Re: Reading lines of a Text File form the end

Tom Anderson <>
Fri, 27 Aug 2010 16:26:13 +0100
On Fri, 27 Aug 2010, Thomas Pornin wrote:

According to Peter Duniho <>:

My understanding is that you can tell from a single byte in UTF-8
whether it's the end of a character or not. But to identify the
beginning of a character, you need to look for the end of the
_previous_ character.

No, that's not how it works in UTF-8:
-- code points which encode into a single byte yield byte values between
0 and 127 (inclusive);
-- other code points become a sequence of bytes:
  ** first byte has value between 192 and 247 (inclusive)
  ** subsequent bytes (one to three extra bytes) have value between
     128 and 191 (inclusive)

The first byte of a multi-byte sequence also encodes how many extra
bytes are to be found afterwards. With Unicode as currently defined, no
code point requires more than four bytes: valid code points are in the
0..1114111 range, while allocated code points use about 10% of that
range (so there is still quite some room). The UTF-8 encoding is good up
to 2097152. If a future Unicode version extends the range, UTF-8
encoding can be extended to up to 6-byte encodings, and the first byte
may then assume values 192 to 253. It is a feature of UTF-8 that byte
values 254 and 255 never appear anywhere (it is used for BOM handling,
so that UTF-8 and UTF-16 can be telled appart unambiguously).

Anyway, the ending byte of the UTF-8 encoding of a code point is not
specially marked; but _starting_ bytes are easy to detect. Hence it is
easy to know whether you are at the start of a code point, or should go
back for at least one byte.


To rephrase Thomas's description in terms of bits, bytes in a UTF-8 stream
look like this:

0xxxxxxx ASCII
10xxxxxx trail byte of multibyte character
110xxxxx start byte of a two-byte character
1110xxxx start byte of a three-byte character

A character starts with a byte which does not start with 10. Those are
pretty easy to spot.

See also:

And everyone should know about this, highly useful:


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