elisp: Buffer Internals
E.8.1 Buffer Internals
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Two structures (see ‘buffer.h’) are used to represent buffers in C. The
‘buffer_text’ structure contains fields describing the text of a buffer;
the ‘buffer’ structure holds other fields. In the case of indirect
buffers, two or more ‘buffer’ structures reference the same
‘buffer_text’ structure.
Here are some of the fields in ‘struct buffer_text’:
‘beg’
The address of the buffer contents.
‘gpt’
‘gpt_byte’
The character and byte positions of the buffer gap. 
Buffer
Gap.
‘z’
‘z_byte’
The character and byte positions of the end of the buffer text.
‘gap_size’
The size of buffer’s gap. Buffer Gap.
‘modiff’
‘save_modiff’
‘chars_modiff’
‘overlay_modiff’
These fields count the number of buffer-modification events
performed in this buffer. ‘modiff’ is incremented after each
buffer-modification event, and is never otherwise changed;
‘save_modiff’ contains the value of ‘modiff’ the last time the
buffer was visited or saved; ‘chars_modiff’ counts only
modifications to the characters in the buffer, ignoring all other
kinds of changes; and ‘overlay_modiff’ counts only modifications to
the overlays.
‘beg_unchanged’
‘end_unchanged’
The number of characters at the start and end of the text that are
known to be unchanged since the last complete redisplay.
‘unchanged_modified’
‘overlay_unchanged_modified’
The values of ‘modiff’ and ‘overlay_modiff’, respectively, after
the last complete redisplay. If their current values match
‘modiff’ or ‘overlay_modiff’, that means ‘beg_unchanged’ and
‘end_unchanged’ contain no useful information.
‘markers’
The markers that refer to this buffer. This is actually a single
marker, and successive elements in its marker ‘chain’ are the other
markers referring to this buffer text.
‘intervals’
The interval tree which records the text properties of this buffer.
Some of the fields of ‘struct buffer’ are:
‘header’
A header of type ‘struct vectorlike_header’ is common to all
vectorlike objects.
‘own_text’
A ‘struct buffer_text’ structure that ordinarily holds the buffer
contents. In indirect buffers, this field is not used.
‘text’
A pointer to the ‘buffer_text’ structure for this buffer. In an
ordinary buffer, this is the ‘own_text’ field above. In an
indirect buffer, this is the ‘own_text’ field of the base buffer.
‘next’
A pointer to the next buffer, in the chain of all buffers,
including killed buffers. This chain is used only for allocation
and garbage collection, in order to collect killed buffers
properly.
‘pt’
‘pt_byte’
The character and byte positions of point in a buffer.
‘begv’
‘begv_byte’
The character and byte positions of the beginning of the accessible
range of text in the buffer.
‘zv’
‘zv_byte’
The character and byte positions of the end of the accessible range
of text in the buffer.
‘base_buffer’
In an indirect buffer, this points to the base buffer. In an
ordinary buffer, it is null.
‘local_flags’
This field contains flags indicating that certain variables are
local in this buffer. Such variables are declared in the C code
using ‘DEFVAR_PER_BUFFER’, and their buffer-local bindings are
stored in fields in the buffer structure itself. (Some of these
fields are described in this table.)
‘modtime’
The modification time of the visited file. It is set when the file
is written or read. Before writing the buffer into a file, this
field is compared to the modification time of the file to see if
the file has changed on disk. Buffer Modification.
‘auto_save_modified’
The time when the buffer was last auto-saved.
‘last_window_start’
The ‘window-start’ position in the buffer as of the last time the
buffer was displayed in a window.
‘clip_changed’
This flag indicates that narrowing has changed in the buffer.
Narrowing.
‘prevent_redisplay_optimizations_p’
This flag indicates that redisplay optimizations should not be used
to display this buffer.
‘overlay_center’
This field holds the current overlay center position.
Managing Overlays.
‘overlays_before’
‘overlays_after’
These fields hold, respectively, a list of overlays that end at or
before the current overlay center, and a list of overlays that end
after the current overlay center. Managing Overlays.
