toybox/www/design.html

<html><head><title>The design of toybox</title></head>
<!--#include file="header.html" -->

<h2>Topics</h2>
<ul>
<li><a href=#goals><h3>Design Goals</h3></a></li>
<li><a href=#portability><h3>Portability Issues</h3></a></li>
<li><a href=#license><h3>License</a></h3></a></li>
<li><a href=#codestyle><h3>Coding Style</h3></a></li>
</ul>
<hr />

<a name="goals"><b><h2><a href="#goals">Design goals</a></h2></b>

<p>Toybox should be simple, small, fast, and full featured. In that order.</p>

<p>It should be possible to get about <a href=https://en.wikipedia.org/wiki/Pareto_principle>80% of the way</a> to each goal
before they really start to fight.
When these goals need to be balanced off against each other, keeping the code
as simple as it can be to do what it does is the most important (and hardest)
goal. Then keeping it small is slightly more important than making it fast.
Features are the reason we write code in the first place but this has all
been implemented before so if we can't do a better job why bother?</p>

<b><h3>Features</h3></b>

<p>Toybox should provide the command line utilities of a build
environment capable of recompiling itself under itself from source code.
This minimal build system conceptually consists of 4 parts: toybox,
a C library, a compiler, and a kernel. Toybox needs to provide all the
commands (with all the behavior) necessary to run the configure/make/install
of each package and boot the resulting system into a usable state.</p>

<p>In addition, it should be possible to bootstrap up to arbitrary complexity
under the result by compiling and installing additional packages into this
minimal system, as measured by building both Linux From Scratch and the
Android Open Source Project under the result. Any "circular dependencies"
should be solved by toybox including the missing dependencies itself
(see "Shared Libraries" below).</p>

<p>Toybox may also provide some "convenience" utilties
like top and vi that aren't necessarily used in a build but which turn
the minimal build environment into a minimal development environment
(supporting edit/compile/test cycles in a text console), configure
network infrastructure for communication with other systems (in a build
cluster), and so on.</p>

<p>And these days toybox is the command line of Android, so anything the android
guys say to do gets at the very least closely listened to.</p>

<p>The hard part is deciding what NOT to include. A project without boundaries
will bloat itself to death. One of the hardest but most important things a
project must do is draw a line and say "no, this is somebody else's problem,
not something we should do."
Some things are simply outside the scope of the project: even though
posix defines commands for compiling and linking, we're not going to include
a compiler or linker (and support for a potentially infinite number of hardware
targets). And until somebody comes up with a ~30k ssh implementation (with
a crypto algorithm that won't need replacing every 5 years), we're
going to point you at dropbear or bearssl.</p>

<p>The <a href=roadmap.html>roadmap</a> has the list of features we're
trying to implement, and the reasons why we decided to include those
features. After the 1.0 release some of that material may get moved here,
but for now it needs its own page. The <a href=status.html>status</a>
page shows the project's progress against the roadmap.</p>

<p>There are potential features (such as a screen/tmux implementation)
that might be worth adding after 1.0, in part because they could share
infrastructure with things like "less" and "vi" so might be less work for
us to do than for an external from scratch implementation. But for now, major
new features outside posix, android's existing commands, and the needs of
development systems, are a distraction from the 1.0 release.</p>

<b><h3>Speed</h3></b>

<p>Quick smoketest: use the "time" command, and if you haven't got a test
case that's embarassing enough to motivate digging, move on.</p>

<p>It's easy to say a lot about optimizing for speed (which is why this section
is so long), but at the same time it's the optimization we care the least about.
The essence of speed is being as efficient as possible, which means doing as
little work as possible.  A design that's small and simple gets you 90% of the
way there, and most of the rest is either fine-tuning or more trouble than
it's worth (and often actually counterproductive).  Still, here's some
advice:</p>

<p>First, understand the darn problem you're trying to solve.  You'd think
I wouldn't have to say this, and yet.  Trying to find a faster sorting
algorithm is no substitute for figuring out a way to skip the sorting step
entirely.  The fastest way to do anything is not to have to do it at all,
and _all_ optimization boils down to avoiding unnecessary work.</p>

<p>Speed is easy to measure; there are dozens of profiling tools for Linux,
but sticking in calls to "millitime()" out of lib.c and subtracting
(or doing two clock_gettime() calls and then nanodiff() on them) is
quick and easy. Don't waste too much time trying to optimize something you
can't measure, and there's no much point speeding up things you don't spend
much time doing anyway.</p>

<p>Understand the difference between throughput and latency.  Faster
processors improve throughput, but don't always do much for latency.
After 30 years of Moore's Law, most of the remaining problems are latency,
not throughput.  (There are of course a few exceptions, like data compression
code, encryption, rsync...)  Worry about throughput inside long-running
loops, and worry about latency everywhere else.  (And don't worry too much
about avoiding system calls or function calls or anything else in the name
of speed unless you are in the middle of a tight loop that's you've already
proven isn't running fast enough.)</p>

