Avoid aborts in cm.c due to too small TTY frame

[emacs.git] / doc / emacs / screen.texi

@c This is part of the Emacs manual.
@c Copyright (C) 1985-1987, 1993-1995, 1997, 2001-2018 Free Software
@c Foundation, Inc.
@c See file emacs.texi for copying conditions.
@node Screen
@chapter The Organization of the Screen
@cindex screen
@cindex frame

  On a graphical display, such as on GNU/Linux using the X Window
System, Emacs occupies a graphical window.  On a text terminal,
Emacs occupies the entire terminal screen.  We will use the term
@dfn{frame} to mean a graphical window or terminal screen occupied by
Emacs.  Emacs behaves very similarly on both kinds of frames.  It
normally starts out with just one frame, but you can create additional
frames if you wish (@pxref{Frames}).

  Each frame consists of several distinct regions.  At the top of the
frame is a @dfn{menu bar}, which allows you to access commands via a
series of menus.  On a graphical display, directly below the menu bar
is a @dfn{tool bar}, a row of icons that perform editing commands when
you click on them.  At the very bottom of the frame is an @dfn{echo
area}, where informative messages are displayed and where you enter
information when Emacs asks for it.

  The main area of the frame, below the tool bar (if one exists) and
above the echo area, is called @dfn{the window}.  Henceforth in this
manual, we will use the word ``window'' in this sense.  Graphical
display systems commonly use the word ``window'' with a different
meaning; but, as stated above, we refer to those graphical windows
as ``frames''.

  An Emacs window is where the @dfn{buffer}---the text or other
graphics you are editing or viewing---is displayed.  On a graphical
display, the window possesses a @dfn{scroll bar} on one side, which
can be used to scroll through the buffer.  The last line of the window
is a @dfn{mode line}.  This displays various information about what is
going on in the buffer, such as whether there are unsaved changes, the
editing modes that are in use, the current line number, and so forth.

  When you start Emacs, there is normally only one window in the
frame.  However, you can subdivide this window horizontally or
vertically to create multiple windows, each of which can independently
display a buffer (@pxref{Windows}).

  At any time, one window is the @dfn{selected window}.  On a
graphical display, the selected window shows a more prominent cursor
(usually solid and blinking); other windows show a less prominent
cursor (usually a hollow box).  On a text terminal, there is only one
cursor, which is shown in the selected window.  The buffer displayed
in the selected window is called the @dfn{current buffer}, and it is
where editing happens.  Most Emacs commands implicitly apply to the
current buffer; the text displayed in unselected windows is mostly
visible for reference.  If you use multiple frames on a graphical
display, selecting a particular frame selects a window in that frame.

@menu
* Point::             The place in the text where editing commands operate.
* Echo Area::         Short messages appear at the bottom of the screen.
* Mode Line::         Interpreting the mode line.
* Menu Bar::          How to use the menu bar.
@end menu

@node Point
@section Point
@cindex point
@cindex cursor

  The cursor in the selected window shows the location where most
editing commands take effect, which is called @dfn{point}@footnote{The
term ``point'' comes from the character @samp{.}, which was the
command in TECO (the language in which the original Emacs was written)
for accessing the editing position.}.  Many Emacs commands move point
to different places in the buffer; for example, you can place point by
clicking mouse button 1 (normally the left button) at the desired
location.

  By default, the cursor in the selected window is drawn as a solid
block and appears to be @emph{on} a character, but you should think of
point as @emph{between} two characters; it is situated @emph{before}
the character under the cursor.  For example, if your text looks like
@samp{frob} with the cursor over the @samp{b}, then point is between
the @samp{o} and the @samp{b}.  If you insert the character @samp{!}
at that position, the result is @samp{fro!b}, with point between the
@samp{!}  and the @samp{b}.  Thus, the cursor remains over the
@samp{b}, as before.

  If you are editing several files in Emacs, each in its own buffer,
each buffer has its own value of point.  A buffer that is not
currently displayed remembers its value of point if you later display
it again.  Furthermore, if a buffer is displayed in multiple windows,
each of those windows has its own value of point.

  @xref{Cursor Display}, for options that control how Emacs displays
the cursor.

@node Echo Area
@section The Echo Area
@cindex echo area

  The line at the very bottom of the frame is the @dfn{echo area}.  It
is used to display small amounts of text for various purposes.

@cindex echoing
  The echo area is so-named because one of the things it is used for
is @dfn{echoing}, which means displaying the characters of a
multi-character command as you type.  Single-character commands are
not echoed.  Multi-character commands (@pxref{Keys}) are echoed if you
pause for more than a second in the middle of a command.  Emacs then
echoes all the characters of the command so far, to prompt you for the
rest.  Once echoing has started, the rest of the command echoes
immediately as you type it.  This behavior is designed to give
confident users fast response, while giving hesitant users maximum
feedback.

