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Q60748: With Invalid COMSPEC, Invoking MS-DOS Shell Hangs Machine

Article: Q60748
Product(s): Microsoft Programming Utilities
Version(s): 2.2,3.0,3.11,3.12,3.14,3.5,4.0,4.01,4.05,4.1
Operating System(s): 
Keyword(s): kb16bitonly
Last Modified: 26-JUN-2001

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The information in this article applies to:

- Microsoft CodeView for MS-DOS, versions 2.2, 3.0, 3.11, 3.14, 4.0, 4.01, 4.05, 4.1 
- Microsoft CodeView for OS/2, versions 2.2, 3.0, 3.11, 3.12, 3.5 
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SUMMARY
=======

If you set your COMSPEC environment variable to point to an invalid command
interpreter, and then shell out of any MS-DOS application, your machine will
hang. This problem occurs because the file that COMSPEC points to is assumed to
be a valid command interpreter and cannot be checked for validity.

MORE INFORMATION
================

This is expected behavior. Because .COM files have no standard file header
structure, they cannot be checked for validity. Therefore, MS-DOS must assume
that whatever the COMSPEC environment variable points to must be a valid command
interpreter, and can do no further error checking.

This behavior can easily be demonstrated in any program that allows you to access
an MS-DOS shell, including CodeView, Programmer's WorkBench (PWB), the Microsoft
Editor (M), and the Quick environments. Type the following line at the MS-DOS
prompt:

  " SET COMPSPEC=a:\junk.c ; Invalid command.com file " (without the quotation
  marks)

Then enter an MS-DOS application and shell out. Your machine will hang, and you
may receive strange error messages.

If you are running under OS/2, you will be warned about an invalid command
interpreter when you attempt to shell to the operating system. Under OS/2, the
system expects an .EXE file to be the command interpreter, and .EXE files have a
standard, recognizable structure that can be checked.

Additional query words: kbinf 2.20 3.00 3.50 4.00 4.10

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Keywords          : kb16bitonly 
Technology        : kbAudDeveloper kbCodeView kbZNotKeyword3 kbCodeView220DOS kbCodeView300DOS kbCodeView311DOS kbCodeView314DOS kbCodeView400DOS kbCodeView401DOS kbCodeView405DOS kbCodeView410DOS kbCodeView220OS2 kbCodeView300OS2 kbCodeView311OS2 kbCodeView312OS2 kbCodeView350OS2
Version           : :2.2,3.0,3.11,3.12,3.14,3.5,4.0,4.01,4.05,4.1

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