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author Rob Landley <rob@landley.net>
date Mon, 29 Apr 2013 16:00:40 -0500
parents 786841fdb1e0
children 27275d0d97e9
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<p><h1>Code style</h1></p>

<p>The primary goal of toybox is _simple_ code. Keeping the code small is
second, with speed and lots of features coming in somewhere after that.
(For more on that, see the <a href=design.html>design</a> page.)</p>

<p>A simple implementation usually takes up fewer lines of source code,
meaning more code can fit on the screen at once, meaning the programmer can
see more of it on the screen and thus keep more if in their head at once.
This helps code auditing and thus reduces bugs. That said, sometimes being
more explicit is preferable to being clever enough to outsmart yourself:
don't be so terse your code is unreadable.</p>

<p>Toybox source uses two spaces per indentation level, and wraps at 80
columns.</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><h1>Building Toybox:</h1></p>

<p>Toybox is configured using the Kconfig language pioneered by the Linux
kernel, and adopted by many other projects (uClibc, OpenEmbedded, etc).
This generates a ".config" file containing the selected options, which
controls which features are included when compiling toybox.</p>

<p>Each configuration option has a default value. The defaults indicate the
"maximum sane configuration", I.E. if the feature defaults to "n" then it
either isn't complete or is a special-purpose option (such as debugging
code) that isn't intended for general purpose use.</p>

<p>The standard build invocation is:</p>

<ul>
<li>make defconfig #(or menuconfig)</li>
<li>make</li>
<li>make install</li>
</ul>

<p>Type "make help" to see all available build options.</p>

<p>The file "configure" contains a number of environment variable definitions
which influence the build, such as specifying which compiler to use or where
to install the resulting binaries. This file is included by the build, but
accepts existing definitions of the environment variables, so it may be sourced
or modified by the developer before building and the definitions exported
to the environment will take precedence.</p>

<p>(To clarify: "configure" describes the build and installation environment,
".config" lists the features selected by defconfig/menuconfig.)</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>
<ul>
<li><a href="#lib_lib">lib/lib.c</a></li>
<li><a href="#lib_llist">lib/llist.c</a></li>
<li><a href="#lib_args">lib/args.c</a></li>
<li><a href="#lib_dirtree">lib/dirtree.c</a></li>
</ul>
<li>The <a href="#toys">toys directory</a> contains the C files implementating
each command. Currently it contains three subdirectories:
posix, lsb, and other.</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>

<a name="adding" />
<p><h1>Adding a new command</h1></p>
<p>To add a new command to toybox, add a C file implementing that command under
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>Currently there are three subdirectories under "toys", one for commands
defined by the POSIX standard, one for commands defined by the Linux Standard
Base, and one for all other commands. (This is just for developer convenience
sorting them, the directories are otherwise functionally identical.)</p>

<p>An easy way to start a new command is copy the file "toys/other/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/other" 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, where applicable.
(Sample links to SUSv4 and LSB are provided, 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 NEWTOY macro fills out this command's <a href="#toy_list">toy_list</a>
structure.  The arguments to the NEWTOY macro are:</p>

<ol>
<li><p>the name used to run your command</p></li>
<li><p>the command line argument <a href="#lib_args">option parsing string</a> (NULL if none)</p></li>
<li><p>a bitfield of TOYFLAG values
(defined in toys.h) providing additional information such as where your
command should be installed on a running system, whether to blank umask
before running, whether or not the command must run as root (and thus should
retain root access if installed SUID), and so on.</p></li>
</ol>
</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",
so "make defconfig" selects all finished commands.  (Note, "finished" means
"ready to be used", not that it'll never change again.)<p>

<p>Each help block should start with a "usage: yourcommand" line explaining
any command line arguments added by this config option.  The "help" command
outputs this text, and scripts/config2help.c in the build infrastructure
collates these usage lines for commands with multiple configuration
options when producing generated/help.h.</p>
</li>

