Methods of Handling Errors

Whenever possible, the Intel® Fortran RTL does certain error handling, such as generating appropriate messages and taking necessary action to recover from errors. You can explicitly supplement or override default actions by using the following methods:

Use the END, EOR, and ERR Branch Specifiers

When a severe error occurs during Intel® Fortran program execution, the default action is to display an error message and terminate the program. To override this default action, there are three branch specifiers you can use in I/O statements to transfer control to a specified point in the program:

If you use the END, EOR, or ERR branch specifiers, no error message is displayed and execution continues at the designated statement, usually error-handling code.

You might encounter an unexpected error that the error-handling routine cannot handle. In this case, do one of the following:

After you modify the source code, compile, link, and run the program to display the error message. For example:

    READ (8,50,ERR=400)

If any severe error occurs during execution of this statement, the Intel® Fortran RTL transfers control to the statement at label 400. Similarly, you can use the END specifier to handle an end-of-file condition that might otherwise be treated as an error. For example:

    READ (12,70,END=550)

When using non-advancing I/O, use the EOR specifier to handle the end-of-record condition. For example:

150 FORMAT (F10.2, F10.2, I6)
    READ (UNIT=20, FMT=150, SIZE=X, ADVANCE='NO', EOR=700) A, F, I

You can also use ERR as a specifier in an OPEN, CLOSE, or INQUIRE statement. For example:

    OPEN (UNIT=10, FILE='FILNAME', STATUS='OLD', ERR=999)

If an error is detected during execution of this OPEN statement, control transfers to the statement at label 999.

Use the IOSTAT and IOMSG Specifiers, and Fortran Exit Codes

The IOSTAT specifier can be used to continue program execution after an I/O error, or an end-of-file or end-of-record condition occurs, and to return information about the status of I/O operations. Certain error conditions are not returned in IOSTAT.

The IOSTAT specifier can supplement or replace the use of the END=, EOR=, and ERR= branch specifiers.

Use of an IOSTAT= specifier in an I/O statement prevents initiation of error termination if an error occurs during the execution of the I/O statement. The integer variable specified in the IOSTAT= specifier becomes defined during the execution of the I/O statement with the following values:

Note that the value assigned to the IOSTAT variable is the same value that would be returned as an exit code if error termination was initiated.

In addition, many I/O statements support an optional IOMSG= specifier. If an error, end of file, or end of record error occurs during the execution of the statement containing the specifier, the default character scalar variable specified in the IOMSG= specifier becomes defined with an explanatory message, truncated or padded as in intrinsic character assignment. If none of these conditions occur, the value of the IOMSG variable is unchanged.

Following the execution of the I/O statement and assignment of an IOSTAT and IOMSG value, control transfers to the END=, EOR=, or ERR= statement label, if any. If there is no control transfer, normal execution continues.

The non-standard include file FOR_ISODEF.FOR and the non-standard module FORISODEF contain symbolic constants for the values returned through an IOSTAT= specifier.

The following example uses the IOSTAT and IOMSG specifier and the module FORIOSDEF to handle an OPEN statement error (in the FILE specifier):


     USE foriosdef
     IMPLICIT NONE
     CHARACTER(LEN=40) :: FILNM
     CHARACTER(LEN=128) :: MSG
     INTEGER IERR
     PRINT *, 'Type file name:'
     READ (*,*) FILNM
     OPEN (UNIT=1, FILE=FILNM, STATUS='OLD', IOSTAT=IERR, ERR=100, IOMSG=MSG)
     PRINT *, 'Opening file: ', FILNM
     ! process the input file
     ...
     CLOSE (UNIT=1)
     STOP
 100 IF(IERR . EQ. FOR$IOS_FILNOTFOU) THEN
        PRINT *, 'File: ', FILNM, ' does not exist '
     ELSE 
        PRINT *, 'Unrecoverable error, code =', IERR
        PRINT *, 'Error description =', MSG
     END IF
     END PROGRAM

Another way to obtain information about an error is by using the ERRSNS subroutine, which allows you to obtain the last I/O system error code associated with an Intel® Fortran RTL error.

Use the STAT or STATUS argument, or STAT and ERRMSG Specifiers

ALLOCATE, DEALLOCATE, and image control statements have optional STAT= and ERRMSG= specifiers. Several intrinsic subroutines have optional STAT or STATUS arguments, and some of these also have an optional ERRMSG argument.

Use STAT specifiers, or STAT and STATUS arguments, to prevent initiation of error termination if an error condition occurs during the execution of the statement or procedure that specifies this specifier or argument.

If an error occurs, the integer status variable is assigned a non-zero value; if no error occurs, it is assigned a value of 0. The status variable can be tested to determine if an error occurred, and the program can contain error handling code if an error did occur. Several intrinsic subroutines and statement descriptions include the meaning of the error code value; for example, IERRNO and CHDIR.

If a statement has an ERRMSG specifier, or an intrinsic subroutine has an ERRMSG argument, the variable associated with it is a default character scalar variable that becomes defined with an explanatory message about the type of error that occurred. The message is truncated or blank padded, according to the rules for intrinsic assignment. If no error occurs, the value of ERRMSG is unchanged.

The following example shows how the use of the STAT= and ERRMSG= can be used on an ALLOCATE statement to handle allocation errors:

     IMPLICIT NONE
     REAL,ALLOCATABLE(:, :, :)  :: A, B
     CHARACTER(LEN=128)         :: MSG
     INTEGER                    :: STATUS
     INTEGER                    :: I, J, K
     PRINT *, 'Enter the 3 extents of the arrays A and B:'
     READ (*,*) I, J, K
     ALLOCATE (A(I, J, K), B(I, J, K), STAT=STATUS, ERRMSG=MSG)
     IF(STATUS .NE.1) THEN
        PRINT *, 'Unrecoverable allocation error', STATUS 
        PRINT *, 'Error description =', MSG
        ERROR STOP STATUS
     END IF
     END PROGRAM

See Also