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31.7 The Mark

Each buffer has a special marker, which is designated the mark. When a buffer is newly created, this marker exists but does not point anywhere; this means that the mark doesn't exist in that buffer yet. Subsequent commands can set the mark.

The mark specifies a position to bound a range of text for many commands, such as kill-region and indent-rigidly. These commands typically act on the text between point and the mark, which is called the region. If you are writing a command that operates on the region, don't examine the mark directly; instead, use interactive with the ‘r’ specification. This provides the values of point and the mark as arguments to the command in an interactive call, but permits other Lisp programs to specify arguments explicitly. See Interactive Codes.

Some commands set the mark as a side-effect. Commands should do this only if it has a potential use to the user, and never for their own internal purposes. For example, the replace-regexp command sets the mark to the value of point before doing any replacements, because this enables the user to move back there conveniently after the replace is finished.

Once the mark exists in a buffer, it normally never ceases to exist. However, it may become inactive, if Transient Mark mode is enabled. The buffer-local variable mark-active, if non-nil, means that the mark is active. A command can call the function deactivate-mark to deactivate the mark directly, or it can request deactivation of the mark upon return to the editor command loop by setting the variable deactivate-mark to a non-nil value.

If Transient Mark mode is enabled, certain editing commands that normally apply to text near point, apply instead to the region when the mark is active. This is the main motivation for using Transient Mark mode. (Another is that this enables highlighting of the region when the mark is active. See Display.)

In addition to the mark, each buffer has a mark ring which is a list of markers containing previous values of the mark. When editing commands change the mark, they should normally save the old value of the mark on the mark ring. The variable mark-ring-max specifies the maximum number of entries in the mark ring; once the list becomes this long, adding a new element deletes the last element.

There is also a separate global mark ring, but that is used only in a few particular user-level commands, and is not relevant to Lisp programming. So we do not describe it here.

— Function: mark &optional force

This function returns the current buffer's mark position as an integer, or nil if no mark has ever been set in this buffer.

If Transient Mark mode is enabled, and mark-even-if-inactive is nil, mark signals an error if the mark is inactive. However, if force is non-nil, then mark disregards inactivity of the mark, and returns the mark position (or nil) anyway.

— Function: mark-marker

This function returns the marker that represents the current buffer's mark. It is not a copy, it is the marker used internally. Therefore, changing this marker's position will directly affect the buffer's mark. Don't do that unless that is the effect you want. 

          (setq m (mark-marker))
               ⇒ #<marker at 3420 in markers.texi>
          (set-marker m 100)
               ⇒ #<marker at 100 in markers.texi>
          (mark-marker)
               ⇒ #<marker at 100 in markers.texi>

Like any marker, this marker can be set to point at any buffer you like. If you make it point at any buffer other than the one of which it is the mark, it will yield perfectly consistent, but rather odd, results. We recommend that you not do it!

— Function: set-mark position

This function sets the mark to position, and activates the mark. The old value of the mark is not pushed onto the mark ring.

Please note: Use this function only if you want the user to see that the mark has moved, and you want the previous mark position to be lost. Normally, when a new mark is set, the old one should go on the mark-ring. For this reason, most applications should use push-mark and pop-mark, not set-mark.

Novice Emacs Lisp programmers often try to use the mark for the wrong purposes. The mark saves a location for the user's convenience. An editing command should not alter the mark unless altering the mark is part of the user-level functionality of the command. (And, in that case, this effect should be documented.) To remember a location for internal use in the Lisp program, store it in a Lisp variable. For example: 

          (let ((beg (point)))
            (forward-line 1)
            (delete-region beg (point))).
— Function: push-mark &optional position nomsg activate

This function sets the current buffer's mark to position, and pushes a copy of the previous mark onto mark-ring. If position is nil, then the value of point is used.

The function push-mark normally does not activate the mark. To do that, specify t for the argument activate.

A ‘Mark set’ message is displayed unless nomsg is non-nil.