<li><p>Change the "#define FOR_hello" line to "#define FOR_yourcommand" right
before the "#include <toys.h>". (This selects the appropriate FLAG_ macros and
does a "#define TT this.yourcommand" so you can access the global variables
out of the space-saving union of structures. If you aren't using any command
flag bits and aren't defining a GLOBAL block, you can delete this line.)</p></li>

<li><p>Update the GLOBALS() macro to contain your command's global
variables. If your command has no global variables, delete this macro.</p>

<p>Variables in the GLOBALS() block are are stored in a space saving
<a href="#toy_union">union of structures</a> format, which may be accessed
using the TT macro as if TT were a global structure (so TT.membername).
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 <a href="#lib_args">lib/args.c</a> for details.)</p></li>

<li><p>Rename hello_main() to yourcommand_main().  This is the main() function
where execution of your command starts. Your command line options are
already sorted into this.optflags, this.optargs, this.optc, and the GLOBALS()
as appropriate by the time this function is called. (See
<a href="#lib_args">get_optflags()</a> for details.</p></li>
</ul>

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

<p>This directory contains global infrastructure.</p>

<h3>toys.h</h3>
<p>Each command #includes "toys.h" as part of its standard prolog. It
may "#define FOR_commandname" before doing so to get some extra entries
specific to this command.</p>

<p>This file sucks in most of the commonly used standard #includes, so
individual files can just #include "toys.h" and not have to worry about
stdargs.h and so on.  Individual commands still need to #include
special-purpose headers that may not be present on all systems (and thus would
prevent toybox from building that command on such a system with that command
enabled).  Examples include regex support, any "linux/" or "asm/" headers, mtab
support (mntent.h and sys/mount.h), and so on.</p>

<p>The toys.h header also defines structures for most of the global variables
provided to each command by toybox_main().  These are described in
detail in the description for main.c, where they are initialized.</p>

<p>The global variables are grouped into structures (and a union) for space
savings, to more easily track the amount of memory consumed by them,
so that they may be automatically cleared/initialized as needed, and so
that access to global variables is more easily distinguished from access to
local variables.</p>

<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[] (via toys.which->toy_main()).
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>
<a name="toy_list" />
<li><p><b>struct toy_list 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 (defined in "toys.h") 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's GLOBALS struct).  When 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>
<li><b>TOYFLAG_STAYROOT</b> - Don't drop permissions for this command if toybox is installed SUID root.</li>
<li><b>TOYFLAG_NEEDROOT</b> - This command cannot function unless run with root access.</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><b>struct toy_context toys</b> - global structure containing information
common to all commands, initializd by toy_init() and defined in "toys.h".
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.  This array is null terminated; a NULL
entry indicates the end of the array.</p>
<p>Most commands don't use this field, instead the use optargs, optflags,
and the fields in the GLOBALS struct initialized by get_optflags().</p>
</li>
<li><p>unsigned <b>optflags</b> - Command line option flags, set by
<a href="#lib_args">get_optflags()</a>.  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.  Punctuation after a letter initializes global variables at the
start of the GLOBALS() block (see <a href="#toy_union">union toy_union this</a>
for details).</p>

<p>The build infrastructure creates FLAG_ macros for each option letter,
corresponding to the bit position, so you can check (toys.optflags & FLAG_x)
to see if a flag was specified. (The correct set of FLAG_ macros is selected
by defining FOR_mycommand before #including toys.h. The macros live in
toys/globals.h which is generated by scripts/make.sh.)</p>

<p>For more information on option parsing, see <a href="#lib_args">get_optflags()</a>.</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>

<a name="toy_union" />
<li><p><b>union toy_union 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 with a single global
instance (called "this").  The GLOBALS() macro contains the global
variables that should go in the current command's global structure.  Each
variable can then be accessed as "this.commandname.varname".
If you #defined FOR_commandname before including toys.h, the macro TT is
#defined to this.commandname so the variable can then be accessed as
"TT.variable".  See toys/hello.c for an example.</p>

