Design objectives


Assuming that findent will be used in serious projects, involving large Fortran programs, the code of findent is kept as simple as possible:

  • Findent handles only two files: standard input and standard output. The input contains the Fortran program to be handled, the output contains the modified program.

  • The programming language is C++, a well maintained and documented language.

  • No multithreading is used.

  • Parsing the input is done with the aid of bison and flex: well known and maintained tools.

  • Findent mostly works on a line-by-line basis. (Exceptions: labelled DO-loops require a simple administration, and relabelling needs the complete source of a program unit.)

  • Findent uses no configuration files: it is steered by command-line parameters and, optionally, an environment variable containing command-line parameters.

  • A comprehensive test-suite is part of the distribution.


Findent is easy to use, yet offers the possibility to tweak the indentation to the user's taste:

  • Easy to use: All options have reasonable defaults, for example, usage can be as simple as:

          findent < program.f90 > newprogram.f90

    Furthermore, the findent distribution comes with a wrapper script wfindent that can be used like (to indent all .f90 files):

          wfindent *.f90

    Normally, findent detects if the input in fixed or free form.

  • All types of indentation (DO, SUBROUTINE, ...) can be specified on the command line, for example to use 5 spaces after a DO:

          findent --indent-do=5 <program.f90 > newprogram.f90
  • Findent ignores white space outside strings and label fields.

  • Fixed and free format Fortran are supported.

  • Conversion from fixed to free form is implemented, as well as the other way around.

  • All kinds of DO-loops are recognized, even nested DO-loops using the same label.

  • Findent has been tested on legacy Fortran sources, going back to Fortran IV. Hollerith's are parsed correctly.

  • Unrecognized constructions are allowed and are written on the output as-is. Incomplete Fortran sources are handled gracefully.

  • Findent can relabel the Fortran source. (The man page contains a warning: 'use this only on correct programs'. If findent detects a problem (missing label definition; incomplete program unit; ...), relabelling is abandoned).

  • High speed: findent indents about 100.000 lines per second.

Building findent

For end users

Building findent is easy and is based on standard tools:

  • The distribution tar ball is based on autoconf, a mature program suite to distribute program sources.

  • The distribution tar ball contains the output files of flex and bison, so the user doesn't need to install these programs. (If they are installed, they will be used).

  • On Linux findent is built by unpacking the tar-ball, and issue the commands:

             cd findent-xx.yy.zz
             make check  # to run the test-suite
             sudo make install
  • For MacOS, building findent is the same as for Linux.

  • A Windows version can be obtained by specifying the flag --with-windows to the configure command. You need to have i686-w64-mingw32-g++ or something like that available. Probably, using WSL or Cygwin on windows should make it possible to do the build on the Windows system.

  • If building as presented above does not succeed, a script simplemake.sh, containing usage instructions can be used.

For program development and maintenance

The following is for developers and maintainers:

  • The script bootstrap runs autoreconf, replaces the copyright statements in nearly all sources and generates the output of the flex and bison. This output will be contained in the distribution tar ball, generated with

       make distcheck
  • An esoteric option is --with-esope. This causes findent to recognize esope constructs, see http://www-cast3m.cea.fr/html/esope/esope.html.

  • In the files src/debug.h and src/debug.cpp some macros and functions are defined to be used when debugging. There is some comment in those files how to use them.

  • Findent comes with a comprehensive test suite, located in the directory test. The tests will exercise every flag, and check if the solved bugs are still solved. Testing is activated by running:

       make check


Flags to influence the working of findent

Options to findent can be given on the command line, like:

   findent -ifixed -ofree -i2 < prog.f > prog.f90

and/or in the environment variable FINDENT_FLAGS, like:

   export FINDENT_FLAGS='-i4 -I8'
   findent -ifixed -ofree < prog.f > prog.f90

Most flags relate to the format of the input file and output file. However, some options arrange that findent does not output an indented Fortran source, but other information. These flags are marked in the man page with the string [NO_ENV] and are ignored when present in the environment variable FINDENT_FLAGS. Invalid flags, both on the command line and in the environment, are silently ignored1. Flags are read first from FINDENT_FLAGS and secondly from the command line. The flags are handled in the files src/flags.cpp and src/flags.h. See the man page for a description of the flags.

Generating documentation

Findent can generate the following documentation:

  • A text file ('help-file'), describing all flags.

  • A man page, suitable for processing with the program man.

  • A text file, containing the ChangeLog.

  • Text files, describing the usage in an editor.

  • Text file describing how to use findent in editor, for example vim.

Miscellaneous other functions

  • Print version of findent.

  • Print 'free' or 'fixed', depending on what findent deduces from the input.

