Design goals

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

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?

It should be possible to get 80% of the way to each goal before they really start to fight. Here they are in reverse order of importance:

Features

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.

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).

Finally, toybox may 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.

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

It's easy to say lots 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, but I do. 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 (although personally I find the "time" command a good starting place). 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.)

"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.

Also, it's 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.)

In general, being simple is better than being clever. 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 and produce laughably bad results. But being simple and efficient is always going to give at least a reasonable result.

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

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.

Understand the difference between binary size, heap size, and stack size. Your binary is the executable file on disk, your heap is where malloc() memory lives, and your stack is where local variables (and function call return addresses) live. Optimizing for binary size is generally good: 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 its successor, MRAM, even more so). 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. Binary size isn't the most accurate indicator of the impact of a given change, because lots of things get combined and rounded during compilation and linking. Matt Mackall's bloat-o-meter is a python script which compares two versions of a program, and shows size changes in each symbol (using the "nm" command behind the scenes). To use this, run "make baseline" to build a baseline version to compare against, and then "make bloatometer" to compare that baseline version against the current code.

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. (This is the most commonly cited trick, which doesn't make it easy. If seeing two lines of code do the same thing makes you slightly uncomfortable, you've got the right mindset.)

Some specific advice: Using a char in place of an int when doing math produces significantly larger code on some platforms (notably arm), because each time the compiler has to emit code to convert it to int, do the math, and convert it back. Bitfields have this problem on most platforms. Because of this, using char to index a for() loop is probably not a net win, although using char (or a bitfield) to store a value in a structure that's repeated hundreds of times can be a good tradeoff of binary size for heap space.

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.

We depend on C99 and 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 such as large environment sizes (linux commit b6a2fea39318, went into 2.6.22 released July 2007, expanding the 128k limit to 2 gigabytes. But it was then 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). In theory this shouldn't prevent us from working on older kernels or other implementations (ala BSD), but we don't police their corner cases.

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 the end of the decade, but 32 bit hardware will continue to be important in embedded devices for several more years.

Toybox relies on the fact that on any Unix-like platform, pointer and long are always the same size (on both 32 and 64 bit). Pointer and int are _not_ the same size on 64 bit systems, but pointer and long are. This is guaranteed by the LP64 memory model, a Unix standard (which Linux and MacOS X both implement, and which modern 64 bit processors such as x86-64 were designed for).

Back before unix.org went down, they hosted the LP64 standard and the LP64 rationale, but the important part is LP64 gives all the basic C integer types defined sizes:

C type 32 bit

sizeof 64 bit

sizeof char 1 byte 1 byte short 2 bytes 2 bytes int 4 bytes 4 bytes long 4 bytes 8 bytes long long 8 bytes 8 bytes

Note that Windows doesn't work like this, and I don't care. The insane legacy reasons why this is broken on Windows are explained here.

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.)

If statements with a single line body go on the same line if the result fits in 80 columns, on a second line if 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.