1<html><head><title>The design of toybox</title></head> 2<!--#include file="header.html" --> 3 4<h2>Topics</h2> 5<ul> 6<li><a href=#goals><h3>Design Goals</h3></a></li> 7<li><a href=#portability><h3>Portability Issues</h3></a></li> 8<li><a href=#license><h3>License</a></h3></a></li> 9<li><a href=#codestyle><h3>Coding Style</h3></a></li> 10</ul> 11<hr /> 12 13<a name="goals"><b><h2><a href="#goals">Design goals</a></h2></b> 14 15<p>Toybox should be simple, small, fast, and full featured. In that order.</p> 16 17<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 18before they really start to fight. 19When these goals need to be balanced off against each other, keeping the code 20as simple as it can be to do what it does is the most important (and hardest) 21goal. Then keeping it small is slightly more important than making it fast. 22Features are the reason we write code in the first place but this has all 23been implemented before so if we can't do a better job why bother?</p> 24 25<b><h3>Features</h3></b> 26 27<p>Toybox should provide the command line utilities of a build 28environment capable of recompiling itself under itself from source code. 29This minimal build system conceptually consists of 4 parts: toybox, 30a C library, a compiler, and a kernel. Toybox needs to provide all the 31commands (with all the behavior) necessary to run the configure/make/install 32of each package and boot the resulting system into a usable state.</p> 33 34<p>In addition, it should be possible to bootstrap up to arbitrary complexity 35under the result by compiling and installing additional packages into this 36minimal system, as measured by building both Linux From Scratch and the 37Android Open Source Project under the result. Any "circular dependencies" 38should be solved by toybox including the missing dependencies itself 39(see "Shared Libraries" below).</p> 40 41<p>Toybox may also provide some "convenience" utilties 42like top and vi that aren't necessarily used in a build but which turn 43the minimal build environment into a minimal development environment 44(supporting edit/compile/test cycles in a text console), configure 45network infrastructure for communication with other systems (in a build 46cluster), and so on.</p> 47 48<p>And these days toybox is the command line of Android, so anything the android 49guys say to do gets at the very least closely listened to.</p> 50 51<p>The hard part is deciding what NOT to include. A project without boundaries 52will bloat itself to death. One of the hardest but most important things a 53project must do is draw a line and say "no, this is somebody else's problem, 54not something we should do." 55Some things are simply outside the scope of the project: even though 56posix defines commands for compiling and linking, we're not going to include 57a compiler or linker (and support for a potentially infinite number of hardware 58targets). And until somebody comes up with a ~30k ssh implementation (with 59a crypto algorithm that won't need replacing every 5 years), we're 60going to point you at dropbear or bearssl.</p> 61 62<p>The <a href=roadmap.html>roadmap</a> has the list of features we're 63trying to implement, and the reasons why we decided to include those 64features. After the 1.0 release some of that material may get moved here, 65but for now it needs its own page. The <a href=status.html>status</a> 66page shows the project's progress against the roadmap.</p> 67 68<p>There are potential features (such as a screen/tmux implementation) 69that might be worth adding after 1.0, in part because they could share 70infrastructure with things like "less" and "vi" so might be less work for 71us to do than for an external from scratch implementation. But for now, major 72new features outside posix, android's existing commands, and the needs of 73development systems, are a distraction from the 1.0 release.</p> 74 75<b><h3>Speed</h3></b> 76 77<p>Quick smoketest: use the "time" command, and if you haven't got a test 78case that's embarassing enough to motivate digging, move on.</p> 79 80<p>It's easy to say a lot about optimizing for speed (which is why this section 81is so long), but at the same time it's the optimization we care the least about. 82The essence of speed is being as efficient as possible, which means doing as 83little work as possible. A design that's small and simple gets you 90% of the 84way there, and most of the rest is either fine-tuning or more trouble than 85it's worth (and often actually counterproductive). Still, here's some 86advice:</p> 87 88<p>First, understand the darn problem you're trying to solve. You'd think 89I wouldn't have to say this, and yet. Trying to find a faster sorting 90algorithm is no substitute for figuring out a way to skip the sorting step 91entirely. The fastest way to do anything is not to have to do it at all, 92and _all_ optimization boils down to avoiding unnecessary work.</p> 93 94<p>Speed is easy to measure; there are dozens of profiling tools for Linux, 95but sticking in calls to "millitime()" out of lib.c and subtracting 96(or doing two clock_gettime() calls and then nanodiff() on them) is 97quick and easy. Don't waste too much time trying to optimize something you 98can't measure, and there's no much point speeding up things you don't spend 99much time doing anyway.</p> 100 101<p>Understand the difference between throughput and latency. Faster 102processors improve throughput, but don't always do much for latency. 