Q84331: BUG: Incorrect Jump to ERR Label in Windows DLLs

Article: Q84331
Product(s): Microsoft Fortran Compiler
Version(s): 5.1
Operating System(s): 
Keyword(s): 
Last Modified: 07-NOV-1999

-------------------------------------------------------------------------------
The information in this article applies to:

- Microsoft FORTRAN for Windows, version 5.1 
-------------------------------------------------------------------------------

SYMPTOMS
========

Opening a file from within a Windows DLL (dynamic-linked library) created with
FORTRAN 5.1 may incorrectly branch to the ERR label specified in the OPEN
statement.

The problem occurs only when all of the following conditions exist:

- The name of the file to be opened is generated within the OPEN statement with
  a substring.

- The substring used for the filename is specified using an integer variable as
  the substring index.

- This substring is concatenated to another string or variable.

- The ERR label has been specified within the OPEN statement.

CAUSE
=====

The problem occurs because of a call within the LDLLFEW.LIB library to an
internal routine called __FCchfr. Following the call to this routine, the AX
register becomes corrupted and the ERR label is branched to, even though the
file was successfully opened. The IOSTAT variable also contains incorrect
information after the OPEN operation. The same code compiled as an MS-DOS
executable, an OS/2 executable, or an OS/2 DLL does not produce the run-time
problem.

RESOLUTION
==========

To avoid this problem, use a temporary CHARACTER variable to assign the result
from the concatenation and substring operations. Then use this temporary
variable as the filename in the OPEN statement.

STATUS
======

Microsoft has confirmed this to be a problem in the library LDLLFEW.LIB shipped
with Microsoft FORTRAN version 5.1.

This is not a problem in FORTRAN PowerStation.

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

Sample codes 1 and 2 together reproduce the problem. The solution suggested is
demonstrated in sample code 3.

Sample Code 1
-------------

This is the source code for the SAMPLE.FOR file:

  c compile line: fl -MW sample.for

        PROGRAM Sample
        INTEGER*4 Init
        CHARACTER*8 String
        String = 'Test'
        PRINT *, Init(String)   ! the Errant DLL is called here.
        END

Sample Code 2
-------------

This is the code for the SAM_DLL.FOR file:

  c compile line: fl -GW -AW sam_dll.for

        INTEGER*4 FUNCTION Init (String1)
        CHARACTER*8 String1
        INTEGER*4 Stat, i
        i = 4
        OPEN (1, FILE = String1(:i) // '.DAT', IOSTAT=Stat,
       +  ERR=20, STATUS='UNKNOWN')
        WRITE (1, *) 'In DLL,  Successful Open' ! Will not be reached.

   20   CONTINUE                        ! Incorrectly, branches to here.

        Init = Stat
        CLOSE (1)
        RETURN
        END

Sample Code 3
-------------

This is the code for the DLL with the suggested solutions:

  c compile line: fl -GW -AW sam_dll.for

        INTEGER*4 FUNCTION Init (String1)
        CHARACTER*8 String1
        CHARACTER*12 FileName
        INTEGER*4 Stat, i
        i = 4
        FileName = String1(:i) // '.DAT'   ! Temporary variable to store
                                           ! the filename.
        OPEN (1, FILE = FileName, IOSTAT=Stat, ERR=20, STATUS='UNKNOWN')
        WRITE (1, *) 'In DLL With Successful Open'

   20   CONTINUE

        Init = Stat
        CLOSE (1)
        RETURN
        END

Additional query words: 5.10

======================================================================
Keywords          :  
Technology        : kbAudDeveloper kbFortranSearch kbFORTRAN510QuickWin
Version           : :5.1

=============================================================================

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