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If you -have additions to this FAQ document, we would love to add them, - -<h2>General questions</h2> -<ol> -<li><a href="#getting_started">How can I get started using BusyBox?</a></li> -<li><a href="#configure">How do I configure busybox?</a></li> -<li><a href="#build">How do I build BusyBox with a cross-compiler?</a></li> -<li><a href="#build_system">How do I build a BusyBox-based system?</a></li> -<li><a href="#kernel">Which Linux kernel versions are supported?</a></li> -<li><a href="#arch">Which architectures does BusyBox run on?</a></li> -<li><a href="#libc">Which C libraries are supported?</a></li> -<li><a href="#commercial">Can I include BusyBox as part of the software on my device?</a></li> -<li><a href="#external">Where can I find other small utilities since busybox does not include the features I want?</a></li> -<li><a href="#demanding">I demand that you to add <favorite feature> right now! How come you don't answer all my questions on the mailing list instantly? I demand that you help me with all of my problems <em>Right Now</em>!</a></li> -<li><a href="#helpme">I need help with BusyBox! What should I do?</a></li> -<li><a href="#contracts">I need you to add <favorite feature>! Are the BusyBox developers willing to be paid in order to fix bugs or add in <favorite feature>? Are you willing to provide support contracts?</a></li> -</ol> - -<h2>Troubleshooting</h2> -<ol> -<li><a href="#bugs">I think I found a bug in BusyBox! What should I do?!</a></li> -<li><a href="#backporting">I'm using an ancient version from the dawn of time and something's broken. Can you backport fixes for free?</a></li> -<li><a href="#init">Busybox init isn't working!</a></li> -<li><a href="#sed">I can't configure busybox on my system.</a></li> -<li><a href="#job_control">Why do I keep getting "sh: can't access tty; job control turned off" errors? Why doesn't Control-C work within my shell?</a></li> -</ol> - -<h2>Misc. questions</h2> -<ol> - <li><a href="#tz">How do I change the time zone in busybox?</a></li> -</ol> - -<h2>Programming questions</h2> -<ol> - <li><a href="#goals">What are the goals of busybox?</a></li> - <li><a href="#design">What is the design of busybox?</a></li> - <li><a href="#source">How is the source code organized?</a> - <ul> - <li><a href="#source_applets">The applet directories.</a></li> - <li><a href="#source_libbb">The busybox shared library (libbb)</a></li> - </ul> - </li> - <li><a href="#optimize">I want to make busybox even smaller, how do I go about it?</a></li> - <li><a href="#adding">Adding an applet to busybox</a></li> - <li><a href="#standards">What standards does busybox adhere to?</a></li> - <li><a href="#portability">Portability.</a></li> - <li><a href="#tips">Tips and tricks.</a> - <ul> - <li><a href="#tips_encrypted_passwords">Encrypted Passwords</a></li> - <li><a href="#tips_vfork">Fork and vfork</a></li> - <li><a href="#tips_short_read">Short reads and writes</a></li> - <li><a href="#tips_memory">Memory used by relocatable code, PIC, and static linking.</a></li> - <li><a href="#tips_kernel_headers">Including Linux kernel headers.</a></li> - </ul> - </li> - <li><a href="#who">Who are the BusyBox developers?</a></li> -</ol> - - -<hr /> -<h1>General questions</h1> - -<hr /> -<h2><a name="getting_started">How can I get started using BusyBox?</a></h2> - -<p> If you just want to try out busybox without installing it, download the - tarball, extract it, run "make defconfig", and then run "make". -</p> -<p> - This will create a busybox binary with almost all features enabled. To try - out a busybox applet, type "./busybox [appletname] [options]", for - example "./busybox ls -l" or "./busybox cat LICENSE". Type "./busybox" - to see a command list, and "busybox appletname --help" to see a brief - usage message for a given applet. -</p> -<p> - BusyBox uses the name it was invoked under to determine which applet is - being invoked. (Try "mv busybox ls" and then "./ls -l".) Installing - busybox consists of creating symlinks (or hardlinks) to the busybox - binary for each applet in busybox, and making sure these links are in - the shell's command $PATH. The special applet name "busybox" (or with - any optional suffix, such as "busybox-static") uses the first argument - to determine which applet to run, as shown above. -</p> -<p> - BusyBox also has a feature called the - <a name="standalone_shell">"standalone shell"</a>, where the busybox - shell runs any built-in applets before checking the command path. This - feature is also enabled by "make allyesconfig", and to try it out run - the command line "PATH= ./busybox ash". This will blank your command path - and run busybox as your command shell, so the only commands it can find - (without an explicit path such as /bin/ls) are the built-in busybox ones. - This is another good way to see what's built into busybox. - Note that the standalone shell requires CONFIG_BUSYBOX_EXEC_PATH - to be set appropriately, depending on whether or not /proc/self/exe is - available or not. If you do not have /proc, then point that config option - to the location of your busybox binary, usually /bin/busybox. - (So if you set it to /proc/self/exe, and happen to be able to chroot into - your rootfs, you must mount /proc beforehand.) -</p> -<p> - A typical indication that you set CONFIG_BUSYBOX_EXEC_PATH to proc but - forgot to mount proc is: -<pre> -$ /bin/echo $PATH -/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/bin/X11 -$ echo $PATH -/bin/sh: echo: not found -</pre> - -<hr /> -<h2><a name="configure">How do I configure busybox?</a></h2> - -<p> Busybox is configured similarly to the linux kernel. Create a default - configuration and then run "make menuconfig" to modify it. The end - result is a .config file that tells the busybox build process what features - to include. So instead of "./configure; make; make install" the equivalent - busybox build would be "make defconfig; make; make install". -</p> - -<p> Busybox configured with all features enabled is a little under a megabyte - dynamically linked on x86. To create a smaller busybox, configure it with - fewer features. Individual busybox applets cost anywhere from a few - hundred bytes to tens of kilobytes. Disable unneeded applets to save, - space, using menuconfig. -</p> - -<p>The most important busybox configurators are:</p> - -<ul> -<li><p>make <b>defconfig</b> - Create the maximum "sane" configuration. This -enables almost all features, minus things like debugging options and features -that require changes to the rest of the system to work (such as selinux or -devfs device names). Use this if you want to start from a full-featured -busybox and remove features until it's small enough.