Q38071: "Permission Denied" If SHELLed Process Accesses Same OPEN File

Article: Q38071
Product(s): See article
Version(s): 4.00 4.00b 4.50
Operating System(s): MS-DOS
Keyword(s): ENDUSER | B_BasicCom | mspl13_basic
Last Modified: 18-SEP-1990

"Permission Denied" normally occurs when you attempt to access a file
that is locked by another process or attempt to write to a
write-protected disk.

For example, if you OPEN a file, LOCK any record, then SHELL to
another program that attempts to access the same file, a "Permission
Denied" error properly occurs.

This information applies to Microsoft QuickBASIC versions 4.00, 4.00b,
and 4.50 for MS-DOS; to Microsoft BASIC Compiler versions 6.00 and
6.00b for MS-DOS and MS OS/2; and to Microsoft BASIC Professional
Development System (PDS) versions 7.00 and 7.10 for MS-DOS and MS
OS/2.

The following program, which SHELLs to a copy of itself, correctly
gives a "Permission Denied" error when run from an .EXE file or from
within the QB.EXE or QBX.EXE editor (if you have first created
TEST.EXE):

   TEST.BAS
   --------

   OPEN "TEST.DAT" FOR RANDOM AS #1
   LOCK #1, 3
   SHELL "TEST.EXE"
   UNLOCK #1, 3
   END

The following program also demonstrates "Permission Denied" if you
attempt to run TEST.EXE from within the SHELL session, before giving
the EXIT command to end the child process (session):

   OPEN "TEST.DAT" FOR RANDOM AS #1
   LOCK #1, 3
   SHELL          ' Before saying EXIT, run TEST.EXE to show the
   UNLOCK #1, 3   ' "Permission Denied" message.
   END

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