# An integer overflow problem **URL:** \ **Category:** Uncategorized\ **Created:** [March 11, 2022, 10:00am UTC](https://fortran-lang.discourse.group/t/an-integer-overflow-problem/2955 "2022-03-11T10:00:35Z")\ **Posts on this page:** 10\ **Page:** 1
**Author:** ![vmagnin](https://yyz2.discourse-cdn.com/free1/user_avatar/fortran-lang.discourse.group/vmagnin/32/28_2.png) [@vmagnin](https://fortran-lang.discourse.group/u/vmagnin)\ **Post date:** [March 11, 2022, 10:00am UTC](https://fortran-lang.discourse.group/t/an-integer-overflow-problem/2955/1 "2022-03-11T10:00:35Z")
The following program compiled with gfortran, ifort and ifx is computing correctly `2**31-1` when `n` is declared as `int32` (although I would not have been surprised if it had failed): ```fortran program main use iso_fortran_env integer(int32) :: n integer :: i i = 31 n = 2**i - 1 print *, n, kind(n) end program main ``` ```bash $ gfortran main.f90 && ./a.out 2147483647 4 $ ifort main.f90 && ./a.out 2147483647 4 $ ifx main.f90 && ./a.out 2147483647 4 ``` but surprisingly with `integer(int64) :: n`, there is an integer overflow with ifort 2021.5.0 (Ubuntu 21.10): ```bash $ gfortran main.f90 && ./a.out 2147483647 8 $ ifort main.f90 && ./a.out -2147483649 8 $ ifx main.f90 && ./a.out 2147483647 8 ``` ---
**Author:** ![Arjen](https://avatars.discourse-cdn.com/v4/letter/a/b9bd4f/32.png) [@Arjen](https://fortran-lang.discourse.group/u/Arjen)\ **Post date:** [March 11, 2022, 10:04am UTC](https://fortran-lang.discourse.group/t/an-integer-overflow-problem/2955/2 "2022-03-11T10:04:59Z")
The calculation is done with an integer of 4 bytes, so the outcome of 2\*\*31 is too big. Apparently there is something odd going on though. ---
**Author:** ![Carltoffel](https://yyz2.discourse-cdn.com/free1/user_avatar/fortran-lang.discourse.group/carltoffel/32/1680_2.png) [@Carltoffel](https://fortran-lang.discourse.group/u/Carltoffel)\ **Post date:** [March 11, 2022, 10:07am UTC](https://fortran-lang.discourse.group/t/an-integer-overflow-problem/2955/3 "2022-03-11T10:07:30Z")
Can you compile your example with warnings, please? Could be interesting… ---
**Author:** ![vmagnin](https://yyz2.discourse-cdn.com/free1/user_avatar/fortran-lang.discourse.group/vmagnin/32/28_2.png) [@vmagnin](https://fortran-lang.discourse.group/u/vmagnin)\ **Post date:** [March 11, 2022, 10:09am UTC](https://fortran-lang.discourse.group/t/an-integer-overflow-problem/2955/4 "2022-03-11T10:09:49Z")
`gfortran -Wall -Wextra main.f90` prints no warning in both cases. Same with `ifort -warn all main.f90`. And if I don’t use the `i` variable and write directly `2**31 - 1`, only gfortran (version 11.2.0) complains: ```bash $ gfortran main.f90 && ./a.out main.f90:7:7: 7 | n = 2**31 - 1 | 1 Error: Result of exponentiation at (1) exceeds the range of INTEGER(4) ``` ---
**Author:** ![interkosmos](https://yyz2.discourse-cdn.com/free1/user_avatar/fortran-lang.discourse.group/interkosmos/32/296_2.png) [@interkosmos](https://fortran-lang.discourse.group/u/interkosmos)\ **Post date:** [March 11, 2022, 10:20am UTC](https://fortran-lang.discourse.group/t/an-integer-overflow-problem/2955/5 "2022-03-11T10:20:03Z")
```fortran n = int(2_8**31 - 1) ``` ---
**Author:** ![vmagnin](https://yyz2.discourse-cdn.com/free1/user_avatar/fortran-lang.discourse.group/vmagnin/32/28_2.png) [@vmagnin](https://fortran-lang.discourse.group/u/vmagnin)\ **Post date:** [March 11, 2022, 10:23am UTC](https://fortran-lang.discourse.group/t/an-integer-overflow-problem/2955/6 "2022-03-11T10:23:58Z")
With ` n = 2_8**i - 1`, ifort is computing correctly in the int64 case. ---
**Author:** ![Carltoffel](https://yyz2.discourse-cdn.com/free1/user_avatar/fortran-lang.discourse.group/carltoffel/32/1680_2.png) [@Carltoffel](https://fortran-lang.discourse.group/u/Carltoffel)\ **Post date:** [March 11, 2022, 10:24am UTC](https://fortran-lang.discourse.group/t/an-integer-overflow-problem/2955/7 "2022-03-11T10:24:33Z")
You can achieve this without int64: ```auto n = -((-2)**31 +1) ``` ---
**Author:** ![msz59](https://avatars.discourse-cdn.com/v4/letter/m/3d9bf3/32.png) [@msz59](https://fortran-lang.discourse.group/u/msz59)\ **Post date:** [March 11, 2022, 2:04pm UTC](https://fortran-lang.discourse.group/t/an-integer-overflow-problem/2955/8 "2022-03-11T14:04:29Z")
Compare following: ```auto program main use iso_fortran_env integer(int32) :: n integer :: i i = 31 n = 2**i print *, n, kind(n) ! ==> -2147483648 4 n = n - 1 print *, n, kind(n) ! ==> 2147483647 4 end program main ``` It seems double overflow occurring here and the result is expected. In 64-bit version it is really odd, looks like `ifort` treats the first result (2\*\*31) as 64-bit value of -2147483648, subtracting 1 gives -2147483649 ---
**Author:** ![vmagnin](https://yyz2.discourse-cdn.com/free1/user_avatar/fortran-lang.discourse.group/vmagnin/32/28_2.png) [@vmagnin](https://fortran-lang.discourse.group/u/vmagnin)\ **Post date:** [March 11, 2022, 2:59pm UTC](https://fortran-lang.discourse.group/t/an-integer-overflow-problem/2955/9 "2022-03-11T14:59:58Z")
> [@msz59](#): > > It seems double overflow occurring here and the result is expected. Interesting, it explains the results with int32. With int64, the three compilers now give the same result, but with an overflow on `2**31`: ```bash $ gfortran main.f90 && ./a.out -2147483648 8 -2147483649 8 $ ifort main.f90 && ./a.out -2147483648 8 -2147483649 8 $ ifx main.f90 && ./a.out -2147483648 8 -2147483649 8 ``` ---
**Author:** ![msz59](https://avatars.discourse-cdn.com/v4/letter/m/3d9bf3/32.png) [@msz59](https://fortran-lang.discourse.group/u/msz59)\ **Post date:** [March 11, 2022, 3:03pm UTC](https://fortran-lang.discourse.group/t/an-integer-overflow-problem/2955/10 "2022-03-11T15:03:48Z")
Which is perfectly okay