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Some MFSs require access to the name of the file as seen by users. For instance, an audit trail MFS will likely wish to stamp the OpenInsight file name on entries in the log. Many MFSs must also use the file name, or information based on it, to open and access related files. An example is SI.MFS, which must open the ! file associated with a data file.
Modifying the File Handle
A second method is to make the OpenInsight file name part of the file handle. This guarantees that the file name is always available whenever the file handle is passed to the MFS.
In this method, the MFS traps the OPEN.FILE call as the BFS is returning the file handle back to BASIC+. During an OPEN.FILE call, the OpenInsight file name is available in the NAME argument. The MFS simply concatenates the file name onto the file handle with a delimiter before returning to the calling program. The BASIC+ program itself is not affected; it merely stores the handle until the next statement requiring it is executed.
When concatenating the two arguments, the MFS should avoid using delimiter characters that might otherwise appear in either argument. Characters to avoid include "!", "_", and "." (all of which frequently appear in the OpenInsight file name), as well as subvalue and value marks, which appear in the handle.
When subsequent file handle-oriented calls are made to the MFS, the handle will be returned to the MFS exactly the way the MFS has constructed it. The MFS must then strip the file name from the handle before passing the handle on for further processing.
The following code fragment illustrates how the OPEN.FILE logic might look in an MFS that is tracking the OpenInsight file handle:
| Code Block |
|---|
OPEN.FILE:
FS = DELETE(BFS,1,1,1)
NEXTFS = FS<1,1,1>
CALL @NEXTFS(CODE, FS, HANDLE, NAME, FMC,RECORD,STATUS)
* add filename onto front of file handle,
* delimit with special character
RECORD = NAME:@TM:RECORD
RETURN |
In this example, the MFS delimits the file name and file handle with a text mark (@TM). The same MFS uses logic as illustrated below to parse the file name out of the handle for the MFS calls that require a file handle:
| Code Block |
|---|
* all following labels share the same logic
CLEARSELECT:
SELECT:
READNEXT:
READ.RECORD:
READO.RECORD:
WRITE.RECORD:
DELETE.RECORD:
LOCK.RECORD:
UNLOCK.RECORD:
* get clear file name, strip from file handle
FILE.NAME = FIELD(HANDLE, @TM , 1)
REAL.HANDLE = FIELD(HANDLE, @TM , 2)
FS = DELETE(BFS, 1, 1, 1)
NEXTFS = FS<1,1,1>
CALL @NEXTFS(CODE,FS,REAL.HANDLE,NAME,FMC,RECORD, STATUS)
RETURN. |
Maintaining a Filename Table
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