How to Retrieve Records That Were Previously Zapped

• Microsoft Visual FoxPro for Windows, version 3.0
• Microsoft FoxPro for MS-DOS, versions 2.0, 2.5, 2.5a, 2.5b, 2.6, 2.6a
• Microsoft FoxPro for Windows, versions 2.5, 2.5a, 2.5b, 2.6, 2.6a

SUMMARY

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

MORE INFORMATION

Step-by-Step Example

1. Open a program file called Lowlevel.prg and type 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))
               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 Lowlevel.prg.

3. Back up the table you are going to experiment on to protect against corruption.

4. Test the function by zapping the back-up of the table and then using unzap() to retrieve the records. The following example uses the Foxuser.dbf table:

   a. In the Command window, type:

         SET PROCEDURE TO LOWLEVEL.PRG
         SET RESOURCE OFF
         USE FOXUSER
   

   b. Take note of how many records are in the foxuser file. For this

         example, assume there are 50 records.
   

   c. In the Command window, type:

         ZAP
         =UNZAP(50)
   

   d. Note that all of the records have been retrieved.

5. It is possible to use UNZAP() to retrieve more records than actually existed, especially if you didn't know how many records to retrieve. After using UNZAP(), you should browse the table to make sure only valid records have been 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. using the correct number of records.

6. If the Foxuser.dbf table is corrupt, delete the file. FoxPro will recreate a new one.