103After 30 years of Moore's Law, most of the remaining problems are latency, 104not throughput. (There are of course a few exceptions, like data compression 105code, encryption, rsync...) Worry about throughput inside long-running 106loops, and worry about latency everywhere else. (And don't worry too much 107about avoiding system calls or function calls or anything else in the name 108of speed unless you are in the middle of a tight loop that's you've already 109proven isn't running fast enough.)</p> 110 111<p>The lowest hanging optimization fruit is usually either "don't make 112unnecessary copies of data" or "use a reasonable block size in your 113I/O transactions instead of byte-at-a-time". 114Start by looking for those, most of the rest of this advice is just explaining 115why they're bad.</p> 116 117<p>"Locality of reference" is generally nice, in all sorts of contexts. 118It's obvious that waiting for disk access is 1000x slower than doing stuff in 119RAM (and making the disk seek is 10x slower than sequential reads/writes), 120but it's just as true that a loop which stays in L1 cache is many times faster 121than a loop that has to wait for a DRAM fetch on each iteration. Don't worry 122about whether "&" is faster than "%" until your executable loop stays in L1 123cache and the data access is fetching cache lines intelligently. (To 124understand DRAM, L1, and L2 cache, read Hannibal's marvelous ram guide at Ars 125Technica: 126<a href=http://arstechnica.com/paedia/r/ram_guide/ram_guide.part1-2.html>part one</a>, 127<a href=http://arstechnica.com/paedia/r/ram_guide/ram_guide.part2-1.html>part two</a>, 128<a href=http://arstechnica.com/paedia/r/ram_guide/ram_guide.part3-1.html>part three</a>, 129plus this 130<a href=http://arstechnica.com/articles/paedia/cpu/caching.ars/1>article on 131cacheing</a>, and this one on 132<a href=http://arstechnica.com/articles/paedia/cpu/bandwidth-latency.ars>bandwidth 133and latency</a>. 134And there's <a href=http://arstechnica.com/paedia/index.html>more where that came from</a>.) 135Running out of L1 cache can execute one instruction per clock cycle, going 136to L2 cache costs a dozen or so clock cycles, and waiting for a worst case dram 137fetch (round trip latency with a bank switch) can cost thousands of 138clock cycles. (Historically, this disparity has gotten worse with time, 139just like the speed hit for swapping to disk. These days, a _big_ L1 cache 140is 128k and a big L2 cache is a couple of megabytes. A cheap low-power 141embedded processor may have 8k of L1 cache and no L2.)</p> 142 143<p>Learn how <a href=http://nommu.org/memory-faq.txt>virtual memory and 144memory managment units work</a>. Don't touch 145memory you don't have to. Even just reading memory evicts stuff from L1 and L2 146cache, which may have to be read back in later. Writing memory can force the 147operating system to break copy-on-write, which allocates more memory. (The 148memory returned by malloc() is only a virtual allocation, filled with lots of 149copy-on-write mappings of the zero page. Actual physical pages get allocated 150when the copy-on-write gets broken by writing to the virtual page. This 151is why checking the return value of malloc() isn't very useful anymore, it 152only detects running out of virtual memory, not physical memory. Unless 153you're using a <a href=http://nommu.org>NOMMU system</a>, where all bets 154are off.)</p> 155 156<p>Don't think that just because you don't have a swap file the system can't 157start swap thrashing: any file backed page (ala mmap) can be evicted, and 158there's a reason all running programs require an executable file (they're 159mmaped, and can be flushed back to disk when memory is short). And long 160before that, disk cache gets reclaimed and has to be read back in. When the 161operating system really can't free up any more pages it triggers the out of 162memory killer to free up pages by killing processes (the alternative is the 163entire OS freezing solid). Modern operating systems seldom run out of 164memory gracefully.</p> 165 166<p>It's usually better to be simple than clever. Many people think that mmap() 167is faster than read() because it avoids a copy, but twiddling with the memory 168management is itself slow, and can cause unnecessary CPU cache flushes. And 169if a read faults in dozens of pages sequentially, but your mmap iterates 170backwards through a file (causing lots of seeks, each of which your program 171blocks waiting for), the read can be many times faster. On the other hand, the 172mmap can sometimes use less memory, since the memory provided by mmap 173comes from the page cache (allocated anyway), and it can be faster if you're 174doing a lot of different updates to the same area. The moral? Measure, then 175try to speed things up, and measure again to confirm it actually _did_ speed 176things up rather than made them worse. (And understanding what's really going 177on underneath is a big help to making it happen faster.)</p> 178 179<p>Another reason to be simple than clever is optimization 180strategies change with time. For example, decades ago precalculating a table 181of results (for things like isdigit() or cosine(int degrees)) was clearly 182faster because processors were so slow. Then processors got faster and grew 183math coprocessors, and calculating the value each time became faster than 184the table lookup (because the calculation fit in L1 cache but the lookup 185had to go out to DRAM). Then cache sizes got bigger (the Pentium M has 1862 megabytes of L2 cache) and the table fit in cache, so the table became 187fast again... Predicting how changes in hardware will affect your algorithm 188is difficult, and using ten year old optimization advice can produce 189laughably bad results. Being simple and efficient should give at least a 190reasonable starting point.