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A general description is here.
The definition of ‘DL_FUNC’ in ‘R_ext/Rdynload.h’ has been changed to be fully C-compliant. This means that functions loaded _via_ for example ‘R_GetCCallable’ need to be cast to an appropriate type if they have any arguments.
‘.Machine’ has a new element ‘sizeof.time_t’ to identify old systems with a 32-bit type and hence a limited range of date-times (and limited support for dates millions of years from present).
As ‘"POSIXlt"’ objects may be “partially filled” and their list components meant to be recycled, ‘length()’ now is the length of the longest component.