How to Truncate a File Using MS-DOS Interrupts from Basic (68159)

MORE INFORMATION

You can invoke MS-DOS service functions using the CALL INTERRUPT statement in Basic.

You can set the size of any file to any arbitrary value by executing MS-DOS Interrupt 21h with service 40h using CX = 00h. The usual technique is to call service 42h to set the file pointer location and then immediately call service 40h with CX = 00h to update the new file size.

The program below is written so that the Truncate% FUNCTION is generic as possible and works with any file. ProcessFile() sets up the file-specific information, which in turn calls the Truncate% FUNCTION. Truncate% generates the specific interrupt calls to truncate the file at the specified record. The two SUB programs PrintFile() and CreateFile() are only needed as an example.

Program Example

To try this example in VBDOS.EXE:

1. From the File menu, choose New Project.
2. Copy the code example to the Code window.
3. Press F5 to run the program.

To run this program in the VBDOS.EXE environment, you must invoke the VBDOS.EXE environment with the /L switch to load the default Quick library. For example: You must run QB or QBX with the /L option to load the QB.QLB or QBX.QLB Quick library which contains the INTERRUPT routine used in this program. You must LINK with QB.LIB or QBX.LIB when making an .EXE program which uses the CALL INTERRUPT statement.

CONST NumRec = 10
TYPE TheType
     i AS INTEGER
END TYPE
DIM SHARED TheDim AS TheType
' Use the following include file for Visual Basic for MS-DOS:
REM $INCLUDE: 'VBDOS.BI'
' Use the following include file for QuickBasic for MS-DOS:
'$INCLUDE: 'QB.BI'
' Use the following include file for Basic PDS for MS-DOS:
'$INCLUDE: 'QBX.BI'
CALL CreateFile
CALL PrintFile
CALL ProcessFile(7)
CALL PrintFile

'################### File specific SUB #############################
SUB ProcessFile (LastRec%)
' Get the information about "TEST.DAT" and also the information
' about the location of the LASTREC% where the file will be truncated.
     OPEN "test.dat" FOR RANDOM AS #1 LEN = LEN(TheDim)
     FilePointer% = LastRec% * LEN(TheDim)
     IF FilePointer% <> Truncate%(FILEATTR(1, 2), FilePointer%) THEN
          PRINT "error..."
     END IF
     CLOSE
END SUB

'################## Generic INT file truncator ####################
FUNCTION Truncate% (handle%, FilePointer%)
' Generic function that will truncate any file at specified offset.
'
' Receives:
'       1. a MS-DOS file handle;
'       2. a file pointer offset in bytes, pointing where to truncate
' Returns:
'       1. If successful, position of file pointer; or
'       2. If error, flags register
'
 DIM Regs AS RegType
     Regs.ax = &H4200                   ' INT 21h, service 42h
     Regs.bx = handle%                  ' MS-DOS Handle
     Regs.dx = FilePointer%             ' Offset in bytes from start.
     CALL Interrupt(&H21, Regs, Regs)
     IF Regs.ax <> FilePointer% THEN    ' AX returns pointer position.
          Truncate% = Regs.flags        '   If error, return flag.
     END IF
     Regs.ax = &H4000                     ' INT 21h, service 40h
     Regs.bx = handle%                    ' MS-DOS handle
     Regs.cx = &H0
     CALL Interrupt(&H21, Regs, Regs)
     IF Regs.ax <> 0 THEN               ' Must be zero bytes written.
          Truncate% = Regs.flags        ' If error, return flag.
     END IF
     Truncate% = FilePointer%          ' Return offset location.
END FUNCTION

'############ Create Sample File using RANDOM Access ################
SUB CreateFile
     KILL "test.dat"
     OPEN "test.dat" FOR RANDOM AS #1 LEN = LEN(TheDim)
     FOR j = 1 TO NumRec
          TheDim.i = j
          PUT #1, j, TheDim
     NEXT j
     CLOSE #1
END SUB

'########### Print contents of TEST.DAT ##################
SUB PrintFile
     OPEN "test.dat" FOR RANDOM AS #1 LEN = LEN(TheDim)
     Max = LOF(1) / LEN(TheDim)
     FOR j = 1 TO Max
          GET #1, j, TheDim
          PRINT TheDim.i
     NEXT j
     CLOSE
END SUB