index.md (6288B)
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