As suggested in getdirentires function definition, the last argument "off_t *basep" is an optional output argument. It returns the value only when the passed-in value(pointer) is non-NULL.
This case is very similiar to sendfile, where the second last argument will get a value only when it is non-NULL. The SAL annotation for this argument in syscall.master is defined as _Out_opt_.
This is a part of the research work at RCSLab, University of Waterloo.