Change variable value when signal is trapped in Fortran - fortran

Question

While developing the program on Fortran, that employs some iteration procedure, I faced the necessity to stop iterations manually (to exit from the iteration loop without program termination).
I decided to do it sending a signal to the process. I have chosen SIGALRM. I have checked that it can be trapped without any unexpected consequences.
When received signal, the flag value is changed. This flag is checked inside the iteration loop and exit if flag is true. The sample of such code is given below.
!file mymod.f90
module mymod
use ifport
integer*4 :: err
integer*4 :: SIGNSET
integer*4, parameter :: mySignal=14
logical*1 :: toStopIteration
contains
! ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ !
integer*4 function setTrap() result(ret)
implicit none
call PXFSTRUCTCREATE('sigset',SIGNSET,err)
call PXFSIGADDSET(SIGNSET,mySignal,err) !add my signal to the set.
ret=0; return
end function setTrap
! ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ !
integer*4 function onTrap(sig_num) result(rcode)
implicit none
integer*4 :: sig_num,err
rcode=0
select case (sig_num)
case(mySignal)
write (*,*) 'Signal occurred. Stop iteration called'
write (*,*) 'flag: ',toStopIteration
toStopIteration=.true.
write (*,*) 'flag: ',toStopIteration
rcode=1
return
case (SIGINT) ; stop
case (SIGTERM); stop
case (SIGABRT); stop
end select
end function onTrap
! ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ !
end module mymod
!file main.f90
program main
use mymod
implicit none
integer*4 :: i,j,N,Niters,sum1
err=setTrap()
err=signal(mySignal, onTrap, -1)
toStopIteration=.false.
open (1,file='output')
write (*,*) 'PID=',getpid()
write (1,*) 'Outside',toStopIteration
N=5000000; Niters=100000
do i = 1,Niters
if (toStopIteration) then
toStopIteration=.false.
exit
endif
sum1=0
do j = 1,N
sum1=sum1+j
enddo
write (1,*) i,toStopIteration,sum1
enddo
write (*,*) 'Procedure was terminated due to signal received. The last iteration was', i
write (*,*) 'Now I will do other job for you.'
stop
end program main
Application was compiled with ifort: ifort -c -O2 -traceback.
When I send signal to the process kill -14 pid,
I get output to the terminal:
Signal occurred. Stop iteration called
flag: F
flag: T
But iteration loop is still running and as written in the file, variable "toStopIteration" is equal false.
Accidentally, I have found out that when compiled with -O0 -traceback parameter, it works fine.
Why does it happen? Does variable "toStopIteration" become local with such optimization level? And what can I do to make it work correctly?
Thanks in advance.
MuKeP.

Answer 1

As Lorri answered (unfortunately that succinct, but correct, answer has been deleted by misdirected reviews) - try the volatile attribute on toStopIteration. This tells the compiler that the variable might be redefined by something else, otherwise from the source visible to the compiler it looks like the value of that variable cannot change in an iteration, and therefore there is no point testing it each iteration.