.F90 File Format – File structure

File extensions

The extension is how most compilers choose the source form, and whether to run the preprocessor. The table lists the extensions in common use. A capital letter in the extension (.F, .F90) is a convention of Unix compilers for “run the C preprocessor first”; the viewer treats upper and lower case alike.

.f is also used for Forth source and, rarely, for other languages. The viewer reads a .f file as Forth, and shows it without colour, when it opens with colon definitions and \ comments and holds no Fortran statements.

GNU Fortran, Intel, Flang and the others let a flag override the extension: -ffree-form and -ffixed-form in GNU Fortran and Flang, -free and -fixed in Intel Fortran, and -qfree and -qfixed in IBM XL Fortran.

Program units

A file holds one or more program units, written one after another. Each unit ends with its own end statement. The kinds are:

• main program: program name … end program name. The program line is optional in every version, so a file of plain statements ending in end is a main program
• external subprogram: a subroutine or function outside any module
• module: module name … contains … end module, holding declarations and module procedures (Fortran 90)
• submodule: submodule (parent) name … end submodule, holding the bodies of a module’s separate module procedures (Fortran 2008)
• block data: block data name … end block data, giving initial values to common blocks (FORTRAN 66 and later; obsolescent from Fortran 2018)

Inside a program unit, contains introduces internal or module subprograms. Interface blocks and derived-type definitions are specifications, not program units. See Program units and procedures.

module shapes
  implicit none
  real, parameter :: pi = 3.14159265
contains
  pure function area(r) result(a)
    real, intent(in) :: r
    real :: a
    a = pi * r**2
  end function area
end module shapes

program demo
  use shapes
  implicit none
  print *, area(2.0)
end program demo

A compiler needs a module’s source compiled before any unit that uses it, so build tools order the files. Common practice is one module per file, named after the module. The Fortran Package Manager (fpm) expects src/, app/ and test/ folders and a fpm.toml manifest.

Include lines

include 'file' is a line, not a statement of the language proper: the compiler replaces it by the text of the named file before reading on. It is part of the standard since Fortran 90 (FORTRAN 77 compilers all had it as an extension). An include line must be alone on its line, and a file may include others up to a compiler-set depth. Separately, the C preprocessor’s #include works in files that are preprocessed (see Directives and preprocessor).

Character set, case and names

• The Fortran character set is the letters, the digits, the underscore, and the special characters blank = + - * / ( ) [ ] , . : ; ' " ! % & < > ? $. Comments, character literals and format texts may hold any character the processor has.
• Upper and lower case are the same everywhere except inside character literals (and H and '…' edit descriptors). PROGRAM, Program and program are one word.
• The viewer reads the bytes as UTF-8 and falls back to Windows-1252, so accented letters in comments and strings show correctly. Old files in other 8-bit sets show the Latin-1 reading.
• Fortran 2003 added the ISO 10646 (Unicode) character kind, selected by selected_char_kind('ISO_10646'); compilers read UTF-8 source for it.
• A tab is not in the standard character set. Every compiler accepts it as a blank in free form, and the DEC compilers give it a meaning in fixed form (tab format). A form feed (Ctrl-L) is blank space to GNU Fortran. A Ctrl-Z (0x1A) at the end of a DOS file is an end-of-file mark.

Line endings, length and the end of the file

Lines end with LF, CR LF or (on classic Mac OS) CR; compilers read them all, and the viewer shows CR LF files with no stray characters. The standard counts characters per line, not bytes:

The last line of a file need not end in a line ending. A file may be empty: a compiler then makes no program unit. Comment lines, blank lines and lines of only ; are allowed between program units.

Preprocessing

The Fortran standard has no preprocessor. Real projects use one of three:

• cpp in traditional mode, or the compiler’s own fpp, which understand #if, #define and #include lines. GNU Fortran runs it for the extensions in the table above and for -cpp; it is switched off by -nocpp
• Coco, the conditional-compilation part of the ISO standard 1539-3, with ?? lines (?? if (…) then). It was never widely used, and is not a part of the language
• Tools that generate Fortran from templates: fypp, pFUnit’s .pf files, and macro systems such as m4

Preprocessor lines are shown as directives. See Directives and preprocessor.

Choosing the source form: how the viewer guesses

A file may be in either form whatever its extension says: .f files in free form are common in newer projects, and fixed-form code with :: and ! comments is common in the middle (the Fortran 90 form of old programs). Compilers trust the extension or a flag. The viewer also reads the text:

• Fixed-form signs: a C, c or * followed by a blank in column 1; a statement label in columns 1 to 5 followed by a blank in column 6; a continuation mark in column 6 with columns 1 to 5 blank; a tab at the start of a line; comments such as cf2py or CDEC$ in column 1
• Free-form signs: a statement that starts before column 7 (program or end in column 1, an indented implicit none), a line ending in &, a line starting with &, :: and a ! after code on such lines

For a free-form extension (.f90 and its relatives) the text is taken as fixed form only when its fixed-form signs clearly outweigh the free-form ones. For .f, .for, .ftn, .f77 and .fpp the text is taken as free form only when the free-form signs clearly win. Text that is neither (Forth, or no Fortran at all) is shown without colour. The overview tab names the form that was chosen.

See also

• Fixed source form and free source form for the line rules
• Versions and standards for what each version of the language added