</p></li> -<li><p>make <b>allnoconfig</b> - Disable everything. This creates a tiny version -of busybox that doesn't do anything. Start here if you know exactly what -you want and would like to select only those features.</p></li> -<li><p>make <b>menuconfig</b> - Interactively modify a .config file through a -multi-level menu interface. Use this after one of the previous two.</p></li> -</ul> - -<p>Some other configuration options are:</p> -<ul> -<li><p>make <b>oldconfig</b> - Update an old .config file for a newer version -of busybox.</p></li> -<li><p>make <b>allyesconfig</b> - Select absolutely everything. This creates -a statically linked version of busybox full of debug code, with dependencies on -selinux, using devfs names... This makes sure everything compiles. Whether -or not the result would do anything useful is an open question.</p></li> -<li><p>make <b>allbareconfig</b> - Select all applets but disable all sub-features -within each applet. More build coverage testing.</p></li> -<li><p>make <b>randconfig</b> - Create a random configuration for test purposes.</p></li> -</ul> - -<p> Menuconfig modifies your .config file through an interactive menu where you can enable or disable - busybox features, and get help about each feature. - -<p> - To build a smaller busybox binary, run "make menuconfig" and disable the - features you don't need. (Or run "make allnoconfig" and then use - menuconfig to add just the features you need. Don't forget to recompile - with "make" once you've finished configuring.) -</p> - -<hr /> -<h2><a name="build">How do I build BusyBox with a cross-compiler?</a></h2> - -<p> - To build busybox with a cross-compiler, specify CROSS_COMPILE=<prefix>. -</p> -<p> - CROSS_COMPILE specifies the prefix used for all executables used - during compilation. Only gcc and related binutils executables - are prefixed with $(CROSS_COMPILE) in the makefiles. - CROSS_COMPILE can be set on the command line: -</p> -<pre> - make CROSS_COMPILE=arm-linux-uclibcgnueabi- -</pre> -<p> - Alternatively CROSS_COMPILE can be set in the environment. - Default value for CROSS_COMPILE is not to prefix executables. -</p> -<p> - To store the cross-compiler in your .config, set the variable - CONFIG_CROSS_COMPILER_PREFIX accordingly in menuconfig or by - editing the .config file. -</p> - -<hr /> -<h2><a name="build_system">How do I build a BusyBox-based system?</a></h2> - -<p> - BusyBox is a package that replaces a dozen standard packages, but it is - not by itself a complete bootable system. Building an entire Linux - distribution from source is a bit beyond the scope of this FAQ, but it - understandably keeps cropping up on the mailing list, so here are some - pointers. -</p> -<p> - Start by learning how to strip a working system down to the bare essentials - needed to run one or two commands, so you know what it is you actually - need. An excellent practical place to do - this is the <a href="http://www.tldp.org/HOWTO/Bootdisk-HOWTO/">Linux - BootDisk Howto</a>, or for a more theoretical approach try - <a href="http://www.tldp.org/HOWTO/From-PowerUp-To-Bash-Prompt-HOWTO.html">From - PowerUp to Bash Prompt</a>. -</p> -<p> - To learn how to build a working Linux system entirely from source code, - the place to go is the <a href="http://www.linuxfromscratch.org/">Linux - From Scratch</a> project. They have an entire book of step-by-step - instructions you can - <a href="http://www.linuxfromscratch.org/lfs/view/stable/">read online</a> - or - <a href="http://www.linuxfromscratch.org/lfs/downloads/stable/">download</a>. - Be sure to check out the other sections of their main page, including - Beyond Linux From Scratch, Hardened Linux From Scratch, their Hints - directory, and their LiveCD project. (They also have mailing lists which - are better sources of answers to Linux-system building questions than - the busybox list.) -</p> -<p> - If you want an automated yet customizable system builder which produces - a BusyBox and uClibc based system, try - <a href="http://buildroot.uclibc.org/">buildroot</a>, which is - another project by the maintainer of the uClibc (Erik Andersen). - Download the tarball, extract it, unset CC, make. - For more instructions, see the website. -</p> - -<hr /> -<h2><a name="kernel">Which Linux kernel versions are supported?</a></h2> - -<p> - Full functionality requires Linux 2.4.x or better. (Earlier versions may - still work, but are no longer regularly tested.) A large fraction of the - code should run on just about anything. While the current code is fairly - Linux specific, it should be fairly easy to port the majority of the code - to support, say, FreeBSD or Solaris, or Mac OS X, or even Windows (if you - are into that sort of thing). -</p> - -<hr /> -<h2><a name="arch">Which architectures does BusyBox run on?</a></h2> - -<p> - BusyBox in general will build on any architecture supported by gcc. - Kernel module loading for 2.4 Linux kernels is currently - limited to ARM, CRIS, H8/300, x86, ia64, x86_64, m68k, MIPS, PowerPC, - S390, SH3/4/5, Sparc, v850e, and x86_64 for 2.4.x kernels. -</p> -<p> - With 2.6.x kernels, module loading support should work on all architectures. -</p> - -<hr /> -<h2><a name="libc">Which C libraries are supported?</a></h2> - -<p> - On Linux, BusyBox releases are tested against uClibc (0.9.27 or later) and - glibc (2.2 or later). Both should provide full functionality with busybox, - and if you find a bug we want to hear about it. -</p> -<p> - Linux-libc5 is no longer maintained (and has no known advantages over - uClibc), dietlibc is known to have numerous unfixed bugs, and klibc is - missing too many features to build BusyBox. If you require a small C - library for Linux, the busybox developers recommend uClibc. -</p> -<p> - Some BusyBox applets have been built and run under a combination - of newlib and libgloss (see - <a href="http://www.busybox.net/lists/busybox/2005-March/013759.html">this thread</a>). - This is still experimental, but may be supported in a future release. -</p> - -<hr /> -<h2><a name="commercial">Can I include BusyBox as part of the software on my device?</a></h2> - -<p> - Yes. As long as you <a href="http://busybox.net/license.html">fully comply - with the generous terms of the GPL BusyBox license</a> you can ship BusyBox - as part of the software on your device. -</p> - -<hr /> -<h2><a name="external">Where can I find other small utilities since busybox - does not include the features i want?</a></h2> - -<p> - we maintain such a <a href="tinyutils.html">list</a> on this site! -</p> - -<hr /> -<h2><a name="demanding">I demand that you to add <favorite feature> right now! How come you don't answer all my questions on the mailing list instantly? I demand that you help me with all of my problems <em>Right Now</em>!</a></h2> - -<p> - You have not paid us a single cent and yet you still have the product of - many years of our work. We are not your slaves! We work on BusyBox - because we find it useful and interesting. If you go off flaming us, we - will ignore you. - -<hr /> -<h2><a name="helpme">I need help with BusyBox! What should I do?