  • Print dependency information, based on:

    • Usage and definitions of modules.

    • Usage of include files.

  • Print a shell script to be used in combination with the dependencies.

  • Print the amount of indentation of the last line read.

  • Print the line number of the last usable line as a start for indenting.

  • Print a report of defined and used labels.

  • Print scripts to incorporate findent in the editors Vim, Emacs or Gedit.

Detailed overview of the internals of findent

Starting the machinery

The main program is in findent.cpp. The flags are read (get_flags()), and if some kind of documentation has to be produced (docs.print()), the program prints the documentation and returns. Otherwise, the class Findent from findentclass.h is instantiated as findent and findent.run() from findentrun.cpp is called.

The main driver: fortran.run()

fortran.run executes the following tasks (some trivia are omitted here):

  • If standard input is connected to a terminal, take appropriate actions.

  • Read all of the input and store the lines in a buffer (input_buffer).

  • If the input format is not forced to be fixed or free, call determine_fix_or_free() to determine the format.

  • Instantiate class indent to either Free or Fixed as appropriate. These are subclasses of class Fortran in fortran.h to define the free or fixed alternatives of certain functions. See free.cpp, free.h, fixed.cpp and fixed.h.

  • Take actions if the user wants to relabel the input, see below.

  • Enter the indenting loop (some trivia omitted here):

    • Call get_full_statement() to create full Fortran line full_statement by collating the possible continuation lines to the first (see below).

    • Call indent_and_output() to determine the indentation and output the lines that define the full Fortran line.

Collecting a full Fortran statement

Collecting a full Fortran statement from the first line and continuation lines is done in src/fortran.cpp: get_full_statement(). This function looks surprisingly complex at first sight. This is because continuation lines can contain:

  • comment lines,

  • blank lines,

  • cpp or coco preprocessor statements,

  • findentfix lines (see the man page).

Furthermore, attention must be paid if we are relabelling or not. The full Fortran statement is stored in full_statement.

In src/fortranline.cpp and src/fortranline.h functions are defined to handle lines with Fortran code. Care has been taken that for fixed format, a tab at the start of a line is handled properly (see do_clean()).

The call to build_statement() has a different implementation for free and fixed format, see src/free.cpp and src/fixed.cpp, respectively. This function performs the following tasks:

  • Add the input Fortranline to c-lines (a list of Fortranline's).

  • Add the line, stripped from all non-fortran stuff to full_statement

  • Signal if there are continuation lines to be expected.

    This is easy in the free-form case, but in the fixed-form case al look-ahead is necessary. This is programmed in wizard() in fixed.cpp.

Preprocessing the full Fortran statement

Once a full_statement has been obtained, this line is preprocessed to make it suitable for parsing using flex and bison. This is done in Line_prep::set_line(), in file src/line_prep.cpp. An example may clarify this.

Below is:

  • s - The full statement.

  • sl - Spaces removed, except in strings and Hollerith's, and after the statement label.

  • sv - Strings, Hollerith's, operators and statement label replaced by a space.

  • sc - Strings etc. replaced by space number space, the number is the index in sv. (sc is used for parsing with bison and flex.)

  • wv - A list, length = sv.size(). Each entry consists of a struct whats (see line_prep.h) which tells (type) what this entry contains: invalid, none, string, statement label or operator. In case of string there is stringtype which discriminates between Hollerith (h), single quoted string (') or double quoted string ("). The value of the entry is contained in value.

s:  [123   call sub(5habcde  , 5,  'f oo', 'ab c' .concat. "def")]
sl: [123 callsub(5habcde,5,'f oo','ab c'.concat."def")]
sv: [ callsub( ,5, ,   )]
sc: [ 0 callsub( 9 ,5, 13 , 15  16  17 )]
wv[0]: statement label       [123]
wv[9]: hollerith string      [abcde]
wv[13]: single quoted string [f oo]
wv[15]: single quoted string [ab c]
wv[16]: operator             [concat]
wv[17]: double quoted string [def]

The other entries have type=none.

Parsing the preprocessed full statement

Parsing the preprocessed full statement is done using bison and flex. Things are arranged that one line at a time is parsed, see lexer_set(Line_prep p, const int state) in lexer.l. The string sc (see above) is used for parsing. Parsing is initiated in indent_and_output() in fortran.cpp by a call to parseline(). This function, returning a propstruct (see prop.h) containing the results, parses the full statement in two passes:

  • The lexer is brought in a state, that it does not recognize Fortran keywords. For example: subroutinesub(x)=10 will return kind=ASSIGNMENT.