<p>The lowest hanging optimization fruit is usually either "don't make
unnecessary copies of data" or "use a reasonable block size in your
I/O transactions instead of byte-at-a-time".
Start by looking for those, most of the rest of this advice is just explaining
why they're bad.</p>

<p>"Locality of reference" is generally nice, in all sorts of contexts.
It's obvious that waiting for disk access is 1000x slower than doing stuff in
RAM (and making the disk seek is 10x slower than sequential reads/writes),
but it's just as true that a loop which stays in L1 cache is many times faster
than a loop that has to wait for a DRAM fetch on each iteration.  Don't worry
about whether "&" is faster than "%" until your executable loop stays in L1
cache and the data access is fetching cache lines intelligently.  (To
understand DRAM, L1, and L2 cache, read Hannibal's marvelous ram guide at Ars
Technica:
<a href=http://arstechnica.com/paedia/r/ram_guide/ram_guide.part1-2.html>part one</a>,
<a href=http://arstechnica.com/paedia/r/ram_guide/ram_guide.part2-1.html>part two</a>,
<a href=http://arstechnica.com/paedia/r/ram_guide/ram_guide.part3-1.html>part three</a>,
plus this
<a href=http://arstechnica.com/articles/paedia/cpu/caching.ars/1>article on
cacheing</a>, and this one on
<a href=http://arstechnica.com/articles/paedia/cpu/bandwidth-latency.ars>bandwidth
and latency</a>.
And there's <a href=http://arstechnica.com/paedia/index.html>more where that came from</a>.)
Running out of L1 cache can execute one instruction per clock cycle, going
to L2 cache costs a dozen or so clock cycles, and waiting for a worst case dram
fetch (round trip latency with a bank switch) can cost thousands of
clock cycles.  (Historically, this disparity has gotten worse with time,
just like the speed hit for swapping to disk.  These days, a _big_ L1 cache
is 128k and a big L2 cache is a couple of megabytes.  A cheap low-power
embedded processor may have 8k of L1 cache and no L2.)</p>

<p>Learn how <a href=http://nommu.org/memory-faq.txt>virtual memory and
memory managment units work</a>.  Don't touch
memory you don't have to.  Even just reading memory evicts stuff from L1 and L2
cache, which may have to be read back in later.  Writing memory can force the
operating system to break copy-on-write, which allocates more memory.  (The
memory returned by malloc() is only a virtual allocation, filled with lots of
copy-on-write mappings of the zero page.  Actual physical pages get allocated
when the copy-on-write gets broken by writing to the virtual page.  This
is why checking the return value of malloc() isn't very useful anymore, it
only detects running out of virtual memory, not physical memory.  Unless
you're using a <a href=http://nommu.org>NOMMU system</a>, where all bets
are off.)</p>

<p>Don't think that just because you don't have a swap file the system can't
start swap thrashing: any file backed page (ala mmap) can be evicted, and
there's a reason all running programs require an executable file (they're
mmaped, and can be flushed back to disk when memory is short).  And long
before that, disk cache gets reclaimed and has to be read back in.  When the
operating system really can't free up any more pages it triggers the out of
memory killer to free up pages by killing processes (the alternative is the
entire OS freezing solid).  Modern operating systems seldom run out of
memory gracefully.</p>

<p>It's usually better to be simple than clever. Many people think that mmap()
is faster than read() because it avoids a copy, but twiddling with the memory
management is itself slow, and can cause unnecessary CPU cache flushes.  And
if a read faults in dozens of pages sequentially, but your mmap iterates
backwards through a file (causing lots of seeks, each of which your program
blocks waiting for), the read can be many times faster.  On the other hand, the
mmap can sometimes use less memory, since the memory provided by mmap
comes from the page cache (allocated anyway), and it can be faster if you're
doing a lot of different updates to the same area.  The moral?  Measure, then
try to speed things up, and measure again to confirm it actually _did_ speed
things up rather than made them worse.  (And understanding what's really going
on underneath is a big help to making it happen faster.)</p>

<p>Another reason to be simple than clever is optimization
strategies change with time.  For example, decades ago precalculating a table
of results (for things like isdigit() or cosine(int degrees)) was clearly
faster because processors were so slow.  Then processors got faster and grew
math coprocessors, and calculating the value each time became faster than
the table lookup (because the calculation fit in L1 cache but the lookup
had to go out to DRAM).  Then cache sizes got bigger (the Pentium M has
2 megabytes of L2 cache) and the table fit in cache, so the table became
fast again...  Predicting how changes in hardware will affect your algorithm
is difficult, and using ten year old optimization advice can produce
laughably bad results.  Being simple and efficient should give at least a
reasonable starting point.</p>