</p> 191 192<p>Even at the design level, a lot of simple algorithms scale terribly but 193perform fine with small data sets. When small datasets are the common case, 194"better" versions that trade higher throughput for worse latency can 195consistently perform worse. 196So if you think you're only ever going to feed the algorithm small data sets, 197maybe just do the simple thing and wait for somebody to complain. For example, 198you probably don't need to sort and binary search the contents of 199/etc/passwd, because even 50k users is still a reasonably manageable data 200set for a readline/strcmp loop, and that's the userbase of a fairly major 201<a href=https://en.wikipedia.org/wiki/List_of_United_States_public_university_campuses_by_enrollment>university</a>. 202Instead commands like "ls" call bufgetpwuid() out of lib/lib.c 203which keeps a linked list of recently seen items, avoiding reparsing entirely 204and trusting locality of reference to bring up the same dozen or so entries 205for "ls -l /dev" or similar. The pathological failure mode of "simple 206linked list" is to perform exactly as badly as constantly rescanning a 207huge /etc/passwd, so this simple optimization shouldn't ever make performance 208worse (modulo possible memory exhaustion and thus swap thrashing). 209On the other hand, toybox's multiplexer does sort and binary 210search its command list to minimize the latency of each command startup, 211because the sort is a compile-time cost done once per build, 212and the whole of command startup 213is a "hot path" that should do as little work as possible because EVERY 214command has to go through it every time before performing any other function 215so tiny gains are worthwhile. (These decisions aren't perfect, the point is 216to show that thought went into them.)</p> 217 218<p>The famous quote from Ken Thompson, "When in doubt, use brute force", 219applies to toybox. Do the simple thing first, do as little of it as possible, 220and make sure it's right. You can always speed it up later.</p> 221 222<b><h3>Size</h3></b> 223<p>Quick smoketest: build toybox with and without the command (or the change), 224and maybe run "nm --size-sort" on files in generated/unstripped. 225(See make bloatcheck below for toybox's built in nm size diff-er.)</p> 226 227<p>Again, being simple gives you most of this. An algorithm that does less work 228is generally smaller. Understand the problem, treat size as a cost, and 229get a good bang for the byte.</p> 230 231<p>What "size" means depends on context: there are at least a half dozen 232different metrics in two broad categories: space used on disk/flash/ROM, 233and space used in memory at runtime.</p> 234 235<p>Your executable file has at least 236four main segments (text = executable code, rodata = read only data, 237data = writeable variables initialized to a value other than zero, 238bss = writeable data initialized to zero). Text and rodata are shared between multiple instances of the program running 239simultaneously, the other 4 aren't. Only text, rodata, and data take up 240space in the binary, bss, stack and heap only matter at runtime. You can 241view toybox's symbols with "nm generated/unstripped/toybox", the T/R/D/B 242lets you know the segment the symbol lives in. (Lowercase means it's 243local/static.)</p> 244 245<p>Then at runtime there's 246heap size (where malloc() memory lives) and stack size (where local 247variables and function call arguments and return addresses live). And 248on 32 bit systems mmap() can have a constrained amount of virtual memory 249(usually a couple gigabytes: the limits on 64 bit systems are generally big 250enough it doesn't come up)</p> 251 252<p>Optimizing for binary size is generally good: less code is less to go 253wrong, and executing fewer instructions makes your program run faster (and 254fits more of it in cache). On embedded systems, binary size is especially 255precious because flash is expensive and code may need binary auditing for 256security. Small stack size 257is important for nommu systems because they have to preallocate their stack 258and can't make it bigger via page fault. And everybody likes a small heap.</p> 259 260<p>Measure the right things. Especially with modern optimizers, expecting 261something to be smaller is no guarantee it will be after the compiler's done 262with it. Will total binary size is the final result, it isn't always the most 263accurate indicator of the impact of a given change, because lots of things 264get combined and rounded during compilation and linking (and things like 265ASAN disable optimization). Toybox has scripts/bloatcheck to compare two versions 266of a program and show size changes in each symbol (using "nm --size-sort"). 267You can "make baseline" to build a baseline version to compare against, 268and then apply your changes and "make bloatcheck" to compare against 269the saved baseline version.</p> 270 271<p>Avoid special cases. Whenever you see similar chunks of code in more than 272one place, it might be possible to combine them and have the users call shared 273code (perhaps out of lib/*.c). This is the most commonly cited trick, which 274doesn't make it easy to work out HOW to share. If seeing two lines of code do 275the same thing makes you slightly uncomfortable, you've got the right mindset, 276but "reuse" requires the "re" to have benefit, and infrastructure in search 277of a user will generally bit-rot before it finds one.