1.. _rfc-23: 2 3================================================================================ 4RFC 23.1: Unicode support in OGR 5================================================================================ 6 7Authors: Frank Warmerdam 8 9Contact: warmerdam@pobox.com 10 11Status: Adopted (implemented) 12 13Summary 14------- 15 16This document proposes preliminary steps towards GDAL/OGR handling 17strings internally in UTF-8, and supporting conversion between different 18encodings. 19 20Main concepts 21------------- 22 23GDAL should be modified in a way to support three following main ideas: 24 251. C Functions will be provided to support a variety of encoding 26 conversions, including conversion between representations (ie. UTF-8 27 to UCS-16/wchar_t). 282. Character encodings will be identified by iconv() style strings. 293. OFTString/OFTStringList feature attributes in OGR will be treated as 30 being in UTF-8. 31 32This RFC specifically does not attempt to address issues of using 33non-ascii filenames. It also does not attempt to make definitions about 34the encoding of other strings used in GDAL/OGR (such as field names, 35metadata, etc). These would presumably be addressed in a later RFC 36building on this one. 37 38CPLRecode API 39------------- 40 41The following three C callable functions will be introduced for recoding 42strings, and for converting between wchar_t (wide character) and char 43(multi-byte) formats: 44 45:: 46 47 char *CPLRecode( const char *pszSource, 48 const char *pszSrcEncoding, const char *pszDstEncoding ); 49 50 char *CPLRecodeFromWChar( const wchar_t *pwszSource, 51 const char *pszSrcEncoding, 52 const char *pszDstEncoding ); 53 wchar_t *CPLRecodeToWChar( const char *pszSource, 54 const char *pszSrcEncoding, 55 const char *pszDstEncoding ); 56 57In each case the returned string is zero terminated, as is the input 58string, and the returned string should be deallocated with CPLFree(). In 59case of error the returned string will be NULL, and the function will 60issue a CPLError(). The functions will be marked with CPL_DLL and 61considered part of the public GDAL/OGR API for use of applications as 62well as internal use. 63 64Encoding Names 65-------------- 66 67It is proposed that the encoding names will be the same sorts of names 68used by iconv(). So stuff like "UTF-8", "LATIN5", "CP850" and 69"ISO_8859-1". It does not appear that these names for encodings are a 701:1 match with C library locale names (like "en_CA.utf8" for instance) 71which may cause some issues. 72 73Some particular names of interest: 74 75- "": The current locale. Use this when converting from/to the users 76 locale. 77- "UTF-8": Unicode in multi-byte encoding. Most of the time this will 78 be our internal linga-franca. 79- "POSIX": I think this is roughly ASCII (perhaps with some extended 80 characters?). 81- "UCS-2": Two byte unicode. This is a wide character format and only 82 suitable for use with the wchar_t methods. 83 84On some systems you can use "iconv --list" to get a list of supported 85encodings. 86 87iconv() 88------- 89 90It is proposed to implement the CPLRecode() method using the iconv() and 91related functions when available. 92 93There is an excellent implementation of this API as GNU libiconv(), 94which is used by the C libraries on Linux. Also some operating systems 95provide the iconv() API as part of the C library (all unix?); however, 96the system iconv() often has a restricted set of conversions supported 97so it may be desirable to use libiconv in preference to the system 98iconv() even when it is available. 99 100If iconv() is not available, a stub implementation of the recode 101services will be provided which: 102 103- implements UCS-2 / UTF-8 interconversion using either mbtowc/wctomb, 104 or an implementation derived from 105 `http://www.cl.cam.ac.uk/~mgk25/unicode.html <http://www.cl.cam.ac.uk/~mgk25/unicode.html>`__. 106- Implements recoding from "" to and from "UTF-8" by doing nothing, but 107 issuing a warning on the first use if the current locale does not 108 appear to be the "C" locale. 109- Implements recoding from "ASCII" to "UTF-8" as a null operation. 110- Implements recoding from "UTF-8" to "ASCII" by turning all non-ASCII 111 multi-byte characters to '?'. 112 113This hopefully gives us a weak operational status when built without 114iconv(), but full operation when it is available. 115 116The --with-iconv= option will be added to configure. The argument can be 117the path to a libiconv installation or the special value 'system' 118indicating that the system lib should be used. Alternatively, 119--without-iconv can be used to avoid using iconv. 120 121OFTString/OFTStringList Fields 122------------------------------ 123 124It is declared that OGR string attribute values will be in UTF-8. This 125means that OGR drivers are responsible for translating format specific 126representations to UTF-8 when reading, and back to the format specific 127representation when writing. In many cases (of simple ASCII text) this 128requires no transformation. 129 130This implies that the arguments to methods like OGRFeature::SetField( 131int i, const char \*) should be UTF-8, and that GetFieldAsString() will 132return UTF-8. 133 134The same issues apply to OFTStringList lists of strings. Each string 135will be assumed to be UTF-8. 136 137OLCStringsAsUTF8 Capability Flag 138-------------------------------- 139 140Some drivers (ie. CSV) can effectively not know the encoding of their 141inputs. Therefore, it isn't always practical to turn things into UTF-8 142in a guaranteed way. So, the new layer level capability called 143"StringsAsUTF8" represented with the macro "OLCStringsAsUTF8" will be 144testable at the layer level with TestCapability(). Drivers which are 145certain to return string attributes as UTF-8 should return TRUE, while 146drivers that do not know the encoding they return should return FALSE. 147Any driver which knows it's encoding should convert to UTF-8. 148 149OGR Driver Updates 150------------------ 151 152The following OGR drivers could benefit immediately from recoding to 153UTF-8 support in one way or another. 154 155- ODBC (add support for wchar_t / NVARSHAR fields) 156- Shapefile 157- GML (I'm not sure how the XML encoding values all map to our concept 158 of encoding) 159- Postgres 160 161I'm sure a number of the other drivers, particularly the RDBMS drivers, 162could benefit from an update. 163 164Implementation 165-------------- 166 167Frank Warmerdam will implement the core iconv() capabilities, the 168CPLRecode() additions and update the ODBC driver. Other OGR drivers 169would be updated as time and demand mandates to conform to the 170definitions in this RFC by interested developers. 171 172The core work will be completed for GDAL/OGR 1.6.0 release. 173 174References 175---------- 176 177- `The Unicode Standard, Version 4.0 - Implementation 178 Guidelines <http://unicode.org/versions/Unicode4.0.0/ch05.pdf>`__ - 179 Chapter 5 (PDF) 180- FAQ on how to use Unicode in software: 181 `http://www.cl.cam.ac.uk/~mgk25/unicode.html <http://www.cl.cam.ac.uk/~mgk25/unicode.html>`__ 182- FLTK implementation of string conversion functions: 183 `http://svn.easysw.com/public/fltk/fltk/trunk/src/utf.c <http://svn.easysw.com/public/fltk/fltk/trunk/src/utf.c>`__ 184- `http://www.easysw.com/~mike/fltk/doc-2.0/html/utf_8h.html <http://www.easysw.com/~mike/fltk/doc-2.0/html/utf_8h.html>`__ 185- Ticket #1494 : UTF-8 encoding for GML output. 186- Libiconv: 187 `http://www.gnu.org/software/libiconv/ <http://www.gnu.org/software/libiconv/>`__ 188- ICU (another i18n library): 189 `http://www.icu-project.org/ <http://www.icu-project.org/>`__ 190