<p>Even at the design level, a lot of simple algorithms scale terribly but
perform fine with small data sets. When small datasets are the common case,
"better" versions that trade higher throughput for worse latency can
consistently perform worse.
So if you think you're only ever going to feed the algorithm small data sets,
maybe just do the simple thing and wait for somebody to complain. For example,
you probably don't need to sort and binary search the contents of
/etc/passwd, because even 50k users is still a reasonably manageable data
set for a readline/strcmp loop, and that's the userbase of a fairly major
<a href=https://en.wikipedia.org/wiki/List_of_United_States_public_university_campuses_by_enrollment>university</a>.
Instead commands like "ls" call bufgetpwuid() out of lib/lib.c
which keeps a linked list of recently seen items, avoiding reparsing entirely
and trusting locality of reference to bring up the same dozen or so entries
for "ls -l /dev" or similar. The pathological failure mode of "simple
linked list" is to perform exactly as badly as constantly rescanning a
huge /etc/passwd, so this simple optimization shouldn't ever make performance
worse (modulo possible memory exhaustion and thus swap thrashing).
On the other hand, toybox's multiplexer does sort and binary
search its command list to minimize the latency of each command startup,
because the sort is a compile-time cost done once per build,
and the whole of command startup
is a "hot path" that should do as little work as possible because EVERY
command has to go through it every time before performing any other function
so tiny gains are worthwhile. (These decisions aren't perfect, the point is
to show that thought went into them.)</p>

<p>The famous quote from Ken Thompson, "When in doubt, use brute force",
applies to toybox.  Do the simple thing first, do as little of it as possible,
and make sure it's right.  You can always speed it up later.</p>

<b><h3>Size</h3></b>
<p>Quick smoketest: build toybox with and without the command (or the change),
and maybe run "nm --size-sort" on files in generated/unstripped.
(See make bloatcheck below for toybox's built in nm size diff-er.)</p>

<p>Again, being simple gives you most of this. An algorithm that does less work
is generally smaller. Understand the problem, treat size as a cost, and
get a good bang for the byte.</p>

<p>What "size" means depends on context: there are at least a half dozen
different metrics in two broad categories: space used on disk/flash/ROM,
and space used in memory at runtime.</p>

<p>Your executable file has at least
four main segments (text = executable code, rodata = read only data,
data = writeable variables initialized to a value other than zero,
bss = writeable data initialized to zero). Text and rodata are shared between multiple instances of the program running
simultaneously, the other 4 aren't. Only text, rodata, and data take up
space in the binary, bss, stack and heap only matter at runtime. You can
view toybox's symbols with "nm generated/unstripped/toybox", the T/R/D/B
lets you know the segment the symbol lives in. (Lowercase means it's
local/static.)</p>

<p>Then at runtime there's
heap size (where malloc() memory lives) and stack size (where local
variables and function call arguments and return addresses live). And
on 32 bit systems mmap() can have a constrained amount of virtual memory
(usually a couple gigabytes: the limits on 64 bit systems are generally big
enough it doesn't come up)</p>

<p>Optimizing for binary size is generally good: less code is less to go
wrong, and executing fewer instructions makes your program run faster (and
fits more of it in cache). On embedded systems, binary size is especially
precious because flash is expensive and code may need binary auditing for
security. Small stack size
is important for nommu systems because they have to preallocate their stack
and can't make it bigger via page fault. And everybody likes a small heap.</p>

<p>Measure the right things. Especially with modern optimizers, expecting
something to be smaller is no guarantee it will be after the compiler's done
with it. Will total binary size is the final result, it isn't always the most
accurate indicator of the impact of a given change, because lots of things
get combined and rounded during compilation and linking (and things like
ASAN disable optimization). Toybox has scripts/bloatcheck to compare two versions
of a program and show size changes in each symbol (using "nm --size-sort").
You can "make baseline" to build a baseline version to compare against,
and then apply your changes and "make bloatcheck" to compare against
the saved baseline version.</p>

<p>Avoid special cases. Whenever you see similar chunks of code in more than
one place, it might be possible to combine them and have the users call shared
code (perhaps out of lib/*.c). This is the most commonly cited trick, which
doesn't make it easy to work out HOW to share. If seeing two lines of code do
the same thing makes you slightly uncomfortable, you've got the right mindset,
but "reuse" requires the "re" to have benefit, and infrastructure in search
of a user will generally bit-rot before it finds one.</p>

<p>The are a lot of potential microoptimizations (on some architectures
using char instead of int as a loop index is noticeably slower, on some
architectures C bitfields are surprisingly inefficient, & is often faster
than % in a tight loop, conditional assignment avoids branch prediction
failures...) but they're generally not worth doing unless you're trying to
speed up the middle of a tight inner loop chewing through a large amount
of data (such as a compression algorithm). For data pumps sane blocking
and fewer system calls (buffer some input/output and do a big read/write
instead of a bunch of little small ones) is usually the big win. But
be careful about cacheing stuff: the two persistently hard problems in computer
science are naming things, cache coherency, and off by one errors.</p>

<b><h3>Simplicity</h3></b>

<p>Complexity is a cost, just like code size or runtime speed. Treat it as
a cost, and spend your complexity budget wisely. (Sometimes this means you
can't afford a feature because it complicates the code too much to be
worth it.)</p>

