@c This is part of the Emacs manual.
@c Copyright (C) 2004--2025 Free Software Foundation, Inc.
@c See file emacs.texi for copying conditions.
@c
@c This file is included either in emacs-xtra.texi (when producing the
@c printed version) or in the main Emacs manual (for the on-line version).
@node Fortran
@section Fortran Mode
@cindex Fortran mode
@cindex mode, Fortran

@cindex Fortran fixed form and free form
@cindex Fortran 77 and Fortran 90, 95, 2003, 2008
@findex f90-mode
@findex fortran-mode
  Fortran mode is meant for editing fixed form (and also tab
format) source code (normally Fortran 77).  For editing more modern
free-form source code (Fortran 90, 95, 2003, 2008), use F90 mode
(@code{f90-mode}).  Emacs normally uses Fortran mode for files with
extension @samp{.f}, @samp{.F} or @samp{.for}, and F90 mode for the
extensions @samp{.f90}, @samp{.f95}, @samp{.f03} and @samp{.f08}.
Customize @code{auto-mode-alist} to add more extensions.  GNU Fortran
supports both free and fixed form.  This manual mainly documents Fortran
mode, but the corresponding F90 mode features are mentioned when
relevant.

  Fortran mode provides special motion commands for Fortran statements
and subprograms, and indentation commands that understand Fortran
conventions of nesting, line numbers and continuation statements.
Fortran mode has support for Auto Fill mode that breaks long lines into
proper Fortran continuation lines.  Fortran mode also supports Hideshow
minor mode
@iftex
(@pxref{Hideshow,,, emacs, the Emacs Manual}),
@end iftex
@ifnottex
(@pxref{Hideshow}),
@end ifnottex
and Imenu
@iftex
(@pxref{Imenu,,, emacs, the Emacs Manual}).
@end iftex
@ifnottex
(@pxref{Imenu}).
@end ifnottex

  Special commands for comments are provided because Fortran comments
are unlike those of other languages.  Built-in abbrevs optionally save
typing when you insert Fortran keywords.

  Use @kbd{M-x fortran-mode} to switch to this major mode.  This
command runs the hook @code{fortran-mode-hook}.
@iftex
@xref{Hooks,,, emacs, the Emacs Manual}.
@end iftex
@ifnottex
@xref{Hooks}.
@end ifnottex

@menu
* Motion: Fortran Motion.        Moving point by statements or subprograms.
* Indent: Fortran Indent.        Indentation commands for Fortran.
* Comments: Fortran Comments.    Inserting and aligning comments.
* Autofill: Fortran Autofill.    Auto fill support for Fortran.
* Columns: Fortran Columns.      Measuring columns for valid Fortran.
* Abbrev: Fortran Abbrev.        Built-in abbrevs for Fortran keywords.
@end menu

@node Fortran Motion
@subsection Motion Commands

  In addition to the normal commands for moving by and operating on
defuns (Fortran subprograms---functions and subroutines, as well
as modules for F90 mode, using the commands @code{fortran-end-of-subprogram}
and @code{fortran-beginning-of-subprogram}), Fortran mode provides
special commands to move by statements and other program units.

@table @kbd
@kindex C-c C-n @r{(Fortran mode)}
@findex fortran-next-statement
@findex f90-next-statement
@item C-c C-n
Move to the beginning of the next statement
(@code{fortran-next-statement}/@code{f90-next-statement}).

@kindex C-c C-p @r{(Fortran mode)}
@findex fortran-previous-statement
@findex f90-previous-statement
@item C-c C-p
Move to the beginning of the previous statement
(@code{fortran-previous-statement}/@code{f90-previous-statement}).
If there is no previous statement (i.e., if called from the first
statement in the buffer), move to the start of the buffer.

@kindex C-c C-e @r{(F90 mode)}
@findex f90-next-block
@item C-c C-e
Move point forward to the start of the next code block, or the end of
the current one, whichever comes first (@code{f90-next-block}).
A code block is a subroutine, @code{if}--@code{endif} statement, and
so forth.  This command exists for F90 mode only, not Fortran mode.
With a numeric argument, it moves forward that many blocks.

