Named common in triggers

Triggers can use named commons to create persistent data across the various operations.

Example(s)

To audit all accesses to a file, the application opens the audit file in the open trigger and stores the file descriptor in the common, along with the file name and an audit message number:

OpenTrigger
   subroutine OpenTrigger( InputFileName )
      common /audit/ AuditFd, FileName, MsgNumber
      open “dm,Audit,” to AuditFd
      FileName = InputFileName
      MsgNumber = 1
   writev “Opening “:FileName to AuditFd, FileName, MsgNumber
   return

The write trigger or the read trigger accesses the audit file using this file descriptor in the common:

ReadTrigger
   subroutine ReadTrigger( Record )
      common /audit/ AuditFd, FileName, MsgNumber
      Msg = “Reading “:Access(10):” from “:FileName
      MsgNumber = MsgNumber + 1
      writev Msg to AuditFd, FileName, MsgNumber
   return

The close trigger closes the audit file:

CloseTrigger
   subroutine CloseTrigger( File )
      common /audit/ AuditFd, FileName, MsgNumber
      Msg = “Closing:FileName
      MsgNumber = MsgNumber + 1
      writev Msg to AuditFd, FileName, MsgNumber
      close AuditFd
   return

Note:

Triggers are attached to the file object, and while the code is shared among all users, the named commons are not. In other words, the data stored in a named common is private to the client application. Also, if a file is opened more than once (aliases), the file descriptors have different named common spaces. Make sure all accesses to the file are done using the same file descriptor.
The named common space is destroyed when the file object is destroyed, not closed. This distinction does not affect VME applications for which closing a file implies the object destruction, but for VB and C++ applications it is possible to close the file without destroying the object.
Commons are shared.