<p>Simplicity has lots of benefits.  Simple code is easy to maintain, easy to
port to new processors, easy to audit for security holes, and easy to
understand.</p>

<p>Simplicity itself can have subtle non-obvious aspects requiring a tradeoff
between one kind of simplicity and another: simple for the computer to
execute and simple for a human reader to understand aren't always the
same thing. A compact and clever algorithm that does very little work may
not be as easy to explain or understand as a larger more explicit version
requiring more code, memory, and CPU time. When balancing these, err on the
side of doing less work, but add comments describing how you
could be more explicit.</p>

<p>In general, comments are not a substitute for good code (or well chosen
variable or function names). Commenting "x += y;" with "/* add y to x */"
can actually detract from the program's readability. If you need to describe
what the code is doing (rather than _why_ it's doing it), that means the
code itself isn't very clear.</p>

<p>Environmental dependencies are another type of complexity, so needing other
packages to build or run is a big downside. For example, we don't use curses
when we can simply output ansi escape sequences and trust all terminal
programs written in the past 30 years to be able to support them. Regularly
testing that we work with C libraries which support static linking (musl does,
glibc doesn't) is another way to be self-contained with known boundaries:
it doesn't have to be the only way to build the project, but should be regularly
tested and supported.</p>

<p>Prioritizing simplicity tends to serve our other goals: simplifying code
generally reduces its size (both in terms of binary size and runtime memory
usage), and avoiding unnecessary work makes code run faster. Smaller code
also tends to run faster on modern hardware due to CPU cacheing: fitting your
code into L1 cache is great, and staying in L2 cache is still pretty good.</p>

<p>But a simple implementation is not always the smallest or fastest, and
balancing simplicity vs the other goals can be difficult. For example, the
atolx_range() function in lib/lib.c always uses the 64 bit "long long" type,
which produces larger and slower code on 32 bit platforms and
often assigned into smaller interger types. Although libc has parallel
implementations for different data sizes (atoi, atol, atoll) we chose a
common codepath which can cover all cases (every user goes through the
same codepath, with the maximum amount of testing and minimum and avoids
surprising variations in behavior).</p>

<p>On the other hand, the "tail" command has two codepaths, one for seekable
files and one for nonseekable files. Although the nonseekable case can handle
all inputs (and is required when input comes from a pipe or similar, so cannot
be removed), reading through multiple gigabytes of data to reach the end of
seekable files was both a common case and hugely penalized by a nonseekable
approach (half-minute wait vs instant results). This is one example
where performance did outweigh simplicity of implementation.</p>

<p><a href=http://www.joelonsoftware.com/articles/fog0000000069.html>Joel
Spolsky argues against throwing code out and starting over</a>, and he has
good points: an existing debugged codebase contains a huge amount of baked
in knowledge about strange real-world use cases that the designers didn't
know about until users hit the bugs, and most of this knowledge is never
explicitly stated anywhere except in the source code.</p>

<p>That said, the Mythical Man-Month's "build one to throw away" advice points
out that until you've solved the problem you don't properly understand it, and
about the time you finish your first version is when you've finally figured
out what you _should_ have done.  (The corrolary is that if you build one
expecting to throw it away, you'll actually wind up throwing away two.  You
don't understand the problem until you _have_ solved it.)</p>

<p>Joel is talking about what closed source software can afford to do: Code
that works and has been paid for is a corporate asset not lightly abandoned.
Open source software can afford to re-implement code that works, over and
over from scratch, for incremental gains.  Before toybox, the unix command line
has already been reimplemented from scratch several times (the
original AT&amp;T Unix command line in assembly and then in C, the BSD
versions, Coherent was the first full from-scratch Unix clone in 1980,
Minix was another clone which Linux was inspired by and developed under,
the GNU tools were yet another rewrite intended for use in the stillborn
"Hurd" project, BusyBox was still another rewrite, and more versions
were written in Plan 9, uclinux, klibc, sash, sbase, s6, and of course
android toolbox...). But maybe toybox can do a better job. :)</p>

<p>As Antoine de St. Exupery (author of "The Little Prince" and an early
aircraft designer) said, "Perfection is achieved, not when there
is nothing left to add, but when there is nothing left to take away."
And Ken Thompson (creator of Unix) said "One of my most productive
days was throwing away 1000 lines of code." It's always possible to
come up with a better way to do it.</p>

<p>P.S. How could I resist linking to an article about
<a href=http://blog.outer-court.com/archive/2005-08-24-n14.html>why
programmers should strive to be lazy and dumb</a>?</p>

<hr>
<a name="portability"><b><h2><a href="#portability">Portability issues</a></h2></b>

<b><h3>Platforms</h3></b>
<p>Toybox should run on Android (all commands with musl-libc, as large a subset
as practical with bionic), and every other hardware platform Linux runs on.
Other posix/susv4 environments (perhaps MacOS X or newlib+libgloss) are vaguely
interesting but only if they're easy to support; I'm not going to spend much
effort on them.</p>