<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 <a href="#lib_args">get_optargs()</a>,
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/aboriginal/new_platform.html#miniconfig>these
instructions</a>.</p>
</li>
</ul>

<a name="generated" />
<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://doc.cat-v.org/henry_spencer/ifdef_considered_harmful.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 attached 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 command 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>

<a name="lib">
<h2>Directory lib/</h2>

<p>TODO: document lots more here.</p>

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

<h3>lib/portability.h</h3>

<p>This file is automatically included from the top of toys.h, and smooths
over differences between platforms (hardware targets, compilers, C libraries,
operating systems, etc).</p>

<p>This file provides SWAP macros (SWAP_BE16(x) and SWAP_LE32(x) and so on).</p>

<p>A macro like SWAP_LE32(x) means "The value in x is stored as a little
endian 32 bit value, so perform the translation to/from whatever the native
32-bit format is".  You do the swap once on the way in, and once on the way
out. If your target is already little endian, the macro is a NOP.</p>

<p>The SWAP macros come in BE and LE each with 16, 32, and 64 bit versions.
In each case, the name of the macro refers to the _external_ representation,
and converts to/from whatever your native representation happens to be (which
can vary depending on what you're currently compiling for).</p>

<a name="lib_llist"><h3>lib/llist.c</h3>

<p>Some generic single and doubly linked list functions, which take
advantage of a couple properties of C:</p>

<ul>
<li><p>Structure elements are laid out in memory in the order listed, and
the first element has no padding. This means you can always treat (typecast)
a pointer to a structure as a pointer to the first element of the structure,
even if you don't know anything about the data following it.</p></li>

<li><p>An array of length zero at the end of a structure adds no space
to the sizeof() the structure, but if you calculate how much extra space
you want when you malloc() the structure it will be available at the end.
Since C has no bounds checking, this means each struct can have one variable
length array.</p></li>
</ul>

<p>Toybox's list structures always have their <b>next</b> pointer as
the first entry of each struct, and singly linked lists end with a NULL pointer.
This allows generic code to traverse such lists without knowing anything
else about the specific structs composing them: if your pointer isn't NULL
typecast it to void ** and dereference once to get the next entry.</p>

<p><b>lib/lib.h</b> defines three structure types:</p>
<ul>
<li><p><b>struct string_list</b> - stores a single string (<b>char str[0]</b>),
memory for which is allocated as part of the node. (I.E. llist_traverse(list,
free); can clean up after this type of list.)</p></li>

<li><p><b>struct arg_list</b> - stores a pointer to a single string
(<b>char *arg</b>) which is stored in a separate chunk of memory.</p></li>

<li><p><b>struct double_list</b> - has a second pointer (<b>struct double_list
*prev</b> along with a <b>char *data</b> for payload.</p></li>
</ul>

<b>List Functions</b>

<ul>
<li><p>void *<b>llist_pop</b>(void **list) - advances through a list ala
<b>node = llist_pop(&list);</b>  This doesn't modify the list contents,
but does advance the pointer you feed it (which is why you pass the _address_
of that pointer, not the pointer itself).</p></li>

<li><p>void <b>llist_traverse</b>(void *list, void (*using)(void *data)) -
iterate through a list calling a function on each node.</p></li>

<li><p>struct double_list *<b>dlist_add</b>(struct double_list **llist, char *data)
- append an entry to a circular linked list.
This function allocates a new struct double_list wrapper and returns the
pointer to the new entry (which you can usually ignore since it's llist->prev,
but if llist was NULL you need it). The argument is the ->data field for the
new node.</p></li>
<ul><li><p>void <b>dlist_add_nomalloc</b>(struct double_list **llist,
struct double_list *new) - append existing struct double_list to
list, does not allocate anything.</p></li></ul>
</ul>

