PRB: Opening a C File When Disk Is Write-Protected

MS-DOS                      | WINDOWS

The information in this article applies to:

• The C Run-time (CRT), included with:

      - Microsoft C for MS-DOS, versions 5.1, 6.0, 6.0a, and 6.0ax
      - Microsoft C/C++ for MS-DOS, version 7.0
      - Microsoft Visual C++ for Windows, versionS 1.0 and 1.5
  

SYMPTOMS

Under MS-DOS, 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 );

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

CAUSE

This is not a problem with the open() function in the Microsoft C run-time library. The low-level MS-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 MS-DOS function can close a file without flushing its associated buffer.

RESOLUTION

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) ;

Sample Code

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

/* Compile options needed: none
*/

#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 */

void main( )

   int FileHandle;

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

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

   if( FileHandle == -1 )          /* check write-protection */
   {
       switch( Flag )              /* may be set by handler */
       {
          case PROTECTED :
             puts( "Disk in drive A: is write-protected." );
             break;
          case OTHER :
             puts( "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 );

NOTE: The argument 0 to _hardretn() is not significant in this program. Please refer to your Microsoft C run-time library reference manual for more specific information regarding the _hardretn() function.