<p>I don't do windows.</p>

<a name="standards" />
<b><h3>Standards</h3></b>

<p>Toybox is implemented with reference to
<a href=https://www.open-std.org/jtc1/sc22/wg14/www/docs/n1548.pdf>c11</a>,
<a href=roadmap.html#susv4>Posix 2008</a>,
<a href=#bits>LP64</a>,
<a href=roadmap.html#sigh>LSB 4.1</a>,
the <a href=https://www.kernel.org/doc/man-pages/>Linux man pages</a>,
various <a href=https://www.rfc-editor.org/rfc-index.html>IETF RFCs</a>,
the linux kernel source's
<a href=https://www.kernel.org/doc/Documentation/>Documentation</a> directory,
utf8 and unicode, and our terminal control outputs ANSI
<a href=https://man7.org/linux/man-pages/man4/console_codes.4.html>escape sequences</a>.
Toybox gets <a href=faq.html#cross>tested</a> with gcc and llvm on glibc,
musl-libc, and bionic, plus occasional <a href=https://github.com/landley/toybox/blob/master/kconfig/freebsd_miniconfig>FreeBSD</a> and
<a href=https://github.com/landley/toybox/blob/master/kconfig/macos_miniconfig>MacOS</a> builds for subsets
of the commands.</p>

<p>For the build environment and runtime environment, toybox depends on
posix-2008 libc features such as the openat() family of
functions. We also root around in the linux /proc directory a lot (no other
way to implement "ps" at the moment), and assume certain "modern" linux kernel
behavior (for example <a href=https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=b6a2fea39318>linux 2.6.22</a>
expanded the 128k process environment size limit to 2 gigabytes, then it was
trimmed back down to 10 megabytes, and when I asked for a way to query the
actual value from the kernel if it was going to keep changing
like that <a href=https://lkml.org/lkml/2017/11/5/204>Linus declined</a>).
We make an effort to support <a href=faq.html#support_horizon>older kernels</a>
and other implementations (primarily MacOS and BSD) but we don't always
police their corner cases very closely.</p>

<p><b>Why not just use the newest version of each standard?</b>

<p>Partly to <a href=faq.html#support_horizon>support older systems</a>:
you can't fix a bug in the old system if you can't build in the old
environment.</p>

<p>Partly because toybox's maintainer has his own corollary to Moore's law:
50% of what you know about programming is obsolete every 18
months, but the advantage of C &amp; Unix it's usually the same 50% cycling
out over and over.</p>

<p>But mostly because the updates haven't added anything we care about.
Posix-2008 switched some things to larger (64 bit) data types and added the
openat() family of functions (which take a directory filehandle instead of
using the Current Working Directory),
but the 2013 and 2018 releases of posix were basically typo fixes: still
release 7, still SUSv4. (An eventual release 8 might be interesting but
it's not out yet.)</p>

<p>We're nominally C11 but mostly just writing good old ANSI C (I.E. C89).
We use a few of the new features like compound literals (6.5.2.5) and structure
initialization by member name with unnamed members zeroed (6.7.9),
but mostly we "officially" went from c99 to C11 to work around a
<a href=https://github.com/landley/toybox/commit/3625a260065b>clang compiler bug</a>.
The main thing we use from c99 that c89 hadn't had was // single line comments.
(We mostly don't even use C99's explicit width data types, ala uint32_t and
friends, because LP64 handles that for us.)</p>

<p>We're ignoring new versions of the Linux Foundation's standards (LSB, FHS)
entirely, for the same reason Debian is: they're not good at maintaining
standards. (The Linux Foundation acquiring the Free Standards Group worked
out about as well as Microsoft buying Nokia, Twitter buying Vine, Yahoo
buying Flickr...)</p>

<p>We refer to current versions of man7.org because it's
not easily versioned (the website updates regularly) and because
Michael Kerrisk does a good job maintaining it so far. That said, we
try to "provide new" in our commands but "depend on old" in our build scripts.
(For example, we didn't start using "wait -n" until it had been in bash for 7
years, and even then people depending on Centos' 10 year support horizon
complained.)</p>

<p>Using newer vs older RFCs, and upgrading between versions, is a per-case
judgement call.</p>

<p><b>How strictly do you adhere to these standards?</b>

<p>...ish? The man pages have a lot of stuff that's not in posix,
and there's no "init" or "mount" in posix, you can't implement "ps"
without replying on non-posix APIs....</p>

<p>When the options a command offers visibly contradict posix, we try to have
a "deviations from posix" section at the top of the source listing the
differences, but that's about what we provide not what we used from the OS
or build environment.</p>

<p>The build needs bash (not a pure-posix sh), and building on MacOS requires
"gsed" (because Mac's sed is terrible), but toybox is explicitly self-hosting
and any failure to build under the tool versions we provide would be a bug
needing to be fixed.</p>

