XoJIG (X/Open-UniForum Joint Internationalization Group) has developed a transformation format for ISO/IEC 10646. We have given this transformation format the name FSS-UTF (File System Safe Universal Character Set Transformation Format). For your information, the initial proposal (September 1992) for FSS-UTF is included below. FSS-UTF has become the preferred ISO/IEC 10646 transformation format in many programming system environments. Also, it is my understanding that FSS-UTF is intended to be included in an addendum to ISO/IEC/10646 as UTF-8. As the addendum to ISO/IEC 10646 may require an extended period of time, I would like to know if it is possible to reserve an escape for FSS-UTF in ISO 2375 for current use and later use this same escape sequence for UTF-8 (or the appropriate name) when the transformation format becomes an official part of the ISO/IEC 10646 standard? ------------------------------------------------------------------------- Gary W. Miller Internet - gwm@austin.ibm.com IBM D04/906 ZIP 9652 X/Open - g.miller@xopen.co.uk 11400 Burnet Road VNET - AUSTIN(GWM) / GWM at AUSTIN Austin, Texas 78758 UUCP: - ...!uunet!aixsm!miller Phone: (512) 838-8297 Fax: (512) 838-8374 ------------------------------------------------------------------------- File System Safe Universal Character Set Transformation Format (FSS-UTF) -------------------------------------------------------------------------- With the approval of ISO/IEC 10646 (Unicode) as an international standard and the anticipated wide spread use of this universal coded character set (UCS), it is necessary for historically ASCII based operating systems to devise ways to cope with representation and handling of the large number of characters that are possible to be encoded by this new standard. There are several challenges presented by UCS which must be dealt with by historical operating systems and the C-language programming environment. The most significant of these challenges is the encoding scheme used by UCS. More precisely, the challenge is the marrying of the UCS standard with existing programming languages and existing operating systems and utilities. The challenges of the programming languages and the UCS standard are being dealt with by other activities in the industry. However, we are still faced with the handling of UCS by historical operating systems and utilities. Prominent among the operating system UCS handling concerns is the representation of the data within the file system. An underlying assumption is that there is an absolute requirement to maintain the existing operating system software investment while at the same time taking advantage of the use the large number of characters provided by the UCS. UCS provides the capability to encode multi-lingual text within a single coded character set. However, UCS and its UTF variant do not protect null bytes and/or the ASCII slash ("/") making these character encodings incompatible with existing Unix implementations. The following proposal provides a Unix compatible transformation format of UCS such that Unix systems can support multi-lingual text in a single encoding. This transformation format encoding is intended to be used as a file code. This transformation format encoding of UCS is intended as an intermediate step towards full UCS support. However, since nearly all Unix implementations face the same obstacles in supporting UCS, this proposal is intended to provide a common and compatible encoding during this transition stage. Goal/Objective -------------- With the assumption that most, if not all, of the issues surrounding the handling and storing of UCS in historical operating system file systems are understood, the objective is to define a UCS transformation format which also meets the requirement of being usable on a historical operating system file system in a non-disruptive manner. The intent is that UCS will be the process code for the transformation format, which is usable as a file code. Criteria for the Transformation Format -------------------------------------- Below are the guidelines that were used in defining the UCS transformation format: 1) Compatibility with historical file systems: Historical file systems disallow the null byte and the ASCII slash character as a part of the file name. 2) Compatibility with existing programs: The existing model for multibyte processing is that ASCII does not occur anywhere in a multibyte encoding. There should be no ASCII code values for any part of a transformation format representation of a character that was not in the ASCII character set in the UCS representation of the character. 3) Ease of conversion from/to UCS. 4) The first byte should indicate the number of bytes to follow in a multibyte sequence. 5) The transformation format should not be extravagant in terms of number of bytes used for encoding. 6) It should be possible to find the start of a character efficiently starting from an arbitrary location in a byte stream. Proposed FSS-UTF ---------------- The proposed UCS transformation format encodes UCS values in the range [0,0x7fffffff] using multibyte characters of lengths 1, 2, 3, 4, 5, and 6 bytes. For all encodings of more than one byte, the initial byte determines the number of bytes used and the high-order bit in each byte is set. Every byte that does not start 10xxxxxx is the start of a UCS character sequence. An easy way to remember this transformation format is to note that the number of high-order 1's in the first byte signifies the number of bytes in the multibyte character: Bits Hex Min Hex Max Byte Sequence in Binary 1 7 00000000 0000007f 0vvvvvvv 2 11 00000080 000007FF 110vvvvv 10vvvvvv 3 16 00000800 0000FFFF 1110vvvv 10vvvvvv 10vvvvvv 4 21 00010000 001FFFFF 11110vvv 10vvvvvv 10vvvvvv 10vvvvvv 5 26 00200000 03FFFFFF 111110vv 10vvvvvv 10vvvvvv 10vvvvvv 10vvvvvv 6 31 04000000 7FFFFFFF 1111110v 10vvvvvv 10vvvvvv 10vvvvvv 10vvvvvv 10vvvvvv The UCS value is just the concatenation of the v bits in the multibyte encoding. When there are multiple ways to encode a value, for example UCS 0, only the shortest encoding is legal. Below are sample implementations of the C standard wctomb() and mbtowc() functions which demonstrate the algorithms for converting from UCS to the transformation format and converting from the transformation format to UCS. The sample implementations include error checks, some of which may not be necessary for conformance: typedef struct { int cmask; int cval; int shift; long lmask; long lval; } Tab; static Tab tab[] = { 0x80, 0x00, 0*6, 0x7F, 0, /* 1 byte sequence */ 0xE0, 0xC0, 1*6, 0x7FF, 0x80, /* 2 byte sequence */ 0xF0, 0xE0, 2*6, 0xFFFF, 0x800, /* 3 byte sequence */ 0xF8, 0xF0, 3*6, 0x1FFFFF, 0x10000, /* 4 byte sequence */ 0xFC, 0xF8, 4*6, 0x3FFFFFF, 0x200000, /* 5 byte sequence */ 0xFE, 0xFC, 5*6, 0x7FFFFFFF, 0x4000000, /* 6 byte sequence */ 0, /* end of table */ }; int mbtowc(wchar_t *p, char *s, size_t n) { long l; int c0, c, nc; Tab *t; if(s == 0) return 0; nc = 0; if(n <= nc) return -1; c0 = *s & 0xff; l = c0; for(t=tab; t->cmask; t++) { nc++; if((c0 & t->cmask) == t->cval) { l &= t->lmask; if(l < t->lval) return -1; *p = l; return nc; } if(n <= nc) return -1; s++; c = (*s ^ 0x80) & 0xFF; if(c & 0xC0) return -1; l = (l<<6) | c; } return -1; } int wctomb(char *s, wchar_t wc) { long l; int c, nc; Tab *t; if(s == 0) return 0; l = wc; nc = 0; for(t=tab; t->cmask; t++) { nc++; if(l <= t->lmask) { c = t->shift; *s = t->cval | (l>>c); while(c > 0) { c -= 6; s++; *s = 0x80 | ((l>>c) & 0x3F); } return nc; } } return -1; }