Fortran style guide

The main principle when contributing code to Fortran projects should be readability. If your code can not be easily read and understood by others it will be hard to maintain and extend. It should also fit well with the existing parts of the code (in style as well as in its programming paradigms) maintaining the principle of least surprise.

Below you will find some explicit coding rules we try to follow. The list can not cover all aspects, so also look at the existing source code and try to follow the conventions being used.

If you use Emacs as editor, consider adding appropriate customisation settings to your config file in order to automatically enforce some of the conventions below.

Line length and indentation

  • Maximal line length is 100 characters. For lines longer than that, use continuation lines.

  • Nested blocks are indented by 2 white spaces:

    write(*, *) "Nested block follows"
    do ii = 1, 100
      write(*, *) "This is the nested block"
      if (ii == 50) then
        write(*, *) "Next nested block"
      end if
    end do
  • Continuation lines are indented by 4 white spaces. Make sure to place continuation characters (&) both at the end of the line as well as at the beginning of the continuation line:

    call someRoutineWithManyParameters(param1, param2, param3, param4,&
        & param5)

    Try to break lines at natural places (e.g. at white space characters) and include one white space character after the opening ampersand in the continuation line.

  • Single line preprocessor directives are indented as normal code:

    call someRoutine(...)
  • Preprocessor block directives (directives with starting and ending constructs) are outdented by 2 characters with respect of the code they enclose. The enclosed code must be aligned as if the preprocessor directives were not present:

      call doSomething()
      call someRoutineScalapackVersion(...)
      call someRoutineSerialVersion(...)
    do iKS = 1, nKS
      call someRoutineScalapackVersion(iKS, ...)
      call someRoutineSerialVersion(iKS, ...)
    end do


The naming conventions basically follow those in the Google Style Guide for Java naming convention, with minor modifications.

  • Variable names follow the lowerCamelCase convention:

    logical :: hasComponent
  • Constants (parameters) use the lowerCamelCase convention similar to variables

    integer, parameter :: maxArraySize = 100

    with the exception of the constants used to define the kind parameter for intrinsic types, which should be all lowercase (and short):

    integer, parameter :: dp = kind(1.0d0)
    real(dp) :: val
  • Subroutine and function names follow also the lowerCamelCase notation:

    subroutine testSomeFunctionality()
    myValue = getSomeValue(...)
  • Type (object) names are written UpperCamelCase:

    type :: TRealList
    type(TRealList) :: myList

    All type names should be prefixed with a capital ‘T’, in order to clarify the distinction between type names and variable names:

    type :: TBroydenMixer
    end type TBroydenMixer
    type(TBroydenMixer) :: broydenMixer
  • Module names follow lower_case_with_underscore convention:

    use dftb_common_accuracy

    Underscores are used for name-spacing only, so the module above would be typically found at the path dftb/common/accuracy.f90. The individual component names (dftb, common, accuracy) may not contain any underscores and must be shorter than 15 characters.

  • Preprocessor variables and macros follow UPPER_CASE_WITH_UNDERSCORE convention:

    #:if WITH_MPI
      withMpi = ${FORTRAN_LOGICAL(WITH_MPI)}$

White spaces

Please use white spaces to make the code readable. In general, you must use white spaces in following situations:

  • Around arithmetic operators:

    2 + 2
  • Around assignment and pointer assignment operators:

    aa = 3 + 2
    pWindow => array(1:3)
  • Around the :: separator in declarations:

    integer :: ind
  • After commas (,) in general and especially in declarations, calls and lists:

    real(wp), allocatable :: array(:)
    type, extends(TBaseType) :: TDerivedType
    subroutine myRoutine(par1, par2)
    call myRoutine(val1, val2)
    print *, 'My value:', val
    do ii = 1, 3
    array(1:3) = [1, 2, 3]
  • When separating array indices, when the actual index value for an index contains an expression:

    myArray(ii + 2, jj) = 12

You may omit white space in following cases:

  • When separating array indices and the actual index values are simple and short (typically two letters) variable names, one or two digit integers or the range operator ::

    myArray(:,1) = vector
    latVecs(1,1) = 1.0_wp
    myArray(ii,jj) = myArray(jj,ii)

You must omit white spaces in following cases:

  • Around opening and closing braces of any kind:

    call mySubroutine(aa, bb)  ! and NOT call mySubroutine( aa, bb )
    myVector(:) = [1, 2, 3]    ! instead of myVector(:) = [ 1, 2, 3 ]
    tmp = 2 * (aa + bb)        ! instead of 2 * ( aa + bb )
  • Around the equal (=) sign, when passing named arguments to a function or subroutine:

    call mySubroutine(aa, optionalArgument=.true.)
  • Around the power operator:

    val = base**power   (instead of val = base ** power)

Avoid white spaces for visual aligning of code, use:

integer, intent(in) :: nNeighbors
real(wp), intent(out) :: interaction

instead of:

integer, intent(in)   :: nNeighbors
real(wp), intent(out) :: energy

Although latter may look more readable, it makes rather difficult to track real changes in the code with the revision control system. For example when a new line is added to the block making the realignment of previous (but otherwise unchanged) lines necessary

integer, intent(in)             :: nNeighbors
real(wp), intent(out)           :: energy
real(wp), intent(out), optional :: forces(:)

the version control system will indicate all of those lines having been modified, although only the alignment (but not the actual instructions) were changed.


  • Module, Subroutine and function comments should be consistent with doxygen / FORD literate comments for publicly visible interfaces and variables.

  • Comments are indented to the same position as the code they document:

    ! Take spin degeneracy into account
    energy = 2.0_wp * energy
  • Generally, write the comment before the code snippet it documents:

    ! Loop over all neighbours
    do iNeigh = 1, nNeighbours
    end do
  • Try to avoid mixing code and comments within one line as this is often hard to read:

    bb = 2 * aa   ! this comment should be before the line.
  • Never use multi-line suffix comments, as an indenting editor would mess up the indentation of subsequent lines:

    bb = 2 * aa  ! This comment goes over multiple lines, therefore, it
                 ! should stay ALWAYS before the code snippet and NOT HERE.
  • Specifically comment any workarounds, include the compiler name and the version number for which the workaround had to be made. Always use the following pattern, so that searching for workarounds which can be possibly removed is easy:

    ! Workaround: gfortran 4.8
    ! Finalisation not working, we have to deallocate explicitly
  • Comments should always start with one bang only. Comments with two bangs are reserved for source code documentation systems:

    ! This block needs a documentation
    do ii = 1, 2
    end do
  • If you need a comment for a longer block of code, consider instead packaging that block of code into a properly named function (if the additional function call would be performance critical, write it as an internal procedure):

    ind = getFirstNonZero(array)

    instead of

    ! Look for the first nonzero element
    found = .false.
    do ind = 1, size(array)
      if (array(ind) > 0) then
        found = .true.
      end if
    end do
    if (.not. found) then
      ind = 0
    end if

Allocation status

At several places, the allocation status of a variable is used to signal choices about logical flow in the code:

!> SCC module internal variables
type(TScc), allocatable :: sccCalc
if (allocated(sccCalc)) then

end if

This is to be preferred to the use of additional logical variables if possible.

Part of the reason for this choice is that from Fortran 2008 onwards, optional arguments to subroutines and functions are treated as not-present if not allocated.