I/O Question using APPEND

[larrybtoys]

Hello all...I am writing a program that will be running on several work stations tracking data and writing it to a data file on a server using the append function. My question is, even though it will take maybe a 10th of a second to open the file, append to it, and close it, what will happen if I while 1 work station is appending to the file another work station tries to do the same operation?


TIA 

[dragonjim]

If you are using Windows on the Workstation, it should lock the file while one user is accessing it, which will cause an I/O error if someone else tries to. This also happens when multithreaded applications try to access the same file at the same time.

The solution is to create error catch code which handles all errors when accessing the file and, in this instance, forces the second user to wait until the operation started by the first user has been completed.

[larrybtoys]

Can you give me an example of this. I don't see anything in the OPEN command help. I looked into GetAttr but it seemed to return 32 regardless of whether or not the file was opened already.

[dragonjim]

$Library "gfawinx"
$Library "UpdateRT"
UpdateRuntime      ' Patches GfaWin23.Ocx

// THIS CODE IS JUST TO SIMULATE ANOTHER USER ACCESSING THE FILE AND SETTING UP A STATUS WINDOW
Ocx Timer tmr : tmr.Interval = 2000 : tmr.Enabled = True        // Set up a Timer to simulate the first user activity
OpenW 1                                                         // Open a window...
Ocx Label lbl = "", 10, 10, 200, 15                             // ...and add a status label
Open App.Path & "\temp.dat" for Output As # 1                   // Simulate the first user accessing the file

// THIS IS WHERE YOUR TRIAL CODE BEGINS
Local fileerror As Boolean                                      /* Create a variable to determine if file error occured */ _
 // - this is needed as you should not jump out of a Try/Catch structure
tryagain:
Try
 Open App.Path & "\temp.dat" for Append As # 2                 // Simulate the second user accessing the file
 fileerror = False                                             // If successful, set error flag to false
Catch                                                           // Otherwise on an error
 If Not fileerror
   lbl.Text = "Error - " & Err$ & " (" & Err.Number & " )"     // [Extra Code] Set Status to show error and code
 EndIf
 fileerror = True                                              // Set error flag to True
 ~DoEvents                                                     // Allow other events to be processed
EndCatch
If fileerror Then GoTo tryagain                                 // If error flag is True, loop back and try again
lbl.Text = "File access OK"                                     // [Extra Code] Set Status to OK
~DoEvents                                                       // [Extra Code] Update label text
//...Add Your code to append data here...//
Close # 2

// THE REST OF THIS CODE IS EXTRA CODE TO TIDY UP
Close # 1
Do : Sleep : Until Win_1 Is Nothing
Sub tmr_Timer
 Close # 1                                                     // [Extra Code] Simulate the first user signing off
 tmr.Enabled = False                                           // [Extra Code] Turn off the Timer object
EndSub


[larrybtoys]

Great.. Thanks for the help. This should work great.