</p> 278 279<p>The are a lot of potential microoptimizations (on some architectures 280using char instead of int as a loop index is noticeably slower, on some 281architectures C bitfields are surprisingly inefficient, & is often faster 282than % in a tight loop, conditional assignment avoids branch prediction 283failures...) but they're generally not worth doing unless you're trying to 284speed up the middle of a tight inner loop chewing through a large amount 285of data (such as a compression algorithm). For data pumps sane blocking 286and fewer system calls (buffer some input/output and do a big read/write 287instead of a bunch of little small ones) is usually the big win. But 288be careful about cacheing stuff: the two persistently hard problems in computer 289science are naming things, cache coherency, and off by one errors.</p> 290 291<b><h3>Simplicity</h3></b> 292 293<p>Complexity is a cost, just like code size or runtime speed. Treat it as 294a cost, and spend your complexity budget wisely. (Sometimes this means you 295can't afford a feature because it complicates the code too much to be 296worth it.)</p> 297 298<p>Simplicity has lots of benefits. Simple code is easy to maintain, easy to 299port to new processors, easy to audit for security holes, and easy to 300understand.</p> 301 302<p>Simplicity itself can have subtle non-obvious aspects requiring a tradeoff 303between one kind of simplicity and another: simple for the computer to 304execute and simple for a human reader to understand aren't always the 305same thing. A compact and clever algorithm that does very little work may 306not be as easy to explain or understand as a larger more explicit version 307requiring more code, memory, and CPU time. When balancing these, err on the 308side of doing less work, but add comments describing how you 309could be more explicit.</p> 310 311<p>In general, comments are not a substitute for good code (or well chosen 312variable or function names). Commenting "x += y;" with "/* add y to x */" 313can actually detract from the program's readability. If you need to describe 314what the code is doing (rather than _why_ it's doing it), that means the 315code itself isn't very clear.</p> 316 317<p>Environmental dependencies are another type of complexity, so needing other 318packages to build or run is a big downside. For example, we don't use curses 319when we can simply output ansi escape sequences and trust all terminal 320programs written in the past 30 years to be able to support them. Regularly 321testing that we work with C libraries which support static linking (musl does, 322glibc doesn't) is another way to be self-contained with known boundaries: 323it doesn't have to be the only way to build the project, but should be regularly 324tested and supported.</p> 325 326<p>Prioritizing simplicity tends to serve our other goals: simplifying code 327generally reduces its size (both in terms of binary size and runtime memory 328usage), and avoiding unnecessary work makes code run faster. Smaller code 329also tends to run faster on modern hardware due to CPU cacheing: fitting your 330code into L1 cache is great, and staying in L2 cache is still pretty good.</p> 331 332<p>But a simple implementation is not always the smallest or fastest, and 333balancing simplicity vs the other goals can be difficult. For example, the 334atolx_range() function in lib/lib.c always uses the 64 bit "long long" type, 335which produces larger and slower code on 32 bit platforms and 336often assigned into smaller interger types. Although libc has parallel 337implementations for different data sizes (atoi, atol, atoll) we chose a 338common codepath which can cover all cases (every user goes through the 339same codepath, with the maximum amount of testing and minimum and avoids 340surprising variations in behavior).</p> 341 342<p>On the other hand, the "tail" command has two codepaths, one for seekable 343files and one for nonseekable files. Although the nonseekable case can handle 344all inputs (and is required when input comes from a pipe or similar, so cannot 345be removed), reading through multiple gigabytes of data to reach the end of 346seekable files was both a common case and hugely penalized by a nonseekable 347approach (half-minute wait vs instant results). This is one example 348where performance did outweigh simplicity of implementation.</p> 349 350<p><a href=http://www.joelonsoftware.com/articles/fog0000000069.html>Joel 351Spolsky argues against throwing code out and starting over</a>, and he has 352good points: an existing debugged codebase contains a huge amount of baked 353in knowledge about strange real-world use cases that the designers didn't 354know about until users hit the bugs, and most of this knowledge is never 355explicitly stated anywhere except in the source code.</p> 356 357<p>That said, the Mythical Man-Month's "build one to throw away" advice points 358out that until you've solved the problem you don't properly understand it, and 359about the time you finish your first version is when you've finally figured 360out what you _should_ have done. (The corrolary is that if you build one 361expecting to throw it away, you'll actually wind up throwing away two. You 362don't understand the problem until you _have_ solved it.)</p> 363 364<p>Joel is talking about what closed source software can afford to do: Code 365that works and has been paid for is a corporate asset not lightly abandoned. 