1 <html><head><title>The design of toybox</title></head>
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4 <b><h2>Design goals</h2></b>
6 <p>Toybox should be simple, small, fast, and full featured. Often, these
7 things need to be balanced off against each other. In general, keeping the
8 code simple the most important (and hardest) goal, and small is slightly more
9 important than fast. Features are the reason we write code in the first
10 place but this has all been implemented before so if we can't do a better
11 job why bother? It should be possible to get 80% of the way to each goal
12 before they really start to fight.</p>
14 <p>Here they are in reverse order of importance:</p>
16 <b><h3>Features</h3></b>
18 <p>The <a href=roadmap.html>roadmap</a> has the list of features we're
19 trying to implement, and the reasons for them. After the 1.0 release
20 some of that material may get moved here.</p>
22 <p>Some things are simply outside the scope of the project: even though
23 posix defines commands for compiling and linking, we're not going to include
24 a compiler or linker (and support for a potentially infinite number of hardware
25 targets). And until somebody comes up with a ~30k ssh implementation, we're
26 going to point you at dropbear or polarssl.</p>
28 <p>Environmental dependencies are a type of complexity, so needing other
29 packages to build or run is a big downside. For example, we don't use curses
30 when we can simply output ansi escape sequences and trust all terminal
31 programs written in the past 30 years to be able to support them. (A common
32 use case is to download a statically linked toybox binary to an arbitrary
33 Linux system, and use it in an otherwise unknown environment; being
34 self-contained helps support this.)</p>
38 <p>It's easy to say lots about optimizing for speed (which is why this section
39 is so long), but at the same time it's the optimization we care the least about.
40 The essence of speed is being as efficient as possible, which means doing as
41 little work as possible. A design that's small and simple gets you 90% of the
42 way there, and most of the rest is either fine-tuning or more trouble than
43 it's worth (and often actually counterproductive). Still, here's some
46 <p>First, understand the darn problem you're trying to solve. You'd think
47 I wouldn't have to say this, but I do. Trying to find a faster sorting
48 algorithm is no substitute for figuring out a way to skip the sorting step
49 entirely. The fastest way to do anything is not to have to do it at all,
50 and _all_ optimization boils down to avoiding unnecessary work.</p>
52 <p>Speed is easy to measure; there are dozens of profiling tools for Linux
53 (although personally I find the "time" command a good starting place).
54 Don't waste too much time trying to optimize something you can't measure,
55 and there's no much point speeding up things you don't spend much time doing
58 <p>Understand the difference between throughput and latency. Faster
59 processors improve throughput, but don't always do much for latency.
60 After 30 years of Moore's Law, most of the remaining problems are latency,
61 not throughput. (There are of course a few exceptions, like data compression
62 code, encryption, rsync...) Worry about throughput inside long-running
63 loops, and worry about latency everywhere else. (And don't worry too much
64 about avoiding system calls or function calls or anything else in the name
65 of speed unless you are in the middle of a tight loop that's you've already
66 proven isn't running fast enough.)</p>
68 <p>"Locality of reference" is generally nice, in all sorts of contexts.
69 It's obvious that waiting for disk access is 1000x slower than doing stuff in
70 RAM (and making the disk seek is 10x slower than sequential reads/writes),
71 but it's just as true that a loop which stays in L1 cache is many times faster
72 than a loop that has to wait for a DRAM fetch on each iteration. Don't worry
73 about whether "&" is faster than "%" until your executable loop stays in L1
74 cache and the data access is fetching cache lines intelligently. (To
75 understand DRAM, L1, and L2 cache, read Hannibal's marvelous ram guide at Ars
77 <a href=http://arstechnica.com/paedia/r/ram_guide/ram_guide.part1-2.html>part one</a>,
78 <a href=http://arstechnica.com/paedia/r/ram_guide/ram_guide.part2-1.html>part two</a>,
79 <a href=http://arstechnica.com/paedia/r/ram_guide/ram_guide.part3-1.html>part three</a>,
81 <a href=http://arstechnica.com/articles/paedia/cpu/caching.ars/1>article on
82 cacheing</a>, and this one on
83 <a href=http://arstechnica.com/articles/paedia/cpu/bandwidth-latency.ars>bandwidth
