Partial update. Needs more work.

[android-x86/external-toybox.git] / www / code.html

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<p><h1>Code style</h1></p>

<p>Toybox source is formatted to be read with 4-space tab stops.  Each file
starts with a special comment telling vi to set the tab stop to 4.  Note that
one of the bugs in Ubuntu 7.10 broke vi's ability to parse these comments; you
must either rebuild vim from source, or go ":ts=4" yourself each time you load
the file.</p>

<p>Gotos are allowed for error handling, and for breaking out of
nested loops.  In general, a goto should only jump forward (not back), and
should either jump to the end of an outer loop, or to error handling code
at the end of the function.  Goto labels are never indented: they override the
block structure of the file.  Putting them at the left edge makes them easy
to spot as overrides to the normal flow of control, which they are.</p>

<p>The primary goal of toybox is _simple_ code.  Small is second,
speed and lots of features come in somewhere after that.  Note that
environmental dependencies are a type of complexity, so needing other packages
to build or run is a downside.  For example, don't use curses when you can
output ansi escape sequences instead.</p>

<p><h1>Infrastructure:</h1></p>

<p>The toybox source code is in following directories:</p>
<ul>
<li>The <a href="#top">top level directory</a> contains the file main.c (were
execution starts), the header file toys.h (included by every command), and
other global infrastructure.</li>
<li>The <a href="#lib">lib directory</a> contains common functions shared by
multiple commands.</li>
<li>The <a href="#toys">toys directory</a> contains the C files implementating
each command.</li>
<li>The <a href="#scripts">scripts directory</a> contains the build and
test infrastructure.</li>
<li>The <a href="#kconfig">kconfig directory</a> contains the configuration
infrastructure implementing menuconfig (copied from the Linux kernel).</li>
<li>The <a href="#generated">generated directory</a> contains intermediate
files generated from other parts of the source code.</li>
</ul>

<p><h1>Adding a new command</h1></p>
<p>To add a new command to toybox, add a C file implementing that command to
the toys directory.  No other files need to be modified; the build extracts
all the information it needs (such as command line arguments) from specially
formatted comments and macros in the C file.  (See the description of the
<a href="#generated">generated directory</a> for details.)</p>

<p>An easy way to start a new command is copy the file "hello.c" to
the name of the new command, and modify this copy to implement the new command.
This file is an example command meant to be used as a "skeleton" for
new commands (more or less by turning every instance of "hello" into the
name of your command, updating the command line arguments, globals, and
help data,  and then filling out its "main" function with code that does
something interesting).  It provides examples of all the build infrastructure
(including optional elements like command line argument parsing and global
variables that a "hello world" program doesn't strictly need).</p>

<p>Here's a checklist of steps to turn hello.c into another command:</p>

<ul>
<li><p>First "cd toys" and "cp hello.c yourcommand.c".  Note that the name
of this file is significant, it's the name of the new command you're adding
to toybox.  Open your new file in your favorite editor.</p></li>

<li><p>Change the one line comment at the top of the file (currently
"hello.c - A hello world program") to describe your new file.</p></li>

<li><p>Change the copyright notice to your name, email, and the current
year.</p></li>

<li><p>Give a URL to the relevant standards document, or say "Not in SUSv3" if
there is no relevant standard.  (Currently both lines are there, delete
whichever is appropriate.)  The existing link goes to the directory of SUSv3
command line utility standards on the Open Group's website, where there's often
a relevant commandname.html file.  Feel free to link to other documentation or
standards as appropriate.</p></li>

<li><p>Update the USE_YOURCOMMAND(NEWTOY(yourcommand,"blah",0)) line.  The
arguments to newtoy are: 1) the name used to run your command, 2)
the command line arguments (NULL if none), and additional information such
as where your command should be installed on a running system.  See [TODO] for
details.</p></li>

<li><p>Change the kconfig data (from "config YOURCOMMAND" to the end of the
comment block) to supply your command's configuration and help
information.  The uppper case config symbols are used by menuconfig, and are
also what the CFG_ and USE_() macros are generated from (see [TODO]).  The
help information here is used by menuconfig, and also by the "help" command to
describe your new command.  (See [TODO] for details.)  By convention,
unfinished commands default to "n" and finished commands default to "y".<p></li>

