Home/ Questions/Q 410089
In Process

The Archive Base Latest Questions

Editorial Team
  • 0
Asked: 2026-05-12T17:51:16+00:00

When writing and calling pure subroutines in Fortran 90 using gfortran, how do I

  • 0

When writing and calling pure subroutines in Fortran 90 using gfortran, how do I find out why the compiler emits this error?

Error: Subroutine call to XXXX at (1) is not PURE

I’ll try to pose my question as specifically as I can while at the same time being general enough to be useful to others, so I’ll avoid pasting in my actual code and instead will sketch what happened.

I understand there are various rules about pure procedures in Fortran 90, which I think basically boil down to not permitting side-effects in either functions or subroutines, and not permitting changes to subroutine parameters declared with intent(in). I’ve a series of subroutines which initially were not declared to be pure, and whose parameters didn’t have declared intent, but which nevertheless didn’t perform side-effects. First, I changed all parameter declarations to have explicitly-declared intent, either in, out, or inout. Then, I declared all the subroutines to be PURE. Naturally, many errors occurred on the first attempt, but the compiler told me what the errors were (such-and-such parameter with intent(in) is being modified, for example), so one-by-one I fixed them all.

There are calls among these procedures, however, and so now I still get many errors of the form shown above: Subroutine call to XXXX at (1) is not PURE. What I don’t understand is why the call is not pure. I’ve done everything I can think of to make XXXX pure, but the compiler still thinks it isn’t.

So my question –rephrased– is: how do I get gfortran to tell me WHY it thinks XXXX is not pure?

Share

Leave an answer

You must login to add an answer.

Need An Account,

1 Answer

  1. Editorial Team

    “Placed all the PURE subroutines in the library I’m working on, into a MODULE (which my client code then USEd). …… Not sure why ….., but after doing this more useful error messages appeared which allowed me to track down the remaining impurities.”

    Placing the subroutines into a module and then using them makes the interface explicit. This allows the compiler to check agreement between the call and the subroutine and generate error messages if there is a discrepancy. Very useful, so placing subroutines and functions into modules good practice.

    The other way of making an interface explicit is to write an interface, but that is extra work and an extra step to get wrong.

    There is a long list of requirements on pure subroutines/functions. If you have Fortran 95/2003 Explained by Metcalf, Reid and Cohen, see section 6.10. For example, no “save” variables, no stop statement, no IO on external file, …

    You can also try other compilers to see if their error message is more helpful. Other free ones, depending on OS, include g95 and Sun Studio.

    • 0