FIX: TMP File Errors If ALTER TABLE Runs Same Time As ODBC DLL

If another process is using the VFP ODBC driver and DLL at the same time as the program is running in Visual FoxPro, and the temp file location is the same for Visual FoxPro as it is for the operating system, then one of the following error messages may occur: NOTE: test_odbc refers to an ODBC data source name.

Furthermore, if two instances of Visual FoxPro 3.0x or 5.0x are running at the same time and are they are creating temp files in the same location, the following errors may occur:

Visual FoxPro and the VFP ODBC driver both use the same naming convention and algorithm for temporary (.tmp) file creation. If both programs run concurrently, there is a conflict in the processes attempting to access the same file or same file name. This conflict creates different error messages.

Visual FoxPro 5.0x uses a tempfile naming scheme based on the system clock. These names are generated for internal use and many times the filename is never actually created on the disk. However, there are many circumstances when FoxPro does create the temporary file on disk, so the name generation scheme could cause two processes or two instances of the run-time in the same process to generate the same temporary file name. If both processes try to create a temporary file on disk later, only the first one succeeds. Most often, the temporary files are used and closed before they are ever created on the disk.

Visual FoxPro creates temporary files that have names like 64382453.tmp. The files are created in the location designated by the parameters given for TMPFILES=, EDITWORK=, PROGWORK=, and SORTWORK= in the Visual FoxPro configuration file, Config.fpw, by default or in the operating system's SET TEMP parameter location.

Each session of Visual FoxPro needs to have a separate location for its temporary files. The steps to implement this are given below.

1. Create a subdirectory on the disk for Visual FoxPro's temporary files similar to D:\vfptemp. Make sure that each session of Visual FoxPro has a unique temporary file location.
2. Create a Config.fpw for each session with parameters that point to the temporary file location:

         TMPFILES=D:\vfptemp
         EDITWORK=D:\vfptemp
         PROGWORK=D:\vfptemp
         SORTWORK=D:\vfptemp
   							

3. Append a -c parameter to the Visual FoxPro startup command line for each session to specify the Config.fpw file to use. An example is below:

         "D:\Visual Studio\VFP\VFP.EXE" "-cd:\Visual Studio\config.fpw"
   						

Microsoft has confirmed this to be a bug in the Microsoft products listed at the beginning of this article. This has been corrected in Visual FoxPro 6.0.

In Visual FoxPro 5.0a, the least-significant-bits of the system clock are mapped into 8 base ten digits. The possible values are 10^8 = 100,000,000. This means that temporary file names are recycled every ~28 hours, since the system clock ticks at 1000 ticks/second. Also, if any two processes or instances generated a name within the same timer tick (easily possible on a fast machine), then the two could have a filename collision, as seen in the scenario below.

Visual FoxPro 5.0x does not delete the .tmp files that are created using the CREATE CURSOR and/or ALTER TABLE commands.

Steps to Reproduce Behavior

1. Start Visual FoxPro 5.0a. This is the first session.
2. Create a program file named Testtmp1.prg and add the following code, but do not run the code yet:

         * Testtmp1.prg
         _SCREEN.CAPTION = "1st Session VFP"
         * FOR ... NEXT loop needs to run long enough that focus
         * may be switched to other FoxPro session and run similar prg.
         * 1000 is arbitrary.
         FOR i = 1 to 1000
            CREATE CURSOR testa (a i, b i)
            ALTER TABLE testa ADD c i
            SELECT testa
            USE
         ENDFOR
   							

3. Start Visual FoxPro 5.0a again. This is the second session.
4. Create a program file named Testtmp2.prg and add the following code, but do not run the code yet:

         * Testtmp2.prg
         _SCREEN.CAPTION = "2nd Session VFP"
         * FOR ... NEXT loop needs to run long enough that focus
         * may be switched to other FoxPro session and run similar prg.
         * 1000 is arbitrary.
         FOR i = 1 to 1000
            CREATE CURSOR testa (a i, b i)
            ALTER TABLE testa ADD c i
            SELECT testa
            USE
         ENDFOR
   							

5. Switch to the first session of Visual FoxPro and run the program.
6. Switch to second session of Visual FoxPro and run the program.
7. One of the error messages listed in the SYMPTOMS section appears in one or both of the Visual FoxPro sessions.
8. Clean the left over temporary files out of the Temp directory.

Microsoft FoxPro Help; search on: "temporary files"

(c) Microsoft Corporation 1998, All Rights Reserved. Contributions by Perry Newton, Microsoft Corporation