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#### E.8.1 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 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. 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.

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