<li><p>Update the DEFINE_GLOBALS() macro to contain your command's global
variables, and also change the name "hello" in the #define TT line afterwards
to the name of your command.  If your command has no global variables, delete
this macro (and the #define TT line afterwards).  Note that if you specified
two-character command line arguments in NEWTOY(), the first few global
variables will be initialized by the automatic argument parsing logic, and
the type and order of these variables must correspond to the arguments
specified in NEWTOY().  See [TODO] for details.</p></li>

<li><p>If you didn't delete the DEFINE_GLOBALS macro, change the "#define TT
this.hello" line to use your command name in place of the "hello".  This is a
shortcut to access your global variables as if they were members of the global
struct "TT".  (Access these members with a period ".", not a right arrow
"->".)</p></li>

<li><p>Rename hello_main() to yourcommand_main().  This is the main() function
where execution of your command starts.  See [TODO] to figure out what
happened to your command line arguments and how to access them.</p></li>
</ul>

<p><a name="top" /><h2>Top level directory.</h2></p>

<p>This directory contains global infrastructure.

<h3>main.c</h3>
<p>Contains the main() function where execution starts, plus
common infrastructure to initialize global variables and select which command
to run.  The "toybox" multiplexer command also lives here.  (This is the
only command defined outside of the toys directory.)</p>

<p>Execution starts in main() which trims any path off of the first command
name and calls toybox_main(), which calls toy_exec(), which calls toy_find()
and toy_init() before calling the appropriate command's function from toy_list.
If the command is "toybox", execution recurses into toybox_main(), otherwise
the call goes to the appropriate commandname_main() from a C file in the toys
directory.</p>

<p>The following global variables are defined in main.c:</p>
<ul>
<li><p>struct toy_list <b>toy_list[]</b> - array describing all the
commands currently configured into toybox.  The first entry (toy_list[0]) is
for the "toybox" multiplexer command, which runs all the other built-in commands
without symlinks by using its first argument as the name of the command to
run and the rest as that command's argument list (ala "./toybox echo hello").
The remaining entries are the commands in alphabetical order (for efficient
binary search).</p>

<p>This is a read-only array initialized at compile time by
defining macros and #including generated/newtoys.h.</p>

<p>Members of struct toy_list include:</p>
<ul>
<li><p>char *<b>name</b> - the name of this command.</p></li>
<li><p>void (*<b>toy_main</b>)(void) - function pointer to run this
command.</p></li>
<li><p>char *<b>options</b> - command line option string (used by
get_optflags() in lib/args.c to intialize toys.optflags, toys.optargs, and
entries in the toy union).  If this is NULL, no option parsing is done before
calling toy_main().</p></li>
<li><p>int <b>flags</b> - Behavior flags for this command.  The following flags are currently understood:</p>

<ul>
<li><b>TOYFLAG_USR</b> - Install this command under /usr</li>
<li><b>TOYFLAG_BIN</b> - Install this command under /bin</li>
<li><b>TOYFLAG_SBIN</b> - Install this command under /sbin</li>
<li><b>TOYFLAG_NOFORK</b> - This command can be used as a shell builtin.</li>
<li><b>TOYFLAG_UMASK</b> - Call umask(0) before running this command.</li>
</ul>
<br>

<p>These flags are combined with | (or).  For example, to install a command
in /usr/bin, or together TOYFLAG_USR|TOYFLAG_BIN.</p>
</ul>
</li>

<li><p>struct toy_context <b>toys</b> - global structure containing information
common to all commands, initializd by toy_init().  Members of this structure
include:</p>
<ul>
<li><p>struct toy_list *<b>which</b> - a pointer to this command's toy_list
structure.  Mostly used to grab the name of the running command
(toys->which.name).</p>
</li>
<li><p>int <b>exitval</b> - Exit value of this command.  Defaults to zero.  The
error_exit() functions will return 1 if this is zero, otherwise they'll
return this value.</p></li>
<li><p>char **<b>argv</b> - "raw" command line options, I.E. the original
unmodified string array passed in to main().  Note that modifying this changes
"ps" output, and is not recommended.</p>
<p>Most commands don't use this field, instead the use optargs, optflags,
and the fields in the toy union initialized by get_optflags().</p>
</li>
<li><p>unsigned <b>optflags</b> - Command line option flags, set by
get_optflags().  Indicates which of the command line options listed in
toys->which.options occurred this time.</p>

<p>The rightmost command line argument listed in toys->which.options sets bit
1, the next one sets bit 2, and so on.  This means the bits are set in the same
order the binary digits would be listed if typed out as a string.  For example,
the option string "abcd" would parse the command line "-c" to set optflags to 2,
"-a" would set optflags to 8, and "-bd" would set optflags to 6 (4|2).</p>