@kindex C-c C-a @r{(F90 mode)}
@findex f90-previous-block
@item C-c C-a
Move point backward to the previous block
(@code{f90-previous-block}).  This is like @code{f90-next-block}, but
moves backwards.

@kindex C-M-n @r{(Fortran mode)}
@findex fortran-end-of-block
@findex f90-end-of-block
@item C-M-n
Move to the end of the current code block
(@code{fortran-end-of-block}/@code{f90-end-of-block}).  With a numeric
argument, move forward that number of blocks.  The mark is set before
moving point.  The F90 mode version of this command checks for
consistency of block types and labels (if present), but it does not
check the outermost block since that may be incomplete.

@kindex C-M-p @r{(Fortran mode)}
@findex fortran-beginning-of-block
@findex f90-beginning-of-block
@item C-M-p
Move to the start of the current code block
(@code{fortran-beginning-of-block}/@code{f90-beginning-of-block}).  This
is like @code{fortran-end-of-block}, but moves backwards.
@end table

The commands @code{fortran-beginning-of-subprogram} and
@code{fortran-end-of-subprogram} move to the start or end of the
current subprogram, respectively.  The commands @code{fortran-mark-do}
and @code{fortran-mark-if} mark the end of the current @code{do} or
@code{if} block, and move point to the start.


@node Fortran Indent
@subsection Fortran Indentation

  Special commands and features are needed for indenting fixed (or tab)
form Fortran code in order to make sure various syntactic entities (line
numbers, comment line indicators and continuation line flags) appear in
the required columns.

@menu
* Commands: ForIndent Commands.  Commands for indenting and filling Fortran.
* Contline: ForIndent Cont.      How continuation lines indent.
* Numbers:  ForIndent Num.       How line numbers auto-indent.
* Conv:     ForIndent Conv.      Conventions you must obey to avoid trouble.
* Vars:     ForIndent Vars.      Variables controlling Fortran indent style.
@end menu

@node ForIndent Commands
@subsubsection Fortran Indentation and Filling Commands

@table @kbd
@item C-M-j
Break the current line at point and set up a continuation line
(@code{fortran-split-line}).

@item M-^
Join this line to the previous line (@code{fortran-join-line}).

@item C-M-q
Indent all the lines of the subprogram that point is in
(@code{fortran-indent-subprogram}).

@item M-q
Fill a comment block or statement (using @code{fortran-fill-paragraph}
or @code{fortran-fill-statement}).
@end table

@kindex C-M-q @r{(Fortran mode)}
@findex fortran-indent-subprogram
  The key @kbd{C-M-q} runs @code{fortran-indent-subprogram}, a command
to reindent all the lines of the Fortran subprogram (function or
subroutine) containing point.

@kindex C-M-j @r{(Fortran mode)}
@findex fortran-split-line
  The key @kbd{C-M-j} runs @code{fortran-split-line}, which splits
a line in the appropriate fashion for Fortran.  In a non-comment line,
the second half becomes a continuation line and is indented
accordingly.  In a comment line, both halves become separate comment
lines.

@kindex M-^ @r{(Fortran mode)}
@kindex C-c C-d @r{(Fortran mode)}
@findex fortran-join-line
  @kbd{M-^} or @kbd{C-c C-d} run the command @code{fortran-join-line},
which joins a continuation line back to the previous line, roughly as
the inverse of @code{fortran-split-line}.  The point must be on a
continuation line when this command is invoked.

@kindex M-q @r{(Fortran mode)}
@kbd{M-q} in Fortran mode fills the comment block or statement that
point is in.  This removes any excess statement continuations.