</a></h2> - -<p> - If you find that you need help with BusyBox, you can ask for help on the - BusyBox mailing list at busybox@busybox.net.</p> - -<p> In addition to the mailing list, Erik Andersen (andersee), Manuel Nova - (mjn3), Rob Landley (landley), Mike Frysinger (SpanKY), - Bernhard Reutner-Fischer (blindvt), and other long-time BusyBox developers - are known to hang out on the uClibc IRC channel: #uclibc on - irc.freenode.net. There is a - <a href="http://ibot.Rikers.org/%23uclibc/">web archive of - daily logs of the #uclibc IRC channel</a> going back to 2002. -</p> - -<p> - <b>Please do not send private email to Rob, Erik, Manuel, or the other - BusyBox contributors asking for private help unless you are planning on - paying for consulting services.</b> -</p> - -<p> - When we answer questions on the BusyBox mailing list, it helps everyone - since people with similar problems in the future will be able to get help - by searching the mailing list archives. Private help is reserved as a paid - service. If you need to use private communication, or if you are serious - about getting timely assistance with BusyBox, you should seriously consider - paying for consulting services. -</p> - -<hr /> -<h2><a name="contracts">I need you to add <favorite feature>! Are the BusyBox developers willing to be paid in order to fix bugs or add in <favorite feature>? Are you willing to provide support contracts?</a></h2> - -<p> - Yes we are. The easy way to sponsor a new feature is to post an offer on - the mailing list to see who's interested. You can also email the project's - maintainer and ask them to recommend someone. -</p> - -<hr /> -<h1>Troubleshooting</h1> - -<hr /> -<h2><a name="bugs">I think I found a bug in BusyBox! What should I do?</a></h2> - -<p> - If you simply need help with using or configuring BusyBox, please submit a - detailed description of your problem to the BusyBox mailing list at <a - href="mailto:busybox@busybox.net">busybox@busybox.net</a>. - Please do not send email to individual developers asking - for private help unless you are planning on paying for consulting services. - When we answer questions on the BusyBox mailing list, it helps everyone, - while private answers help only you... -</p> - -<p> - Bug reports and new feature patches sometimes get lost when posted to the - mailing list, because the developers of BusyBox are busy people and have - only so much they can keep in their brains at a time. You can post a - polite reminder after 2-3 days without offending anybody. If that doesn't - result in a solution, please use the - <a href="https://bugs.busybox.net/">BusyBox Bug - and Patch Tracking System</a> to submit a detailed explanation and we'll - get to it as soon as we can. -</p> - -<p> - Note that bugs entered into the bug system without being mentioned on the - mailing list first may languish there for months before anyone even notices - them. We generally go through the bug system when preparing for new - development releases, to see what fell through the cracks while we were - off writing new features. (It's a fast/unreliable vs slow/reliable thing. - Saves retransits, but the latency sucks.) -</p> - -<hr /> -<h2><a name="backporting">I'm using an ancient version from the dawn of time and something's broken. Can you backport fixes for free?</a></h2> - -<p>Variants of this one get asked a lot.</p> - -<p>The purpose of the BusyBox mailing list is to develop and improve BusyBox, -and we're happy to respond to our users' needs. But if you're coming to the -list for free tech support we're going to ask you to upgrade to a current -version before we try to diagnose your problem.</p> - -<p>If you're building BusyBox 0.50 with uClibc 0.9.19 and gcc 1.27 there's a -fairly large chance that whatever problem you're seeing has already been fixed. -To get that fix, all you have to do is upgrade to a newer version. If you -don't at least _try_ that, you're wasting our time.</p> - -<p>The volunteers are happy to fix any bugs you point out in the current -versions because doing so helps everybody and makes the project better. We -want to make the current version work for you. But diagnosing, debugging, and -backporting fixes to old versions isn't something we do for free, because it -doesn't help anybody but you. The cost of volunteer tech support is using a -reasonably current version of the project.</p> - -<p>If you don't want to upgrade, you have the complete source code and thus -the ability to fix it yourself, or hire a consultant to do it for you. If you -got your version from a vendor who still supports the older version, they can -help you. But there are limits as to what the volunteers will feel obliged to -do for you.</p> - -<p>As a rule of thumb, volunteers will generally answer polite questions about -a given version for about three years after its release before it's so old -we don't remember the answer off the top of our head. And if you want us to -put any _effort_ into tracking it down, we want you to put in a little effort -of your own by confirming it's still a problem with the current version. It's -also hard for us to fix a problem of yours if we can't reproduce it because -we don't have any systems running an environment that old.</p> - -<p>A consultant will happily set up a special environment just to reproduce -your problem, and you can always ask on the list if any of the developers -have consulting rates.</p> - -<hr /> -<h2><a name="init">Busybox init isn't working!</a></h2> - -<p> - Init is the first program that runs, so it might be that no programs are - working on your new system because of a problem with your cross-compiler, - kernel, console settings, shared libraries, root filesystem... To rule all - that out, first build a statically linked version of the following "hello - world" program with your cross compiler toolchain: -</p> -<pre> -#include <stdio.h> - -int main(int argc, char *argv) -{ - printf("Hello world!\n"); - sleep(999999999); -} -</pre> - -<p> - Now try to boot your device with an "init=" argument pointing to your - hello world program. Did you see the hello world message? Until you - do, don't bother messing with busybox init. -</p> - -<p> - Once you've got it working statically linked, try getting it to work - dynamically linked. Then read the FAQ entry <a href="#build_system">How - do I build a BusyBox-based system?</a>, and the - <a href="/downloads/BusyBox.html#item_init">documentation for BusyBox - init</a>. -</p> - -<hr /> -<h2><a name="sed">I can't configure busybox on my system.