PRB: fclose() on Unopened Files Causes Protection Violation

• The C Run-time (CRT), included with: - Microsoft C for MS-DOS, versions 5.1, 6.0, 6.0a, 6.0ax - Microsoft C/C++ for MS-DOS, version 7.0 - Microsoft Visual C++ for Windows, versions 1.0 1.5 - Microsoft Visual C++ 32-bit Edition, versions 1.0, 2.0, 4.0, 5.0

SYMPTOMS

An attempt to close a file with the fclose() function fails and one of the following occurs:

    - A protection violation or application error occurs
    - A "R6001 - null pointer assignment" error occurs

CAUSE

The specified file was not open.

16-bit applications: The source code is compiled in the compact or large memory models. In the small memory model, fclose() may return EOF as expected or it may generate an error cited above.

RESOLUTION

Modify the source code to only close files that are already open.

MORE INFORMATION

An attempt to close a file that is not open is a user error. The C run- time library does not verify the validity of file handles passed to the fclose() function.

The code example below demonstrates this behavior.

Sample Code

   /*
    * Compile options needed - 16-bit: /AL
    *                        - 32-bit: none
    */

   #include <stdio.h>
   FILE *myfile;
   int status;

   main()
   {
      myfile = fopen("myfile.dat", "r");
      if (myfile == NULL)
         printf("file open error\n");
      status = fclose(myfile);
      printf("file close status = %d\n", status);
   }

Keywords          : CRTIss kbfasttip
Version           : MS-DOS:5.1,6.0,6.00a,6.00ax,7.0; WINDOWS:1.0,1.5; WINDOWS  NT:1.0,2.0,2.1,4.0,5.0
Platform          : MS-DOS NT WINDOWS
Issue type        : kbprb