Redirecting a TSR May Leave File Handle Open (84718)


The information in this article applies to:

  • Microsoft MS-DOS operating system 3.1
  • Microsoft MS-DOS operating system 3.2
  • Microsoft MS-DOS operating system 3.21
  • Microsoft MS-DOS operating system 3.3
  • Microsoft MS-DOS operating system 3.3a
  • Microsoft MS-DOS operating system 4.0
  • Microsoft MS-DOS operating system 4.01
  • Microsoft MS-DOS operating system 5.0
  • Microsoft MS-DOS operating system 5.0a
  • Microsoft MS-DOS operating system 6.0
  • Microsoft MS-DOS operating system 6.2
  • Microsoft MS-DOS operating system 6.21
  • Microsoft MS-DOS operating system 6.22
This article was previously published under Q84718

SUMMARY

If you run a terminate-and-stay-resident (TSR) program and redirect the output to a file using a command similar to

TSR > file

the file handle of the redirected file is not closed automatically by MS-DOS. In this case, you can run out of file handles if the TSR reinstalls itself each time it is invoked.

MORE INFORMATION

When a program is executed, it inherits all the files its "parent" (usually COMMAND.COM) had open. At a minimum, this usually includes the file handles for the standard devices: STDIN, STDOUT, STDAUX, STDERR, and STDPRN.

If the input or output of a program is redirected, MS-DOS opens the specified file(s) and redirects STDIN or STDOUT to the file specified before executing the program. The program will input and output information as normal, but anything read from STDIN or written to STDOUT will actually go to the specified file instead. The program is not aware of this redirection (and if the program does not use STDIN or STDOUT, the redirection will appear "fail").

When the program exits, MS-DOS automatically closes the handles that were opened due to redirection. TSR programs stay resident instead of exiting, thus the redirected files are not closed automatically by MS-DOS. If MS-DOS closed file handles when a program terminates but stays resident, problems could occur if the TSR tried to use the file(s) at a later time.

TSR programs can avoid this problem by closing file handle(s) when the TSR program is finished using them. For example, if the program has sent all its startup information to STDOUT and will not be using STDOUT in the future, the program can close STDOUT before terminating. If the program has read all necessary input from STDIN, close STDIN.
Modification Type:MajorLast Reviewed:5/12/2003
Keywords:KB84718
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