85 And there's <a href=http://arstechnica.com/paedia/index.html>more where that came from</a>.)
86 Running out of L1 cache can execute one instruction per clock cycle, going
87 to L2 cache costs a dozen or so clock cycles, and waiting for a worst case dram
88 fetch (round trip latency with a bank switch) can cost thousands of
89 clock cycles. (Historically, this disparity has gotten worse with time,
90 just like the speed hit for swapping to disk. These days, a _big_ L1 cache
91 is 128k and a big L2 cache is a couple of megabytes. A cheap low-power
92 embedded processor may have 8k of L1 cache and no L2.)</p>
94 <p>Learn how virtual memory and memory managment units work. Don't touch
95 memory you don't have to. Even just reading memory evicts stuff from L1 and L2
96 cache, which may have to be read back in later. Writing memory can force the
97 operating system to break copy-on-write, which allocates more memory. (The
98 memory returned by malloc() is only a virtual allocation, filled with lots of
99 copy-on-write mappings of the zero page. Actual physical pages get allocated
100 when the copy-on-write gets broken by writing to the virtual page. This
101 is why checking the return value of malloc() isn't very useful anymore, it
102 only detects running out of virtual memory, not physical memory. Unless
103 you're using a NOMMU system, where all bets are off.)</p>
105 <p>Don't think that just because you don't have a swap file the system can't
106 start swap thrashing: any file backed page (ala mmap) can be evicted, and
107 there's a reason all running programs require an executable file (they're
108 mmaped, and can be flushed back to disk when memory is short). And long
109 before that, disk cache gets reclaimed and has to be read back in. When the
110 operating system really can't free up any more pages it triggers the out of
111 memory killer to free up pages by killing processes (the alternative is the
112 entire OS freezing solid). Modern operating systems seldom run out of
113 memory gracefully.</p>
115 <p>Also, it's better to be simple than clever. Many people think that mmap()
116 is faster than read() because it avoids a copy, but twiddling with the memory
117 management is itself slow, and can cause unnecessary CPU cache flushes. And
118 if a read faults in dozens of pages sequentially, but your mmap iterates
119 backwards through a file (causing lots of seeks, each of which your program
120 blocks waiting for), the read can be many times faster. On the other hand, the
121 mmap can sometimes use less memory, since the memory provided by mmap
122 comes from the page cache (allocated anyway), and it can be faster if you're
123 doing a lot of different updates to the same area. The moral? Measure, then
124 try to speed things up, and measure again to confirm it actually _did_ speed
125 things up rather than made them worse. (And understanding what's really going
126 on underneath is a big help to making it happen faster.)</p>
128 <p>In general, being simple is better than being clever. Optimization
129 strategies change with time. For example, decades ago precalculating a table
130 of results (for things like isdigit() or cosine(int degrees)) was clearly
131 faster because processors were so slow. Then processors got faster and grew
132 math coprocessors, and calculating the value each time became faster than
133 the table lookup (because the calculation fit in L1 cache but the lookup
134 had to go out to DRAM). Then cache sizes got bigger (the Pentium M has
135 2 megabytes of L2 cache) and the table fit in cache, so the table became
136 fast again... Predicting how changes in hardware will affect your algorithm
137 is difficult, and using ten year old optimization advice and produce
138 laughably bad results. But being simple and efficient is always going to
139 give at least a reasonable result.</p>
141 <p>The famous quote from Ken Thompson, "When in doubt, use brute force",
142 applies to toybox. Do the simple thing first, do as little of it as possible,
143 and make sure it's right. You can always speed it up later.</p>
146 <p>Again, simple gives you most of this. An algorithm that does less work
147 is generally smaller. Understand the problem, treat size as a cost, and
148 get a good bang for the byte.</p>
150 <p>Understand the difference between binary size, heap size, and stack size.
