Simultaneously write file

[Michael Mattias]

In another executable you cannot use the same handle, because they don't share the same memory space...

You would not want to use the same handle (although you actually could do that by passing a duplicated handle, but that is esoteric by anyone's standards).

Any "handle" of any type is always specific to and valid only in the process in which it was obtained or for which it was duplicated.

Not quite on topic but.....

You know a long time ago I wrote an MS-DOS demo to test/review file status in the shared/locked file environment. You could do things like select what file to open in what mode with what locking and sharing , then lock / free individual records and try writing, reading, etc and query the status codes and data.

I wonder if maybe something like this for Windows - using two separate processes - might not be a handy thing? It could allow 'network testing' without having two computers.

(Since MS-DOS was single-tasking only, the only way the old tool was really useful was to run two separate copies of the program on two seprate computers and have each computer open the same network file).

Maybe this is another possibility for one of Mr. Purvis' contests? Surely it could be useful to "newbies to multiuser programming."

MCM

[Michael Mattias]

Well, in a real application, I need that 2 programs write in the same file at same time.
The threads were a simulation in fact.

Two threads of the same process do not in any way simulate two processes writing to the same file.

For data integrity issues involved in multi-user programming, here's an article and a little code you can download from my website:

http://www.talsystems.com/tsihome_html/downloads/multiuser_programming.rtf

MCM

[Peter Lameijn]

In a DLL used by the program, you can just pass the handle to the DLL.
In another executable you cannot use the same handle, because they don't share the same memory space...
Using several threads is something completely different from using several executables. You cannot use the threaded example to simulate multiple executables...

[Paulo Ferreira]

Well, in a real application, I need that 2 programs write in the same
file at same time.
The threads were a simulation in fact.
Like propose P.Lameijn, open "Teste.txt" previously, then all threads
writes at the file "simultaneously".
Well, if a program open a file, and other program (or dll) need to write
in this file, I want to know the file handle.
I can get the file open status.
Do I get file handle with its name ?

[Peter Lameijn]

I changed the code for hf to be a global long, that will work. (You don't need separate handles in the threads, because they share the global vars)
I can imagine that it's used for logging some data on several threads running to handle user requests. (I use a similar thing with a voice response system serving 60 lines on an ISDN system with equal threads, all logging to a shared logfile)

[Richard Angell]

Richard,
Your example won't work in all cases. (Because it's not assured that thread #1 will run as first one...)

Yes, so I suggested a delay before starting the other threads, yet even at 60 x 3, no observed problems. Yet the example is *not* real world. But I agree, Peter, that doing an OPEN or KILL in the MAIN should effecively begin a new file as well, then start the threads.

Your code does not do that however, and the threads do not know the handle as it is not passed to them. Setting it global does not work either. In fact each thread wants it's file handle for APPEND, otherwise nothing gets written to the file.

I suppose if one wants to have threads monitoring separate 'ports" that you want to grab the data from and file in one file maybe the situation might be real world, but as Michael correctly points out that would need for a critcal section approach (to avoid lost data). You can easily simulate such by introducing random delays in each thread before a write. Some data gets to the file, some does not.

[Michael Mattias]

It makes no sense to do these writes in a separate thread of execution. But let's just say this is a demo, where the ordinary rules don't apply... in that case....

You'd want to use a critical section as a synchronization object. (q.v. in your WinApi reference or in at least two thousand six hundred forty-one places here).

[Peter Lameijn]

No, but every time you open the file for output, the previous contents gets lost because it is overwritten. Just open the file for append, or open the file only once for output in PbMain() before starting the threads, and close the file if the threeads are closed...

#COMPILE EXE

GLOBAL hf AS LONG

FUNCTION PBMAIN () AS LONG

LOCAL x AS LONG
LOCAL s AS LONG

DIM idThread(10) AS LONG

hF&=FREEFILE
ERRCLEAR
OPEN "TESTE.txt" FOR OUTPUT LOCK SHARED AS #hF&
IF ERR THEN MSGBOX STR$(ERR)

FOR x = 1 TO 3

  THREAD CREATE MyThread(x) TO idThread(x)
  SLEEP 0

NEXT

MSGBOX "3 Threads Started!" & $CRLF & "Wait for them to finish!"