@cindex error message
@cindex echo area message
  The echo area is also used to display an @dfn{error message} when a
command cannot do its job.  Error messages may be accompanied by
beeping or by flashing the screen.

  Some commands display informative messages in the echo area to tell
you what the command has done, or to provide you with some specific
information.  These @dfn{informative} messages, unlike error messages,
are not accompanied with a beep or flash.  For example, @kbd{C-x =}
(hold down @key{Ctrl} and type @kbd{x}, then let go of @key{Ctrl} and
type @kbd{=}) displays a message describing the character at point,
its position in the buffer, and its current column in the window.
Commands that take a long time often display messages ending in
@samp{...} while they are working (sometimes also indicating how much
progress has been made, as a percentage), and add @samp{done} when
they are finished.

@cindex @file{*Messages*} buffer
@cindex saved echo area messages
@cindex messages saved from echo area
@vindex message-log-max
  Informative echo area messages are saved in a special buffer named
@file{*Messages*}.  (We have not explained buffers yet; see
@ref{Buffers}, for more information about them.)  If you miss a
message that appeared briefly on the screen, you can switch to the
@file{*Messages*} buffer to see it again.  The @file{*Messages*}
buffer is limited to a certain number of lines, specified by the
variable @code{message-log-max}.  (We have not explained variables
either; see @ref{Variables}, for more information about them.)  Beyond
this limit, one line is deleted from the beginning whenever a new
message line is added at the end.

  @xref{Display Custom}, for options that control how Emacs uses the
echo area.

  The echo area is also used to display the @dfn{minibuffer}, a
special window where you can input arguments to commands, such as the
name of a file to be edited.  When the minibuffer is in use, the text
displayed in the echo area begins with a @dfn{prompt string}, and the
active cursor appears within the minibuffer, which is temporarily
considered the selected window.  You can always get out of the
minibuffer by typing @kbd{C-g}.  @xref{Minibuffer}.

@node Mode Line
@section The Mode Line
@cindex mode line
@cindex top level

  At the bottom of each window is a @dfn{mode line}, which describes
what is going on in the current buffer.  When there is only one
window, the mode line appears right above the echo area; it is the
next-to-last line in the frame.  On a graphical display, the mode line
is drawn with a 3D box appearance.  Emacs also usually draws the mode
line of the selected window with a different color than that of
unselected windows, in order to make it stand out.

  The text displayed in the mode line has the following format:

@example
 @var{cs}:@var{ch}-@var{fr}  @var{buf}      @var{pos} @var{line}   (@var{major} @var{minor})
@end example

@noindent
On a text terminal, this text is followed by a series of dashes
extending to the right edge of the window.  These dashes are omitted
on a graphical display.

The @var{cs} string and the colon character after it describe the
character set and newline convention used for the current buffer.
Normally, Emacs automatically handles these settings for you, but it
is sometimes useful to have this information.

  @var{cs} describes the character set of the text in the buffer
(@pxref{Coding Systems}).  If it is a dash (@samp{-}), that indicates
no special character set handling (with the possible exception of
end-of-line conventions, described in the next paragraph).  @samp{=}
means no conversion whatsoever, and is usually used for files
containing non-textual data.  Other characters represent various
@dfn{coding systems}---for example, @samp{1} represents ISO Latin-1.

  On a text terminal, @var{cs} is preceded by two additional
characters that describe the coding systems for keyboard input and
terminal output.  Furthermore, if you are using an input method,
@var{cs} is preceded by a string that identifies the input method
(@pxref{Input Methods}).

@cindex end-of-line convention, mode-line indication
  The character after @var{cs} is usually a colon.  If a different
string is displayed, that indicates a nontrivial end-of-line
convention for encoding a file.  Usually, lines of text are separated
by @dfn{newline characters} in a file, but two other conventions are
sometimes used.  The MS-DOS convention uses a carriage-return
character followed by a linefeed character; when editing such
files, the colon changes to either a backslash (@samp{\}) or
@samp{(DOS)}, depending on the operating system.  Another convention,
employed by older Macintosh systems, uses a carriage-return
character instead of a newline; when editing such files, the colon
changes to either a forward slash (@samp{/}) or @samp{(Mac)}.  On some
systems, Emacs displays @samp{(Unix)} instead of the colon for files
that use newline as the line separator.

  On frames created for @command{emacsclient} (@pxref{Invoking
emacsclient}), the next character is @samp{@@}.  This indication is
typical for frames of an Emacs process running as a daemon
(@pxref{Emacs Server}).

  The next element on the mode line is the string indicated by
@var{ch}.  This shows two dashes (@samp{--}) if the buffer displayed
in the window has the same contents as the corresponding file on the
disk; i.e., if the buffer is unmodified.  If the buffer is
modified, it shows two stars (@samp{**}).  For a read-only buffer, it
shows @samp{%*} if the buffer is modified, and @samp{%%} otherwise.

  The character after @var{ch} is normally a dash (@samp{-}).
However, if @code{default-directory} (@pxref{File Names}) for the
current buffer is on a remote machine, @samp{@@} is displayed instead.