‘overlays_before’ is sorted in order of decreasing end position,
and ‘overlays_after’ is sorted in order of increasing beginning
position.
‘name’
A Lisp string that names the buffer. It is guaranteed to be
unique. Buffer Names.
‘save_length’
The length of the file this buffer is visiting, when last read or
saved. This and other fields concerned with saving are not kept in
the ‘buffer_text’ structure because indirect buffers are never
saved.
‘directory’
The directory for expanding relative file names. This is the value
of the buffer-local variable ‘default-directory’ (File Name
Expansion).
‘filename’
The name of the file visited in this buffer, or ‘nil’. This is the
value of the buffer-local variable ‘buffer-file-name’ (Buffer
File Name).
‘undo_list’
‘backed_up’
‘auto_save_file_name’
‘auto_save_file_format’
‘read_only’
‘file_format’
‘file_truename’
‘invisibility_spec’
‘display_count’
‘display_time’
These fields store the values of Lisp variables that are
automatically buffer-local (Buffer-Local Variables), whose
corresponding variable names have the additional prefix ‘buffer-’
and have underscores replaced with dashes. For instance,
‘undo_list’ stores the value of ‘buffer-undo-list’.
‘mark’
The mark for the buffer. The mark is a marker, hence it is also
included on the list ‘markers’. The Mark.
‘local_var_alist’
The association list describing the buffer-local variable bindings
of this buffer, not including the built-in buffer-local bindings
that have special slots in the buffer object. (Those slots are
omitted from this table.) Buffer-Local Variables.
‘major_mode’
Symbol naming the major mode of this buffer, e.g., ‘lisp-mode’.
‘mode_name’
Pretty name of the major mode, e.g., ‘"Lisp"’.
‘keymap’
‘abbrev_table’
‘syntax_table’
‘category_table’
‘display_table’
These fields store the buffer’s local keymap (Keymaps),
DONTPRINTYET abbrev table (Abbrev Tables), syntax table (*noteSyntax
DONTPRINTYET abbrev table (Abbrev Tables), syntax table (Syntax
Tables), category table (Categories), and display table
(Display Tables).
‘downcase_table’
‘upcase_table’
‘case_canon_table’
These fields store the conversion tables for converting text to
lower case, upper case, and for canonicalizing text for case-fold
search. Case Tables.
‘minor_modes’
An alist of the minor modes of this buffer.
‘pt_marker’
‘begv_marker’
‘zv_marker’
These fields are only used in an indirect buffer, or in a buffer
that is the base of an indirect buffer. Each holds a marker that
records ‘pt’, ‘begv’, and ‘zv’ respectively, for this buffer when
the buffer is not current.
‘mode_line_format’
‘header_line_format’
‘case_fold_search’
‘tab_width’
‘fill_column’
‘left_margin’
‘auto_fill_function’
‘truncate_lines’
‘word_wrap’
‘ctl_arrow’
‘bidi_display_reordering’
‘bidi_paragraph_direction’
‘selective_display’
‘selective_display_ellipses’
‘overwrite_mode’
‘abbrev_mode’
‘mark_active’
‘enable_multibyte_characters’
‘buffer_file_coding_system’
‘cache_long_line_scans’
‘point_before_scroll’
‘left_fringe_width’
‘right_fringe_width’
‘fringes_outside_margins’
‘scroll_bar_width’
‘indicate_empty_lines’
‘indicate_buffer_boundaries’
‘fringe_indicator_alist’
‘fringe_cursor_alist’
‘scroll_up_aggressively’
‘scroll_down_aggressively’
‘cursor_type’
‘cursor_in_non_selected_windows’
These fields store the values of Lisp variables that are
automatically buffer-local (Buffer-Local Variables), whose
corresponding variable names have underscores replaced with dashes.
For instance, ‘mode_line_format’ stores the value of
‘mode-line-format’.
‘last_selected_window’
This is the last window that was selected with this buffer in it,
or ‘nil’ if that window no longer displays this buffer.
Info Catalog
elisp: Object Internals
elisp: Window Internals