docs/busybox.net/FAQ.html

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<h3>Frequently Asked Questions</h3>

This is a collection of some of the more frequently asked questions
about BusyBox.  Some of the questions even have answers. 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><a href="#build_system">How do I build a BusyBox-based system?</a>
<li><a href="#kernel">Which Linux kernel versions are supported?</a>
<li><a href="#arch">Which architectures does BusyBox run on?</a>
<li><a href="#libc">Which C libraries are supported?</a>
<li><a href="#commercial">Can I include BusyBox as part of the software on my device?</a>
<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 &lt;favorite feature&gt; 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><a href="#helpme">I need help with BusyBox!  What should I do?</a>
<li><a href="#contracts">I need you to add &lt;favorite feature&gt;!  Are the BusyBox developers willing to be paid in order to fix bugs or add in &lt;favorite feature&gt;?  Are you willing to provide support contracts?</a>
</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="#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>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></li>
  <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><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></li>
  <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><a href="#who">Who are the BusyBox developers?</a></li>
</ul>


</ol>

<h1>General questions</h1>

<hr />
<p>
<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 allyesconfig", and then run "make".
</p>
<p>
    This will create a busybox binary with 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 "standalone shell", 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 is dependent on the existence of /proc/self/exe, so before
    using it in a chroot environment you must mount /proc.)
</p>
<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/>
<p/>
<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 />
<p>
<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 />
<p>
<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 />
<p>
<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 />
<p>
<h2><a name="commercial">Can I include BusyBox as part of the software on my device?</a></h2>
<p>

<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 />
<p>
<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 />
<p>
<h2><a name="demanding">I demand that you to add &lt;favorite feature&gt; 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 />
<p>
<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 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 />
<p>
<h2><a name="contracts">I need you to add &lt;favorite feature&gt;!  Are the BusyBox developers willing to be paid in order to fix bugs or add in &lt;favorite feature&gt;?  Are you willing to provide support contracts?</a></h2>
</p>

<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>

<p> If you prefer to deal with an organization rather than an individual, Rob
    Landley (the current BusyBox maintainer) works for
    <a http://www.timesys.com>TimeSys</a>, and Eric Andersen (the previous
    busybox maintainer and current uClibc maintainer) owns
    <a href="http://codepoet-consulting.com/">CodePoet Consulting</a>.  Both
    companies offer support contracts and handle new development, and there
    are plenty of other companies that do the same.
</p>




<h1>Troubleshooting</h1>

<hr />
<p></p>
<h2><a name="bugs">I think I found a bug in BusyBox!  What should I do?</a></h2>
<p></p>

<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>
    The developers of BusyBox are busy people, and have only so much they can
    keep in their brains at a time.  As a result, bug reports and new feature
    patches sometimes get lost when posted to the mailing list.  To prevent
    your bug report from getting lost, if you find a bug in BusyBox that isn't
    immediately addressed, please use the <a
    href="http://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>

<hr />
<p>
<h2><a name="init">Busybox init isn't working!</a></h2>
<p>
    Build a statically linked version of the following "hello world" program
    with your cross compiler toolchain.
</p>
<pre>
#include &lt;stdio.h&gt;

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>
</p>

<hr />
<p>
<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 />
<p>
<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>

<h1>Development</h1>

<h2><b><a name="goals">What are the goals of busybox?</a></b></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>

<h2><b><a name="design">What is the design of busybox?</a></b></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>

<h2><a name="source_applets"><b>The applet directories</b></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 bb_applet_name to argv[0] and calls
run_applet_by_name() 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 applets/busybox.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>

<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 (getopt_ulflags.c), and a whole lot more.</p>

<hr />
<p>
<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, rename busybox_unstripped to
        busybox_old, and then build a 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"><b>Adding an applet to busybox</b></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>

<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>

<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>

<h2><a name="tips" />Programming tips and tricks.</a></h2>

<p>Various things busybox uses that aren't particularly well documented
elsewhere.</p>

<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>

<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>

<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 &gt; 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>

<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>

<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>

<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 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        <- BusyBox maintainer
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
</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.  (If anybody would like to make
a stab at it...)</p>

<pre>
aaronl
beppu
dwhedon
erik    : Also Erik Andersen?
gfeldman
jimg
kraai
markw
miles
proski
rjune
tausq
vodz      :Vladimir N. Oleynik
</pre>


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