Character encoding
Index
The character encoding problem
Developers are usually familiar with the ASCII character set. This
is a character set that assigns a unique number to some characters, e.g.
an "A" has ASCII code 65 (or 0x41 in hex), and an "a" has ASCII code 97 (or
0x61 in hex). Some people may also have used ASCII codes for several
drawing characters (such as a horizontal bar, a vertical bar, or a top-right
corner) in the old DOS days, to be able to draw nice windows in text mode.
However, these last characters are strictly spoken not part of the ASCII
set. The standard ASCII set contains only the character positions from
0 to 127 (i.e. anything that fits into an integer that is 7 bits wide). An
example of this table can be found here. Anything that has an ASCII code between 128 and 255 is in principle undefined.
Now, several systems (including the old DOS) have defined those character
positions anyway, but usually in totally different ways. Some well
known extensions are:
- the DOS
character set, nowadays usually known as Code Page 437, but sometimes also
named LatinUS, ECS (Extended Character Set) or PC-8; note that the table
displayed in the link also contains the standard ASCII part
- the ANSI character set, also known as Code Page 1252, and usually the default on Windows
- the ISO-8859-1 character set (also called Latin-1), which is an ISO standard for Western European languages, mostly used on various Unices
- the Adobe Standard Encoding, which is by default used in Postscript, unless overridden
And these are only examples of character sets used in West-European languages.
For Japanese, Chinese, Korean, Vietnamese, ... there are separate character
sets in which one byte's meaning can even be influenced by what the previous
byte was, i.e. these are multi-byte character sets. This is because
even 256 characters (the maximum for 8 bits) is totally inadequate to represent all characters in
such languages.
So, summarizing, if a text file contains a byte that has a value 65, it is
pretty safe to assume that this byte represents an "A", if we ignore the
multi-byte character sets spoken of before. However, a value 233 cannot
be interpreted without knowing in which character set the text file is written.
In Latin-1, it happens to be the character "é", but in another
character set it can be something totally different (e.g. in the DOS character
set it is the Greek letter theta).
Vice versa, if you need to write a character "é" to a file, it depends
on the character set you will use what the numerical value will be in the
file: in Latin-1 it will be 233, but if you use the DOS character set it
will be 130.
Unicode
Enter the Unicode standard...
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