PRB: Data Corruption with Row Buffering in Visual FoxPro 3.0

• Microsoft Visual FoxPro for Windows, versions 3.0, 3.0b

SYMPTOMS

Using optimistic or pessimistic table buffering with multiple users trying to simultaneously update a table, located on a Windows 95 machine, certain records in the table may become corrupted. This is sometimes seen more often with optimistic row buffering. This behavior does not happen in Visual FoxPro 5.0 or 5.0a.

RESOLUTION

Place the table(s) on a computer running Windows NT 4.0 using either the NT File System (NTFS) or File Allocation Table (FAT) hard disk partitions.

STATUS

Microsoft is researching this problem and will post new information here in the Microsoft Knowledge Base as it becomes available.

MORE INFORMATION

Steps to Reproduce Behavior

The example below uses a program to simulate 10 users opening the same table and simultaneously updating individual records.

1. Create a table called ztest using the following command:

         CREATE TABLE ztest (datadir c(100), inidir c(100), inifilenam c(12),;
         databasenm c(12), ihq_path c(100), intrqlpath c(100))
   

2. Add a record to the table using the following commands:

         APPEND BLANK
         REPLACE datadir WITH "First test record"
   

3. Create two programs called Usera.prg and Userb.prg that contains the following code:

         LOCAL laAlias, lnX
   

         SET EXCLUSIVE OFF
         SET MULTILOCKS ON
         SET REPROCESS TO 5
   

         DIMENSION laAlias[10]
         laAlias[1] = "A"
         laAlias[2] = "B"
         laAlias[3] = "C"
         laAlias[4] = "D"
         laAlias[5] = "E"
         laAlias[6] = "F"
         laAlias[7] = "G"
         laAlias[8] = "H"
         laAlias[9] = "I"
         laAlias[10] = "J"
   

         FOR lnX = 1 TO ALEN(laAlias)
            USE ztest again ALIAS &laAlias[lnX] in 0 SHARED
            SELECT &laAlias[lnX]
            =CURSORSETPROP("BUFFERING",3)
            APPEND BLANK
         ENDFOR
   
         FOR lnX = 1 TO ALEN(laAlias)
            SELECT &laAlias[lnX]
            REPLACE DataDir WITH STR(RECNO())+ "  UserA" + STR(RECCOUNT())
            llError=TABLEUPDATE()
            ?llERROR
            IF !LLERROR
               =MESSAGEBOX("Update Error",0,"Test")
            ENDIF
            USE
         ENDFOR
   
   

4. Place Ztest.dbf, Usera.prg, and Userb.prg in the same directory on a machine running Windows 95.

5. Share the folder and map the drive on another machine. This machine can be running either a Windows 95 or Windows NT 4.0.

6. Two users must work at the same time. One user will run Usera.prg on one machine and one user will run Userb.prg on another machine.

7. Both users run their programs from their respective Command windows simultaneously. For best results, run the programs two or three times. The corruption does not always show up on the first run.

8. When the programs finish, look at the Ztest.dbf. Notice some of the records contain corrupted data.

Keywords          : FxprgTable vfoxwin kbcode
Version           : WINDOWS:3.0,3.0b
Platform          : WINDOWS
Issue type        : kbprb