<b>Trivia questions:</b>

<ul>
<li><p><b>Why do arg_list and double_list contain a char * payload instead of
a void *?</b> - Because you always have to typecast a void * to use it, and
typecasting a char * does no harm. Thus having it default to the most common
pointer type saves a few typecasts (strings are the most common payload),
and doesn't hurt anything otherwise.</p>
</li>

<li><p><b>Why do the names ->str, ->arg, and ->data differ?</b> - To force
you to keep track of which one you're using, calling free(node->str) would
be bad, and _failing_ to free(node->arg) leaks memory.</p></li>

<li><p><b>Why does llist_pop() take a void * instead of void **?</b> -
because the stupid compiler complains about "type punned pointers" when
you typecast and dereference ont he same line,
due to insane FSF developers hardwiring limitations of their optimizer
into gcc's warning system. Since C automatically typecasts any other
pointer _down_ to a void *, the current code works fine. It's sad that it
won't warn you if you forget the &, but the code crashes pretty quickly in
that case.</p></li>

<li><p><b>How do I assemble a singly-linked-list in order?</b> - use
a double_list, dlist_add() your entries, and then break the circle with
<b>list->prev->next = NULL;</b> when done.</li>
</ul>

<a name="lib_args"><h3>lib/args.c</h3>

<p>Toybox's main.c automatically parses command line options before calling the
command's main function.  Option parsing starts in get_optflags(), which stores
results in the global structures "toys" (optflags and optargs) and "this".</p>

<p>The option parsing infrastructure stores a bitfield in toys.optflags to
indicate which options the current command line contained.  Arguments
attached to those options are saved into the command's global structure
("this").  Any remaining command line arguments are collected together into
the null-terminated array toys.optargs, with the length in toys.optc.  (Note
that toys.optargs does not contain the current command name at position zero,
use "toys.which->name" for that.)  The raw command line arguments get_optflags()
parsed are retained unmodified in toys.argv[].</p>

<p>Toybox's option parsing logic is controlled by an "optflags" string, using
a format reminiscent of getopt's optargs but has several important differences.
Toybox does not use the getopt()
function out of the C library, get_optflags() is an independent implementation
which doesn't permute the original arguments (and thus doesn't change how the
command is displayed in ps and top), and has many features not present in
libc optargs() (such as the ability to describe long options in the same string
as normal options).</p>

<p>Each command's NEWTOY() macro has an optflags string as its middle argument,
which sets toy_list.options for that command to tell get_optflags() what
command line arguments to look for, and what to do with them.
If a command has no option
definition string (I.E. the argument is NULL), option parsing is skipped
for that command, which must look at the raw data in toys.argv to parse its
own arguments.  (If no currently enabled command uses option parsing,
get_optflags() is optimized out of the resulting binary by the compiler's
--gc-sections option.)</p>

<p>You don't have to free the option strings, which point into the environment
space (I.E. the string data is not copied).  A TOYFLAG_NOFORK command
that uses the linked list type "*" should free the list objects but not
the data they point to, via "llist_free(TT.mylist, NULL);".  (If it's not
NOFORK, exit() will free all the malloced data anyway unless you want
to implement a CONFIG_TOYBOX_FREE cleanup for it.)</p>

<h4>Optflags format string</h4>

<p>Note: the optflags option description string format is much more
concisely described by a large comment at the top of lib/args.c.</p>

<p>The general theory is that letters set optflags, and punctuation describes
other actions the option parsing logic should take.</p>

<p>For example, suppose the command line <b>command -b fruit -d walrus -a 42</b>
is parsed using the optflags string "<b>a#b:c:d</b>".  (I.E.
toys.which->options="a#b:c:d" and argv = ["command", "-b", "fruit", "-d",
"walrus", "-a", "42"]).  When get_optflags() returns, the following data is
available to command_main():