  • If parsing does is not successful (kind = UNCLASSIFIED), full statement is parsed again, but now recognizing relevant Fortran keywords. For example: subroutinesub(x) will succeed and returning kind=SUBROUTINE.

Keeping track of indents

In indent_and_output() (fortran.cpp), a stack is maintained containing the indents, along with the current index. The actions are in principle quite simple: If, after parsing a relevant keyword is found (SUBROUTINE, DO, ...) the indent is changed as appropriate and put on the stack. If a kind of END (ENDIF, END SUBROUTINE, ...) is found, the indent is pulled from the stack. Some constructs deserve extra attention:

  • Labelled DO-loops: if a labelled DO-loop is encountered, the label involved is stored on a stack. When a corresponding statement label is encountered, appropriate action is taken, also in the case of nested DO-loops sharing to the same label.2

  • MODULE PROCEDURE statements: at encountering a MODULE PROCEDURE, indentation if the next full statement is classified as an executable statement.

  • An ambiguity: MODULEPROCEDUREmyproc Should this be interpreted as: MODULE PROCEDUREmyproc or: MODULE PROCEDURE myproc Findent assumes the last is correct.3

Handling cpp and coco preprocessor statements

It was a design goal that findent should handle macro's more or less intelligent.

For example:

| **Input**       | **Desired**              | **Not desired**             |
|                 |                          |                             |
|     #ifdef DIM2 |     #ifdef DIM2          |     #ifdef DIM2             |
|     do y=1,ny   |        do y=1,ny         |        do y=1,ny            |
|     #else       |     #else                |     #else                   |
|     do y=1,1    |        do y=1,1          |           do y=1,1          |
|     #endif      |     #endif               |     #endif                  |
|     do x=1,nx   |           do x=1,nx      |              do x=1,nx      |
|     call s(x,y) |              call s(x,y) |                 call s(x,y) |
|     enddo       |           enddo          |              enddo          |
|     enddo       |        enddo             |           enddo             |

recognizes the following preprocessor statements (defined in lexer.l):

cpp coco
# if ?? if
# endif ?? endif
# else ?? else
# elif ?? elseif
# include "..." ?? include...||
# include <...> ?? include...||

Note1: the rest of the preprocessing line is ignored, so, for example, #if has the same effect as #ifdef. Note2: the include's are only used when generating dependencies, and are ignored when indenting.

Most of the preprocessor-handling code is reached via handle_pre() in Fortran.cpp and Pre_analyzer() in pre_analyzer.cpp. The strategy is as follows:

  • A stack is maintained to store the relevant items (e.g. the indentation level and the stack of indentations) (see push_all(), top_all() and pop_all() in fortran.h.

  • The relevant items are pushed on this stack after a #if

  • The relevant items are popped of the stack if appropriate after #endif, #else and #elif.

  • Handling the preprocessor statements is done recursively.

  • After a construct like

          #if ...
          <fortran statements>

    the indentation continues from the state before the #if, but after a construct like

          #if ...
          <fortran statements>
          <fortran statements>

    the indentation continues from the state after the #else.

In this way, most of the times findent will generate sensible indentation. If findent makes an error, this can easily be fixed by inclusion of a findentfix statement, for example (admittedly somewhat constructed):

Original Corrected
    program p
#ifdef A
       do i=1,10
       i = 1
       x = x+i
#ifdef A
    program p
#ifdef A
       do i=1,10
       i = 1
       x = x+i
#ifdef A
!findentfix: do


Relabelling (Renumbering of labels) is done in the following stages:

  • Scan the input until a complete program unit (program, subroutine, function) is obtained, collecting the labels defined and used.

  • Check if all used labels are defined.

  • Regenerate the program unit, now with the renumbered labels.

  • Indent and output the renumbered program unit.

  • Go to the first step.

If some error is detected, (not defined label, label spanning continuation lines, ...) relabelling is abandoned for the current and following program units, however, indentation proceeds as normal. If relabelling fails, one can run findent with the flag --query-relabel, to see the reason.

Generate miscellaneous text files

Help files, man page, scripts for usage in editors etc. are generated in the file src/docs.cpp. For generating the man page and the help-file, the function manout() is used so that generating these files is based on the same input.

The other files are include files, generated from the original text files. For example: vim_fortran.inc is generated from vim/fortran.vim, using the script src/tocpp. Details are available in the file src/Makefile.am.

  1. Since findent only writes to standard output, error messages would clutter the indented Fortran program↩︎

  2. Shared DO-termination is flagged as a 'deleted feature' by gfortran.↩︎

  3. This ambiguity arises from the fact that all spaces are removed in the preprocessing phase. In fixed format (where spaces do not count), however, this ambiguity is also present for the compiler.↩︎