@node ForIndent Cont
@subsubsection Continuation Lines
@cindex Fortran continuation lines

@vindex fortran-continuation-string
  Most Fortran 77 compilers allow two ways of writing continuation lines.
If the first non-space character on a line is in column 5, then that
line is a continuation of the previous line.  We call this @dfn{fixed
form}.  (In GNU Emacs we always count columns from 0; but note that
the Fortran standard counts from 1.  You can customize the variable
@code{column-number-indicator-zero-based} to make the column display
@iftex
Fortran-like; @pxref{Optional Mode Line,,, emacs, the Emacs Manual}.)
@end iftex
@ifnottex
Fortran-like; @pxref{Optional Mode Line}.)
@end ifnottex
The variable
@code{fortran-continuation-string} specifies what character to put in
column 5.  A line that starts with a tab character followed by any digit
except @samp{0} is also a continuation line.  We call this style of
continuation @dfn{tab format}.  (Fortran 90 introduced free-form
continuation lines.)

@vindex indent-tabs-mode @r{(Fortran mode)}
@vindex fortran-analyze-depth
@vindex fortran-tab-mode-default
  Fortran mode can use either style of continuation line.  When you
enter Fortran mode, it tries to deduce the proper continuation style
automatically from the buffer contents.  It does this by scanning up to
@code{fortran-analyze-depth} (default 100) lines from the start of the
buffer.  The first line that begins with either a tab character or six
spaces determines the choice.  If the scan fails (for example, if the
buffer is new and therefore empty), the value of
@code{fortran-tab-mode-default} (@code{nil} for fixed form, and
non-@code{nil} for tab format) is used.  @samp{/t}
(@code{fortran-tab-mode-string}) in the mode line indicates tab format
is selected.  Fortran mode sets the value of @code{indent-tabs-mode}
accordingly.

  If the text on a line starts with the Fortran continuation marker
@samp{$}, or if it begins with any non-whitespace character in column
5, Fortran mode treats it as a continuation line.  When you indent a
continuation line with @kbd{@key{TAB}}, it converts the line to the
current continuation style.  When you split a Fortran statement with
@kbd{C-M-j}, the continuation marker on the newline is created
according to the continuation style.

  The setting of continuation style affects several other aspects of
editing in Fortran mode.  In fixed form mode, the minimum column
number for the body of a statement is 6.  Lines inside of Fortran
blocks that are indented to larger column numbers must use only the
space character for whitespace.  In tab format mode, the minimum
column number for the statement body is 8, and the whitespace before
column 8 must consist of one tab character.

@node ForIndent Num
@subsubsection Line Numbers

  If a number is the first non-whitespace in the line, Fortran
indentation assumes it is a line number and moves it to columns 0
through 4.  (Columns always count from 0 in Emacs, but setting
@code{column-number-indicator-zero-based} to @code{nil} can change
that,
@iftex
@pxref{Optional Mode Line,,, emacs, the Emacs Manual}.)
@end iftex
@ifnottex
@pxref{Optional Mode Line}.)
@end ifnottex

@vindex fortran-line-number-indent
  Line numbers of four digits or less are normally indented one space.
The variable @code{fortran-line-number-indent} controls this; it
specifies the maximum indentation a line number can have.  The default
value of the variable is 1.  Fortran mode tries to prevent line number
digits passing column 4, reducing the indentation below the specified
maximum if necessary.  If @code{fortran-line-number-indent} has the
value 5, line numbers are right-justified to end in column 4.

@vindex fortran-electric-line-number
  Simply inserting a line number is enough to indent it according to
these rules.  As each digit is inserted, the indentation is recomputed.
To turn off this feature, set the variable
@code{fortran-electric-line-number} to @code{nil}.


@node ForIndent Conv
@subsubsection Syntactic Conventions

  Fortran mode assumes that you follow certain conventions that simplify
the task of understanding a Fortran program well enough to indent it
properly:

@itemize @bullet
@item
Two nested @samp{do} loops never share a @samp{continue} statement.

@item
Fortran keywords such as @samp{if}, @samp{else}, @samp{then}, @samp{do}
and others are written without embedded whitespace or line breaks.

