Segfault when calling a function with a constant argument

Question

I have written this very simple code in Fortran:

program su
  implicit none
  real ran3
  write(*,*) ran3(0)
end program su

real*8 function ran3(iseed)
  im         


        

Answer 1

If you want, for whatever reason, that your iseed is modified by the function, you should mark it with intent(in out). If you do so, the compiler will trigger an error at compile time when you call the function using a literal constant. If you want to use the parameter just as input, you can mark it as intent(in), and you will get again an error since you are assigning iseed inside your function.

I think it can be a good idea to get the habit of declaring the intent.

Your code could look like

program su
    implicit none

    write(*, *) ran3(0)

contains

  function ran3(iseed)
    implicit none
    real :: ran3
    integer, intent(in) :: iseed
    ! or intent(in out) :: iseed

    iseed = iseed*153941+1
    ran3 = float(iseed)*2.328+0.5     
  end function ran3

end program su

(this won't compile no matter if you use "in" or "in out" as intent, because of what explaned early).

The following instead will compile (and should work, too)

program su
    implicit none

    write(*, *) ran3(0)

contains

  function ran3(iseed)
    implicit none
    real :: ran3
    integer, intent(in) :: iseed

    ran3 = real(iseed*153941+1)*2.328+0.5     
  end function ran3

end program su