</a></h2> - -<p> - Configuring Busybox depends on a recent version of sed. Older - distributions (Red Hat 7.2, Debian 3.0) may not come with a - usable version. Luckily BusyBox can use its own sed to configure itself, - although this leads to a bit of a chicken and egg problem. - You can work around this by hand-configuring busybox to build with just - sed, then putting that sed in your path to configure the rest of busybox - with, like so: -</p> - -<pre> - tar xvjf sources/busybox-x.x.x.tar.bz2 - cd busybox-x.x.x - make allnoconfig - make include/bb_config.h - echo "CONFIG_SED=y" >> .config - echo "#undef ENABLE_SED" >> include/bb_config.h - echo "#define ENABLE_SED 1" >> include/bb_config.h - make - mv busybox sed - export PATH=`pwd`:"$PATH" -</pre> - -<p>Then you can run "make defconfig" or "make menuconfig" normally.</p> - -<hr /> -<h2><a name="job_control">Why do I keep getting "sh: can't access tty; job control turned off" errors? Why doesn't Control-C work within my shell?</a></h2> - -<p> - Job control will be turned off since your shell can not obtain a controlling - terminal. This typically happens when you run your shell on /dev/console. - The kernel will not provide a controlling terminal on the /dev/console - device. Your should run your shell on a normal tty such as tty1 or ttyS0 - and everything will work perfectly. If you <em>REALLY</em> want your shell - to run on /dev/console, then you can hack your kernel (if you are into that - sortof thing) by changing drivers/char/tty_io.c to change the lines where - it sets "noctty = 1;" to instead set it to "0". I recommend you instead - run your shell on a real console... -</p> - -<hr /> -<h1>Misc. questions</h1> - -<hr /> -<h2><a name="tz">How do I change the time zone in busybox?</a></h2> - -<p>Busybox has nothing to do with the timezone. Please consult your libc -documentation. (<a href="http://google.com/search?q=uclibc+glibc+timezone">http://google.com/search?q=uclibc+glibc+timezone</a>).</p> - -<hr /> -<h1>Development</h1> - -<hr /> -<h2><a name="goals">What are the goals of busybox?</a></h2> - -<p>Busybox aims to be the smallest and simplest correct implementation of the -standard Linux command line tools. First and foremost, this means the -smallest executable size we can manage. We also want to have the simplest -and cleanest implementation we can manage, be <a href="#standards">standards -compliant</a>, minimize run-time memory usage (heap and stack), run fast, and -take over the world.</p> - -<hr /> -<h2><a name="design">What is the design of busybox?</a></h2> - -<p>Busybox is like a swiss army knife: one thing with many functions. -The busybox executable can act like many different programs depending on -the name used to invoke it. Normal practice is to create a bunch of symlinks -pointing to the busybox binary, each of which triggers a different busybox -function. (See <a href="FAQ.html#getting_started">getting started</a> in the -FAQ for more information on usage, and <a href="BusyBox.html">the -busybox documentation</a> for a list of symlink names and what they do.) - -<p>The "one binary to rule them all" approach is primarily for size reasons: a -single multi-purpose executable is smaller then many small files could be. -This way busybox only has one set of ELF headers, it can easily share code -between different apps even when statically linked, it has better packing -efficiency by avoding gaps between files or compression dictionary resets, -and so on.</p> - -<p>Work is underway on new options such as "make standalone" to build separate -binaries for each applet, and a "libbb.so" to make the busybox common code -available as a shared library. Neither is ready yet at the time of this -writing.</p> - -<a name="source"></a> - -<hr /> -<h2><a name="source_applets">The applet directories</a></h2> - -<p>The directory "applets" contains the busybox startup code (applets.c and -busybox.c), and several subdirectories containing the code for the individual -applets.</p> - -<p>Busybox execution starts with the main() function in applets/busybox.c, -which sets the global variable applet_name to argv[0] and calls -run_applet_and_exit() in applets/applets.c. That uses the applets[] array -(defined in include/busybox.h and filled out in include/applets.h) to -transfer control to the appropriate APPLET_main() function (such as -cat_main() or sed_main()). The individual applet takes it from there.</p> - -<p>This is why calling busybox under a different name triggers different -functionality: main() looks up argv[0] in applets[] to get a function pointer -to APPLET_main().</p> - -<p>Busybox applets may also be invoked through the multiplexor applet -"busybox" (see busybox_main() in libbb/appletlib.c), and through the -standalone shell (grep for STANDALONE_SHELL in applets/shell/*.c). -See <a href="FAQ.html#getting_started">getting started</a> in the -FAQ for more information on these alternate usage mechanisms, which are -just different ways to reach the relevant APPLET_main() function.</p> - -<p>The applet subdirectories (archival, console-tools, coreutils, -debianutils, e2fsprogs, editors, findutils, init, loginutils, miscutils, -modutils, networking, procps, shell, sysklogd, and util-linux) correspond -to the configuration sub-menus in menuconfig. Each subdirectory contains the -code to implement the applets in that sub-menu, as well as a Config.in -file defining that configuration sub-menu (with dependencies and help text -for each applet), and the makefile segment (Makefile.in) for that -subdirectory.</p> - -<p>The run-time --help is stored in usage_messages[], which is initialized at -the start of applets/applets.c and gets its help text from usage.h. During the -build this help text is also used to generate the BusyBox documentation (in -html, txt, and man page formats) in the docs directory. See -<a href="#adding">adding an applet to busybox</a> for more -information.</p> - -<hr /> -<h2><a name="source_libbb"><b>libbb</b></a></h2> - -<p>Most non-setup code shared between busybox applets lives in the libbb -directory. It's a mess that evolved over the years without much auditing -or cleanup. For anybody looking for a great project to break into busybox -development with, documenting libbb would be both incredibly useful and good -experience.</p> - -<p>Common themes in libbb include allocation functions that test -for failure and abort the program with an error message so the caller doesn't -have to test the return value (xmalloc(), xstrdup(), etc), wrapped versions -of open(), close(), read(), and write() that test for their own failures -and/or retry automatically, linked list management functions (llist.c), -command line argument parsing (getopt32.c), and a whole lot more.</p> - -<hr /> -<h2><a name="optimize">I want to make busybox even smaller, how do I go about it?