Each simple character set has a configuration file located in
the sql/share/charsets directory. The file
is named
MYSET.xml<map> array elements to list
character set properties. <map>
elements appear within these elements:
<ctype>defines attributes for each character<lower>and<upper>list the lowercase and uppercase characters<unicode>maps 8-bit character values to Unicode values<collation>elements indicate character ordering for comparisons and sorts, one element per collation (binary collations need no<map>element because the character codes themselves provide the ordering)
For a complex character set as implemented in a
ctype-
file in the MYSET.cstrings directory, there are
corresponding arrays:
ctype_,
MYSET[]to_lower_,
and so forth. Not every complex character set has all of the
arrays. See the existing MYSET[]ctype-*.c files
for examples. See the CHARSET_INFO.txt file
in the strings directory for additional
information.
The ctype array is indexed by character value
+ 1 and has 257 elements. This is an old legacy convention for
handling EOF. The other arrays are indexed by
character value and have 256 elements.
ctype array elements are bit values. Each
element describes the attributes of a single character in the
character set. Each attribute is associated with a bitmask, as
defined in include/m_ctype.h:
#define _MY_U 01 /* Upper case */ #define _MY_L 02 /* Lower case */ #define _MY_NMR 04 /* Numeral (digit) */ #define _MY_SPC 010 /* Spacing character */ #define _MY_PNT 020 /* Punctuation */ #define _MY_CTR 040 /* Control character */ #define _MY_B 0100 /* Blank */ #define _MY_X 0200 /* heXadecimal digit */
The ctype value for a given character should
be the union of the applicable bitmask values that describe the
character. For example, 'A' is an uppercase
character (_MY_U) as well as a hexadecimal
digit (_MY_X), so its
ctype value should be defined like this:
ctype['A'+1] = _MY_U | _MY_X = 01 | 0200 = 0201
The bitmask values in m_ctype.h are octal
values, but the elements of the ctype array
in MYSET.xml
The lower and upper arrays
hold the lowercase and uppercase characters corresponding to
each member of the character set. For example:
lower['A'] should contain 'a' upper['a'] should contain 'A'
Each collation array is a map indicating how
characters should be ordered for comparison and sorting
purposes. MySQL sorts characters based on the values of this
information. In some cases, this is the same as the
upper array, which means that sorting is
case-insensitive. For more complicated sorting rules (for
complex character sets), see the discussion of string collating
in Section 9.3.2, “String Collating Support”.