Free-form Fortran with Abaqus

Free-form Fortran with Abaqus#

Free form vs. Fixed-form#

Fortran has two standard formats for code layout:

  • Fixed-form (legacy)

  • Free-form

The default in Abaqus is to expect Fortran user subroutines to be written in fixed-form Fortran. However, fixed-form Fortran is a legacy format that should be avoided for a number of reasons:

  • it reduces readability of the code

  • it needlessly increases developer workload

  • it permits ambiguities that lead to obscure errors

For these reasons, it is recommended that free-form Fortran is used as much as possible.

This guide will demonstrate how to override the default Abaqus behaviour and allow the use of Free-form Fortran in your user subroutines.

Further reading: Learn more about Fixed-form vs Free-form in “Source Form Just Wants to be Free”[1]

Using Free-form with Abaqus#

Caution

The information in this guide refers to the Intel Fortran compiler only.

There are two approaches to using free-form Fortran with Abaqus:

  1. Add the -free//free flag to the compiler options in the abaqus_v6.env file

  2. Use compiler directives to indicate free-form source files

The second approach is recommended here since it does not introduce reliance of the user subroutine on the contents of the abaqus_v6.env file.

See also

See the sample repository on Github which gives a complete working example of the concepts presented in this guide.

Indicating Free-form with Compiler Directives#

Note

Compiler directives are special comments added to code, that are not part of the Fortran code, but which convey information to the compiler about the code.

The compiler directive for indicating that code is free-form is:

!DIR$ FREEFORM

This is placed at the start of the file to indicate that all code following the directive is in free-form.

We also need to place the following directives at the end of the file to ‘deactivate’ the free-form mode when this file is included with other files:

!DIR$ NOFREEFORM
!DIR$ FIXEDFORMLINESIZE:132

Example:

!DIR$ FREEFORM

! This subroutine is written in free-form
subroutine free_form_example(a,b)
  integer, intent(in) :: a
  integer, intent(out) :: b

  if (a > 0) then
    b = a
  else
    b = 0
  end if

end subroutine free_form

!DIR$ NOFREEFORM
!DIR$ FIXEDFORMLINESIZE:132

Abaqus Include Files#

Abaqus user subroutines typically need to include one or more Abaqus definition files such as aba_param.inc/vaba_param.inc and SMAAspUserSubroutines.hdr.

These files are written in fixed-form and so errors will occur if you include them in a source file marked as free-form.

We can remedy this by using compiler directives around the include statements to temporarily enter fixed-form mode:

subroutine free_form_example(a)

!DIR$ NOFREEFORM
!DIR$ FIXEDFORMLINESIZE:132
      include "aba_param.inc"
!DIR$ FREEFORM

end subroutine free_form_example

See also

See the guide on Explicit Typing with Abaqus for how to avoid Abaqus include files completely.

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