VMS Help CC, Language topics, Builtin Functions, Probe Write Accessibility  *Conan The Librarian (sorry for the slow response - running on an old VAX)

  The _PROBEW function checks the write accessibility of the first
  and last byte of the given address and length pair.

  Syntax:

       typedef enum {_probe_not_accessible, _probe_accessible}
                     _PROBE_STATUS;

       _PROBE_STATUS _PROBEW(char __mode, unsigned short __length,
                             const void *__address);