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The portable SUBLIB is completely independent from the old VMS-specific SUBLIB. As a matter of fact, an application cannot use both at the same time. If it is a portable application, it must use the new library. If it has not yet been ported, it must use the old one.
There are many reasons for the complete split between the old and the new subroutines. The major reason is that it allows programmers to change the calling sequences easily for the SUBLIB routines, while still retaining the old names. Since the names are duplicated, an attempt to link to both libraries would cause an error. Another reason is that it ensures portable applications will not attempt to use non-portable subroutines, an error that would not show up on the VMS system. Finally, it makes configuration management of the system much easier, since portable subroutines are clearly isolated and identified as such.