encoding::warnings - man - phpman

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NAME
encoding::warnings - Warn on implicit encoding conversions

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

NOTICE
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.

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 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:

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, 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.