— Function: pop-mark

This function pops off the top element of mark-ring and makes that mark become the buffer's actual mark. This does not move point in the buffer, and it does nothing if mark-ring is empty. It deactivates the mark.

— User Option: transient-mark-mode

This variable, if non-nil, enables Transient Mark mode. In Transient Mark mode, every buffer-modifying primitive sets deactivate-mark. As a consequence, most commands that modify the buffer also deactivate the mark.

When Transient Mark mode is enabled and the mark is active, many commands that normally apply to the text near point instead apply to the region. Such commands should use the function use-region-p to test whether they should operate on the region. See The Region.

Lisp programs can set transient-mark-mode to non-nil, non-t values to enable Transient Mark mode temporarily. If the value is lambda, Transient Mark mode is automatically turned off after any action, such as buffer modification, that would normally deactivate the mark. If the value is (only . oldval), then transient-mark-mode is set to the value oldval after any subsequent command that moves point and is not shift-translated (see shift-translation), or after any other action that would normally deactivate the mark.

— User Option: mark-even-if-inactive

If this is non-nil, Lisp programs and the Emacs user can use the mark even when it is inactive. This option affects the behavior of Transient Mark mode. When the option is non-nil, deactivation of the mark turns off region highlighting, but commands that use the mark behave as if the mark were still active.

— Variable: deactivate-mark

If an editor command sets this variable non-nil, then the editor command loop deactivates the mark after the command returns (if Transient Mark mode is enabled). All the primitives that change the buffer set deactivate-mark, to deactivate the mark when the command is finished. Setting this variable makes it buffer-local.

To write Lisp code that modifies the buffer without causing deactivation of the mark at the end of the command, bind deactivate-mark to nil around the code that does the modification. For example: 

          (let (deactivate-mark)
            (insert " "))
— Function: deactivate-mark &optional force

If Transient Mark mode is enabled or force is non-nil, this function deactivates the mark and runs the normal hook deactivate-mark-hook. Otherwise, it does nothing.

— Variable: mark-active

The mark is active when this variable is non-nil. This variable is always buffer-local in each buffer. Do not use the value of this variable to decide whether a command that normally operates on text near point should operate on the region instead. Use the function use-region-p for that (see The Region).

— Variable: activate-mark-hook
— Variable: deactivate-mark-hook

These normal hooks are run, respectively, when the mark becomes active and when it becomes inactive. The hook activate-mark-hook is also run at the end of the command loop if the mark is active and it is possible that the region may have changed.

— Function: handle-shift-selection

This function implements the shift-selection behavior of point-motion commands. See Shift Selection. It is called automatically by the Emacs command loop whenever a command with a ‘^’ character in its interactive spec is invoked, before the command itself is executed (see ^).

If shift-select-mode is non-nil and the current command was invoked via shift translation (see shift-translation), this function sets the mark and temporarily activates the region, unless the region was already temporarily activated in this way. Otherwise, if the region has been activated temporarily, it deactivates the mark and restores the variable transient-mark-mode to its earlier value.

— Variable: mark-ring

The value of this buffer-local variable is the list of saved former marks of the current buffer, most recent first. 

          mark-ring
          ⇒ (#<marker at 11050 in markers.texi>
              #<marker at 10832 in markers.texi>
              ...)
— User Option: mark-ring-max

The value of this variable is the maximum size of mark-ring. If more marks than this are pushed onto the mark-ring, push-mark discards an old mark when it adds a new one.

When Delete Selection mode (see Delete Selection) is enabled, commands that operate on the active region (a.k.a. “selection”) behave slightly differently. This works by adding the function delete-selection-pre-hook to the pre-command-hook (see Command Overview). That function calls delete-selection-helper to delete the selection as appropriate for the command. If you want to adapt a command to Delete Selection mode, put the delete-selection property on the function's symbol (see Symbol Plists); commands that don't have this property on their symbol won't delete the selection. This property can have one of several values to tailor the behavior to what the command is supposed to do; see the doc strings of delete-selection-pre-hook and delete-selection-helper for the details.