encoding::warnings(3) Perl Programmers Reference Guide encoding::warnings(3)NAMEencoding::warnings - Warn on implicit encoding conversions
This document describes version 0.11 of encoding::warnings, released
June 5, 2007.
use encoding::warnings; # or 'FATAL' to raise fatal exceptions
utf8::encode($a = chr(20000)); # a byte-string (raw bytes)
$b = chr(20000); # a unicode-string (wide characters)
# "Bytes implicitly upgraded into wide characters as iso-8859-1"
$c = $a . $b;
Overview of the problem
By default, there is a fundamental asymmetry in Perl's unicode model:
implicit upgrading from byte-strings to unicode-strings assumes that
they were encoded in ISO 8859-1 (Latin-1), but unicode-strings are
downgraded with UTF-8 encoding. This happens because the first 256
codepoints in Unicode happens to agree with Latin-1.
However, this silent upgrading can easily cause problems, if you happen
to mix unicode strings with non-Latin1 data -- i.e. byte-strings
encoded in UTF-8 or other encodings. The error will not manifest until
the combined string is written to output, at which time it would be
impossible to see where did the silent upgrading occur.
Detecting the problem
This module simplifies the process of diagnosing such problems. Just
put this line on top of your main program:
Afterwards, implicit upgrading of high-bit bytes will raise a warning.
Ex.: "Bytes implicitly upgraded into wide characters as iso-8859-1 at -
However, strings composed purely of ASCII code points (0x00..0x7F) will
not trigger this warning.
You can also make the warnings fatal by importing this module as:
use encoding::warnings 'FATAL';
Solving the problem
Most of the time, this warning occurs when a byte-string is
concatenated with a unicode-string. There are a number of ways to
· Upgrade both sides to unicode-strings
If your program does not need compatibility for Perl 5.6 and
earlier, the recommended approach is to apply appropriate IO
disciplines, so all data in your program become unicode-strings.
See encoding, open and "binmode" in perlfunc for how.
· Downgrade both sides to byte-strings
The other way works too, especially if you are sure that all your
data are under the same encoding, or if compatibility with older
versions of Perl is desired.
You may downgrade strings with "Encode::encode" and "utf8::encode".
See Encode and utf8 for details.
· Specify the encoding for implicit byte-string upgrading
If you are confident that all byte-strings will be in a specific
encoding like UTF-8, and need not support older versions of Perl,
use the "encoding" pragma:
use encoding 'utf8';
Similarly, this will silence warnings from this module, and
preserve the default behaviour:
use encoding 'iso-8859-1';
However, note that "use encoding" actually had three distinct
· PerlIO layers for STDIN and STDOUT
This is similar to what open pragma does.
· Literal conversions
This turns all literal string in your program into unicode-
strings (equivalent to a "use utf8"), by decoding them using
the specified encoding.
· Implicit upgrading for byte-strings
This will silence warnings from this module, as shown above.
Because literal conversions also work on empty strings, it may
surprise some people:
use encoding 'big5';
my $byte_string = pack("C*", 0xA4, 0x40);
print length $a; # 2 here.
$a .= ""; # concatenating with a unicode string...
print length $a; # 1 here!
In other words, do not "use encoding" unless you are certain that
the program will not deal with any raw, 8-bit binary data at all.
However, the "Filter => 1" flavor of "use encoding" will not affect
implicit upgrading for byte-strings, and is thus incapable of
silencing warnings from this module. See encoding for more
For Perl 5.9.4 or later, this module's effect is lexical.
For Perl versions prior to 5.9.4, this module affects the whole script,
instead of inside its lexical block.
open, utf8, encoding, Encode
Copyright 2004, 2005, 2006, 2007 by Audrey Tang <email@example.com>.
This program is free software; you can redistribute it and/or modify it
under the same terms as Perl itself.
perl v5.10.0 2007-12-18 encoding::warnings(3)