How to stop a subroutine and raise a flag? - fortran

Question

I am writing a program in Fortran 95 (to be compiled with with gfortran) containing a subroutine that performs a certain computation. As suggested in "Fortran 95/2003 for Scientists & Engineers" by S. J. Chapman, I am trying to stop the subroutine when an error is encountered and "throw"[1] an error flag that is "catch"ed[1] by the calling program, that will take all the necessary actions. Ideally, I am going for something like:
! Pseudo-code
PROGRAM my_prog
integer :: error_flag
CALL my_subr (<input_args>, <output_args>, error_flag)
! Also error_flag is an output: 0 -> everything OK, 1 -> error
IF (error_flag /= 0) THEN
WRITE (*,*) 'Error during execution of "my_subr"'
ELSE
... do something ...
END IF
END PROGRAM my_prog
How can I stop the subroutine and gracefully handle the errors?
Here is an example: the subroutine "division" takes an integer input value and iteratively divides it by a value that is the input value decremented by the number of steps-1. When such value reaches zero, a flag should be raised and the subroutine should be exited without performing the division by zero.
SUBROUTINE division (inval, outval, error_flag)
IMPLICIT NONE
INTEGER, INTENT(IN) :: inval
REAL, INTENT(OUT) :: outval
INTEGER, INTENT(OUT) :: error_flag ! 0 -> OK, 1 -> error
INTEGER :: i
REAL :: x
error_flag = 0
x = REAL(inval)
DO i = 0, 10
IF (inval-i == 0) error_flag = 1
! How can I gracefully exit now?
x = x / REAL(inval-i)
END DO
END SUBROUTINE division
PROGRAM my_prog
IMPLICIT NONE
REAL :: outval
INTEGER :: error_flag
CALL division (8, outval, error_flag)
IF (error_flag == 1) THEN
WRITE (*,*) 'Division by zero'
ELSE
WRITE (*,*) 'Output value:', outval
END IF
END PROGRAM my_prog
Notes:
[1] I am borrowing (in a probably inappropriate way) C++'s jargon.

Answer 1

Seeing your example it seems that you are just missing the return statement:
error_flag = 0
x = REAL(inval)
DO i = 0, 10
IF (inval-i == 0) then
error_flag = 1
return
END IF
x = x / REAL(inval-i)
END DO

Answer 2

One possibility would be to change
DO i = 0, 10
IF (inval-i == 0) error_flag = 1
! How can I gracefully exit now?
x = x / REAL(inval-i)
END DO
to
DO i = 0, 10
IF (inval-i == 0) THEN
error_flag = 1
EXIT
END IF
! Now you have gracefully exited
x = x / REAL(inval-i)
END DO
! Code to tidy up if the error flag was set
Here the EXIT statement exits the loop -- Vladimir's answer shows you how to use RETURN to exit the subroutine more immediately. Whichever approach you choose don't forget to assign to outval before leaving the subroutine.