xref: /aosp_15_r20/external/libxml2/os400/iconv/README.iconv (revision 7c5688314b92172186c154356a6374bf7684c3ca)

IBM OS/400 implements iconv in an odd way:
- Type iconv_t is a structure: therefore objects of this type cannot be
  compared to (iconv_t) -1.
- Supported character sets names are all of the form IBMCCSIDccsid..., where
  ccsid is a decimal 5-digit integer identifying an IBM coded character set.
  In addition, character set names have to be given in EBCDIC.
  Standard character set names like "UTF-8" are NOT recognized.
- The prototype of iconv_open() does not declare parameters as const, although
  they are not altered.

 Since libiconv does not support EBCDIC, use of this package here as a
replacement is not a solution.

 For these reasons, the code in this directory implements a wrapper to the
OS/400 iconv implementation. The wrapper performs the following transformations:
- Type iconv_t is an pointer. Although OS/400 pointers are odd, comparing
  with (iconv_t) -1 is OK.
- All IANA character set names are recognized in a coding- and case-insensitive
  way, providing an equivalent CCSID exists. see
  http://www.iana.org/assignments/character-sets/character-sets.xhtml
- All CCSIDs from the association file can be expressed as IBMCCSIDxxxxx where
  xxxxx is the 5 digit CCSID; no null terminator is required. Alternate codes
  are of the form ibm-xxx (null-terminated), where xxx is the integer CCSID with
  leading zeroes stripped.
- If a IANA BIBenum is defined for a CCSID, the name iana-xxx can be used,
  where xxx is the integer MIBenum without leading zeroes.
- In addition, some aliases are also taken from the association file. Examples
  are: ASCII, EBCDIC, UTF8.
- Prototype of iconv_open() has const parameters.
- Character code names can be given in any code.

Character set names to CCSID conversion.
- http://www.iana.org/assignments/character-sets/character-sets.xhtml provides
  all IANA registered character set names and aliases associated with a
  MIBenum, that is a unique character set identifier.
- A hand-maintained file ccsid_mibenum.xml associates IBM CCSIDs to
  IANA MBenums.
- An OS/400 C program (in subdirectory bldcsndfa) generates a deterministic
  finite automaton from the files mentioned above into a C file for all
  possible character set name and associating each of them with its
  corresponding CCSID. This program can only be run on OS/400 since it uses
  the native iconv support for EBCDIC.
- Since these operations are tedious and the table generation needs bootstraping
  with libxml2, the generated automaton is stored within sources and need not
  be rebuilt at each compilation. However, source is provided here to allow
  new table generation with conversion tables that were not available at the
  time of original generation.