</a></h2> - -<p> - To conserve bytes it's good to know where they're being used, and the - size of the final executable isn't always a reliable indicator of - the size of the components (since various structures are rounded up, - so a small change may not even be visible by itself, but many small - savings add up). -</p> - -<p> The busybox Makefile builds two versions of busybox, one of which - (busybox_unstripped) has extra information that various analysis tools - can use. (This has nothing to do with CONFIG_DEBUG, leave that off - when trying to optimize for size.) -</p> - -<p> The <b>"make bloatcheck"</b> option uses Matt Mackall's bloat-o-meter - script to compare two versions of busybox (busybox_unstripped vs - busybox_old), and report which symbols changed size and by how much. - To use it, first build a base version with <b>"make baseline"</b>. - (This creates busybox_old, which should have the original sizes for - comparison purposes.) Then build the new version with your changes - and run "make bloatcheck" to see the size differences from the old - version. -</p> -<p> - The first line of output has totals: how many symbols were added or - removed, how many symbols grew or shrank, the number of bytes added - and number of bytes removed by these changes, and finally the total - number of bytes difference between the two files. The remaining - lines show each individual symbol, the old and new sizes, and the - increase or decrease in size (which results are sorted by). -</p> -<p> - The <b>"make sizes"</b> option produces raw symbol size information for - busybox_unstripped. This is the output from the "nm --size-sort" - command (see "man nm" for more information), and is the information - bloat-o-meter parses to produce the comparison report above. For - defconfig, this is a good way to find the largest symbols in the tree - (which is a good place to start when trying to shrink the code). To - take a closer look at individual applets, configure busybox with just - one applet (run "make allnoconfig" and then switch on a single applet - with menuconfig), and then use "make sizes" to see the size of that - applet's components. -</p> -<p> - The "showasm" command (in the scripts directory) produces an assembly - dump of a function, providing a closer look at what changed. Try - "scripts/showasm busybox_unstripped" to list available symbols, and - "scripts/showasm busybox_unstripped symbolname" to see the assembly - for a sepecific symbol. -</p> - -<hr /> -<h2><a name="adding">Adding an applet to busybox</a></h2> - -<p>To add a new applet to busybox, first pick a name for the applet and -a corresponding CONFIG_NAME. Then do this:</p> - -<ul> -<li>Figure out where in the busybox source tree your applet best fits, -and put your source code there. Be sure to use APPLET_main() instead -of main(), where APPLET is the name of your applet.</li> - -<li>Add your applet to the relevant Config.in file (which file you add -it to determines where it shows up in "make menuconfig"). This uses -the same general format as the linux kernel's configuration system.</li> - -<li>Add your applet to the relevant Makefile.in file (in the same -directory as the Config.in you chose), using the existing entries as a -template and the same CONFIG symbol as you used for Config.in. (Don't -forget "needlibm" or "needcrypt" if your applet needs libm or -libcrypt.)</li> - -<li>Add your applet to "include/applets.h", using one of the existing -entries as a template. (Note: this is in alphabetical order. Applets -are found via binary search, and if you add an applet out of order it -won't work.)</li> - -<li>Add your applet's runtime help text to "include/usage.h". You need -at least appname_trivial_usage (the minimal help text, always included -in the busybox binary when this applet is enabled) and appname_full_usage -(extra help text included in the busybox binary with -CONFIG_FEATURE_VERBOSE_USAGE is enabled), or it won't compile. -The other two help entry types (appname_example_usage and -appname_notes_usage) are optional. They don't take up space in the binary, -but instead show up in the generated documentation (BusyBox.html, -BusyBox.txt, and the man page BusyBox.1).</li> - -<li>Run menuconfig, switch your applet on, compile, test, and fix the -bugs. Be sure to try both "allyesconfig" and "allnoconfig" (and -"allbareconfig" if relevant).</li> - -</ul> - -<hr /> -<h2><a name="standards">What standards does busybox adhere to?</a></h2> - -<p>The standard we're paying attention to is the "Shell and Utilities" -portion of the <a href="http://www.opengroup.org/onlinepubs/009695399/">Open -Group Base Standards</a> (also known as the Single Unix Specification version -3 or SUSv3). Note that paying attention isn't necessarily the same thing as -following it.</p> - -<p>SUSv3 doesn't even mention things like init, mount, tar, or losetup, nor -commonly used options like echo's '-e' and '-n', or sed's '-i'. Busybox is -driven by what real users actually need, not the fact the standard believes -we should implement ed or sccs. For size reasons, we're unlikely to include -much internationalization support beyond UTF-8, and on top of all that, our -configuration menu lets developers chop out features to produce smaller but -very non-standard utilities.</p> - -<p>Also, Busybox is aimed primarily at Linux. Unix standards are interesting -because Linux tries to adhere to them, but portability to dozens of platforms -is only interesting in terms of offering a restricted feature set that works -everywhere, not growing dozens of platform-specific extensions. Busybox -should be portable to all hardware platforms Linux supports, and any other -similar operating systems that are easy to do and won't require much -maintenance.</p> - -<p>In practice, standards compliance tends to be a clean-up step once an -applet is otherwise finished. When polishing and testing a busybox applet, -we ensure we have at least the option of full standards compliance, or else -document where we (intentionally) fall short.</p> - -<hr /> -<h2><a name="portability">Portability.</a></h2> - -<p>Busybox is a Linux project, but that doesn't mean we don't have to worry -about portability. First of all, there are different hardware platforms, -different C library implementations, different versions of the kernel and -build toolchain... The file "include/platform.h" exists to centralize and -encapsulate various platform-specific things in one place, so most busybox -code doesn't have to care where it's running.</p> - -<p>To start with, Linux runs on dozens of hardware platforms. We try to test -each release on x86, x86-64, arm, power pc, and mips. (Since qemu can handle -all of these, this isn't that hard.) This means we have to care about a number -of portability issues like endianness, word size, and alignment, all of which -belong in platform.h. That header handles conditional #includes and gives -us macros we can use in the rest of our code. At some point in the future -we might grow a platform.c, possibly even a platform subdirectory. As long -as the applets themselves don't have to care.