BUG: Incorrect Jump to ERR Label in Windows DLLs

ID: Q84331

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 :
Version : :5.1
Platform :
Issue type :