NAME

Sys::Binmode - A fix for Perl’s system call encoding bug.

SYNOPSIS

use Sys::Binmode;

my $foo = "é";
$foo .= "\x{100}";
chop $foo;

# Prints “é”:
print $foo, $/;

# In Perl 5.32 this may print mojibake,
# but with Sys::Binmode it always prints “é”:
exec 'echo', $foo;

DESCRIPTION

tl;dr: Use this module in all new code. Seriously.

BACKGROUND

Ideally, a Perl application doesn’t need to know how the interpreter stores a given string internally. Perl can thus store any Unicode code point while still optimizing for size and speed when storing “bytes-compatible” strings—i.e., strings whose code points all lie below 256. Perl’s “optimized” string storage format is faster and less memory-hungry, but it can only store code points 0-255. The “unoptimized” format, on the other hand, can store any Unicode code point.

Of course, Perl doesn’t always optimize “bytes-compatible” strings; Perl can also, if it wants, store such strings “unoptimized” (i.e., in Perl’s internal “loose UTF-8” format), too. For code points 0-127 there’s actually no difference between the two forms, but for 128-255 the formats differ. (cf. "The "Unicode Bug"" in perlunicode) This means that anything that reads Perl’s internals MUST differentiate between the two forms in order to use the string correctly.

Alas, that differentiation doesn’t always happen. Thus, Perl can output a string that stores one or more 128-255 code points differently depending on whether Perl has “optimized” that string or not. Remember, though, that Perl applications should not care about Perl’s string storage internals. (This is why, for example, the bytes pragma is discouraged.) But without that knowledge, the application can’t know what it actually says to the outside world! Thus we have unpredictable behaviour, which is categorically bad.

HOW THIS MODULE (PARTLY) FIXES THE PROBLEM

This module provides predictable behaviour for Perl’s built-in functions by downgrading all strings before giving them to the operating system. It’s equivalent to—but faster than!—prefixing your system calls with utf8::downgrade() (cf. utf8) on all arguments.

Predictable behaviour is always a good thing; ergo, you should use this module in all new code.

CAVEATS

If you apply this module injudiciously to existing code you may see exceptions thrown where previously things worked just fine. This can happen if you’ve neglected to encode one or more strings before sending them to the OS; if Perl has such a string stored upgraded then Perl will, under default behaviour, send a UTF-8-encoded version of that string to the OS. In essence, it’s an implicit UTF-8 auto-encode.

The fix is to apply an explicit UTF-8 encode prior to the system call that throws the error. This is what we should do anyway; Sys::Binmode just enforces that better.

WHERE ELSE THIS PROBLEM CAN APPEAR

This problem is widespread in XS modules due to rampant use of the SvPV macro and variants. SvPV is like the bytes pragma in C: it gives you the string’s internal bytes with no regard for what those bytes represent. XS authors generally should avoid SvPV unless the C code in question ensures that the string is in the intended format. (If in doubt, prefer SvPVbyte or SvPVutf8.)

Note in particular that, as of Perl 5.32, the default XS typemap converts scalars to C char * and const char * via an SvPV variant. This means that any module that uses that conversion logic also has this problem. So XS authors should also avoid the default typemap for such conversions.

LIMITATIONS

• This module will cause an exception to be thrown whenever an application tries to send a string with a >255 code point to the operating system. This exception is a GOOD THING! because it means you’ve neglected to encode your string appropriately for output, and Perl now points you to the bug.
• This module works by replacing the affected ops’ default handlers with a wrapper function that downgrades the strings then calls the default handler. If, though, an op’s default handler was already overwritten, we don’t want to clobber that. This will trigger a compile-time exception. (It may be possible to accommodate such cases, but currently that doesn’t happen.)

LEXICAL SCOPING

If, for some reason, you want Perl’s unpredictable default behaviour, you can disable this module for a given block via no Sys::Binmode, thus:

use Sys::Binmode;

system 'echo', $foo;        # predictable/sane/happy

{
    no Sys::Binmode;

    system 'echo', $foo;    # nasal demons
}

AFFECTED BUILT-INS

• exec and system
• do and require
• File tests (e.g., -e) and the following: chdir, chmod, chown, chroot, fcntl, glob, ioctl, link, lstat, mkdir, open, opendir, readlink, rename, rmdir, select, stat, symlink, sysopen, truncate, umask, unlink, utime
• bind, connect, and setsockopt
• gethostbyaddr and getnetbyaddr
• syscall

TODO

• dbmopen and the System V IPC functions aren’t covered here. If you’d like them, ask.
• There’s room for optimization, if that’s gainful.
• Ideally this behaviour should be in Perl’s core distribution.
• Even more ideally, this behaviour should be Perl’s default. Maybe someday!

LICENSE & COPYRIGHT

Copyright 2021 Gasper Software Consulting. All rights reserved.

This library is licensed under the same license as Perl.