</p> - -<p>On a related note, we made the "default signedness of char varies" problem -go away by feeding the compiler -funsigned-char. This gives us consistent -behavior on all platforms, and defaults to 8-bit clean text processing (which -gets us halfway to UTF-8 support). NOMMU support is less easily separated -(see the tips section later in this document), but we're working on it.</p> - -<p>Another type of portability is build environments: we unapologetically use -a number of gcc and glibc extensions (as does the Linux kernel), but these have -been picked up by packages like uClibc, TCC, and Intel's C Compiler. As for -gcc, we take advantage of newer compiler optimizations to get the smallest -possible size, but we also regression test against an older build environment -using the Red Hat 9 image at "http://busybox.net/downloads/qemu". This has a -2.4 kernel, gcc 3.2, make 3.79.1, and glibc 2.3, and is the oldest -build/deployment environment we still put any effort into maintaining. (If -anyone takes an interest in older kernels you're welcome to submit patches, -but the effort would probably be better spent -<a href="http://www.selenic.com/linux-tiny/">trimming -down the 2.6 kernel</a>.) Older gcc versions than that are uninteresting since -we now use c99 features, although -<a href="http://fabrice.bellard.free.fr/tcc/">tcc</a> might be worth a -look.</p> - -<p>We also test busybox against the current release of uClibc. Older versions -of uClibc aren't very interesting (they were buggy, and uClibc wasn't really -usable as a general-purpose C library before version 0.9.26 anyway).</p> - -<p>Other unix implementations are mostly uninteresting, since Linux binaries -have become the new standard for portable Unix programs. Specifically, -the ubiquity of Linux was cited as the main reason the Intel Binary -Compatability Standard 2 died, by the standards group organized to name a -successor to ibcs2: <a href="http://www.telly.org/86open/">the 86open -project</a>. That project disbanded in 1999 with the endorsement of an -existing standard: Linux ELF binaries. Since then, the major players at the -time (such as <a -href="http://www-03.ibm.com/servers/aix/products/aixos/linux/index.html">AIX</a>, <a -href="http://www.sun.com/software/solaris/ds/linux_interop.jsp#3">Solaris</a>, and -<a href="http://www.onlamp.com/pub/a/bsd/2000/03/17/linuxapps.html">FreeBSD</a>) -have all either grown Linux support or folded.</p> - -<p>The major exceptions are newcomer MacOS X, some embedded environments -(such as newlib+libgloss) which provide a posix environment but not a full -Linux environment, and environments like Cygwin that provide only partial Linux -emulation. Also, some embedded Linux systems run a Linux kernel but amputate -things like the /proc directory to save space.</p> - -<p>Supporting these systems is largely a question of providing a clean subset -of BusyBox's functionality -- whichever applets can easily be made to -work in that environment. Annotating the configuration system to -indicate which applets require which prerequisites (such as procfs) is -also welcome. Other efforts to support these systems (swapping #include -files to build in different environments, adding adapter code to platform.h, -adding more extensive special-case supporting infrastructure such as mount's -legacy mtab support) are handled on a case-by-case basis. Support that can be -cleanly hidden in platform.h is reasonably attractive, and failing that -support that can be cleanly separated into a separate conditionally compiled -file is at least worth a look. Special-case code in the body of an applet is -something we're trying to avoid.</p> - -<hr /> -<h2><a name="tips">Programming tips and tricks.</a></h2> - -<p>Various things busybox uses that aren't particularly well documented -elsewhere.</p> - -<hr /> -<h2><a name="tips_encrypted_passwords">Encrypted Passwords</a></h2> - -<p>Password fields in /etc/passwd and /etc/shadow are in a special format. -If the first character isn't '$', then it's an old DES style password. If -the first character is '$' then the password is actually three fields -separated by '$' characters:</p> -<pre> - <b>$type$salt$encrypted_password</b> -</pre> - -<p>The "type" indicates which encryption algorithm to use: 1 for MD5 and 2 for SHA1.</p> - -<p>The "salt" is a bunch of ramdom characters (generally 8) the encryption -algorithm uses to perturb the password in a known and reproducible way (such -as by appending the random data to the unencrypted password, or combining -them with exclusive or). Salt is randomly generated when setting a password, -and then the same salt value is re-used when checking the password. (Salt is -thus stored unencrypted.)</p> - -<p>The advantage of using salt is that the same cleartext password encrypted -with a different salt value produces a different encrypted value. -If each encrypted password uses a different salt value, an attacker is forced -to do the cryptographic math all over again for each password they want to -check. Without salt, they could simply produce a big dictionary of commonly -used passwords ahead of time, and look up each password in a stolen password -file to see if it's a known value. (Even if there are billions of possible -passwords in the dictionary, checking each one is just a binary search against -a file only a few gigabytes long.) With salt they can't even tell if two -different users share the same password without guessing what that password -is and decrypting it. They also can't precompute the attack dictionary for -a specific password until they know what the salt value is.</p> - -<p>The third field is the encrypted password (plus the salt). For md5 this -is 22 bytes.</p> - -<p>The busybox function to handle all this is pw_encrypt(clear, salt) in -"libbb/pw_encrypt.c". The first argument is the clear text password to be -encrypted, and the second is a string in "$type$salt$password" format, from -which the "type" and "salt" fields will be extracted to produce an encrypted -value. (Only the first two fields are needed, the third $ is equivalent to -the end of the string.) The return value is an encrypted password in -/etc/passwd format, with all three $ separated fields. It's stored in -a static buffer, 128 bytes long.</p> - -<p>So when checking an existing password, if pw_encrypt(text, -old_encrypted_password) returns a string that compares identical to -old_encrypted_password, you've got the right password. When setting a new -password, generate a random 8 character salt string, put it in the right -format with sprintf(buffer, "$%c$%s", type, salt), and feed buffer as the -second argument to pw_encrypt(text,buffer).