toybox

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Design goals

Toybox should be simple, small, fast, and full featured. In that order.

It should be possible to get about 80% of the way 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?

Features

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.

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).

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.

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.

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.

The roadmap 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 status page shows the project's progress against the roadmap.

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.

Speed

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

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:

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.

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() cals 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.

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.)

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.

"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: part one, part two, part three, plus this article on cacheing, and this one on bandwidth and latency. And there's more where that came from.) 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.)

Learn how virtual memory and memory managment units work. 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 NOMMU system, where all bets are off.)

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.

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.)

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.

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 university. 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.)

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.

Size

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.)

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.

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.

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.)

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)

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.

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.

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.

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 had problems in computer science are naming things, cache coherency, and off by one errors.

Simplicity

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.)

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.

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.

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.

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.

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.

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).

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.

Joel Spolsky argues against throwing code out and starting over, 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.

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.)

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&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. :)

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.S. How could I resist linking to an article about why programmers should strive to be lazy and dumb?


Portability issues

Platforms

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.

I don't do windows.

Standards

Toybox is implemented with reference to c99, Posix 2008, LP64, LSB 4.1, the Linux man pages, various IETF RFCs, the linux kernel source's Documentation directory, utf8 and unicode, and our terminal control outputs ANSI escape sequences. Toybox gets tested with gcc and llvm on glibc, musl-libc, and bionic, plus occasional FreeBSD and MacOS builds for subsets of the commands.

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 linux 2.6.22 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 Linus declined). We make an effort to support older kernels and other implementations (primarily MacOS and BSD) but we don't always police their corner cases very closely.

Why not just use the newest version of each standard?

Partly to support older systems: you can't fix a bug in the old system if you can't build in the old enviornment.

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

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.) We use C99 instead of C11 or newer because the new stuff was mostly about threading (atomic variables and such), and except for using // style single line comments we're more or less writing C89 code anyway. The main other new thing of interest in C99 was explicit width data types (uint32_t and friends), which LP64 handles for us.

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.)

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.)

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

How strictly do you adhere to these standards?

...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....

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.

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.

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].

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 <linux/*.h> (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.

32/64 bit

Toybox should work on both 32 bit and 64 bit systems. 64 bit desktop hardware went mainstream in 2005 and was essentially ubiquitous by 2012, but 32 bit hardware will continue to be important in embedded devices for years to come.

Toybox relies on the LP64 standard which Linux, MacOS X, and BSD all implement, and which modern 64 bit processors such as x86-64 were explicitly designed to support. (Here's the original LP64 white paper.)

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.

C typecharshortintlonglong long
32 bit
sizeof
8 bits16 bits32 bits32 bits64 bits
64 bit
sizeof
8 bits16 bits32 bits64 bits64 bits

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.

Note that Windows doesn't work like this, and I don't care, but if you're curious here are the insane legacy reasons why this is broken on Windows.

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 proprietary unix wars (all those lawsuits between AT&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 until 2053 (because 2005+32*1.5), and with the S-curve of Moore's Law slowly bending back down as atomic limits and exponential cost increases 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.

Signedness of char

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.

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

Error messages and internationalization:

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.)

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.

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".

All commands should be 8-bit clean, with explicit UTF-8 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.)

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

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

Shared Libraries

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

Toybox should provide the command line utilities for self-hosting development envirionments, 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.

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.

(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.)


License

Toybox is licensed 0BSD, which is a public domain equivalent license approved by SPDX. 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.)

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.

The exception to this is code under an existing public domain equivalent license, such as the xz decompressor or libtommath and libtomcrypt.


Coding style

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.

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.)

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:

sed -i ':loop;s/^\( *\)  /\1\t/;t loop' filename

And here's the sed invocation to convert leading tabs to two spaces each:

sed -i ':loop;s/^\( *\)\t/\1  /;t loop' filename

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;".

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.)

Variable declarations go at the start of blocks, with a blank line between them and other code. Yes, c99 allows 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?

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.)

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 exasperates Linus torvalds (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.)

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).

I.E.

if (thingy) thingy;
else thingy;

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

if (blah blah blah...
    && blah blah blah)
{
  thingy;
}

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.

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.

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.

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.