# 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.

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Source: https://d3codex.com/pickbasic-flashbasic/named-common-in-triggers/ - part of the D3Codex reference.