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1 LIBGRAPHEME(7) - Miscellaneous Information Manual
2
3 # NAME
4
5 **libgrapheme** - unicode string library
6
7 # SYNOPSIS
8
9 **#include <grapheme.h>**
10
11 # DESCRIPTION
12
13 The
14 **libgrapheme**
15 library provides functions to properly handle Unicode strings according
16 to the Unicode specification in regard to character, word, sentence and
17 line segmentation and case detection and conversion.
18
19 Unicode strings are made up of user-perceived characters (so-called
20 "grapheme clusters",
21 see
22 *MOTIVATION*)
23 that are composed of one or more Unicode codepoints, which in turn
24 are encoded in one or more bytes in an encoding like UTF-8.
25
26 There is a widespread misconception that it was enough to simply
27 determine codepoints in a string and treat them as user-perceived
28 characters to be Unicode compliant.
29 While this may work in some cases, this assumption quickly breaks,
30 especially for non-Western languages and decomposed Unicode strings
31 where user-perceived characters are usually represented using multiple
32 codepoints.
33
34 Despite this complicated multilevel structure of Unicode strings,
35 **libgrapheme**
36 provides methods to work with them at the byte-level (i.e. UTF-8
37 'char'
38 arrays) while also offering codepoint-level methods.
39 Additionally, it is a
40 "freestanding"
41 library (see ISO/IEC 9899:1999 section 4.6) and thus does not depend on
42 a standard library. This makes it easy to use in bare metal environments.
43
44 Every documented function's manual page provides a self-contained
45 example illustrating the possible usage.
46
47 # SEE ALSO
48
49 grapheme\_decode\_utf8(3),
50 grapheme\_encode\_utf8(3),
51 grapheme\_is\_character\_break(3),
52 grapheme\_is\_lowercase(3),
53 grapheme\_is\_lowercase\_utf8(3),
54 grapheme\_is\_titlecase(3),
55 grapheme\_is\_titlecase\_utf8(3),
56 grapheme\_is\_uppercase(3),
57 grapheme\_is\_uppercase\_utf8(3),
58 grapheme\_next\_character\_break(3),
59 grapheme\_next\_character\_break\_utf8(3),
60 grapheme\_next\_line\_break(3),
61 grapheme\_next\_line\_break\_utf8(3),
62 grapheme\_next\_sentence\_break(3),
63 grapheme\_next\_sentence\_break\_utf8(3),
64 grapheme\_next\_word\_break(3),
65 grapheme\_next\_word\_break\_utf8(3),
66 grapheme\_to\_lowercase(3),
67 grapheme\_to\_lowercase\_utf8(3),
68 grapheme\_to\_titlecase(3),
69 grapheme\_to\_titlecase\_utf8(3)
70 grapheme\_to\_uppercase(3),
71 grapheme\_to\_uppercase\_utf8(3),
72
73 # STANDARDS
74
75 **libgrapheme**
76 is compliant with the Unicode 15.0.0 specification.
77
78 # MOTIVATION
79
80 The idea behind every character encoding scheme like ASCII or Unicode
81 is to express abstract characters (which can be thought of as shapes
82 making up a written language). ASCII for instance, which comprises the
83 range 0 to 127, assigns the number 65 (0x41) to the abstract character
84 'A'.
85 This number is called a
86 "codepoint",
87 and all codepoints of an encoding make up its so-called
88 "code space".
89
90 Unicode's code space is much larger, ranging from 0 to 0x10FFFF, but its
91 first 128 codepoints are identical to ASCII's. The additional code
92 points are needed as Unicode's goal is to express all writing systems
93 of the world.
94 To give an example, the abstract character
95 'Ä'
96 is not expressable in ASCII, given no ASCII codepoint has been assigned
97 to it.
98 It can be expressed in Unicode, though, with the codepoint 196 (0xC4).
99
100 One may assume that this process is straightfoward, but as more and
101 more codepoints were assigned to abstract characters, the Unicode
102 Consortium (that defines the Unicode standard) was facing a problem:
103 Many (mostly non-European) languages have such a large amount of
104 abstract characters that it would exhaust the available Unicode code
105 space if one tried to assign a codepoint to each abstract character.
106 The solution to that problem is best introduced with an example: Consider
107 the abstract character
108 'Ǟ',
109 which is
110 'A'
111 with an umlaut and a macron added to it.
112 In this sense, one can consider
113 'Ǟ'
114 as a two-fold modification (namely
115 "add umlaut"
116 and
117 "add macron")
118 of the
119 "base character"
120 'A'.
121
122 The Unicode Consortium adapted this idea by assigning codepoints to
123 modifications.
124 For example, the codepoint 0x308 represents adding an umlaut and 0x304
125 represents adding a macron, and thus, the codepoint sequence
126 "0x41 0x308 0x304",
127 namely the base character
128 'A'
129 followed by the umlaut and macron modifiers, represents the abstract
130 character
131 'Ǟ'.
132 As a side-note, the single codepoint 0x1DE was also assigned to
133 'Ǟ',
134 which is a good example for the fact that there can be multiple
135 representations of a single abstract character in Unicode.
136
137 Expressing a single abstract character with multiple codepoints solved
138 the code space exhaustion-problem, and the concept has been greatly
139 expanded since its first introduction (emojis, joiners, etc.). A sequence
140 (which can also have the length 1) of codepoints that belong together
141 this way and represents an abstract character is called a
142 "grapheme cluster".
143
144 In many applications it is necessary to count the number of
145 user-perceived characters, i.e. grapheme clusters, in a string.
146 A good example for this is a terminal text editor, which needs to
147 properly align characters on a grid.
148 This is pretty simple with ASCII-strings, where you just count the number
149 of bytes (as each byte is a codepoint and each codepoint is a grapheme
150 cluster).
151 With Unicode-strings, it is a common mistake to simply adapt the
152 ASCII-approach and count the number of code points.
153 This is wrong, as, for example, the sequence
154 "0x41 0x308 0x304",
155 while made up of 3 codepoints, is a single grapheme cluster and
156 represents the user-perceived character
157 'Ǟ'.
158
159 The proper way to segment a string into user-perceived characters
160 is to segment it into its grapheme clusters by applying the Unicode
161 grapheme cluster breaking algorithm (UAX #29).
162 It is based on a complex ruleset and lookup-tables and determines if a
163 grapheme cluster ends or is continued between two codepoints.
164 Libraries like ICU and libunistring, which also offer this functionality,
165 are often bloated, not correct, difficult to use or not reasonably
166 statically linkable.
167
168 Analogously, the standard provides algorithms to separate strings by
169 words, sentences and lines, convert cases and compare strings.
170 The motivation behind
171 **libgrapheme**
172 is to make unicode handling suck less and abide by the UNIX philosophy.
173
174 # AUTHORS
175
176 Laslo Hunhold ([dev@frign.de](mailto:dev@frign.de))
177
178 suckless.org - 2022-10-06