151 Your binary is the executable file on disk, your heap is where malloc() memory
152 lives, and your stack is where local variables (and function call return
153 addresses) live. Optimizing for binary size is generally good: executing
154 fewer instructions makes your program run faster (and fits more of it in
155 cache). On embedded systems, binary size is especially precious because
156 flash is expensive (and its successor, MRAM, even more so). Small stack size
157 is important for nommu systems because they have to preallocate their stack
158 and can't make it bigger via page fault. And everybody likes a small heap.</p>
160 <p>Measure the right things. Especially with modern optimizers, expecting
161 something to be smaller is no guarantee it will be after the compiler's done
162 with it. Binary size isn't the most accurate indicator of the impact of a
163 given change, because lots of things get combined and rounded during
164 compilation and linking. Matt Mackall's bloat-o-meter is a python script
165 which compares two versions of a program, and shows size changes in each
166 symbol (using the "nm" command behind the scenes). To use this, run
167 "make baseline" to build a baseline version to compare against, and
168 then "make bloatometer" to compare that baseline version against the current
171 <p>Avoid special cases. Whenever you see similar chunks of code in more than
172 one place, it might be possible to combine them and have the users call shared
173 code. (This is the most commonly cited trick, which doesn't make it easy. If
174 seeing two lines of code do the same thing makes you slightly uncomfortable,
175 you've got the right mindset.)</p>
177 <p>Some specific advice: Using a char in place of an int when doing math
178 produces significantly larger code on some platforms (notably arm),
179 because each time the compiler has to emit code to convert it to int, do the
180 math, and convert it back. Bitfields have this problem on most platforms.
181 Because of this, using char to index a for() loop is probably not a net win,
182 although using char (or a bitfield) to store a value in a structure that's
183 repeated hundreds of times can be a good tradeoff of binary size for heap
186 <b><h3>Simple</h3></b>
188 <p>Complexity is a cost, just like code size or runtime speed. Treat it as
189 a cost, and spend your complexity budget wisely. (Sometimes this means you
190 can't afford a feature because it complicates the code too much to be
193 <p>Simplicity has lots of benefits. Simple code is easy to maintain, easy to
194 port to new processors, easy to audit for security holes, and easy to
197 <p>Simplicity itself can have subtle non-obvious aspects requiring a tradeoff
198 between one kind of simplicity and another: simple for the computer to
199 execute and simple for a human reader to understand aren't always the
200 same thing. A compact and clever algorithm that does very little work may
201 not be as easy to explain or understand as a larger more explicit version
202 requiring more code, memory, and CPU time. When balancing these, err on the
203 side of doing less work, but add comments describing how you
204 could be more explicit.</p>
206 <p>In general, comments are not a substitute for good code (or well chosen
207 variable or function names). Commenting "x += y;" with "/* add y to x */"
208 can actually detract from the program's readability. If you need to describe
209 what the code is doing (rather than _why_ it's doing it), that means the
210 code itself isn't very clear.</p>
212 <p>Prioritizing simplicity tends to serve our other goals: simplifying code
213 generally reduces its size (both in terms of binary size and runtime memory
214 usage), and avoiding unnecessary work makes code run faster. Smaller code
215 also tends to run faster on modern hardware due to CPU cacheing: fitting your
216 code into L1 cache is great, and staying in L2 cache is still pretty good.</p>
218 <p><a href=http://www.joelonsoftware.com/articles/fog0000000069.html>Joel
219 Spolsky argues against throwing code out and starting over</a>, and he has
220 good points: an existing debugged codebase contains a huge amount of baked
221 in knowledge about strange real-world use cases that the designers didn't
222 know about until users hit the bugs, and most of this knowledge is never
223 explicitly stated anywhere except in the source code.</p>
225 <p>That said, the Mythical Man-Month's "build one to throw away" advice points
226 out that until you've solved the problem you don't properly understand it, and
227 about the time you finish your first version is when you've finally figured
228 out what you _should_ have done. (The corrolary is that if you build one
229 expecting to throw it away, you'll actually wind up throwing away two. You
230 don't understand the problem until you _have_ solved it.)</p>
232 <p>Joel is talking about what closed source software can afford to do: Code
233 that works and has been paid for is a corporate asset not lightly abandoned.
