# NAME Sys::Binmode - A fix for Perl’s system call character encoding
Coverage Status
# SYNOPSIS use Sys::Binmode; my $foo = "\xff"; $foo .= "\x{100}"; chop $foo; # Prints a single octet (0xFF) and a newline: print $foo, $/; # In Perl 5.32 this may print the same single octet, or it may # print UTF-8-encoded U+00FF. With Sys::Binmode, though, it always # gives the single octet, just like print: exec 'echo', $foo; # DESCRIPTION tl;dr: Use this module in **all** new code. # 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 (ASCII printables, controls, and DEL) there’s actually no difference between the two forms, but for 128-255 the formats differ. (cf. ["The "Unicode Bug"" in perlunicode](https://metacpan.org/pod/perlunicode#The-Unicode-Bug)) 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. When it doesn’t, Perl outputs code points 128-255 differently depending on whether the containing string is “optimized” or not. Remember, though: Perl applications _should_ _not_ _care_ about Perl’s string storage internals like optimized/unoptimized. (This is why, for example, the [bytes](https://metacpan.org/pod/bytes) pragma is discouraged.) The catch, though, is that without that knowledge, **the** **application** **can’t** **know** **what** **it** **actually** **says** **to** **the** **outside** **world!** Thus, applications must either monitor Perl’s string-storage internals or accept unpredictable behavior, both of which are categorically bad. (Perl’s documentation calls the “unoptimized” format “upgraded”, while it calls the “optimized” format “downgraded”. The rest of this document will favor Perl’s terms.) # HOW THIS MODULE (PARTLY) FIXES THE PROBLEM This module provides predictable behavior 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](https://metacpan.org/pod/utf8)) on all arguments. Predictable behavior is **always** a good thing; ergo, you should use this module in **all** new code. # CAVEAT: CHARACTER ENCODING If you apply this module injudiciously to existing code you may see exceptions or character corruption where previously things worked fine. This can happen if you’ve neglected to encode one or more strings before sending them to the OS. Without Sys::Binmode, Perl sends upgraded strings to the OS in UTF-8 encoding. In essence, it’s an implicit UTF-8 auto-encode, which is kind of nice, except that it depends on Perl’s internals, which are unpredictable. Sys::Binmode removes that implicit UTF-8 auto-encode, which of course will break things that need it. 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. ## Example: The [utf8](https://metacpan.org/pod/utf8) Pragma The widely-used [utf8](https://metacpan.org/pod/utf8) pragma particularly exemplifies this problem. If you have code like this: use utf8; mkdir "épée"; … then adding this module will change your program’s behavior in ways you’ll probably dislike. Consider the string `épée`. Without the `utf8` pragma (but assuming that the code _is_ actually written in UTF-8) this is 6 characters because the two `é`s are 2 bytes each (so 2 + 1 + 2 + 1), and without the `utf8` pragma each byte in a string constant becomes its own character, even if multiple bytes make up a single UTF-8 character. Since nothing _probably_ upgrades that string on its way to `mkdir()`, the OS will receive the intended 6 bytes and create a directory with a UTF-8-encoded name. _With_ `utf8`, though, `épée` is **4** characters, not 6, because this string is now UTF-8-decoded. Those 4 characters all lie beneath 256, so the string is still bytes-compatible. Thus, if you `print()` that string you’ll get 4 bytes of Latin-1, which probably **isn’t** what you want. `mkdir()`, though, _probably_ still creates a directory with a 6-byte (UTF-8) name. This happens when Perl itself stores `épée` in upgraded (i.e., “unoptimized”) form. If that’s the case, that means Perl’s _internal_ buffer of `épée` is still the 6 bytes of UTF-8, even though to the Perl _application_ it’s a 4-character string. Perl’s `mkdir()` doesn’t care about characters, though; it just gives Perl’s internal buffer to the OS’s create-directory function. So by violating its own abstraction, Perl happens to achieve something that is _sometimes_ useful. There are still two problems, though: - 1. Inconsistency: `print()` sends 4 bytes to the OS while `mkdir()` (again, _probably_) outputs 6. - 2. Uncertainty: `épée` _could_ be stored downgraded rather than upgraded, which would cause `mkdir()` to send 4 bytes instead. `print()`’s outputting of 4 bytes here is actually the **correct** behavior because it doesn’t depend on whether Perl stores the string upgraded or downgraded. Sys::Binmode extends that correct behavior to `mkdir()` and other such Perl commands. To get what you want, just encode your string for output before you give it to the OS (as you should do anyway): use utf8; use Encode; mkdir encode_utf8("épée"); Now adding Sys::Binmode to your module will change nothing. It _will_, though, make any future omitted-encoding bugs more apparent. ## Non-POSIX Operating Systems (e.g., Windows) In a POSIX operating system, an application’s communication with the OS happens entirely through byte strings. Thus, treating all OS-destined strings as byte strings is good and natural. In Windows, though, things are weirder. For example, Windows exposes multiple APIs for creating a directory, and the one Perl uses (as of 5.32, anyway) only accepts code points 0-255. In this context Sys::Binmode doesn’t _break_ anything, but it does reinforce one of Perl’s unfortunate limitations on Windows. Sys::Binmode is a good idea anywhere that Perl sends byte strings to the OS. As far as I know, that’s everywhere that Perl runs. If that’s not true, please file a bug. # WHERE ELSE THIS PROBLEM CAN APPEAR The unpredictable-behavior problem that this module fixes in core Perl is also common in XS modules due to rampant use of [the SvPV macro](https://perldoc.perl.org/perlapi#SvPV) and variants. SvPV is like the [bytes](https://metacpan.org/pod/bytes) pragma in C: it gives you the string’s internal bytes with no regard for what those bytes represent. XS authors _generally_ should prefer [SvPVbyte](https://perldoc.perl.org/perlapi#SvPVbyte) or [SvPVutf8](https://perldoc.perl.org/perlapi#SvPVutf8) in lieu of SvPV unless the C code in question deals with Perl’s encoding abstraction. 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. # LEXICAL SCOPING If, for some reason, you _want_ Perl’s unpredictable default behavior, you can disable this module for a given block via `no Sys::Binmode`, thus: use Sys::Binmode; system 'echo', $foo; # predictable/sane/happy { # You should probably explain here why you’re doing this. 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`, `link`, `lstat`, `mkdir`, `open`, `opendir`, `readlink`, `rename`, `rmdir`, `stat`, `symlink`, `sysopen`, `truncate`, `unlink`, `utime` - `bind`, `connect`, and `setsockopt` - `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 behavior should be in Perl’s core distribution. - Even more ideally, Perl should adopt this behavior as _default_. Maybe someday! # ACKNOWLEDGEMENTS Thanks to Leon Timmermans (LEONT) and Paul Evans (PEVANS) for some debugging and design help. # LICENSE & COPYRIGHT Copyright 2021 Gasper Software Consulting. All rights reserved. This library is licensed under the same license as Perl.