366Open source software can afford to re-implement code that works, over and 367over from scratch, for incremental gains. Before toybox, the unix command line 368has already been reimplemented from scratch several times (the 369original AT&T Unix command line in assembly and then in C, the BSD 370versions, Coherent was the first full from-scratch Unix clone in 1980, 371Minix was another clone which Linux was inspired by and developed under, 372the GNU tools were yet another rewrite intended for use in the stillborn 373"Hurd" project, BusyBox was still another rewrite, and more versions 374were written in Plan 9, uclinux, klibc, sash, sbase, s6, and of course 375android toolbox...). But maybe toybox can do a better job. :)</p> 376 377<p>As Antoine de St. Exupery (author of "The Little Prince" and an early 378aircraft designer) said, "Perfection is achieved, not when there 379is nothing left to add, but when there is nothing left to take away." 380And Ken Thompson (creator of Unix) said "One of my most productive 381days was throwing away 1000 lines of code." It's always possible to 382come up with a better way to do it.</p> 383 384<p>P.S. How could I resist linking to an article about 385<a href=http://blog.outer-court.com/archive/2005-08-24-n14.html>why 386programmers should strive to be lazy and dumb</a>?</p> 387 388<hr> 389<a name="portability"><b><h2><a href="#portability">Portability issues</a></h2></b> 390 391<b><h3>Platforms</h3></b> 392<p>Toybox should run on Android (all commands with musl-libc, as large a subset 393as practical with bionic), and every other hardware platform Linux runs on. 394Other posix/susv4 environments (perhaps MacOS X or newlib+libgloss) are vaguely 395interesting but only if they're easy to support; I'm not going to spend much 396effort on them.</p> 397 398<p>I don't do windows.</p> 399 400<a name="standards" /> 401<b><h3>Standards</h3></b> 402 403<p>Toybox is implemented with reference to 404<a href=http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1256.pdf>c99</a>, 405<a href=roadmap.html#susv4>Posix 2008</a>, 406<a href=#bits>LP64</a>, 407<a href=roadmap.html#sigh>LSB 4.1</a>, 408the <a href=https://www.kernel.org/doc/man-pages/>Linux man pages</a>, 409various <a href=https://www.rfc-editor.org/rfc-index.html>IETF RFCs</a>, 410the linux kernel source's 411<a href=https://www.kernel.org/doc/Documentation/>Documentation</a> directory, 412utf8 and unicode, and our terminal control outputs ANSI 413<a href=https://man7.org/linux/man-pages/man4/console_codes.4.html>escape sequences</a>. 414Toybox gets <a href=faq.html#cross>tested</a> with gcc and llvm on glibc, 415musl-libc, and bionic, plus occasional <a href=https://github.com/landley/toybox/blob/master/kconfig/freebsd_miniconfig>FreeBSD</a> and 416<a href=https://github.com/landley/toybox/blob/master/kconfig/macos_miniconfig>MacOS</a> builds for subsets 417of the commands.</p> 418 419<p>For the build environment and runtime environment, toybox depends on 420posix-2008 libc features such as the openat() family of 421functions. We also root around in the linux /proc directory a lot (no other 422way to implement "ps" at the moment), and assume certain "modern" linux kernel 423behavior (for example <a href=https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=b6a2fea39318>linux 2.6.22</a> 424expanded the 128k process environment size limit to 2 gigabytes, then it was 425trimmed back down to 10 megabytes, and when I asked for a way to query the 426actual value from the kernel if it was going to keep changing 427like that <a href=https://lkml.org/lkml/2017/11/5/204>Linus declined</a>). 428We make an effort to support <a href=faq.html#support_horizon>older kernels</a> 429and other implementations (primarily MacOS and BSD) but we don't always 430police their corner cases very closely.</p> 431 432<p><b>Why not just use the newest version of each standard?</b> 433 434<p>Partly to <a href=faq.html#support_horizon>support older systems</a>: 435you can't fix a bug in the old system if you can't build in the old 436environment.</p> 437 438<p>Partly because toybox's maintainer has his own corollary to Moore's law: 43950% of what you know about programming the hardware is obsolete every 18 440months, but the advantage of C & Unix it's usually the same 50% cycling 441out over and over.</p> 442 443<p>But mostly because the updates haven't added anything we care about. 444Posix-2008 switched some things to larger (64 bit) data types and added the 445openat() family of functions (which take a directory filehandle instead of 446using the Current Working Directory), 447but the 2013 and 2018 releases of posix were basically typo fixes: still 448release 7, still SUSv4. (An eventual release 8 might be interesting but 449it's not out yet.) We use C99 instead of C11 or newer because the new stuff 450was mostly about threading (atomic variables and such), and except for using 451// style single line comments we're more or less writing C89 code anyway. 452The main other new thing of interest in C99 was explicit width data 453types (uint32_t and friends), which LP64 handles for us.</p> 454 455<p>We're ignoring new versions of the Linux Foundation's standards (LSB, FHS) 456entirely, for the same reason Debian is: they're not good at maintaining 457standards. (The Linux Foundation acquiring the Free Standards Group worked 458out about as well as Microsoft buying Nokia.)</p> 459 460<p>We refer to current versions of man7.org because it's 461not easily versioned (the website updates regularly) and because 462Michael Kerrisk does a good job maintaining it so far. That said, we 463try to "provide new" in our commands but "depend on old" in our build scripts. 464(For example, we didn't start using "wait -n" until it had been in bash for 7 465years, and even then people depending on Centos' 10 year support horizon 466complained.)