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E.6.1 Buffer Internals

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. See 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. See 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 struct vectorlike_header structure where header.next points to the next buffer, in the chain of all buffers (including killed buffers). This chain is used only for garbage collection, in order to collect killed buffers properly. Note that vectors, and most kinds of objects allocated as vectors, are all on one chain, but buffers are on a separate chain of their own.
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.
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. See 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. See 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. See 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. See 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. See 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 (see 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 (see 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 (see 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. See 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.) See 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 (see Keymaps), abbrev table (see Abbrev Tables), syntax table (see Syntax Tables), category table (see Categories), and display table (see 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. See 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 (see 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.