<p>Only letters are relevant to optflags.  In the string "a*b:c#d", d=1, c=2,
b=4, a=8.  The punctuation after a letter initializes global variables
(see [TODO] DECLARE_GLOBALS() for details).</p>

<p>For more information on option parsing, see [TODO] get_optflags().</p>

</li>
<li><p>char **<b>optargs</b> - Null terminated array of arguments left over
after get_optflags() removed all the ones it understood.  Note: optarg[0] is
the first argument, not the command name.  Use toys.which->name for the command
name.</p></li>
<li><p>int <b>optc</b> - Optarg count, equivalent to argc but for
optargs[].<p></li>
<li><p>int <b>exithelp</b> - Whether error_exit() should print a usage message
via help_main() before exiting.  (True during option parsing, defaults to
false afterwards.)</p></li>
</ul><br>

<li><p>union toy_union <b>this</b> - Union of structures containing each
command's global variables.</p>

<p>Global variables are useful: they reduce the overhead of passing extra
command line arguments between functions, they conveniently start prezeroed to
save initialization costs, and the command line argument parsing infrastructure
can also initialize global variables with its results.</p>

<p>But since each toybox process can only run one command at a time, allocating
space for global variables belonging to other commands you aren't currently
running would be wasteful.</p>

<p>Toybox handles this by encapsulating each command's global variables in
a structure, and declaring a union of those structures.  The DECLARE_GLOBALS()
macro contains the global variables that should go in a command's global
structure.  Each variable can then be accessed as "this.commandname.varname".
Generally, the macro TT is #defined to this.commandname so the variable
can then be accessed as "TT.variable".</p>

A command that needs global variables should declare a structure to
contain them all, and add that structure to this union.  A command should never
declare global variables outside of this, because such global variables would
allocate memory when running other commands that don't use those global
variables.</p>

<p>The first few fields of this structure can be intialized by get_optargs(),
as specified by the options field off this command's toy_list entry.  See
the get_optargs() description in lib/args.c for details.</p>
</li>

<li><b>char toybuf[4096]</b> - a common scratch space buffer so
commands don't need to allocate their own.  Any command is free to use this,
and it should never be directly referenced by functions in lib/ (although
commands are free to pass toybuf in to a library function as an argument).</li>
</ul>

<p>The following functions are defined in main.c:</p>
<ul>
<li><p>struct toy_list *<b>toy_find</b>(char *name) - Return the toy_list
structure for this command name, or NULL if not found.</p></li>
<li><p>void <b>toy_init</b>(struct toy_list *which, char *argv[]) - fill out
the global toys structure, calling get_optargs() if necessary.</p></li>
<li><p>void <b>toy_exec</b>(char *argv[]) - Run a built-in command with
arguments.</p>
<p>Calls toy_find() on argv[0] (which must be just a command name
without path).  Returns if it can't find this command, otherwise calls
toy_init(), toys->which.toy_main(), and exit() instead of returning.</p>

<p>Use the library function xexec() to fall back to external executables
in $PATH if toy_exec() can't find a built-in command.  Note that toy_exec()
does not strip paths before searching for a command, so "./command" will
never match an internal command.</li>

<li><p>void <b>toybox_main</b>(void) - the main function for the multiplexer
command (I.E. "toybox").  Given a command name as its first argument, calls
toy_exec() on its arguments.  With no arguments, it lists available commands.
If the first argument starts with "-" it lists each command with its default
install path prepended.</p></li>

</ul>

<h3>Config.in</h3>

<p>Top level configuration file in a stylized variant of
<a href=http://kernel.org/doc/Documentation/kbuild/kconfig-language.txt>kconfig</a> format.  Includes generated/Config.in.</p>

<p>These files are directly used by "make menuconfig" to select which commands
to build into toybox (thus generating a .config file), and by
scripts/config2help.py to create generated/help.h.</p>

<h3>Temporary files:</h3>

<p>There is one temporary file in the top level source directory:</p>
<ul>
<li><p><b>.config</b> - Configuration file generated by kconfig, indicating
which commands (and options to commands) are currently enabled.  Used
to make generated/config.h and determine which toys/*.c files to build.</p>

