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

encoding::warnings(3p) Perl Programmers Reference Guide #

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

NNAAMMEE #

 encoding::warnings - Warn on implicit encoding conversions

VVEERRSSIIOONN #

 This document describes version 0.13 of encoding::warnings, released June
 20, 2016.

NNOOTTIICCEE #

 As of Perl 5.26.0, this module has no effect.  The internal Perl feature
 that was used to implement this module has been removed.  In recent
 years, much work has been done on the Perl core to eliminate
 discrepancies in the treatment of upgraded versus downgraded strings.  In
 addition, the encoding pragma, which caused many of the problems, is no
 longer supported.  Thus, the warnings this module produced are no longer
 necessary.

 Hence, if you load this module on Perl 5.26.0, you will get one warning
 that the module is no longer supported; and the module will do nothing
 thereafter.

SSYYNNOOPPSSIISS #

     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;

DDEESSCCRRIIPPTTIIOONN #

OOvveerrvviieeww ooff tthhee pprroobblleemm 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 _I_S_O _8_8_5_9_-_1 _(_L_a_t_i_n_-_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.

DDeetteeccttiinngg tthhee pprroobblleemm 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
 _n_o_t trigger this warning.

 You can also make the warnings fatal by importing this module as:

     use encoding::warnings 'FATAL';

SSoollvviinngg tthhee pprroobblleemm 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, _a_n_d 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 SSTTDDIINN and SSTTDDOOUUTT

         This is similar to what open pragma does.

     •   Literal conversions

         This turns _a_l_l 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 _n_o_t affect
     implicit upgrading for byte-strings, and is thus incapable of
     silencing warnings from this module.  See encoding for more details.

CCAAVVEEAATTSS #

 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.

SSEEEE AALLSSOO #

 perlunicode, perluniintro

 open, utf8, encoding, Encode

AAUUTTHHOORRSS #

 Audrey Tang

CCOOPPYYRRIIGGHHTT #

 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 v5.36.3 2019-02-13 encoding::warnings(3p)