Fortran compilers generally ignore whitespace outside of string
constants, but Fortran mode does not recognize these keywords if they
are not contiguous.  Constructs such as @samp{else if} or @samp{end do}
are acceptable, but the second word should be on the same line as the
first and not on a continuation line.
@end itemize

@noindent
If you fail to follow these conventions, the indentation commands may
indent some lines unaesthetically.  However, a correct Fortran program
retains its meaning when reindented even if the conventions are not
followed.

@node ForIndent Vars
@subsubsection Variables for Fortran Indentation

@vindex fortran-do-indent
@vindex fortran-if-indent
@vindex fortran-structure-indent
@vindex fortran-continuation-indent
@vindex fortran-check-all-num@dots{}
@vindex fortran-minimum-statement-indent@dots{}
  Several additional variables control how Fortran indentation works:

@table @code
@item fortran-do-indent
Extra indentation within each level of @samp{do} statement (default 3).

@item fortran-if-indent
Extra indentation within each level of @samp{if}, @samp{select case}, or
@samp{where} statements (default 3).

@item fortran-structure-indent
Extra indentation within each level of @samp{structure}, @samp{union},
@samp{map}, or @samp{interface} statements (default 3).

@item fortran-continuation-indent
Extra indentation for bodies of continuation lines (default 5).

@item fortran-check-all-num-for-matching-do
In Fortran 77, a numbered @samp{do} statement is terminated by any statement
with a matching line number.  It is common (but not compulsory) to use a
@samp{continue} statement for this purpose.  If this variable has a
non-@code{nil} value, indenting any numbered statement must check for a
@samp{do} that ends there.  If you always end @samp{do} statements with
a @samp{continue} line (or if you use the more modern @samp{enddo}),
then you can speed up indentation by setting this variable to
@code{nil} (the default).

@item fortran-blink-matching-if
If this is @code{t}, indenting an @samp{endif} (or @samp{enddo})
statement moves the cursor momentarily to the matching @samp{if} (or
@samp{do}) statement to show where it is.  The default is @code{nil}.

@item fortran-minimum-statement-indent-fixed
Minimum indentation for Fortran statements when using fixed form
continuation line style.  Statement bodies are never indented by less than
this.  The default is 6.

@item fortran-minimum-statement-indent-tab
Minimum indentation for Fortran statements for tab format continuation line
style.  Statement bodies are never indented by less than this.  The
default is 8.
@end table

The following section describes the variables controlling the
indentation of comments.

@node Fortran Comments
@subsection Fortran Comments

  The usual Emacs comment commands assume that a comment can follow a
line of code.  In Fortran 77, the standard comment syntax requires an
entire line to be just a comment.  Therefore, Fortran mode replaces the
standard Emacs comment commands and defines some new variables.

@vindex fortran-comment-line-start
  Fortran mode can also handle the Fortran 90 comment syntax where
comments start with @samp{!} and can follow other text.  Because only
some Fortran 77 compilers accept this syntax, Fortran mode will not
insert such comments unless you have said in advance to do so.  To do
this, set the variable @code{fortran-comment-line-start} to @samp{"!"}.
If you use an unusual value, you may need to change
@code{fortran-comment-line-start-skip}.


@table @kbd
@item M-;
Align comment or insert new comment (@code{comment-dwim}).

@item C-x ;
Applies to nonstandard @samp{!} comments only (@code{comment-set-column}).

@item C-c ;
Turn all lines of the region into comments, or (with argument) turn them back
into real code (@code{fortran-comment-region}).
@end table

  @kbd{M-;} in Fortran mode runs the standard @code{comment-dwim}.
This recognizes any kind of existing comment and aligns its text
appropriately; if there is no existing comment, a comment is inserted
and aligned.  Inserting and aligning comments are not the same in
Fortran mode as in other modes.

  When a new comment must be inserted, if the current line is blank, a
full-line comment is inserted.  On a non-blank line, a nonstandard @samp{!}
comment is inserted if you have said you want to use them.  Otherwise, a
full-line comment is inserted on a new line before the current line.

