Q68159: How to Truncate a File Using DOS Interrupts from BASIC

Article: Q68159
Product(s): See article
Version(s): 4.00 4.00b 4.50
Operating System(s): MS-DOS
Keyword(s): ENDUSER | SR# S901228-74 B_BASICCOM | mspl13_basic
Last Modified: 15-JAN-1991

The following BASIC code shows how to truncate a file using DOS
functions. A more clumsy approach in BASIC would be to read a
specified amount of records one at a time, writing these records to
another file, deleting the first file, and then renaming the new file
to the old file. The approach using DOS functions (below) is quicker
and more efficient.

This information applies to QuickBASIC versions 4.00, 4.00b, and 4.50
for MS-DOS; to Microsoft BASIC Compiler versions 6.00 and 6.00b for
MS-DOS; and to Microsoft BASIC Professional System (PDS) versions 7.00
and 7.10 for MS-DOS.

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 21 hex with service 40 hex using CX = 00 hex. The
usual technique is to call service 42 hex to set the file pointer
location and then immediately call service 40 hex with CX = 00 hex 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
---------------

' 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
' In the following line, use 'QBX.BI' for BASIC PDS, or use 'QB.BI'
' for QuickBASIC 4.x:
'$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 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 21 hex, service 42 hex
     Regs.bx = handle%                    '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 21 hex, service 40 hex
     Regs.bx = handle%                    '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

References:

The INTERRUPT routine is located in the files QB.LIB and QB.QLB for
QuickBASIC 4.x and in QBX.LIB and QBX.QLB in BASIC PDS 7.00/7.10.
Programs that execute a CALL INTERRUPT statement when compiled in the
QB.EXE/QBX.EXE editor require the presence of the QB.QLB/QBX.QLB Quick
library. This means that QB.EXE or QBX.EXE must be invoked with the /L
option, which automatically loads the correct Quick library. Compiled
programs that execute CALL INTERRUPT must be linked with the LINK
library QB.LIB or QBX.LIB. More information on the use of CALL
INTERRUPT can be found under the CALL statement in the language
reference manual or online help. For more information on how to use
CALL INTERRUPT, query on the following words:

   CALL and INTERRUPT and application and note and QuickBASIC

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