Compose and dead-keys support

## Data Structures

struct | xkb_compose_table
struct | xkb_compose_state

## Enumerations

enum | xkb_compose_compile_flags { XKB_COMPOSE_COMPILE_NO_FLAGS = 0 }
enum | xkb_compose_format { XKB_COMPOSE_FORMAT_TEXT_V1 = 1 }
enum | xkb_compose_state_flags { XKB_COMPOSE_STATE_NO_FLAGS = 0 }
enum | xkb_compose_status { XKB_COMPOSE_NOTHING, XKB_COMPOSE_COMPOSING, XKB_COMPOSE_COMPOSED, XKB_COMPOSE_CANCELLED }
enum | xkb_compose_feed_result { XKB_COMPOSE_FEED_IGNORED, XKB_COMPOSE_FEED_ACCEPTED }

## Functions

struct xkb_compose_table * | xkb_compose_table::xkb_compose_table_new_from_locale (struct xkb_context *context, const char *locale, enum xkb_compose_compile_flags flags)
struct xkb_compose_table * | xkb_compose_table::xkb_compose_table_new_from_file (struct xkb_context *context, FILE *file, const char *locale, enum xkb_compose_format format, enum xkb_compose_compile_flags flags)
struct xkb_compose_table * | xkb_compose_table::xkb_compose_table_new_from_buffer (struct xkb_context *context, const char *buffer, size_t length, const char *locale, enum xkb_compose_format format, enum xkb_compose_compile_flags flags)
struct xkb_compose_table * | xkb_compose_table::xkb_compose_table_ref (struct xkb_compose_table *table)
void | xkb_compose_table::xkb_compose_table_unref (struct xkb_compose_table *table)
struct xkb_compose_state * | xkb_compose_state::xkb_compose_state_new (struct xkb_compose_table *table, enum xkb_compose_state_flags flags)
struct xkb_compose_state * | xkb_compose_state::xkb_compose_state_ref (struct xkb_compose_state *state)
void | xkb_compose_state::xkb_compose_state_unref (struct xkb_compose_state *state)
struct xkb_compose_table * | xkb_compose_state::xkb_compose_state_get_compose_table (struct xkb_compose_state *state)
enum xkb_compose_feed_result | xkb_compose_state::xkb_compose_state_feed (struct xkb_compose_state *state, xkb_keysym_t keysym)
void | xkb_compose_state::xkb_compose_state_reset (struct xkb_compose_state *state)
enum xkb_compose_status | xkb_compose_state::xkb_compose_state_get_status (struct xkb_compose_state *state)
int | xkb_compose_state::xkb_compose_state_get_utf8 (struct xkb_compose_state *state, char *buffer, size_t size)
xkb_keysym_t | xkb_compose_state::xkb_compose_state_get_one_sym (struct xkb_compose_state *state)

## Detailed Description

Support for Compose and dead-keys.

Since
  0.5.0

### Overview

Compose and dead-keys are a common feature of many keyboard input systems. They extend the range of the keysysm that can be produced directly from a keyboard by using a sequence of key strokes, instead of just one.

Here are some example sequences, in the libX11 Compose file format:

    <dead_acute> <a>         : "á"   aacute # LATIN SMALL LETTER A WITH ACUTE
    <Multi_key> <A> <T>      : "@"   at # COMMERCIAL AT

When the user presses a key which produces the <dead_acute> keysym, nothing initially happens (thus the key is dubbed a "dead-key"). But when the user enters , "á" is "composed", in place of "a". If instead the user had entered a keysym which does not follow <dead_acute> in any compose sequence, the sequence is said to be "cancelled".

Compose files define many such sequences. For a description of the common file format for Compose files, see the Compose(5) man page.

A successfuly-composed sequence has two results: a keysym and a UTF-8 string. At least one of the two is defined for each sequence. If only a keysym is given, the keysym's string representation is used for the result string (using xkb_keysym_to_utf8()).

This library provides low-level support for Compose file parsing and processing. Higher-level APIs (such as libX11's Xutf8LookupString(3)) may be built upon it, or it can be used directly.

### Conflicting Sequences

To avoid ambiguity, a sequence is not allowed to be a prefix of another. In such a case, the conflict is resolved thus:

1. A longer sequence overrides a shorter one.
2. An equal sequence overrides an existing one.
3. A shorter sequence does not override a longer one.

Sequences of length 1 are allowed, although they are not common.