234 Open source software can afford to re-implement code that works, over and
235 over from scratch, for incremental gains. Before toybox, the unix command line
236 has already been reimplemented from scratch several times in a row (the
237 original AT&T Unix command line in assembly and then in C, the BSD
238 versions, the GNU tools, BusyBox...) but maybe toybox can do a better job. :)</p>
240 <p>P.S. How could I resist linking to an article about
241 <a href=http://blog.outer-court.com/archive/2005-08-24-n14.html>why
242 programmers should strive to be lazy and dumb</a>?</p>
244 <b><h2>Portability issues</h2></b>
246 <b><h3>Platforms</h3></b>
247 <p>Toybox should run on Android (all commands with musl-libc, as large a subset
248 as practical with bionic), and every other hardware platform Linux runs on.
249 Other posix/susv4 environments (perhaps MacOS X or newlib+libgloss) are vaguely
250 interesting but only if they're easy to support; I'm not going to spend much
253 <p>I don't do windows.</p>
255 <b><h3>32/64 bit</h3></b>
256 <p>Toybox should work on both 32 bit and 64 bit systems. By the end of 2008
257 64 bit hardware will be the new desktop standard, but 32 bit hardware will
258 continue to be important in embedded devices for years to come.</p>
260 <p>Toybox relies on the fact that on any Unix-like platform, pointer and long
261 are always the same size (on both 32 and 64 bit). Pointer and int are _not_
262 the same size on 64 bit systems, but pointer and long are.</p>
264 <p>This is guaranteed by the LP64 memory model, a Unix standard (which Linux
265 and MacOS X both implement, and which modern 64 bit processors such as
266 x86-64 were <a href=http://www.pagetable.com/?p=6>designed for</a>). See
267 <a href=http://www.unix.org/whitepapers/64bit.html>the LP64 standard</a> and
268 <a href=http://www.unix.org/version2/whatsnew/lp64_wp.html>the LP64
269 rationale</a> for details.</p>
271 <p>Note that Windows doesn't work like this, and I don't care.
272 <a href=http://blogs.msdn.com/oldnewthing/archive/2005/01/31/363790.aspx>The
273 insane legacy reasons why this is broken on Windows are explained here.</a></p>
275 <b><h3>Signedness of char</h3></b>
276 <p>On platforms like x86, variables of type char default to unsigned. On
277 platforms like arm, char defaults to signed. This difference can lead to
278 subtle portability bugs, and to avoid them we specify which one we want by
279 feeding the compiler -funsigned-char.</p>
281 <p>The reason to pick "unsigned" is that way we're 8-bit clean by default.</p>
283 <p><h3>Error messages and internationalization:</h3></p>
284 <p>Error messages are extremely terse not just to save bytes, but because we
285 don't use any sort of _("string") translation infrastructure.</p>
287 <p>Thus "bad -A '%c'" is
288 preferable to "Unrecognized address base '%c'", because a non-english speaker
289 can see that -A was the problem, and with a ~20 word english vocabulary is
290 more likely to know (or guess) "bad" than the longer message.</p>
292 <p>The help text might someday have translated versions, and strerror()
293 messages produced by perror_exit() and friends can be expected to be
294 localized by libc. Our error functions also prepend the command name,
295 which non-english speakers can presumably recognize already.</p>
297 <p>An enventual goal is <a href=http://yarchive.net/comp/linux/utf8.html>UTF-8</a> support, although it isn't a priority for the
298 first pass of each command. (All commands should at least be 8-bit clean.)</p>
300 <p>Locale support isn't currently a goal; that's a presentation layer issue,
301 X11 or Dalvik's problem.</p>
303 <a name="codestyle" />
304 <h2>Coding style</h2>
306 <p>The real coding style holy wars are over things that don't matter
307 (whitespace, indentation, curly bracket placement...) and thus have no
308 obviously correct answer. As in academia, "the fighting is so vicious because
309 the stakes are so small". That said, being consistent makes the code readable,
310 so here's how to make toybox code look like other toybox code.</p>
312 <p>Toybox source uses two spaces per indentation level, and wraps at 80
313 columns. (Indentation of continuation lines is awkward no matter what
314 you do, sometimes two spaces looks better, sometimes indenting to the
315 contents of a parentheses looks better.)</p>
317 <p>There's a space after C flow control statements that look like functions, so
318 "if (blah)" instead of "if(blah)". (Note that sizeof is actually an
319 operator, so we don't give it a space for the same reason ++ doesn't get
320 one. Yeah, it doesn't need the parentheses either, but it gets them.
