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encoding::warnings(3pm)               Perl Programmers Reference Guide               encoding::warnings(3pm)



NAME
       encoding::warnings - Warn on implicit encoding conversions

VERSION
       This document describes version 0.11 of encoding::warnings, released June 5, 2007.

SYNOPSIS
           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;

DESCRIPTION
   Overview of the problem
       By default, there is a fundamental asymmetry in Perl's unicode model: implicit upgrading from byte-strings bytestrings
       strings to unicode-strings assumes that they were encoded in ISO 8859-1 (Latin-1), but unicode-strings unicodestrings
       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:

           use encoding::warnings;

       Afterwards, implicit upgrading of high-bit bytes will raise a warning.  Ex.: "Bytes implicitly
       upgraded into wide characters as iso-8859-1 at - line 7".

       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 solve it:

       •   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 effects:

           •   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, bytestrings,
           strings, and is thus incapable of silencing warnings from this module.  See encoding for more
           details.

CAVEATS
       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.

SEE ALSO
       perlunicode, perluniintro

       open, utf8, encoding, Encode

AUTHORS
       Audrey Tang

COPYRIGHT
       Copyright 2004, 2005, 2006, 2007 by Audrey Tang <cpan@audreyt.org>.

       This program is free software; you can redistribute it and/or modify it under the same terms as Perl
       itself.

       See <http://www.perl.com/perl/misc/Artistic.html>



perl v5.12.5                                     2012-10-11                          encoding::warnings(3pm)