<p>Within the code, everything in main.c and lib/*.c has to build
on every supported Linux version, compiler, and library, plus BSD and MacOS.
We mostly try to keep #if/else staircases for portability issues to
lib/portability.[ch].</p>

<p>Portability of individual commands varies: we sometimes program directly
against linux kernel APIs (unavoidable when accessing /proc and /sys),
individual commands are allowed to #include &lt;linux/*.h&gt; (common
headers and library files are not, except maybe lib/portability.* within an
appropriate #ifdef),  we only really test against Linux errno values
(unless somebody on BSD submits a bug), and a few commands outright cheat
(the way ifconfig checks for ioctl numbers in the 0x89XX range). This is
the main reason some commands build on BSD/MacOS and some don't.</p>

<a name="bits" />
<b><h3>32/64 bit</h3></b>
<p>Toybox should work on both 32 bit and 64 bit systems. 64 bit desktop
hardware went mainstream <a href=https://web.archive.org/web/20040307000108mp_/http://developer.intel.com/technology/64bitextensions/faq.htm>in 2005</a>
and was essentially ubiquitous <a href=faq.html#support_horizon>by 2012</a>,
but 32 bit hardware will continue to be important in embedded devices for years to come.</p>

<p>Toybox relies on the
<a href=http://archive.opengroup.org/public/tech/aspen/lp64_wp.htm>LP64 standard</a>
which Linux, MacOS X, and BSD all implement, and which modern 64 bit processors such as
x86-64 were <a href=http://www.pagetable.com/?p=6>explicitly designed to
support</a>. (Here's the original <a href=https://web.archive.org/web/20020905181545/http://www.unix.org/whitepapers/64bit.html>LP64 white paper</a>.)</p>

<p>LP64 defines explicit sizes for all the basic C integer types, and
guarantees that on any Unix-like platform "long" and "pointer" types
are always the same size (the processor's register size).
This means it's safe to assign pointers into
longs and vice versa without losing data: on 32 bit systems both are 32 bit,
on 64 bit systems both are 64 bit.</p>

<table border=1 cellpadding=10 cellspacing=2>
<tr><td>C type</td><td>char</td><td>short</td><td>int</td><td>long</td><td>long long</td></tr>
<tr><td>32 bit<br />sizeof</td><td>8 bits</td><td>16 bits</td><td>32 bits</td><td>32 bits</td><td>64 bits</td></tr>
<tr><td>64 bit<br />sizeof</td><td>8 bits</td><td>16 bits</td><td>32 bits</td><td>64 bits</td><td>64 bits</td></tr>
</table>

<p>LP64 eliminates the need to use c99 "uint32_t" and friends: the basic
C types all have known size/behavior, and the only type whose
size varies is "long", which is the natural register size of the processor.</p>

<p>Note that Windows doesn't work like this, and I don't care, but if you're
curious here are <a href=https://devblogs.microsoft.com/oldnewthing/20050131-00/?p=36563>the insane legacy reasons why this is broken on Windows</a>.</a></p>

<p>The main squishy bit in LP64 is that "long long" was defined as
"at least" 64 bits instead of "exactly" 64 bits, and the standards body
that issued it collapsed in the wake of the <a href=https://en.wikipedia.org/wiki/Unix_wars>proprietary unix wars</a> (all
those lawsuits between AT&amp;T/BSDI/Novell/Caldera/SCO), so is
not available to issue an official correction. Then again a processor
with 128-bit general purpose registers wouldn't be commercially viable
<a href=https://landley.net/notes-2011.html#26-06-2011>until 2053</a>
(because 2005+32*1.5), and with the S-curve of Moore's Law slowly
<a href=http://www.acm.org/articles/people-of-acm/2016/david-patterson>bending back down</a> as
atomic limits and <a href=http://www.cnet.com/news/end-of-moores-law-its-not-just-about-physics/>exponential cost increases</a> produce increasing
drag.... (The original Moore's Law curve would mean that in the year 2022
a high end workstation would have around 8 terabytes of RAM, available retail.
Most don't even come with
that much disk space.) At worst we don't need to care for decades, the
S-curve bending down means probably not in our lifetimes, and
atomic limits may mean "never". So I'm ok treating "long long" as exactly 64 bits.</p>

<b><h3>Signedness of char</h3></b>
<p>On platforms like x86, variables of type char default to unsigned.  On
platforms like arm, char defaults to signed.  This difference can lead to
subtle portability bugs, and to avoid them we specify which one we want by
feeding the compiler -funsigned-char.</p>

<p>The reason to pick "unsigned" is that way char strings are 8-bit clean by
default, which makes UTF-8 support easier.</p>

<p><h3>Error messages and internationalization</h3></p>

<p>Error messages are extremely terse not just to save bytes, but because we
don't use any sort of _("string") translation infrastructure. (We're not
translating the command names themselves, so we must expect a minimum amount of
english knowledge from our users, but let's keep it to a minimum.)</p>

