unicode_canonical - Man Page

unicode canonical normalization and denormalization


#include <courier-unicode.h>

unicode_canonical_t unicode_canonical(char32_t c);

uint8_t unicode_ccc(char32_t c);

void unicode_decomposition_init(unicode_decomposition_t *info, char32_t *string, size_t *string_size, void *arg);

int unicode_decompose(unicode_decomposition_t *info);

void unicode_decomposition_deinit(unicode_decomposition_t *info);

size_t unicode_decompose_reallocate_size(unicode_decomposition_t *info, const size_t *sizes, size_t n);

int unicode_compose(char32_t *string, size_t string_size, int flags, size_t *new_size);

int unicode_composition_init(const char32_t *string, size_t string_size, int flags, unicode_composition_t *compositions);

void unicode_composition_deinit(unicode_composition_t *compositions);

size_t unicode_composition_apply(char32_t *string, size_t string_size, unicode_composition_t *compositions);


These functions compose or decompose a Unicode string into a canonical or a compatible normalized form.

unicode_canonical() looks up the character's canonical and compatibility mapping[1]. unicode_canonical() returns a structure with the following fields:


A pointer to the canonical or equivalent representation of the character.


Number of characters in the canonical_chars.


A value of UNICODE_CANONICAL_FMT_NONE indicates a canonical mapping, other values indicate a compatibility equivalent mapping.

A NULL canonical_chars (with a 0 n_canonical_chars) indicates that the character has no canonical or compatibility equivalence.

unicode_ccc() returns the character's canonical combining class value.

unicode_decomposition_init(), unicode_decompose() and unicode_decomposition_deinit() implement a complete interface for decomposing a Unicode string:

unicode_decomposition_t info;

unicode_decomposition_init(&info, before, (size_t)-1, NULL);

unicode_decomposition_init() initializes a new unicode_decomposition_t structure, that gets passed in as its first parameter. The second parameter is a pointer to a Unicode string, with the number of characters in the string in the third parameter. A string size of -1 indicates a \0-terminated string and calculates its string_size (which does not include the trailing \0. The last parameter is a void *, an opaque pointer that gets stored in the initialized unicode_decomposition_t object:

typedef struct unicode_decomposition {
   struct unicode_decomposition*info,
   const size_t*offsets,
   const size_t*sizes,
} unicode_decomposition_t;

unicode_decompose() proceeds and decomposes the string and replaces it with its decomposed string version.

unicode_decomposition_t's string, string_size and arg are copies of unicode_decomposition_init's parameters. unicode_decomposition_init initializes all other fields to their default values.

The decompose_flags bitmask gets initialized to 0, and is a bit mask:


Check each character's appropriate “quick check” property and skip decomposing Unicode characters that would get re-composed by unicode_composition_apply().


Perform a compatibility decomposition instead of a canonical decomposition.

reallocate is a pointer to a function that gets called to reallocate a larger string. unicode_decompose() determines which characters in the string need decomposing and calls the reallocate function pointer zero or more times. Each call to reallocate passes information about where new characters will get inserted into the string.

reallocate only needs to grow the size of the buffer where string points so that it's big enough to hold a larger, decomposed string; then update string accordingly. reallocate should not update string_size or make any changes to the existing string, that's unicode_decompose()'s job (after reallocate returns).

The reallocate callback function receives the following parameters.

reallocate must update the string if necessary to hold at least the number of characters that's the sum total of the initial string_size and the sum total of al sizes.

unicode_decomposition_init() initializes the reallocate pointer to a default implementation that uses realloc(3) and updates string with its return value. The application can use its own reallocate to handle this task on its own, and use unicode_decompose_reallocate_size to compute the minimum string size:

size_t unicode_decompose_reallocate_size(unicode_decomposition_t *info,
                                         const size_t *sizes,
                                         size_t n)
    size_t i;
    size_t new_size=info->string_size;

    for (i=0; i<n; ++i)
        new_size += sizes[i];

    return new_size;

The reallocate function returns 0 on success and a non-0 error code to report a failure; and unicode_decompose() does the same. The only error condition from unicode_decompose() is a non-0 error code from the reallocate function. Otherwise: a successful decomposition results in unicode_decompose() returning 0 and unicode_decomposition_init()'s string pointing to the decomposed string and string_size giving the number of characters in the decomposed string.


string_size does not include the trailing \0 character. The input string also has its string_size specified without counting its \0 character. The default implementation of reallocate allocates an extra char32_t ands sets it to a \0. Therefore:

  • If the Unicode string before decomposition has a trailing \0 and no decomposition occurs, and no calls to reallocate takes place: the string in the unicode_decomposition_t is unchanged and it's still \0-terminated.
  • The default reallocate allocates an extra char32_t ands sets it to a \0; and it takes care of that for the decomposed string.
  • An application that provides its own replacement reallocate is responsible for doing the same, if it wants the decomposed string to be \0 terminated.


