Fortran issues with renaming subroutines when using from modules

Fortran-issues-renaming-use.md

Fortran issues with renaming subroutines when using from modules

There are issues with naming a subroutine the same as a subroutine that is used from another module but is renamed. See the code for details.

Instructions

Download all files in this gist.

To see the bug in the code, compile with gfortran-7:

make FC=gfortran-7
./prog

It should hang, and leak memory.

Try again with a newer gfortran:

make FC=gfortran-8
./prog

This should finish immediately and print

   2.00000000

Issue

The issue is that even when renaming a subroutine used from a module you cannot name a subroutine, locally, with the same original name as the used subroutine. At least with a gfortran version earlier than 8.

Solution

Use gfortran-8 or greater. Or don't use name a subroutine with the same name as another even if renaming when using from the other module.

In the latter case, in lib.f90 replace

subroutine subr

with, for example,

subroutine subr_c

And in prog.f90 replace subr with subr_c.

lib.f90

module lib
  use modu, only: subr_f => subr
  use iso_c_binding, only: c_float
  implicit none

  public :: subr

contains

subroutine subr(x, y) bind(c)

  real(c_float), intent(in) :: x
  real(c_float), intent(out) :: y

  call subr_f(x, y)

end subroutine subr

end module lib

Makefile

FC      = gfortran
FCFLAGS = -O3

default: prog

mod:
	$(FC) $(FCFLAGS) -c mod.f90

lib: mod
	$(FC) $(FCFLAGS) -c lib.f90

prog: mod lib
	$(FC) $(FCFLAGS) -o prog mod.o lib.o prog.f90

.PHONY:
clean:
	\rm -rf *.o *.mod

mod.f90

module modu
  implicit none

  public :: subr

contains

subroutine subr(x, y)

  real, intent(in) :: x
  real, intent(out) :: y

  y = 2*x

end subroutine subr

end module modu

prog.f90

program prog
  use lib, only: subr
  implicit none

  real :: x, y

  x = 1.0

  call subr(x, y)

  print*,y

end program prog