<p>Thus "bad -A '%c'" is
preferable to "Unrecognized address base '%c'", because a non-english speaker
can see that -A was the problem (giving back the command line argument they
supplied). A user with a ~20 word english vocabulary is
more likely to know (or guess) "bad" than the longer message, and you can
use "bad" in place of "invalid", "inappropriate", "unrecognized"...
Similarly when atolx_range() complains about range constraints with
"4 < 17" or "12 > 5", it's intentional: those don't need to be translated.</p>

<p>The strerror() messages produced by perror_exit() and friends should be
localized by libc, and our error functions also prepend the command name
(which non-english speakers can presumably recognize already). Keep the
explanation in between to a minimum, and where possible feed back the values
they passed in to identify _what_ we couldn't process.
If you say perror_exit("setsockopt"), you've identified the action you
were trying to take, and the perror gives a translated error message (from libc)
explaining _why_ it couldn't do it, so you probably don't need to add english
words like "failed" or "couldn't assign".</p>

<p>All commands should be 8-bit clean, with explicit
<a href=http://yarchive.net/comp/linux/utf8.html>UTF-8</a> support where
necessary. Assume all input data might be utf8, and at least preserve
it and pass it through. (For this reason, our build is -funsigned-char on
all architectures; "char" is unsigned unless you stick "signed" in front
of it.)</p>

<p>Locale support isn't currently a goal; that's a presentation layer issue
(I.E. a GUI problem).</p>

<p>Someday we should probably have translated --help text, but that's a
post-1.0 issue.</p>

<p><h3>Help text</h3></p>

<p>Each command's help text tries to briefly answer the questions "what does
this command do" and "how do I use it". There's a usage: line, basic
description, list of command line options (mostly in alphabetical order),
and sometimes additional explanation at the end. Default values and --longopts
are usually in parentheses on the end of an option's explanation line.</p>

<p>Toybox silently accepts a lot of compatibility flags like <b>patch -u</b>
that aren't in the help text to work with existing scripts, but may not
mention options that don't help write new scripts (mostly synonyms and NOPs).</p>

<p><h3>Shared Libraries</h3></p>

<p>Toybox's policy on shared libraries is that they should never be
required, but can optionally be used to improve performance.</p>

<p>Toybox should provide the command line utilities for
<a href=roadmap.html#dev_env>self-hosting development envirionments</a>,
and an easy way to set up "hermetic builds" (I.E. builds which provide
their own dependencies, isolating the build logic from host command version
skew with a simple known build environment). In both cases, external
dependencies defeat the purpose.</p>

<p>This means toybox should provide full functionality without relying
on any external dependencies (other than libc). But toybox may optionally use
libraries such as zlib and openssl to improve performance for things like
deflate and sha1sum, which lets the corresponding built-in implementations
be simple (and thus slow). But the built-in implementations need to exist and
work.</p>

<p>(This is why we use an external https wrapper program, because depending on
openssl or similar to be linked in would change the behavior of toybox.)</p>

<hr /><a name="license" /><h2>License</h2>

<p>Toybox is licensed <a href=license.html>0BSD</a>, which is a public domain
equivalent license approved by <a href=https://spdx.org/licenses/0BSD.html>SPDX</a>. This works like other BSD licenses except that it doesn't
require copying specific license text into the resulting project when
you copy code. (We care about attribution, not ownership, and the internet's
really good at pointing out plagiarism.)</p>

<p>This means toybox usually can't use external code contributions, and must
implement new versions of everything unless the external code's original
author (and any additional contributors) grants permission to relicense.
Just as a GPLv2 project can't incorporate GPLv3 code and a BSD-licensed
project can't incorporate either kind of GPL code, we can't incorporate
most BSD or Apache licensed code without changing our license terms.</p>

<p>The exception to this is code under an existing public domain equivalent
license, such as the xz decompressor or
<a href=https://github.com/mkj/dropbear/blob/master/libtommath/LICENSE>libtommath</a> and <a href=https://github.com/mkj/dropbear/blob/master/libtomcrypt/LICENSE>libtomcrypt</a>.</p>

<hr /><a name="codestyle" /><h2>Coding style</h2>

<p>The real coding style holy wars are over things that don't matter
(whitespace, indentation, curly bracket placement...) and thus have no
obviously correct answer. As in academia, "the fighting is so vicious because
the stakes are so small". That said, being consistent makes the code readable,
so here's how to make toybox code look like other toybox code.</p>

<p>Toybox source uses two spaces per indentation level, and wraps at 80
columns. (Indentation of continuation lines is awkward no matter what
you do, sometimes two spaces looks better, sometimes indenting to the
contents of a parentheses looks better.)</p>

<p>I'm aware this indentation style creeps some people out, so here's
the sed invocation to convert groups of two leading spaces to tabs:</p>
<blockquote><pre>
sed -i ':loop;s/^\( *\)  /\1\t/;t loop' filename
</pre></blockquote>

<p>And here's the sed invocation to convert leading tabs to two spaces each:</p>
<blockquote><pre>
sed -i ':loop;s/^\( *\)\t/\1  /;t loop' filename
</pre></blockquote>