</p> 467 468<p>Using newer vs older RFCs, and upgrading between versions, is a per-case 469judgement call.</p> 470 471<p><b>How strictly do you adhere to these standards?</b> 472 473<p>...ish? The man pages have a lot of stuff that's not in posix, 474and there's no "init" or "mount" in posix, you can't implement "ps" 475without replying on non-posix APIs....</p> 476 477<p>When the options a command offers visibly contradict posix, we try to have 478a "deviations from posix" section at the top of the source listing the 479differences, but that's about what we provide not what we used from the OS 480or build environment.</p> 481 482<p>The build needs bash (not a pure-posix sh), and building on MacOS requires 483"gsed" (because Mac's sed is terrible), but toybox is explicitly self-hosting 484and any failure to build under the tool versions we provide would be a bug 485needing to be fixed.</p> 486 487<p>Within the code, everything in main.c and lib/*.c has to build 488on every supported Linux version, compiler, and library, plus BSD and MacOS. 489We mostly try to keep #if/else staircases for portability issues to 490lib/portability.[ch].</p> 491 492<p>Portability of individual commands varies: we sometimes program directly 493against linux kernel APIs (unavoidable when accessing /proc and /sys), 494individual commands are allowed to #include <linux/*.h> (common 495headers and library files are not, except maybe lib/portability.* within an 496appropriate #ifdef), we only really test against Linux errno values 497(unless somebody on BSD submits a bug), and a few commands outright cheat 498(the way ifconfig checks for ioctl numbers in the 0x89XX range). This is 499the main reason some commands build on BSD/MacOS and some don't.</p> 500 501<a name="bits" /> 502<b><h3>32/64 bit</h3></b> 503<p>Toybox should work on both 32 bit and 64 bit systems. 64 bit desktop 504hardware went mainstream <a href=https://web.archive.org/web/20040307000108mp_/http://developer.intel.com/technology/64bitextensions/faq.htm>in 2005</a> 505and was essentially ubiquitous <a href=faq.html#support_horizon>by 2012</a>, 506but 32 bit hardware will continue to be important in embedded devices for years to come.</p> 507 508<p>Toybox relies on the 509<a href=http://archive.opengroup.org/public/tech/aspen/lp64_wp.htm>LP64 standard</a> 510which Linux, MacOS X, and BSD all implement, and which modern 64 bit processors such as 511x86-64 were <a href=http://www.pagetable.com/?p=6>explicitly designed to 512support</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> 513 514<p>LP64 defines explicit sizes for all the basic C integer types, and 515guarantees that on any Unix-like platform "long" and "pointer" types 516are always the same size (the processor's register size). 517This means it's safe to assign pointers into 518longs and vice versa without losing data: on 32 bit systems both are 32 bit, 519on 64 bit systems both are 64 bit.</p> 520 521<table border=1 cellpadding=10 cellspacing=2> 522<tr><td>C type</td><td>char</td><td>short</td><td>int</td><td>long</td><td>long long</td></tr> 523<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> 524<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> 525</table> 526 527<p>LP64 eliminates the need to use c99 "uint32_t" and friends: the basic 528C types all have known size/behavior, and the only type whose 529size varies is "long", which is the natural register size of the processor.</p> 530 531<p>Note that Windows doesn't work like this, and I don't care, but if you're 532curious 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> 533 534<p>The main squishy bit in LP64 is that "long long" was defined as 535"at least" 64 bits instead of "exactly" 64 bits, and the standards body 536that issued it collapsed in the wake of the <a href=https://en.wikipedia.org/wiki/Unix_wars>proprietary unix wars</a> (all 537those lawsuits between AT&T/BSDI/Novell/Caldera/SCO), so is 538not available to issue an official correction. Then again a processor 539with 128-bit general purpose registers wouldn't be commercially viable 540<a href=https://landley.net/notes-2011.html#26-06-2011>until 2053</a> 541(because 2005+32*1.5), and with the S-curve of Moore's Law slowly 542<a href=http://www.acm.org/articles/people-of-acm/2016/david-patterson>bending back down</a> as 543atomic limits and <a href=http://www.cnet.com/news/end-of-moores-law-its-not-just-about-physics/>exponential cost increases</a> produce increasing 544drag.... (The original Moore's Law curve would mean that in the year 2022 545a high end workstation would have around 8 terabytes of RAM, available retail. 546Most don't even come with 547that much disk space.) At worst we don't need to care for decades, the 548S-curve bending down means probably not in our lifetimes, and 549atomic limits may mean "never". So I'm ok treating "long long" as exactly 64 bits.</p> 550 551<b><h3>Signedness of char</h3></b> 552<p>On platforms like x86, variables of type char default to unsigned. On 553platforms like arm, char defaults to signed. This difference can lead to 554subtle portability bugs, and to avoid them we specify which one we want by 555feeding the compiler -funsigned-char.</p> 556 557<p>The reason to pick "unsigned" is that way char strings are 8-bit clean by 558default, which makes UTF-8 support easier.</p> 559 560<p><h3>Error messages and internationalization:</h3></p> 561 562<p>Error messages are extremely terse not just to save bytes, but because we 563don't use any sort of _("string") translation infrastructure. (We're not 564translating the command names themselves, so we must expect a minimum amount of 565english knowledge from our users, but let's keep it to a minimum.)