321 These rules are mostly to make the code look consistent, and thus easier
322 to read.) We also put a space around assignment operators (on both sides),
325 <p>Blank lines (vertical whitespace) go between thoughts. "We were doing that,
326 now we're doing this. (Not a hard and fast rule about _where_ it goes,
327 but there should be some.)"</p>
329 <p>Variable declarations go at the start of blocks, with a blank line between
330 them and other code. Yes, c99 allows you to put them anywhere, but they're
331 harder to find if you do that. If there's a large enough distance between
332 the declaration and the code using it to make you uncomfortable, maybe the
333 function's too big, or is there an if statement or something you can
334 use as an excuse to start a new closer block?</p>
336 <p>If statments with a single line body go on the same line if the result
337 fits in 80 columns, on a second line if it doesn't. We usually only use
338 curly brackets if we need to, either because the body is multiple lines or
339 because we need to distinguish which if an else binds to. Curly brackets go
340 on the same line as the test/loop statement. The exception to both cases is
341 if the test part of an if statement is long enough to split into multiple
342 lines, then we put the curly bracket on its own line afterwards (so it doesn't
343 get lost in the multple line variably indented mess), and we put it there
344 even if it's only grouping one line (because the indentation level is not
345 providing clear information in that case).</p>
359 if (blah blah blah...
366 <p>Gotos are allowed for error handling, and for breaking out of
367 nested loops. In general, a goto should only jump forward (not back), and
368 should either jump to the end of an outer loop, or to error handling code
369 at the end of the function. Goto labels are never indented: they override the
370 block structure of the file. Putting them at the left edge makes them easy
371 to spot as overrides to the normal flow of control, which they are.</p>
373 <p>When there's a shorter way to say something, we tend to do that for
374 consistency. For example, we tend to say "*blah" instead of "blah[0]" unless
375 we're referring to more than one element of blah. Similarly, NULL is
376 really just 0 (and C will automatically typecast 0 to anything, except in
377 varargs), "if (function() != NULL)" is the same as "if (function())",
378 "x = (blah == NULL);" is "x = !blah;", and so on.</p>
381 concise, not cryptic: if you're worried about the code being hard to
382 understand, splitting it to multiple steps on multiple lines is
383 better than a NOP operation like "!= NULL". A common sign of trying to
384 hard is nesting ? : three levels deep, sometimes if/else and a temporary
385 variable is just plain easier to read. If you think you need a comment,
386 you may be right.</p>
388 <p>Comments are nice, but don't overdo it. Comments should explain _why_,
389 not how. If the code doesn't make the how part obvious, that's a problem with
390 the code. Sometimes choosing a better variable name is more revealing than a
391 comment. Comments on their own line are better than comments on the end of
392 lines, and they usually have a blank line before them. Most of toybox's
393 comments are c99 style // single line comments, even when there's more than
394 one of them. The /* multiline */ style is used at the start for the metadata,
395 but not so much in the code itself. They don't nest cleanly, are easy to leave
396 accidentally unterminated, need extra nonfunctional * to look right, and if
397 you need _that_ much explanation maybe what you really need is a URL citation
398 linking to a standards document? Long comments can fall out of sync with what
399 the code is doing. Comments do not get regression tested. There's no such
400 thing as self-documenting code (if nothing else, code with _no_ comments
401 is a bit unfriendly to new readers), but "chocolate sauce isn't the answer
402 to bad cooking" either. Don't use comments as a crutch to explain unclear
403 code if the code can be fixed.</p>
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