<p>There's a space after C flow control statements that look like functions, so
"if (blah)" instead of "if(blah)". (Note that sizeof is actually an
operator, so we don't give it a space for the same reason ++ doesn't get
one. Yeah, it doesn't need the parentheses either, but it gets them.
These rules are mostly to make the code look consistent, and thus easier
to read.) We also put a space around assignment operators (on both sides),
so "int x = 0;".</p>

<p>Blank lines (vertical whitespace) go between thoughts. "We were doing that,
now we're doing this." (Not a hard and fast rule about _where_ it goes,
but there should be some for the same reason writing has paragraph breaks.)</p>

<p>Variable declarations go at the start of blocks, with a blank line between
them and other code. Yes, c99 allowed you to put them anywhere, but they're
harder to find if you do that. If there's a large enough distance between
the declaration and the code using it to make you uncomfortable, maybe the
function's too big, or is there an if statement or something you can
use as an excuse to start a new closer block? Use a longer variable name
that's easier to search for perhaps?</p>

<p>An * binds to a variable name not a type name, so space it that way.
(In C "char *a, b;" and "char* a, b;" mean the same thing: "a" is a pointer
but "b" is not. Spacing it the second way is not how C works.)</p>

<p>We wrap lines at 80 columns. Part of the reason for this I (toybox's
founder Rob) have mediocre eyesight (so tend to increase the font size in
terminal windows and web browsers), and program in a lot of coffee shops
on laptops with a smallish sceen. I'm aware this <a href=http://lkml.iu.edu/hypermail/linux/kernel/2005.3/08168.html>exasperates Linus torvalds</a>
(with his 8-character tab indents where just being in a function eats 8 chars
and 4 more indent levels eats half of an 80 column terminal), but you've
gotta break somewhere and even Linus admits there isn't another obvious
place to do so. (80 columns came from punched cards, which came
from civil war era dollar bill sorting boxes IBM founder Herman Hollerith
bought secondhand when bidding to run the 1890 census. "Totally arbitrary"
plus "100 yeas old" = standard.)</p>

<p>If statements with a single line body go on the same line when the result
fits in 80 columns, on a second line when it doesn't. We usually only use
curly brackets if we need to, either because the body is multiple lines or
because we need to distinguish which if an else binds to. Curly brackets go
on the same line as the test/loop statement. The exception to both cases is
if the test part of an if statement is long enough to split into multiple
lines, then we put the curly bracket on its own line afterwards (so it doesn't
get lost in the multple line variably indented mess), and we put it there
even if it's only grouping one line (because the indentation level is not
providing clear information in that case).</p>

<p>I.E.</p>

<blockquote>
<pre>
if (thingy) thingy;
else thingy;

if (thingy) {
  thingy;
  thingy;
} else thingy;

if (blah blah blah...
    && blah blah blah)
{
  thingy;
}
</pre></blockquote>

<p>Gotos are allowed for error handling, and for breaking out of
nested loops. In general, a goto should only jump forward (not back), and
should either jump to the end of an outer loop, or to error handling code
at the end of the function. Goto labels are never indented: they override the
block structure of the file. Putting them at the left edge makes them easy
to spot as overrides to the normal flow of control, which they are.</p>

<p>When there's a shorter way to say something, we tend to do that for
consistency. For example, we tend to say "*blah" instead of "blah[0]" unless
we're referring to more than one element of blah. Similarly, NULL is
really just 0 (and C will automatically typecast 0 to anything, except in
varargs), "if (function() != NULL)" is the same as "if (function())",
"x = (blah == NULL);" is "x = !blah;", and so on.</p>

<p>The goal is to be
concise, not cryptic: if you're worried about the code being hard to
understand, splitting it to multiple steps on multiple lines is
better than a NOP operation like "!= NULL". A common sign of trying too
hard is nesting ? : three levels deep, sometimes if/else and a temporary
variable is just plain easier to read. If you think you need a comment,
you may be right.</p>

<p>Comments are nice, but don't overdo it. Comments should explain _why_,
not how. If the code doesn't make the how part obvious, that's a problem with
the code. Sometimes choosing a better variable name is more revealing than a
comment. Comments on their own line are better than comments on the end of
lines, and they usually have a blank line before them. Most of toybox's
comments are c99 style // single line comments, even when there's more than
one of them. The /* multiline */ style is used at the start for the metadata,
but not so much in the code itself. They don't nest cleanly, are easy to leave
accidentally unterminated, need extra nonfunctional * to look right, and if
you need _that_ much explanation maybe what you really need is a URL citation
linking to a standards document? Long comments can fall out of sync with what
the code is doing. Comments do not get regression tested. There's no such
thing as self-documenting code (if nothing else, code with _no_ comments
is a bit unfriendly to new readers), but "chocolate sauce isn't the answer
to bad cooking" either. Don't use comments as a crutch to explain unclear
code if the code can be fixed.</p>

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