</p> 566 567<p>Thus "bad -A '%c'" is 568preferable to "Unrecognized address base '%c'", because a non-english speaker 569can see that -A was the problem (giving back the command line argument they 570supplied). A user with a ~20 word english vocabulary is 571more likely to know (or guess) "bad" than the longer message, and you can 572use "bad" in place of "invalid", "inappropriate", "unrecognized"... 573Similarly when atolx_range() complains about range constraints with 574"4 < 17" or "12 > 5", it's intentional: those don't need to be translated.</p> 575 576<p>The strerror() messages produced by perror_exit() and friends should be 577localized by libc, and our error functions also prepend the command name 578(which non-english speakers can presumably recognize already). Keep the 579explanation in between to a minimum, and where possible feed back the values 580they passed in to identify _what_ we couldn't process. 581If you say perror_exit("setsockopt"), you've identified the action you 582were trying to take, and the perror gives a translated error message (from libc) 583explaining _why_ it couldn't do it, so you probably don't need to add english 584words like "failed" or "couldn't assign".</p> 585 586<p>All commands should be 8-bit clean, with explicit 587<a href=http://yarchive.net/comp/linux/utf8.html>UTF-8</a> support where 588necessary. Assume all input data might be utf8, and at least preserve 589it and pass it through. (For this reason, our build is -funsigned-char on 590all architectures; "char" is unsigned unless you stick "signed" in front 591of it.)</p> 592 593<p>Locale support isn't currently a goal; that's a presentation layer issue 594(I.E. a GUI problem).</p> 595 596<p>Someday we should probably have translated --help text, but that's a 597post-1.0 issue.</p> 598 599<p><h3>Shared Libraries</h3></p> 600 601<p>Toybox's policy on shared libraries is that they should never be 602required, but can optionally be used to improve performance.</p> 603 604<p>Toybox should provide the command line utilities for 605<a href=roadmap.html#dev_env>self-hosting development envirionments</a>, 606and an easy way to set up "hermetic builds" (I.E. builds which provide 607their own dependencies, isolating the build logic from host command version 608skew with a simple known build environment). In both cases, external 609dependencies defeat the purpose.</p> 610 611<p>This means toybox should provide full functionality without relying 612on any external dependencies (other than libc). But toybox may optionally use 613libraries such as zlib and openssl to improve performance for things like 614deflate and sha1sum, which lets the corresponding built-in implementations 615be simple (and thus slow). But the built-in implementations need to exist and 616work.</p> 617 618<p>(This is why we use an external https wrapper program, because depending on 619openssl or similar to be linked in would change the behavior of toybox.)</p> 620 621<hr /><a name="license" /><h2>License</h2> 622 623<p>Toybox is licensed <a href=license.html>0BSD</a>, which is a public domain 624equivalent license approved by <a href=https://spdx.org/licenses/0BSD.html>SPDX</a>. This works like other BSD licenses except that it doesn't 625require copying specific license text into the resulting project when 626you copy code. (We care about attribution, not ownership, and the internet's 627really good at pointing out plagiarism.)</p> 628 629<p>This means toybox usually can't use external code contributions, and must 630implement new versions of everything unless the external code's original 631author (and any additional contributors) grants permission to relicense. 632Just as a GPLv2 project can't incorporate GPLv3 code and a BSD-licensed 633project can't incorporate either kind of GPL code, we can't incorporate 634most BSD or Apache licensed code without changing our license terms.</p> 635 636<p>The exception to this is code under an existing public domain equivalent 637license, such as the xz decompressor or 638<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> 639 640<hr /><a name="codestyle" /><h2>Coding style</h2> 641 642<p>The real coding style holy wars are over things that don't matter 643(whitespace, indentation, curly bracket placement...) and thus have no 644obviously correct answer. As in academia, "the fighting is so vicious because 645the stakes are so small". That said, being consistent makes the code readable, 646so here's how to make toybox code look like other toybox code.</p> 647 648<p>Toybox source uses two spaces per indentation level, and wraps at 80 649columns. (Indentation of continuation lines is awkward no matter what 650you do, sometimes two spaces looks better, sometimes indenting to the 651contents of a parentheses looks better.)</p> 652 653<p>I'm aware this indentation style creeps some people out, so here's 654the sed invocation to convert groups of two leading spaces to tabs:</p> 655<blockquote><pre> 656sed -i ':loop;s/^\( *\) /\1\t/;t loop' filename 657</pre></blockquote> 658 659<p>And here's the sed invocation to convert leading tabs to two spaces each:</p> 660<blockquote><pre> 661sed -i ':loop;s/^\( *\)\t/\1 /;t loop' filename 662</pre></blockquote> 663 664<p>There's a space after C flow control statements that look like functions, so 665"if (blah)" instead of "if(blah)". (Note that sizeof is actually an 666operator, so we don't give it a space for the same reason ++ doesn't get 667one. Yeah, it doesn't need the parentheses either, but it gets them. 668These rules are mostly to make the code look consistent, and thus easier 669to read.) We also put a space around assignment operators (on both sides), 670so "int x = 0;".