`random_seed(get)` immediately after `random_seed(put)`: behavior of various compilers

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Consider the following piece of code. It gets the current random seed (= seed0), modifies it (=seed1), put the modified seed back, and then get immediately the random seed again (=seed2). As far as I understand, we should have seed1 == seed2 so that the program should print 0, unless seed1 is not a valid seed.

program testrand
implicit none
integer :: n
integer, allocatable :: seed0(:), seed1(:), seed2(:)

! Initialize the seed, and get its size.
call random_seed()
call random_seed(size=n)
allocate (seed0(n), seed1(n), seed2(n))

! Get the current seed.
call random_seed(get=seed0)

! Modify the seed, and put it as the new seed.
seed1 = max(seed0 - 1, 1)   
! In most cases, `seed1` should still be a valid seed, no？
call random_seed(put=seed1)

! Get the new seed.
call random_seed(get=seed2)

! Check whether the new seed is the one we put.
print *, maxval(abs(seed2 - seed1))

end program testrand

The aforementioned speculation seems correct with gfortran, ifort, g95, flang, nvfortran, af95 (Absoft), and sunf95 (Oracle). However, with nagfor (NAG Fortran Compiler Release 7.0(Yurakucho) Build 7036), the printed number is never 0 on my machine (Ubuntu 20.04).

If we do not modify seed0 but set seed1=seed0, then all the compilers will print 0 as expected.

Did I overlook something about random_seed?

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3

Thanks, @kargl. I see. So, when receiving a seed from random_seed(put), the compiler may translate it into another one that has a different value but the same effect as a seed.

Totally unrelated, but this reminds me of the following remark made by Johann Wolfgang von Goethe about mathematicians.

(Mathematicians are like Frenchmen: whatever you say to them they translate into their own language and forthwith it is something entirely different.)

It is clear now. Thanks.

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