DO
  FOR x = 1 TO 3
    THREAD STATUS idThread(x) TO s
    IF s THEN ITERATE DO
  NEXT
LOOP WHILE s

CLOSE #hf&

MSGBOX "Finished!"

END FUNCTION

FUNCTION MyThread (BYVAL x AS LONG) AS LONG

LOCAL n AS LONG
LOCAL t AS SINGLE

FOR n = 1 TO 20
  
  PRINT #hF&, FORMAT$(n,"00")+" "+STR$(x)
 
NEXT n

MSGBOX "end : " & STR$(x) 

END FUNCTION

[Paulo Ferreira]

Ok. Thanks friends.
So, you say me that "simultaneous" write is not possible.
I have to wait the end of a process and so start other to write access.
Is it ?

[Peter Lameijn]

In the first post you read: "File "Teste.txt" would save 60 regs. (20 each thread)."
So it needs appending.

[edit]
Richard,
Your example won't work in all cases. (Because it's not assured that thread #1 will run as first one...)
[/edit]

[Richard Angell]

Peter may be totally or partially correct. It depends on whether this file needs to continually grow or start anew each time. The code below starts anew with the first thread, so may want a slight delay in starting the threads:

FUNCTION MyThread (BYVAL x AS LONG) AS LONG
    LOCAL n AS LONG
    LOCAL t AS SINGLE
    LOCAL hF AS LONG
    LOCAL a$
    hF&=FREEFILE
    IF x = 1 THEN
        OPEN "TESTE.txt" FOR OUTPUT LOCK SHARED AS #hF&
        IF ERR THEN
            ? STR$(ERR)
            EXIT FUNCTION
        END IF
    ELSE
        OPEN "TESTE.txt" FOR APPEND LOCK SHARED AS #hF&
        IF ERR THEN
            ? STR$(ERR)
            EXIT FUNCTION
        END IF
    END IF
    FOR n = 1 TO 20
      PRINT #hF&,FORMAT$(n,"00")+" "+STR$(x)
    NEXT n
    CLOSE #hF&
    ? "end : " & STR$(x)
END FUNCTION

[Elias Montoya]

True.

[Peter Lameijn]

If you open the file for output, it will get overwritten by each thread. Open the file in APPEND mode...

[Elias Montoya]

Try this:

DO 
 hF&=FREEFILE
 ERRCLEAR
 OPEN "TESTE.txt" FOR APPEND LOCK SHARED AS #hF&
 IF ERR THEN SLEEP 256
LOOP UNTIL ISFALSE(ERR)

[Paulo Ferreira]

File "Teste.txt" would save 60 regs. (20 each thread).
What is wrong in this code ?
Thanks

#COMPILE EXE

FUNCTION PBMAIN () AS LONG

LOCAL x AS LONG
LOCAL s AS LONG

DIM idThread(10) AS LONG

FOR x = 1 TO 3

  THREAD CREATE MyThread(x) TO idThread(x)
  SLEEP 0

NEXT

MSGBOX "3 Threads Started!" & $CRLF & "Wait for them to finish!"

DO
  FOR x = 1 TO 3
    THREAD STATUS idThread(x) TO s
    IF s THEN ITERATE DO
  NEXT
LOOP WHILE s

MSGBOX "Finished!"


END FUNCTION

FUNCTION MyThread (BYVAL x AS LONG) AS LONG

LOCAL n AS LONG
LOCAL t AS SINGLE

hF&=FREEFILE
ERRCLEAR
OPEN "TESTE.txt" FOR OUTPUT LOCK SHARED AS #hF&
IF ERR THEN MSGBOX STR$(ERR)

FOR n = 1 TO 20
  
  PRINT #hF&, FORMAT$(n,"00")+" "+STR$(x)
 
NEXT n

MSGBOX "end : " & STR$(x) 

END FUNCTION