mirror of
https://github.com/sheumann/hush.git
synced 2024-12-27 01:32:08 +00:00
Start of developer documentation for busybox.
This commit is contained in:
Rob Landley
parent
f9d40d6815
commit
aaffef4d33
179
docs/busybox.net/programming.html
Normal file
179
docs/busybox.net/programming.html
Normal file
@ -0,0 +1,179 @@
|
||||
<!--#include file="header.html" -->
|
||||
|
||||
<h2>Rob's notes on programming busybox.</h2>
|
||||
|
||||
<ul>
|
||||
<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="#adding">Adding an applet to busybox</a></li>
|
||||
<li><a href="#standards">What standards does busybox adhere to?</a></li>
|
||||
</ul>
|
||||
|
||||
<h2><b><a name="goals" />What are the goals of busybox?</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?</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" />
|
||||
|
||||
<h2><a name="source_applets" /><b>The applet directories</b></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></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>
|
||||
|
||||
<h2><a name="adding" /><b>Adding an applet to busybox</b></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>
|
||||
|
||||
<br>
|
||||
<br>
|
||||
<br>
|
||||
|
||||
<!--#include file="footer.html" -->
|
||||
Loading…
Reference in New Issue
Block a user