<ul>
<li><p>In <b>struct toys</b>:
<ul>
<li>toys.optflags = 13; // -a = 8 | -b = 4 | -d = 1</li>
<li>toys.optargs[0] = "walrus"; // leftover argument</li>
<li>toys.optargs[1] = NULL; // end of list</li>
<li>toys.optc=1; // there was 1 leftover argument</li>
<li>toys.argv[] = {"-b", "fruit", "-d", "walrus", "-a", "42"}; // The original command line arguments
</ul>
<p></li>

<li><p>In <b>union this</b> (treated as <b>long this[]</b>):
<ul>
<li>this[0] = NULL; // -c didn't get an argument this time, so get_optflags() didn't change it and toys_init() zeroed "this" during setup.)</li>
<li>this[1] = (long)"fruit"; // argument to -b</li>
<li>this[2] = 42; // argument to -a</li>
</ul>
</p></li>
</ul>

<p>If the command's globals are:</p>

<blockquote><pre>
GLOBALS(
	char *c;
	char *b;
	long a;
)
</pre></blockquote>
<p>That would mean TT.c == NULL, TT.b == "fruit", and TT.a == 42.  (Remember,
each entry that receives an argument must be a long or pointer, to line up
with the array position.  Right to left in the optflags string corresponds to
top to bottom in GLOBALS().</p>

<p><b>long toys.optflags</b></p>

<p>Each option in the optflags string corresponds to a bit position in
toys.optflags, with the same value as a corresponding binary digit.  The
rightmost argument is (1<<0), the next to last is (1<<1) and so on.  If
the option isn't encountered while parsing argv[], its bit remains 0.</p>

<p>For example,
the optflags string "abcd" would parse the command line argument "-c" to set
optflags to 2, "-a" would set optflags to 8, "-bd" would set optflags to
6 (I.E. 4|2), and "-a -c" would set optflags to 10 (2|8).</p>

<p>Only letters are relevant to optflags, punctuation is skipped: in the
string "a*b:c#d", d=1, c=2, b=4, a=8.  The punctuation after a letter
usually indicate that the option takes an argument.</p>

<p>Since toys.optflags is an unsigned int, it only stores 32 bits.  (Which is
the amount a long would have on 32-bit platforms anyway; 64 bit code on
32 bit platforms is too expensive to require in common code used by almost
all commands.)  Bit positions beyond the 1<<31 aren't recorded, but
parsing higher options can still set global variables.</p>

<p><b>Automatically setting global variables from arguments (union this)</b></p>

<p>The following punctuation characters may be appended to an optflags
argument letter, indicating the option takes an additional argument:</p>

<ul>
<li><b>:</b> - plus a string argument, keep most recent if more than one.</li>
<li><b>*</b> - plus a string argument, appended to a linked list.</li>
<li><b>@</b> - plus an occurrence counter (stored in a long)</li>
<li><b>#</b> - plus a signed long argument.
<li><b>.</b> - plus a floating point argument (if CFG_TOYBOX_FLOAT).</li>
<ul>The following can be appended to a float or double:
<li><b>&lt;123</b> - error if argument is less than this</li>
<li><b>&gt;123</b> - error if argument is greater than this</li>
<li><b>=123</b> - default value if argument not supplied</li>
</ul>
<ul><li>Option parsing only understands <>= after . when CFG_TOYBOX_FLOAT
is enabled. (Otherwise the code to determine where floating point constants
end drops out.  When disabled, it can reserve a global data slot for the
argument so offsets won't change, but will never fill it out.). You can handle
this by using the USE_BLAH() macros with C string concatenation, ala:
"abc." USE_TOYBOX_FLOAT("<1.23>4.56=7.89") "def"</li></ul>
</ul>

<p>Arguments may occur with or without a space (I.E. "-a 42" or "-a42").
The command line argument "-abc" may be interepreted many different ways:
the optflags string "cba" sets toys.optflags = 7, "c:ba" sets toys.optflags=4
and saves "ba" as the argument to -c, and "cb:a" sets optflags to 6 and saves
"c" as the argument to -b.</p>