### Cancellation Behavior

What should happen when a sequence is cancelled? For example, consider there are only the above sequences, and the input kesysms are <dead_acute> <b>. There are a few approaches:

1. Swallow the cancelling keysym; that is, no keysym is produced. This is the approach taken by libX11.
2. Let the cancelling keysym through; that is, <b> is produced.
3. Replay the entire sequence; that is, <dead_acute> <b> is produced. This is the approach taken by Microsoft Windows (approximately; instead of <dead_acute>, the underlying key is used. This is difficult to simulate with XKB keymaps).

You can program whichever approach best fits users' expectations.

### Compose Locale

Compose files are locale dependent:

- Compose files are written for a locale, and the locale is used when searching for the appropriate file to use.
- Compose files may reference the locale internally, with directives such as L. As such, functions like xkb_compose_table_new_from_locale() require a `locale` parameter. This will usually be the current locale (see locale(7) for more details). You may also want to allow the user to explicitly configure it, so he can use the Compose file of a given locale, but not use that locale for other things.

You may query the current locale as follows:

const char *locale;
locale = setlocale(LC_CTYPE, NULL);

This will only give useful results if the program had previously set the current locale using setlocale(3), with LC_CTYPE or LC_ALL and a non-NULL argument.

If you prefer not to use the locale system of the C runtime library, you may nevertheless obtain the user's locale directly using environment variables, as described in locale(7). For example,

locale = getenv("LC_ALL");
if (!locale)
locale = getenv("LC_CTYPE");
if (!locale)
locale = getenv("LANG");
if (!locale)
locale = "C";

Note that some locales supported by the C standard library may not have a Compose file assigned.

## Enumeration Type Documentation

enum xkb_compose_compile_flags

Flags affecting Compose file compilation.

Enumerator
XKB_COMPOSE_COMPILE_NO_FLAGS | Do not apply any flags.

enum xkb_compose_format

The recognized Compose file formats.

Enumerator
XKB_COMPOSE_FORMAT_TEXT_V1 | The classic libX11 Compose text format, described in Compose(5).

enum xkb_compose_state_flags

Flags for compose state creation.

Enumerator
XKB_COMPOSE_STATE_NO_FLAGS | Do not apply any flags.

enum xkb_compose_status

Status of the Compose sequence state machine.

Enumerator
XKB_COMPOSE_NOTHING | The initial state; no sequence has started yet.
XKB_COMPOSE_COMPOSING | In the middle of a sequence.
XKB_COMPOSE_COMPOSED | A complete sequence has been matched.
XKB_COMPOSE_CANCELLED | The last sequence was cancelled due to an unmatched keysym.

enum xkb_compose_feed_result

The effect of a keysym fed to xkb_compose_state_feed().

Enumerator
XKB_COMPOSE_FEED_IGNORED | The keysym had no effect - it did not affect the status.
XKB_COMPOSE_FEED_ACCEPTED | The keysym started, advanced or cancelled a sequence.

## Function Documentation

struct xkb_compose_table * xkb_compose_table_new_from_locale | ( | struct xkb_context * | context,
|  | const char * | locale,
|  | enum xkb_compose_compile_flags | flags
| ) |  |

Create a compose table for a given locale.

The locale is used for searching the file-system for an appropriate Compose file. The search order is described in Compose(5). It is affected by the following environment variables: XCOMPOSEFILE, HOME, XLOCALEDIR.

Parameters

  context | The library context in which to create the compose table.
  locale | The current locale. See Compose Locale.
  flags | Optional flags for the compose table, or 0.

Returns
  A compose table for the given locale, or NULL if the compilation failed or a Compose file was not found.

struct xkb_compose_table * xkb_compose_table_new_from_file | ( | struct xkb_context * | context,
|  | FILE * | file,
|  | const char * | locale,
|  | enum xkb_compose_format | format,
|  | enum xkb_compose_compile_flags | flags
| ) |  |

Create a new compose table from a Compose file.

Parameters

  context | The library context in which to create the compose table.
  file | The Compose file to compile.
  locale | The current locale. See Compose Locale.
  format | The text format of the Compose file to compile.
  flags | Optional flags for the compose table, or 0.

Returns
  A compose table compiled from the given file, or NULL if the compilation failed.

struct xkb_compose_table * xkb_compose_table_new_from_buffer | ( | struct xkb_context * | context,
|  | const char * | buffer,
|  | size_t | length,
|  | const char * | locale,
|  | enum xkb_compose_format | format,
|  | enum xkb_compose_compile_flags | flags
| ) |  |

Create a new compose table from a memory buffer.

This is just like xkb_compose_table_new_from_file(), but instead of a file, gets the table as one enormous string.

See also
  xkb_compose_table_new_from_file()

struct xkb_compose_table * xkb_compose_table_ref | ( | struct xkb_compose_table * | table | ) |

Take a new reference on a compose table.

