How to Retrieve Records That Were Previously Zapped (135905)


The information in this article applies to:

  • Microsoft Visual FoxPro for Windows 3.0
  • Microsoft FoxPro for MS-DOS 2.0
  • Microsoft FoxPro for MS-DOS 2.5
  • Microsoft FoxPro for MS-DOS 2.5a
  • Microsoft FoxPro for MS-DOS 2.5b
  • Microsoft FoxPro for MS-DOS 2.6
  • Microsoft FoxPro for MS-DOS 2.6a
  • Microsoft FoxPro for Windows 2.5
  • Microsoft FoxPro for Windows 2.5a
  • Microsoft FoxPro for Windows 2.5b
  • Microsoft FoxPro for Windows 2.6
  • Microsoft FoxPro for Windows 2.6a
This article was previously published under Q135905

SUMMARY

Sometimes, Microsoft Visual FoxPro's ZAP command can be reversed and you can retrieve all the records by using a low-level file I/O. This method works only if the data was not overwritten by other files. The approximate number of records to retrieve is also important.

MORE INFORMATION

Step-by-Step Example

  1. Open a program file named Lowlevel.prg and type in the following function: 
       FUNCTION UNZAP
   PARAMETER Y
   IF Y>0 .AND. USED()
      IF RECCOUNT()=0
         FILENAME=DBF()
         USE
         HANDLE=FOPEN(FILENAME,2)
         IF HANDLE>0
            BYTE=FREAD(HANDLE,32)
            BKUP_BYTE=BYTE
            FIELD_SIZE=ASC(SUBSTR(BYTE,11,1))+(ASC(SUBSTR(BYTE,12,1))*256)
            FILE_SIZE=FSEEK(HANDLE,0,2)
            BYTE8=CHR(INT(Y/(256*256*256)))
            BYTE7=CHR(INT(Y/(256*256)))                        BYTE6=CHR(INT(Y/256)-IIF(Y/256>256,(INT(INT(Y/256)/256)*256),0))
            BYTE5=CHR(MOD(Y,256))
            BYTE=SUBSTR(BYTE,1,4)+BYTE5+BYTE6+BYTE7+BYTE8+SUBSTR(BYTE,9)
            =FSEEK(HANDLE,0)
            =FWRITE(HANDLE,BYTE)
            =FCHSIZE(HANDLE,FILE_SIZE+(FIELD_SIZE*Y))
            =FCLOSE(HANDLE)
         ENDIF
         USE &FILENAME
      ENDIF
   ENDIF
  2. Save and close the Lowlevel.prg program file.
  3. Back up the table that you are going to experiment on to protect against data corruption.
  4. Test the function by zapping the back-up of the table and then use the UNZAP function to retrieve the records. The following example uses the Foxuser.dbf table:
    1. In the Command window, type: 
            SET PROCEDURE TO LOWLEVEL.PRG
      SET RESOURCE OFF
      USE FOXUSER
    2. Note how many records are in the foxuser file. For this example, assume that there are 50 records.
    3. In the Command window, type: 
            ZAP
      =UNZAP(50)
    4. Note that all of the records are retrieved.
  5. It is possible to use UNZAP to retrieve more records than what actually existed, especially if you do not know exactly how many records to retrieve.

    After you use UNZAP, you should browse the table to make sure only valid records were retrieved. Invalid records will immediately be apparent by the incomprehensible data in all the fields. Locate the last valid record and repeat step 4(c) and use the correct number of records.
  6. If the Foxuser.dbf table is corrupt, delete the file. FoxPro will create a new one.
Modification Type:MinorLast Reviewed:3/14/2005
Keywords:KB135905
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