer copied to the installed ‘include’ 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’.