Returns
  The passed in object.

void xkb_compose_table_unref | ( | struct xkb_compose_table * | table | ) |

Release a reference on a compose table, and possibly free it.

Parameters

  table | The object. If it is NULL, this function does nothing.

struct xkb_compose_state * xkb_compose_state_new | ( | struct xkb_compose_table * | table,
|  | enum xkb_compose_state_flags | flags
| ) |  |

Create a new compose state object.

Parameters

  table | The compose table the state will use.
  flags | Optional flags for the compose state, or 0.

Returns
  A new compose state, or NULL on failure.

struct xkb_compose_state * xkb_compose_state_ref | ( | struct xkb_compose_state * | state | ) |

Take a new reference on a compose state object.

Returns
  The passed in object.

void xkb_compose_state_unref | ( | struct xkb_compose_state * | state | ) |

Release a reference on a compose state object, and possibly free it.

Parameters

  state | The object. If NULL, do nothing.

struct xkb_compose_table * xkb_compose_state_get_compose_table | ( | struct xkb_compose_state * | state | ) |

Get the compose table which a compose state object is using.

Returns
  The compose table which was passed to xkb_compose_state_new() when creating this state object.

This function does not take a new reference on the compose table; you must explicitly reference it yourself if you plan to use it beyond the lifetime of the state.

enum xkb_compose_feed_result xkb_compose_state_feed | ( | struct xkb_compose_state * | state,
|  | xkb_keysym_t | keysym
| ) |  |

Feed one keysym to the Compose sequence state machine.

This function can advance into a compose sequence, cancel a sequence, start a new sequence, or do nothing in particular . The resulting status may be observed with xkb_compose_state_get_status().

Some keysyms, such as keysysm for modifier keys, are ignored - they have no effect on the status or otherwise.

The following is a description of the possible status transitions, in the format CURRENT STATUS => NEXT STATUS, given a non-ignored input keysym `keysym:`

    NOTHING or CANCELLED or COMPOSED =>
       NOTHING   if keysym does not start a sequence.
       COMPOSING if keysym starts a sequence.
       COMPOSED  if keysym starts and terminates a single-keysym sequence.

    COMPOSING =>
       COMPOSING if keysym advances any of the currently possible
                 sequences but does not terminate any of them.
       COMPOSED  if keysym terminates one of the currently possible
                 sequences.
       CANCELLED if keysym does not advance any of the currently
                 possible sequences.

The current Compose formats do not support multiple-keysyms. Therefore, if you are using a function such as xkb_state_key_get_syms() and it returns more than one keysym, consider feeding `XKB_KEY_NoSymbol` instead.

Parameters

  state | The compose state object.
  keysym | A keysym, usually obtained after a key-press event, with a function such as xkb_state_key_get_one_sym().

Returns
  Whether the keysym was ignored. This is useful, for example, if you want to keep a record of the sequence matched thus far.

void xkb_compose_state_reset | ( | struct xkb_compose_state * | state | ) |

Reset the Compose sequence state machine.

The status is set to XKB_COMPOSE_NOTHING, and the current sequence is discarded.

enum xkb_compose_status xkb_compose_state_get_status | ( | struct xkb_compose_state * | state | ) |

Get the current status of the compose state machine.

See also
  xkb_compose_status

int xkb_compose_state_get_utf8 | ( | struct xkb_compose_state * | state,
|  | char * | buffer,
|  | size_t | size
| ) |  |

Get the result Unicode/UTF-8 string for a composed sequence.

See Overview for more details. This function is only useful when the status is XKB_COMPOSE_COMPOSED.

Parameters

  [in] | state | The compose state.
  [out] | buffer | A buffer to write the string into.
  [in] | size | Size of the buffer.

Warning
  If the buffer passed is too small, the string is truncated (though still NUL-terminated).

Returns
  The number of bytes required for the string, excluding the NUL byte. If the sequence is not complete, or does not have a viable result string, returns 0, and sets `buffer` to the empty string (if possible).
  You may check if truncation has occurred by comparing the return value with the size of `buffer`, similarly to the snprintf(3) function. You may safely pass NULL and 0 to `buffer` and `size` to find the required size (without the NUL-byte).

xkb_keysym_t xkb_compose_state_get_one_sym | ( | struct xkb_compose_state * | state | ) |

Get the result keysym for a composed sequence.

See Overview for more details. This function is only useful when the status is XKB_COMPOSE_COMPOSED.

Returns
  The result keysym. If the sequence is not complete, or does not specify a result keysym, returns XKB_KEY_NoSymbol.