Segfault when calling a function with a constant argument

I see two problems with the code. The first is the one which I think is the cause of the error. The function ran3 is referenced with the constant 0 as the actual argument, but the corresponding dummy argument iseed is used on the left side of an assignment statement in the function. This is an error: you can't change the value of zero.

The second error is that ran3 returns a real*8 (whatever that may be; it's a non-standard declaration), but in the main program ran3 is declared as being a default real.

The following program and function compile with gfortran 4.7.2.

program su
    implicit none
    real :: ran3

    write(*, *) ran3(0)
end program su

function ran3(iseed)
    implicit none
    integer :: iseed, temp
    real :: ran3

    temp = iseed * 153941 + 1
    ran3 = temp * 2.328 + 0.5
end function ran3

While there are many good points made above, most of the solutions above defeat the immediate purpose of the function. Notably, random number generators need to return also the "new" value of iSeed in many cases (though the OP's post does not say so explicitly), since often on the next call to the Ran s/r, the "new" value of iSeed is required.

As a basic rule, constants should be passed as Args ONLY to Intent(In) dummy's.

In a sense, the OP was "lucky" to get a segv, since in the (bad) old days it was possible to send the "number" "0" in as "0", but on return "0" everywhere else would contain the value of iSeed and no segv, but lots of bad arithmetic.

A more appropriate solution in this particular case would be NOT to pass a constant at all, but rather as:

program su
implicit none
Integer      :: iSeed
!
iSeed = 0   ! or whatever iSeed is requried
!
write(*, *) ran3(iSeed)

contains

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


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

end program su

Now, the call/use of Ran3() can be iterative, e.g. via a loop, or Elemental etc, to create a (quasi) random series.

There are other possibilities using External, etc, but that's for another day.

Your code has done nothing to tell the compiler that the declaration

real ran3

refers to the function you define later in your source file. To the compiler you have declared a real variable called ran3. Once the compiler has read the end statement at the end of the program it can bugger off and drink mojitos if it wants to, it is not bound to do any more compilation -- though you might find that some compilers do.

A general rule in structuring Fortran programs is that the compiler must encounter the definition of an entity (variable, function, subroutine, derived-type, what-have-you) before it encounters any use thereof. Your code has broken this rule.

Once the code has declared a real variable it tries, in this statement,

write(*,*) ran3(0)

to access the 0-th element of an array called ran3 and it all ends in tears.

The quick fix would be to move end program su to the end of the source file, and to put a line containing the keyword contains before the definition of the function. You could then delete the declaration real ran3 as the compiler will take care of any linking that needs to be done.

Oh, and while I'm writing, you could do yourself, and those trying to comprehend your code, a favour by paying more attention to formatting what you have posted. Personally (opinion coming up, look away now if you are easily upset) I would fire any programmer who turned in code looking like that on the grounds that anyone who pays so little attention to the small stuff probably doesn't pay much attention to the big stuff either.

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

First you have to define idum as an integer.

  program su
  implicit none
  integer idum
  real ran3
  idum = 334
  write(*,*) ran3(idum)
  end program su

then your code will work