<p>You can create a human readable "miniconfig" version of this file using
<a href=http://landley.net/code/firmware/new_platform.html#miniconfig>these
instructions</a>.</p>
</li>
</ul>

<p>The "generated/" directory contains files generated from other source code
in toybox.  All of these files can be recreated by the build system, although
some (such as generated/help.h) are shipped in release versions to reduce
environmental dependencies (I.E. so you don't need python on your build
system).</p>

<ul>
<li><p><b>generated/config.h</b> - list of CFG_SYMBOL and USE_SYMBOL() macros,
generated from .config by a sed invocation in the top level Makefile.</p>

<p>CFG_SYMBOL is a comple time constant set to 1 for enabled symbols and 0 for
disabled symbols.  This allows the use of normal if() statements to remove
code at compile time via the optimizer's dead code elimination (which removes
from the binary any code that cannot be reached).  This saves space without
cluttering the code with #ifdefs or leading to configuration dependent build
breaks.  (See the 1992 Usenix paper
<a href=http://www.chris-lott.org/resources/cstyle/ifdefs.pdf>#ifdef
Considered Harmful</a> for more information.)</p>

<p>USE_SYMBOL(code) evaluates to the code in parentheses when the symbol
is enabled, and nothing when the symbol is disabled.  This can be used
for things like varargs or variable declarations which can't always be
eliminated by a simple test on CFG_SYMBOL.  Note that
(unlike CFG_SYMBOL) this is really just a variant of #ifdef, and can
still result in configuration dependent build breaks.  Use with caution.</p>
</li>
</ul>

<p><h2>Directory toys/</h2></p>

<h3>toys/Config.in</h3>

<p>Included from the top level Config.in, contains one or more
configuration entries for each command.</p>

<p>Each command has a configuration entry matching the command name (although
configuration symbols are uppercase and command names are lower case).
Options to commands start with the command name followed by an underscore and
the option name.  Global options are attachd to the "toybox" command,
and thus use the prefix "TOYBOX_".  This organization is used by
scripts/cfg2files to select which toys/*.c files to compile for a given
.config.</p>

<p>A commands with multiple names (or multiple similar commands implemented in
the same .c file) should have config symbols prefixed with the name of their
C file.  I.E. config symbol prefixes are NEWTOY() names.  If OLDTOY() names
have config symbols they're options (symbols with an underscore and suffix)
to the NEWTOY() name.  (See toys/toylist.h)</p>

<h3>toys/toylist.h</h3>
<p>The first half of this file prototypes all the structures to hold
global variables for each command, and puts them in toy_union.  These
prototypes are only included if the macro NEWTOY isn't defined (in which
case NEWTOY is defined to a default value that produces function
prototypes).</p>

<p>The second half of this file lists all the commands in alphabetical
order, along with their command line arguments and install location.
Each command has an appropriate configuration guard so only the commands that
are enabled wind up in the list.</p>

<p>The first time this header is #included, it defines structures and
produces function prototypes for the commands in the toys directory.</p>


<p>The first time it's included, it defines structures and produces function
prototypes.
  This
is used to initialize toy_list in main.c, and later in that file to initialize
NEED_OPTIONS (to figure out whether the command like parsing logic is needed),
and to put the help entries in the right order in toys/help.c.</p>

<h3>toys/help.h</h3>

<p>#defines two help text strings for each command: a single line
command_help and an additinal command_help_long.  This is used by help_main()
in toys/help.c to display help for commands.</p>

<p>Although this file is generated from Config.in help entries by
scripts/config2help.py, it's shipped in release tarballs so you don't need
python on the build system.  (If you check code out of source control, or
modify Config.in, then you'll need python installed to rebuild it.)</p>

<p>This file contains help for all commands, regardless of current
configuration, but only the currently enabled ones are entered into help_data[]
in toys/help.c.</p>

<h2>Directory lib/</h2>

<p>lib: llist, getmountlist(), error_msg/error_exit, xmalloc(),
strlcpy(), xexec(), xopen()/xread(), xgetcwd(), xabspath(), find_in_path(),
itoa().</p>



<h2>Directory scripts/</h2>

<h3>scripts/cfg2files.sh</h3>

<p>Run .config through this filter to get a list of enabled commands, which
is turned into a list of files in toys via a sed invocation in the top level
Makefile.
</p>

<h2>Directory kconfig/</h2>

<p>Menuconfig infrastructure copied from the Linux kernel.  See the
Linux kernel's Documentation/kbuild/kconfig-language.txt</p>

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