Multiple calls to the reallocate callback are possible. Each call to reallocate reflect the prior calls' decompositions. Example: the original string has five characters and the first call to reallocate had two offsets, at position 1 and 3, with a value of 1 for their both sizes. This effects transforming an original Unicode string "AAAAA" into "AXAAXAA" (with “A” representing unspecified characters in the original string, and “X” showing the two characters added in the first call to reallocate.

A second call to varname with am offset at position 4, and a size of 1, results in the updated string of "AXAAYXAA" (with “Y”) marking an unspecified character inserted by the second call.


Unicode string decomposition involves replacing a given Unicode character with one or more other characters. The sizes given to reallocate reflect the net addition to the Unicode string. For example: decomposing one Unicode character into three decomposed characters results in a call to reallocate reporting an insert of two more characters.


offsets actually report the indices of each Unicode character that's getting decomposed. A 1:1 decomposition of a Unicode Character gets reported as an additional sizes entry of 0.

unicode_decomposition_deinit() releases all resources and destroys the unicode_decomposition_t; it is no longer valid.


unicode_decomposition_deinit() does not free(3) the string. The original string gets passed in to unicode_decomposition_init() and the decomposed string is left in the string.

The default implementation of the reallocate function assumes the string is a malloc(3)-ed string, and reallocs it.


At this time unicode_decomposition_deinit() does nothing. All code should explicitly call it in order to remain forward-compatible (at the source level).

unicode_compose() performs a canonical composition of a decomposed string. Its parameters are:

  • A flags bitmask, which can have the following values:

Remove all combining marks after doing all canonical compositions. Normally any unused combining marks are left in place, in the combined text. This option removes them.


Perform canonical composition once per character, and do not attempt to combine any resulting combined characters again.

unicode_composition_init(), unicode_composition_apply() and unicode_composition_deinit() implement a detailed interface for canonical composition of a decomposed Unicode string:

unicode_compositions_t compositions;

if (unicode_composition_init(str, strsize, flags, &compositions) == 0)
    size_t new_size=unicode_composition_apply(str, strsize, &compositions);


The first two parameters to both unicode_composition_init() and unicode_composition_apply() are the same: the Unicode string and the number of characters (not including any trailing \0 character) in the Unicode string.

unicode_composition_init()'s additional parameters are: any optional flags (see unicode_compose() for a list of available flags), and the address of a unicode_composition_t object. A non-0 return from unicode_composition_init() indicates an error. unicode_composition_init() indicates success by returning 0 and initializing the unicode_composition_t's object which contains a pointer to an array of pointers to of unicode_compose_info objects, and the number of pointers. unicode_composition_init() does not change the string; the only thing it does is initialize the unicode_composition_t object.

unicode_composition_apply() applies the compositions to the string, in place, and returns the new size of the string (also not including the \0 byte, however it does append one if the composed string is smaller, so the composed string is \0-terminated if the decomposed string was).

It is necessary to call unicode_composition_deinit() to free all memory that was allocated for the unicode_composition_t object:

struct unicode_compose_info {
typedef struct {
 struct unicode_compose_info**compositions;
} unicode_composition_t;

index gives the character index in the string where each composition occurs. n_composed gives the number of characters in the original string that get composed. The composed characters are the composition; and n_composition gives the number of composed characters.

Effectively: at the index position in the original string, #n_composed characters get removed and there are #n_composition characters that replace them (always n_composed or less).


The UNICODE_COMPOSE_FLAG_REMOVEUNUSED flag has the effect of including the combining marks that did not get combined in the n_composed count. It's possible that, in this case, n_composition is 0. This indicates complete removal of the combining marks, without anything getting combined in their place.

unicode_composition_init() sets unicode_composition_t's compositions pointer to an array of pointers to unicode_compose_infos that are sorted according to their index. n_compositions gives the number of pointers in the array, and is 0 if there are no compositions, the array is empty. The empty array gets interpreted accordingly when it gets passed to unicode_composition_apply() and unicode_composition_deinit(): nothing happens. unicode_composition_apply() simply returns the size of the unchanged string, and unicode_composition_deinit() does a pro-forma cleanup.

See Also

TR-15[2], courier-unicode(7), unicode::canonical(3).


Sam Varshavchik



  1. canonical and compatibility mapping
  2. TR-15

Referenced By

courier-unicode(7), unicode::canonical(3).

The man pages unicode_ccc(3), unicode_compose(3), unicode_composition_apply(3), unicode_composition_deinit(3), unicode_composition_init(3), unicode_decompose(3), unicode_decompose_reallocate_size(3), unicode_decomposition_deinit(3) and unicode_decomposition_init(3) are aliases of unicode_canonical(3).

04/16/2022 Courier Unicode Library