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.