<p>Options which have an argument fill in the corresponding slot in the global
union "this" (see generated/globals.h), treating it as an array of longs
with the rightmost saved in this[0].  Again using "a*b:c#d", "-c 42" would set
this[0]=42; and "-b 42" would set this[1]="42"; each slot is left NULL if
the corresponding argument is not encountered.</p>

<p>This behavior is useful because the LP64 standard ensures long and pointer
are the same size. C99 guarantees structure members will occur in memory
in the same order they're declared, and that padding won't be inserted between
consecutive variables of register size.  Thus the first few entries can
be longs or pointers corresponding to the saved arguments.</p>

<p><b>char *toys.optargs[]</b></p>

<p>Command line arguments in argv[] which are not consumed by option parsing
(I.E. not recognized either as -flags or arguments to -flags) will be copied
to toys.optargs[], with the length of that array in toys.optc.
(When toys.optc is 0, no unrecognized command line arguments remain.)
The order of entries is preserved, and as with argv[] this new array is also
terminated by a NULL entry.</p>

<p>Option parsing can require a minimum or maximum number of optargs left
over, by adding "<1" (read "at least one") or ">9" ("at most nine") to the
start of the optflags string.</p>

<p>The special argument "--" terminates option parsing, storing all remaining
arguments in optargs.  The "--" itself is consumed.</p>

<p><b>Other optflags control characters</b></p>

<p>The following characters may occur at the start of each command's
optflags string, before any options that would set a bit in toys.optflags:</p>

<ul>
<li><b>^</b> - stop at first nonoption argument (for nice, xargs...)</li>
<li><b>?</b> - allow unknown arguments (pass non-option arguments starting
with - through to optargs instead of erroring out).</li>
<li><b>&amp;</b> - the first argument has imaginary dash (ala tar/ps.  If given twice, all arguments have imaginary dash.)</li>
<li><b>&lt;</b> - must be followed by a decimal digit indicating at least this many leftover arguments are needed in optargs (default 0)</li>
<li><b>&gt;</b> - must be followed by a decimal digit indicating at most this many leftover arguments allowed (default MAX_INT)</li>
</ul>

<p>The following characters may be appended to an option character, but do
not by themselves indicate an extra argument should be saved in this[].
(Technically any character not recognized as a control character sets an
optflag, but letters are never control characters.)</p>

<ul>
<li><b>^</b> - stop parsing options after encountering this option, everything else goes into optargs.</li>
<li><b>|</b> - this option is required.  If more than one marked, only one is required.</li>
<li><b>+X</b> enabling this option also enables option X (switch bit on).</li>
<li><b>~X</b> enabling this option disables option X (switch bit off).</li>
<li><b>!X</b> this option cannot be used in combination with X (die with error).</li>
<li><b>[yz]</b> this option requires at least one of y or z to also be enabled.</li>
</ul>

<p>The following may be appended to a float or double:</p>

<ul>
<li><b>&lt;123</b> - error if argument is less than this</li>
<li><b>&gt;123</b> - error if argument is greater than this</li>
<li><b>=123</b> - default value if argument not supplied</li>
</ul>

<p>Option parsing only understands <>= after . when CFG_TOYBOX_FLOAT
is enabled. (Otherwise the code to determine where floating point constants
end drops out.  When disabled, it can reserve a global data slot for the
argument so offsets won't change, but will never fill it out.). You can handle
this by using the USE_BLAH() macros with C string concatenation, ala:</p>

<blockquote>"abc." USE_TOYBOX_FLOAT("<1.23>4.56=7.89") "def"</blockquote>

<p><b>--longopts</b></p>

<p>The optflags string can contain long options, which are enclosed in
parentheses.  They may be appended to an existing option character, in
which case the --longopt is a synonym for that option, ala "a:(--fred)"
which understands "-a blah" or "--fred blah" as synonyms.</p>

