Q42773: Opening a C File When Disk Is Write Protected

Article: Q42773
Product(s): See article
Version(s): 5.00 5.10
Operating System(s): MS-DOS
Keyword(s): ENDUSER | s_quickc | mspl13_c
Last Modified: 17-MAY-1989

With the run-time library in the Microsoft C Optimizing Compiler, a
program can open an existing file for both read and write when the
floppy disk is write protected. The following statement may be used to
open the file:

   handle = open ("a:test.dat", O_RDWR | O_TRUNC) ;

No error condition is returned and no hard error occurs. However,
later, when the program tries to write to the file handle or even to
close the file without writing, a hard error will occur with the
following message:

   Writing protect error writing drive A:
   Abort, Retry, Fail?

This is not a problem with the open() function in the Microsoft C
run-time library. The low-level DOS function call that is used to
implement open() does not check for a write-protect error. When the
file is to be closed by close(), the internal buffer has to be flushed
to the disk. No low-level DOS function can close a file without
flushing its associated buffer.

Workaround

There is no direct way to detect the write-protect condition. An
indirect workaround is to open a file with the mode to be O_CREAT, as
follows:

    open ( "a:chk00000.xxx", O_CREAT, S_IWRITE | S_IREAD) ;

A hard error will occur, which can be captured by a user-implemented
and installed hard-error handler. This handler will override the
printing of the hard-error message on the user screen. The open()
function does return -1 when it regains the control from the
hard-error handle. If the file was opened successfully, it may be
removed at the end of the program.

The following sample program demonstrates checking of a
write-protected disk:

/* sample program */
#include <fcntl.h>
#include <sys\types.h>
#include <sys\stat.h>
#include <io.h>
#include <stdio.h>
#include <dos.h>
void far handler ( unsigned, unsigned, unsigned far * ) ;

#define PROTECTED 1
#define OTHER     2

int Flag = 0 ;

char * ChkName = "a:qwlbqwsi.ufp" ;     /* dummy file name */

/* Note:
** If the file happens to exist, the hard error will not occur.
** The program will output "Disk is not write-protected."
*/

void main(void)
{
int FileHandle;

_harderr ( handler ) ;              /* set up hard error handler */

FileHandle = open ( ChkName, O_CREAT, S_IWRITE | S_IREAD ) ;

if ( FileHandle == -1 )             /* check write-protect */
     {
     switch ( Flag ) {              /* may be set by the handler */
        case PROTECTED :
            puts ( "Disk in drive A: is write-protected." ) ;
            break ;
        case OTHER :
            puts ( "A another hard error has occurred." ) ;
            break ;
        default :
            puts ( "Error opening file (non hard error.)" ) ;
        }
     }
else {
     puts ( "Disk is not write-protected." ) ;
     close ( FileHandle ) ;
     remove ( ChkName ) ;           /* delete the file */
     }
}

/*
        Hard error routine should be as short as possible
*/
void far handler ( unsigned deverror, unsigned errcode,
                   unsigned far *devhdr )
{
if ( errcode == 0 )
    Flag = PROTECTED ;
else
    Flag = OTHER ;              /* like drive door is open */

_hardretn ( 0 ) ;
}
/* end of sample */

Note: The argument 0 to _hardretn() is not significant in this
program. Please refer to Page 351 of the "Microsoft C for the MS-DOS
Operating System: Run-Time Library Reference" for Version 5.10 for
more specific information regarding the _hardretn() function.

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