</p> 671 672<p>Blank lines (vertical whitespace) go between thoughts. "We were doing that, 673now we're doing this." (Not a hard and fast rule about _where_ it goes, 674but there should be some for the same reason writing has paragraph breaks.)</p> 675 676<p>Variable declarations go at the start of blocks, with a blank line between 677them and other code. Yes, c99 allows you to put them anywhere, but they're 678harder to find if you do that. If there's a large enough distance between 679the declaration and the code using it to make you uncomfortable, maybe the 680function's too big, or is there an if statement or something you can 681use as an excuse to start a new closer block? Use a longer variable name 682that's easier to search for perhaps?</p> 683 684<p>An * binds to a variable name not a type name, so space it that way. 685(In C "char *a, b;" and "char* a, b;" mean the same thing: "a" is a pointer 686but "b" is not. Spacing it the second way is not how C works.)</p> 687 688<p>We wrap lines at 80 columns. Part of the reason for this I (toybox's 689founder Rob) have mediocre eyesight (so tend to increase the font size in 690terminal windows and web browsers), and program in a lot of coffee shops 691on 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> 692(with his 8-character tab indents where just being in a function eats 8 chars 693and 4 more indent levels eats half of an 80 column terminal), but you've 694gotta break somewhere and even Linus admits there isn't another obvious 695place to do so. (80 columns came from punched cards, which came 696from civil war era dollar bill sorting boxes IBM founder Herman Hollerith 697bought secondhand when bidding to run the 1890 census. "Totally arbitrary" 698plus "100 yeas old" = standard.)</p> 699 700<p>If statements with a single line body go on the same line when the result 701fits in 80 columns, on a second line when it doesn't. We usually only use 702curly brackets if we need to, either because the body is multiple lines or 703because we need to distinguish which if an else binds to. Curly brackets go 704on the same line as the test/loop statement. The exception to both cases is 705if the test part of an if statement is long enough to split into multiple 706lines, then we put the curly bracket on its own line afterwards (so it doesn't 707get lost in the multple line variably indented mess), and we put it there 708even if it's only grouping one line (because the indentation level is not 709providing clear information in that case).</p> 710 711<p>I.E.</p> 712 713<blockquote> 714<pre> 715if (thingy) thingy; 716else thingy; 717 718if (thingy) { 719 thingy; 720 thingy; 721} else thingy; 722 723if (blah blah blah... 724 && blah blah blah) 725{ 726 thingy; 727} 728</pre></blockquote> 729 730<p>Gotos are allowed for error handling, and for breaking out of 731nested loops. In general, a goto should only jump forward (not back), and 732should either jump to the end of an outer loop, or to error handling code 733at the end of the function. Goto labels are never indented: they override the 734block structure of the file. Putting them at the left edge makes them easy 735to spot as overrides to the normal flow of control, which they are.</p> 736 737<p>When there's a shorter way to say something, we tend to do that for 738consistency. For example, we tend to say "*blah" instead of "blah[0]" unless 739we're referring to more than one element of blah. Similarly, NULL is 740really just 0 (and C will automatically typecast 0 to anything, except in 741varargs), "if (function() != NULL)" is the same as "if (function())", 742"x = (blah == NULL);" is "x = !blah;", and so on.</p> 743 744<p>The goal is to be 745concise, not cryptic: if you're worried about the code being hard to 746understand, splitting it to multiple steps on multiple lines is 747better than a NOP operation like "!= NULL". A common sign of trying too 748hard is nesting ? : three levels deep, sometimes if/else and a temporary 749variable is just plain easier to read. If you think you need a comment, 750you may be right.</p> 751 752<p>Comments are nice, but don't overdo it. Comments should explain _why_, 753not how. If the code doesn't make the how part obvious, that's a problem with 754the code. Sometimes choosing a better variable name is more revealing than a 755comment. Comments on their own line are better than comments on the end of 756lines, and they usually have a blank line before them. Most of toybox's 757comments are c99 style // single line comments, even when there's more than 758one of them. The /* multiline */ style is used at the start for the metadata, 759but not so much in the code itself. They don't nest cleanly, are easy to leave 760accidentally unterminated, need extra nonfunctional * to look right, and if 761you need _that_ much explanation maybe what you really need is a URL citation 762linking to a standards document? Long comments can fall out of sync with what 763the code is doing. Comments do not get regression tested. There's no such 764thing as self-documenting code (if nothing else, code with _no_ comments 765is a bit unfriendly to new readers), but "chocolate sauce isn't the answer 766to bad cooking" either. Don't use comments as a crutch to explain unclear 767code if the code can be fixed.</p> 768 769<!--#include file="footer.html" --> 770