  Nonstandard @samp{!} comments are aligned like comments in other
languages, but full-line comments are different.  In a standard full-line
comment, the comment delimiter itself must always appear in column zero.
What can be aligned is the text within the comment.  You can choose from
three styles of alignment by setting the variable
@code{fortran-comment-indent-style} to one of these values:

@vindex fortran-comment-indent-style
@vindex fortran-comment-line-extra-indent
@table @code
@item fixed
Align the text at a fixed column, which is the sum of
@code{fortran-comment-line-extra-indent} and the minimum statement
indentation.  This is the default.

The minimum indentation is
@code{fortran-minimum-statement-indent-tab} for tab format
continuation line style and @code{fortran-minimum-statement-indent-fixed}
for fixed form style.

@item relative
Align the text as if it were a line of code, but with an additional
@code{fortran-comment-line-extra-indent} columns of indentation.

@item nil
Don't move text in full-line comments automatically.
@end table

@vindex fortran-comment-indent-char
  In addition, you can specify the character to be used to indent within
full-line comments by setting the variable
@code{fortran-comment-indent-char} to the single-character string you want
to use.

@vindex fortran-directive-re
  Compiler directive lines, or preprocessor lines, have much the same
appearance as comment lines.  It is important, though, that such lines
never be indented at all, no matter what the value of
@code{fortran-comment-indent-style}.  The variable
@code{fortran-directive-re} is a regular expression that specifies which
lines are directives.  Matching lines are never indented, and receive
distinctive font-locking.

  The normal Emacs comment command @kbd{C-x ;} (@code{comment-set-column})
has not been redefined.  If you use @samp{!} comments, this command
can be used with them.  Otherwise, it is useless in Fortran mode.

@kindex C-c ; @r{(Fortran mode)}
@findex fortran-comment-region
@vindex fortran-comment-region
  The command @kbd{C-c ;} (@code{fortran-comment-region}) turns all the
lines of the region into comments by inserting the string @samp{c$$$} at
the front of each one.  With a numeric argument, it turns the region
back into live code by deleting @samp{c$$$} from the front of each line
in it.  The string used for these comments can be controlled by setting
the variable @code{fortran-comment-region}.  Note that here we have an
example of a command and a variable with the same name; these two uses
of the name never conflict because in Lisp and in Emacs it is always
clear from the context which one is meant.

@node Fortran Autofill
@subsection Auto Fill in Fortran Mode

  Fortran mode has specialized support for Auto Fill mode, which is a
minor mode that automatically splits statements as you insert them
when they become too wide.  Splitting a statement involves making
continuation lines using @code{fortran-continuation-string}
(@pxref{ForIndent Cont}).  This splitting happens when you type
@kbd{@key{SPC}}, @kbd{@key{RET}}, or @kbd{@key{TAB}}, and also in the
Fortran indentation commands.  You activate Auto Fill in Fortran mode
in the normal way.
@iftex
@xref{Auto Fill,,, emacs, the Emacs Manual}.
@end iftex
@ifnottex
@xref{Auto Fill}.
@end ifnottex

@vindex fortran-break-before-delimiters
   Auto Fill breaks lines at spaces or delimiters when the lines get
longer than the desired width (the value of @code{fill-column}).  The
delimiters (besides whitespace) that Auto Fill can break at are
@samp{+}, @samp{-}, @samp{/}, @samp{*}, @samp{=}, @samp{<}, @samp{>},
and @samp{,}.  The line break comes after the delimiter if the
variable @code{fortran-break-before-delimiters} is @code{nil}.
Otherwise (and by default), the break comes before the delimiter.

  To enable Auto Fill in all Fortran buffers, add
@code{auto-fill-mode} to @code{fortran-mode-hook}.
@iftex
@xref{Hooks,,, emacs, the Emacs Manual}.
@end iftex
@ifnottex
@xref{Hooks}.
@end ifnottex

@node Fortran Columns
@subsection Checking Columns in Fortran

@vindex fortran-line-length
In standard Fortran 77, anything beyond column 72 is ignored.
Most compilers provide an option to change this (for example,
@samp{-ffixed-line-length-N} in gfortran).  Customize the variable
@code{fortran-line-length} to change the line length in Fortran mode.
Anything beyond this point is font-locked as a comment.  (Unless it is
inside a string: strings that extend beyond @code{fortran-line-length}
will confuse font-lock.)

