FIX: READ Does Not Pause or F6501 Error After CTRL+Z (81326)


The information in this article applies to:

  • Microsoft FORTRAN Compiler for MS-DOS 4.0
  • Microsoft FORTRAN Compiler for MS-DOS 4.01
  • Microsoft FORTRAN Compiler for MS-DOS 4.1
  • Microsoft FORTRAN Compiler for MS-DOS 5.0
  • Microsoft FORTRAN Compiler for MS-DOS 5.1
This article was previously published under Q81326

SYMPTOMS

In an application compiled with Microsoft FORTRAN, when the user responds to a keyboard READ statement with a CTRL+Z character, subsequent READ statements do not pause for input or the run-time library generates the following message:
run-time error F6501: READ(CON)
- end of file encountered

CAUSE

When the user enters a CTRL+Z character to signify the end-of-file (EOF) condition, the run-time library sets an internal variable to indicate that the EOF has occurred. This variable remains set until the application executes a PRINT or WRITE statement. When the READ statement includes the END modifier and the internal EOF variable is set, execution branches immediately to the statement specified in the END modifier. When the READ statement does not include the END modifier and the internal EOF variable is set, the F6501 error occurs.

RESOLUTION

To address this problem, use the PRINT or WRITE statement to send information to the screen and to reset the internal EOF flag prior to each READ that may receive the CTRL+Z. Using the graphics function OUTTEXT to send output to the screen does not clear the internal EOF flag.

STATUS

Microsoft has confirmed this to be a problem in FORTRAN versions 4.0, 4.01, 4.1, 5.0 and 5.1 for MS-DOS. This problem was corrected in FORTRAN PowerStation, version 1.0.

MORE INFORMATION

The following code sample demonstrates this problem. If the user enters a CTRL+Z for the first READ statement, the application goes into an infinite loop.

Sample Code #1

C Compile options needed: None

      INCLUDE 'FGRAPH.FI'
      INCLUDE 'FGRAPH.FD'
      CHARACTER*1 CHR

10    READ (*, '(A)', END = 20) CHR
20    CALL OUTTEXT('Fill the screen!')
      READ (*, '(A)', END = 10) CHR
      PRINT *, CHR
      END
The following code example uses the WRITE statement to clear the internal EOF flag before the application encounters any EOF condition. When the user enters CTRL+Z, the application immediately resets the EOF flag and the each READ statement in the application pauses for input as expected.

Sample Code #2

C Compile options needed: None

      INCLUDE 'FGRAPH.FI'
      INCLUDE 'FGRAPH.FD'
      CHARACTER*1 CHR

      WRITE (*, *)
10    READ (*, '(A)', END = 20) CHR
20    CALL OUTTEXT('WRITES ONE LINE!')
      READ (*, '(A)', END = 10) CHR
      PRINT *, CHR
      END
Modification Type:MajorLast Reviewed:9/30/2003
Keywords:kbfix KB81326
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