File In Use Determination

[Bob Scott]

I'm new to PowerBasic but like it very much. I have a question
about the proper way to determine if a file is in use. I have
been Opening the file with LOCK READ WRITE and this seems to
work but it gets ugly in NT4 because I have to actually read
a few bytes from the file and write them back and that changes
the file date etc, etc, etc. So, I decided it was time to ask
for the correct way or at least a better way.

Thanks,

Bob


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[Phil Tippit]

Bob,

In a multi-user environment, where several users are accessing
the same data files, the standard way to open files would
be the following:

OPEN FNAME$ FOR BINARY ACCESS READ WRITE SHARED AS #F
OR
OPEN FNAME$ FOR INPUT ACCESS READ SHARED AS #F

using this type of open commands, you need to review
the LOCK and UNLOCK commands

Then if you wanted to test to see if another user has
a file open.

OPEN fname$ for binary as #F
ON ERROR GOTO ERRORHANDLER
A$=SPACE$(1):GET #F,1,A$ ' THIS will result a in error

or

OPEN FNAME$ FOR INPUT AS #F
ON ERROR GOTO ERRORHANDLER
line input #f,a$ ' this will result in a error


Phil

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E-Mail: [email protected]

[Bob Scott]

Phil,

Thanks for the reply. I'm doing exactly the way you describe by
opening the file in binary (the default being LOCK READ WRITE) and
in W95/W98 I don't even have to do a GET but in NT 4.0 in addition
to a GET I have to do a PUT to create the ERROR condition. And of
course that changes the file date etc. Isn't there a better way??

Thanks,

Bob

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[Greg Turgeon]

%HFILE_ERROR = -1
'====================
FUNCTION IsFileOpen&(The_File AS ASCIIZ)
LOCAL result&, ecode&
   
'_lopen(filename, OF_READ | OF_SHARE_EXCLUSIVE);
result& = lopen&(The_File, %OF_READ OR %OF_SHARE_EXCLUSIVE)
   
if (result& = %HFILE_ERROR) then
   '-- Save last error
   ecode& = GetLastError
else
   '-- If the open was successful, close the file
   lclose result&
end if
   
''msgbox str$(result&) + $CR + str$(ecode&)
'-- Return TRUE if sharing violation
function = ((result& = %HFILE_ERROR) AND (ecode& = %ERROR_SHARING_VIOLATION))
END FUNCTION

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-- Greg
[email protected]

[Tom Hanlin]

In general, it's not your business whether the file is in use, just whether you can
access it as needed. To that end, an appropriate OPEN should suffice. If you can
explain in more detail what you aim to accomplish, perhaps we can help you better.

Given appropriate access handling, you may not be able to detect whether a file is
in use, by the way... that depends as much on how others open the file as how you
open the file.


------------------
Tom Hanlin
PowerBASIC Staff

[Bob Scott]

Tom,

That's more than fair.

I am trying to find out when a particular third party program is
running. When started, the program awaits user information to
continue or even exit. My program was developed using W95 and
sucessfully uses a BINARY file OPEN on the third party program
(default LOCK READ WRITE)to trigger the "Permission Denied"
response without even having to resort to a GET to input a few
bytes.

My problem is making the program work in NT 4.0 where not only
do I need to use GET but I also need to use PUT to write the bytes
back to the file before "Permission Denied" is triggered. This
then results in a Last Write Time change for the third party file
and begins to get messy. So, that's why I am asking - is there a
better way??

Thanks,

~Bob


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[Tom Hanlin]

I'd expect an OPEN with LOCK READ WRITE to fail if the file is open, but that depends
on whether the other program also employs file locking and on the way the operating
system implements file-level locking errors. In this case, it sounds as if you've run
afoul of one and/or the other. You might look for other techniques. Barring that, the
pragmatic solution would be to save and restore the file time/date stamp, which is
simple using Windows API calls.

It generally isn't difficult to tell whether a given program is running by other
methods: window name, classes loaded, or whatever. Some such approach may work better
for you. If you're trying to detect a console app, that may be more of a problem, as
Windows seems to keep console functionality to a bare minimum.


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Tom Hanlin
PowerBASIC Staff

[Bob Scott]

I ran several tests and MS QBasic/QuickBasic responded identically
to PowerBasic - that is they all caught the "Permission Denied" response
in W95 when an in use file was opened for either INPUT or BINARY
regardless if any data was actually input or output to/from the file.
However, neither MS QuickBasic or PowerBasic trapped "Permission
Denied" in when running under NT 4.0.

The solution for this case turned out to be quite simple and works both
in W95 and NT 4.0. Attempt to OPEN the file for WRITE access using
the Win32API instead of PowerBasic per the following:

Thanks to all for your help!!

FUNCTION IsBusy(F1$) AS LONG

  DIM OFS AS OFSTRUCT

  h& = OpenFile(BYCOPY F1$, OFS, %OF_WRITE)
  IF h& > 0 THEN
    ' Not Busy
    x& = CloseHandle(h&)
    FUNCTION = FALSE
  ELSE
    ' Busy
    FUNCTION = TRUE
  END IF

END FUNCTION

[This message has been edited by Bob Scott (edited March 19, 2001).]

[Tom Hanlin]

To keep the formatting, start a code block with [ code] and end it with
[ /code] (leave out the space after the left bracket!).

The OpenFile API has a long history of bugs. You might find CreateFile a
safer choice.

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Tom Hanlin
PowerBASIC Staff

[Bob Scott]

Tom,

Thanks for all your help. I will try the CreateFile.

~Bob

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