</p> - -<hr /> -<h2><a name="tips_vfork">Fork and vfork</a></h2> - -<p>On systems that haven't got a Memory Management Unit, fork() is unreasonably -expensive to implement (and sometimes even impossible), so a less capable -function called vfork() is used instead. (Using vfork() on a system with an -MMU is like pounding a nail with a wrench. Not the best tool for the job, but -it works.)</p> - -<p>Busybox hides the difference between fork() and vfork() in -libbb/bb_fork_exec.c. If you ever want to fork and exec, use bb_fork_exec() -(which returns a pid and takes the same arguments as execve(), although in -this case envp can be NULL) and don't worry about it. This description is -here in case you want to know why that does what it does.</p> - -<p>Implementing fork() depends on having a Memory Management Unit. With an -MMU then you can simply set up a second set of page tables and share the -physical memory via copy-on-write. So a fork() followed quickly by exec() -only copies a few pages of the parent's memory, just the ones it changes -before freeing them.</p> - -<p>With a very primitive MMU (using a base pointer plus length instead of page -tables, which can provide virtual addresses and protect processes from each -other, but no copy on write) you can still implement fork. But it's -unreasonably expensive, because you have to copy all the parent process' -memory into the new process (which could easily be several megabytes per fork). -And you have to do this even though that memory gets freed again as soon as the -exec happens. (This is not just slow and a waste of space but causes memory -usage spikes that can easily cause the system to run out of memory.)</p> - -<p>Without even a primitive MMU, you have no virtual addresses. Every process -can reach out and touch any other process' memory, because all pointers are to -physical addresses with no protection. Even if you copy a process' memory to -new physical addresses, all of its pointers point to the old objects in the -old process. (Searching through the new copy's memory for pointers and -redirect them to the new locations is not an easy problem.)</p> - -<p>So with a primitive or missing MMU, fork() is just not a good idea.</p> - -<p>In theory, vfork() is just a fork() that writeably shares the heap and stack -rather than copying it (so what one process writes the other one sees). In -practice, vfork() has to suspend the parent process until the child does exec, -at which point the parent wakes up and resumes by returning from the call to -vfork(). All modern kernel/libc combinations implement vfork() to put the -parent to sleep until the child does its exec. There's just no other way to -make it work: the parent has to know the child has done its exec() or exit() -before it's safe to return from the function it's in, so it has to block -until that happens. In fact without suspending the parent there's no way to -even store separate copies of the return value (the pid) from the vfork() call -itself: both assignments write into the same memory location.</p> - -<p>One way to understand (and in fact implement) vfork() is this: imagine -the parent does a setjmp and then continues on (pretending to be the child) -until the exec() comes around, then the _exec_ does the actual fork, and the -parent does a longjmp back to the original vfork call and continues on from -there. (It thus becomes obvious why the child can't return, or modify -local variables it doesn't want the parent to see changed when it resumes.) - -<p>Note a common mistake: the need for vfork doesn't mean you can't have two -processes running at the same time. It means you can't have two processes -sharing the same memory without stomping all over each other. As soon as -the child calls exec(), the parent resumes.</p> - -<p>If the child's attempt to call exec() fails, the child should call _exit() -rather than a normal exit(). This avoids any atexit() code that might confuse -the parent. (The parent should never call _exit(), only a vforked child that -failed to exec.)</p> - -<p>(Now in theory, a nommu system could just copy the _stack_ when it forks -(which presumably is much shorter than the heap), and leave the heap shared. -Even with no MMU at all -In practice, you've just wound up in a multi-threaded situation and you can't -do a malloc() or free() on your heap without freeing the other process' memory -(and if you don't have the proper locking for being threaded, corrupting the -heap if both of you try to do it at the same time and wind up stomping on -each other while traversing the free memory lists). The thing about vfork is -that it's a big red flag warning "there be dragons here" rather than -something subtle and thus even more dangerous.)</p> - -<hr /> -<h2><a name="tips_sort_read">Short reads and writes</a></h2> - -<p>Busybox has special functions, bb_full_read() and bb_full_write(), to -check that all the data we asked for got read or written. Is this a real -world consideration? Try the following:</p> - -<pre>while true; do echo hello; sleep 1; done | tee out.txt</pre> - -<p>If tee is implemented with bb_full_read(), tee doesn't display output -in real time but blocks until its entire input buffer (generally a couple -kilobytes) is read, then displays it all at once. In that case, we _want_ -the short read, for user interface reasons. (Note that read() should never -return 0 unless it has hit the end of input, and an attempt to write 0 -bytes should be ignored by the OS.)</p> - -<p>As for short writes, play around with two processes piping data to each -other on the command line (cat bigfile | gzip > out.gz) and suspend and -resume a few times (ctrl-z to suspend, "fg" to resume). The writer can -experience short writes, which are especially dangerous because if you don't -notice them you'll discard data. They can also happen when a system is under -load and a fast process is piping to a slower one. (Such as an xterm waiting -on x11 when the scheduler decides X is being a CPU hog with all that -text console scrolling...)</p> - -<p>So will data always be read from the far end of a pipe at the -same chunk sizes it was written in? Nope. Don't rely on that. For one -counterexample, see <a href="http://www.faqs.org/rfcs/rfc896.html">rfc 896 -for Nagle's algorithm</a>, which waits a fraction of a second or so before -sending out small amounts of data through a TCP/IP connection in case more -data comes in that can be merged into the same packet. (In case you were -wondering why action games that use TCP/IP set TCP_NODELAY to lower the latency -on their their sockets, now you know.)</p> - -<hr /> -<h2><a name="tips_memory">Memory used by relocatable code, PIC, and static linking.