  @var{fr} gives the selected frame name (@pxref{Frames}).  It appears
only on text terminals.  The initial frame's name is @samp{F1}.

  @var{buf} is the name of the buffer displayed in the window.
Usually, this is the same as the name of a file you are editing.
@xref{Buffers}.

  @var{pos} tells you whether there is additional text above the top
of the window, or below the bottom.  If your buffer is small and all
of it is visible in the window, @var{pos} is @samp{All}.  Otherwise,
it is @samp{Top} if you are looking at the beginning of the buffer,
@samp{Bot} if you are looking at the end of the buffer, or
@samp{@var{nn}%}, where @var{nn} is the percentage of the buffer above
the top of the window.  With Size Indication mode, you can display the
size of the buffer as well.  @xref{Optional Mode Line}.

  @var{line} is the character @samp{L} followed by the line number at
point.  (You can display the current column number too, by turning on
Column Number mode.  @xref{Optional Mode Line}.)

  @var{major} is the name of the @dfn{major mode} used in the buffer.
A major mode is a principal editing mode for the buffer, such as Text
mode, Lisp mode, C mode, and so forth.  @xref{Major Modes}.  Some
major modes display additional information after the major mode name.
For example, Compilation buffers and Shell buffers display the status
of the subprocess.

  @var{minor} is a list of some of the enabled @dfn{minor modes},
which are optional editing modes that provide additional features on
top of the major mode.  @xref{Minor Modes}.

  Some features are listed together with the minor modes whenever they
are turned on, even though they are not really minor modes.
@samp{Narrow} means that the buffer being displayed has editing
restricted to only a portion of its text (@pxref{Narrowing}).
@samp{Def} means that a keyboard macro is currently being defined
(@pxref{Keyboard Macros}).

  In addition, if Emacs is inside a recursive editing level, square
brackets (@samp{[@dots{}]}) appear around the parentheses that
surround the modes.  If Emacs is in one recursive editing level within
another, double square brackets appear, and so on.  Since recursive
editing levels affect Emacs globally, such square brackets appear in
the mode line of every window.  @xref{Recursive Edit}.

  You can change the appearance of the mode line as well as the format
of its contents.  @xref{Optional Mode Line}.  In addition, the mode
line is mouse-sensitive; clicking on different parts of the mode line
performs various commands.  @xref{Mode Line Mouse}.  Also, hovering
the mouse pointer above mouse-sensitive portions of the mode line
shows tooltips (@pxref{Tooltips}) with information about commands you
can invoke by clicking on the mode line.

@node Menu Bar
@section The Menu Bar
@cindex menu bar

  Each Emacs frame normally has a @dfn{menu bar} at the top which you
can use to perform common operations.  There's no need to list them
here, as you can more easily see them yourself.

  On a display that supports a mouse, you can use the mouse to choose a
command from the menu bar.  An arrow on the right edge of a menu item
means it leads to a subsidiary menu, or @dfn{submenu}.  A @samp{...}
at the end of a menu item means that the command will prompt you for
further input before it actually does anything.

  Some of the commands in the menu bar have ordinary key bindings as
well; if so, a key binding is shown after the item itself.  To view
the full command name and documentation for a menu item, type
@kbd{C-h k}, and then select the menu bar with the mouse in the usual
way (@pxref{Key Help}).

@kindex F10
@findex menu-bar-open
@cindex menu bar access using keyboard
  Instead of using the mouse, you can also invoke the first menu bar
item by pressing @key{F10} (to run the command @code{menu-bar-open}).
You can then navigate the menus with the arrow keys or with @kbd{C-b},
@kbd{C-f} (left/right), @kbd{C-p}, and @kbd{C-n} (up/down).  To
activate a selected menu item, press @key{RET}; to cancel menu
navigation, press @kbd{C-g} or @kbd{@key{ESC} @key{ESC} @key{ESC}}.
(However, note that when Emacs was built with a GUI toolkit, the menus
are drawn and controlled by the toolkit, and the key sequences to
cancel menu navigation might be different from the above description.)

@kindex M-`
@findex tmm-menubar
@vindex tty-menu-open-use-tmm
  On a text terminal, you can optionally access the menu-bar menus in
the echo area.  To this end, customize the variable
@code{tty-menu-open-use-tmm} to a non-@code{nil} value.  Then typing
@key{F10} will run the command @code{tmm-menubar} instead of dropping
down the menu.  (You can also type @kbd{M-`}, which always invokes
@code{tmm-menubar}.)  @code{tmm-menubar} lets you select a menu item
with the keyboard.  A provisional choice appears in the echo area.
You can use the up and down arrow keys to move through the menu to
different items, and then you can type @key{RET} to select the item.
Each menu item is also designated by a letter or digit (usually the
initial of some word in the item's name).  This letter or digit is
separated from the item name by @samp{==>}.  You can type the item's
letter or digit to select the item.