@table @kbd
@item C-c C-r
Display a column ruler momentarily above the current line
(@code{fortran-column-ruler}).

@item C-c C-w
Split the current window horizontally temporarily so that it is
@code{fortran-line-length} columns wide
(@code{fortran-window-create-momentarily}).  This may help you avoid
making lines longer than the limit imposed by your Fortran compiler.

@item C-u C-c C-w
Split the current window horizontally so that it is
@code{fortran-line-length} columns wide (@code{fortran-window-create}).
You can then continue editing.

@item M-x fortran-strip-sequence-nos
Delete all text in column @code{fortran-line-length} and beyond.
@end table

@kindex C-c C-r @r{(Fortran mode)}
@findex fortran-column-ruler
  The command @kbd{C-c C-r} (@code{fortran-column-ruler}) shows a column
ruler momentarily above the current line.  The comment ruler is two lines
of text that show you the locations of columns with special significance in
Fortran programs.  Square brackets show the limits of the columns for line
numbers, and curly brackets show the limits of the columns for the
statement body.  Column numbers appear above them.

  Note that the column numbers count from zero, as always in GNU Emacs
(but customizing @code{column-number-indicator-zero-based} can change
column display to match that of Fortran;
@iftex
@pxref{Optional Mode Line,,, emacs, the Emacs Manual}.)
@end iftex
@ifnottex
@pxref{Optional Mode Line}.)
@end ifnottex
As a result, the numbers may be one less than those you are familiar
with; but the positions they indicate in the line are standard for
Fortran.

@vindex fortran-column-ruler-fixed
@vindex fortran-column-ruler-tabs
  The text used to display the column ruler depends on the value of the
variable @code{indent-tabs-mode}.  If @code{indent-tabs-mode} is
@code{nil}, then the value of the variable
@code{fortran-column-ruler-fixed} is used as the column ruler.
Otherwise, the value of the variable @code{fortran-column-ruler-tab} is
displayed.  By changing these variables, you can change the column ruler
display.

@kindex C-c C-w @r{(Fortran mode)}
@findex fortran-window-create-momentarily
  @kbd{C-c C-w} (@code{fortran-window-create-momentarily}) temporarily
splits the current window horizontally, making a window
@code{fortran-line-length} columns wide, so you can see any lines that
are too long.  Type a space to restore the normal width.

@kindex C-u C-c C-w @r{(Fortran mode)}
@findex fortran-window-create
  You can also split the window horizontally and continue editing with
the split in place.  To do this, use @kbd{C-u C-c C-w} (@code{M-x
fortran-window-create}).  By editing in this window you can
immediately see when you make a line too wide to be correct Fortran.

@findex fortran-strip-sequence-nos
  The command @kbd{M-x fortran-strip-sequence-nos} deletes all text in
column @code{fortran-line-length} and beyond, on all lines in the
current buffer.  This is the easiest way to get rid of old sequence
numbers.

@node Fortran Abbrev
@subsection Fortran Keyword Abbrevs

  Fortran mode provides many built-in abbrevs for common keywords and
declarations.  These are the same sort of abbrev that you can define
yourself.  To use them, you must turn on Abbrev mode.
@iftex
@xref{Abbrevs,,, emacs, the Emacs Manual}.
@end iftex
@ifnottex
@xref{Abbrevs}.
@end ifnottex

  The built-in abbrevs are unusual in one way: they all start with a
semicolon.  For example, one built-in Fortran abbrev is @samp{;c} for
@samp{continue}.  If you insert @samp{;c} and then insert a punctuation
character such as a space or a newline, the @samp{;c} expands automatically
to @samp{continue}, provided Abbrev mode is enabled.

  Type @samp{;?} or @samp{;C-h} to display a list of all the built-in
Fortran abbrevs and what they stand for.