</a></h2> - -<p>The downside of standard dynamic linking is that it results in self-modifying -code. Although each executable's pages are mmaped() into a process' address -space from the executable file and are thus naturally shared between processes -out of the page cache, the library loader (ld-linux.so.2 or ld-uClibc.so.0) -writes to these pages to supply addresses for relocatable symbols. This -dirties the pages, triggering copy-on-write allocation of new memory for each -processes' dirtied pages.</p> - -<p>One solution to this is Position Independent Code (PIC), a way of linking -a file so all the relocations are grouped together. This dirties fewer -pages (often just a single page) for each process' relocations. The down -side is this results in larger executables, which take up more space on disk -(and a correspondingly larger space in memory). But when many copies of the -same program are running, PIC dynamic linking trades a larger disk footprint -for a smaller memory footprint, by sharing more pages.</p> - -<p>A third solution is static linking. A statically linked program has no -relocations, and thus the entire executable is shared between all running -instances. This tends to have a significantly larger disk footprint, but -on a system with only one or two executables, shared libraries aren't much -of a win anyway.</p> - -<p>You can tell the glibc linker to display debugging information about its -relocations with the environment variable "LD_DEBUG". Try -"LD_DEBUG=help /bin/true" for a list of commands. Learning to interpret -"LD_DEBUG=statistics cat /proc/self/statm" could be interesting.</p> - -<p>For more on this topic, here's Rich Felker:</p> -<blockquote> -<p>Dynamic linking (without fixed load addresses) fundamentally requires -at least one dirty page per dso that uses symbols. Making calls (but -never taking the address explicitly) to functions within the same dso -does not require a dirty page by itself, but will with ELF unless you -use -Bsymbolic or hidden symbols when linking.</p> - -<p>ELF uses significant additional stack space for the kernel to pass all -the ELF data structures to the newly created process image. These are -located above the argument list and environment. This normally adds 1 -dirty page to the process size.</p> - -<p>The ELF dynamic linker has its own data segment, adding one or more -dirty pages. I believe it also performs relocations on itself.</p> - -<p>The ELF dynamic linker makes significant dynamic allocations to manage -the global symbol table and the loaded dso's. This data is never -freed. It will be needed again if libdl is used, so unconditionally -freeing it is not possible, but normal programs do not use libdl. Of -course with glibc all programs use libdl (due to nsswitch) so the -issue was never addressed.</p> - -<p>ELF also has the issue that segments are not page-aligned on disk. -This saves up to 4k on disk, but at the expense of using an additional -dirty page in most cases, due to a large portion of the first data -page being filled with a duplicate copy of the last text page.</p> - -<p>The above is just a partial list of the tiny memory penalties of ELF -dynamic linking, which eventually add up to quite a bit. The smallest -I've been able to get a process down to is 8 dirty pages, and the -above factors seem to mostly account for it (but some were difficult -to measure).</p> -</blockquote> - -<hr /> -<h2><a name="tips_kernel_headers"></a>Including kernel headers</h2> - -<p>The "linux" or "asm" directories of /usr/include -contain Linux kernel -headers, so that the C library can talk directly to the Linux kernel. In -a perfect world, applications shouldn't include these headers directly, but -we don't live in a perfect world.</p> - -<p>For example, Busybox's losetup code wants linux/loop.c because nothing else -#defines the structures to call the kernel's loopback device setup ioctls. -Attempts to cut and paste the information into a local busybox header file -proved incredibly painful, because portions of the loop_info structure vary by -architecture, namely the type __kernel_dev_t has different sizes on alpha, -arm, x86, and so on. Meaning we either #include <linux/posix_types.h> or -we hardwire #ifdefs to check what platform we're building on and define this -type appropriately for every single hardware architecture supported by -Linux, which is simply unworkable.</p> - -<p>This is aside from the fact that the relevant type defined in -posix_types.h was renamed to __kernel_old_dev_t during the 2.5 series, so -to cut and paste the structure into our header we have to #include -<linux/version.h> to figure out which name to use. (What we actually -do is -check if we're building on 2.6, and if so just use the new 64 bit structure -instead to avoid the rename entirely.) But we still need the version -check, since 2.4 didn't have the 64 bit structure.</p> - -<p>The BusyBox developers spent <u>two years</u> trying to figure -out a clean way to do all this. There isn't one. The losetup in the -util-linux package from kernel.org isn't doing it cleanly either, they just -hide the ugliness by nesting #include files. Their mount/loop.h -#includes "my_dev_t.h", which #includes <linux/posix_types.h> -and <linux/version.h> just like we do. There simply is no alternative. -</p> - -<p>Just because directly #including kernel headers is sometimes -unavoidable doesn't me we should include them when there's a better -way to do it. However, block copying information out of the kernel headers -is not a better way.</p> - -<hr /> -<h2><a name="who">Who are the BusyBox developers?</a></h2> - -<p>The following login accounts currently exist on busybox.net. (I.E. these -people can commit <a href="http://busybox.net/downloads/patches/">patches</a> -into subversion for the BusyBox, uClibc, and buildroot projects.)</p> - -<pre> -aldot :Bernhard Reutner-Fischer -andersen :Erik Andersen - uClibc and BuildRoot maintainer. -bug1 :Glenn McGrath -davidm :David McCullough -gkajmowi :Garrett Kajmowicz - uClibc++ maintainer -jbglaw :Jan-Benedict Glaw -jocke :Joakim Tjernlund -landley :Rob Landley -lethal :Paul Mundt -mjn3 :Manuel Novoa III -osuadmin :osuadmin -pgf :Paul Fox -pkj :Peter Kjellerstedt -prpplague :David Anders -psm :Peter S. Mazinger -russ :Russ Dill -sandman :Robert Griebl -sjhill :Steven J. Hill -solar :Ned Ludd -timr :Tim Riker -tobiasa :Tobias Anderberg -vapier :Mike Frysinger -vda :Denys Vlasenko - BusyBox maintainer -</pre> - -<p>The following accounts used to exist on busybox.net, but don't anymore so -I can't ask /etc/passwd for their names. Rob Wentworth -<robwen at gmail.com> asked Google and recovered the names:</p> - -<pre> -aaronl :Aaron Lehmann -beppu :John Beppu -dwhedon :David Whedon -erik :Erik Andersen -gfeldman :Gennady Feldman -jimg :Jim Gleason -kraai :Matt Kraai -markw :Mark Whitley -miles :Miles Bader -proski :Pavel Roskin -rjune :Richard June -tausq :Randolph Chung -vodz :Vladimir N. Oleynik -</pre> - - -<br> -<br> -<br> - -<!--#include file="footer.html" --> |