<p>Longopts may also appear before any other options in the optflags string,
in which case they have no corresponding short argument, but instead set
their own bit based on position.  So for "(walrus)#(blah)xy:z" "command
--walrus 42" would set toys.optflags = 16 (-z = 1, -y = 2, -x = 4, --blah = 8)
and would assign this[1] = 42;</p>

<p>A short option may have multiple longopt synonyms, "a(one)(two)", but
each "bare longopt" (ala "(one)(two)abc" before any option characters)
always sets its own bit (although you can group them with +X).</p>

<a name="lib_dirtree"><h3>lib/dirtree.c</h3>

<p>The directory tree traversal code should be sufficiently generic
that commands never need to use readdir(), scandir(), or the fts.h family
of functions.</p>

<p>These functions do not call chdir() or rely on PATH_MAX. Instead they
use openat() and friends, using one filehandle per directory level to
recurseinto subdirectories. (I.E. they can descend 1000 directories deep
if setrlimit(RLIMIT_NOFILE) allows enough open filehandles, and the default
in /proc/self/limits is generally 1024.)</p>

<p>The basic dirtree functions are:</p>

<ul>
<li><p><b>dirtree_read(char *path, int (*callback)(struct dirtree node))</b> -
recursively read directories, either applying callback() or returning
a tree of struct dirtree if callback is NULL.</p></li>

<li><p><b>dirtree_path(struct dirtree *node, int *plen)</b> - malloc() a
string containing the path from the root of this tree to this node. If
plen isn't NULL then *plen is how many extra bytes to malloc at the end
of string.</p></li>

<li><p><b>dirtree_parentfd(struct dirtree *node)</b> - return fd of
containing directory, for use with openat() and such.</p></li>
</ul>

<p>The <b>dirtree_read()</b> function takes two arguments, a starting path for
the root of the tree, and a callback function. The callback takes a
<b>struct dirtree *</b> (from lib/lib.h) as its argument. If the callback is
NULL, the traversal uses a default callback (dirtree_notdotdot()) which
recursively assembles a tree of struct dirtree nodes for all files under
this directory and subdirectories (filtering out "." and ".." entries),
after which dirtree_read() returns the pointer to the root node of this
snapshot tree.</p>

<p>Otherwise the callback() is called on each entry in the directory,
with struct dirtree * as its argument. This includes the initial
node created by dirtree_read() at the top of the tree.</p>

<p><b>struct dirtree</b></p>

<p>Each struct dirtree node contains <b>char name[]</b> and <b>struct stat
st</b> entries describing a file, plus a <b>char *symlink</b>
which is NULL for non-symlinks.</p>

<p>During a callback function, the <b>int data</b> field of directory nodes
contains a dirfd (for use with the openat() family of functions). This is
generally used by calling dirtree_parentfd() on the callback's node argument.
For symlinks, data contains the length of the symlink string. On the second
callback from DIRTREE_COMEAGAIN (depth-first traversal) data = -1 for
all nodes (that's how you can tell it's the second callback).</p>

<p>Users of this code may put anything they like into the <b>long extra</b>
field. For example, "cp" and "mv" use this to store a dirfd for the destination
directory (and use DIRTREE_COMEAGAIN to get the second callback so they can
close(node->extra) to avoid running out of filehandles).
This field is not directly used by the dirtree code, and
thanks to LP64 it's large enough to store a typecast pointer to an
arbitrary struct.</p>

<p>The return value of the callback combines flags (with boolean or) to tell
the traversal infrastructure how to behave:</p>

<ul>
<li><p><b>DIRTREE_SAVE</b> - Save this node, assembling a tree. (Without
this the struct dirtree is freed after the callback returns. Filtering out
siblings is fine, but discarding a parent while keeping its child leaks
memory.)</p></li>
<li><p><b>DIRTREE_ABORT</b> - Do not examine any more entries in this
directory. (Does not propagate up tree: to abort entire traversal,
return DIRTREE_ABORT from parent callbacks too.)</p></li>
<li><p><b>DIRTREE_RECURSE</b> - Examine directory contents. Ignored for
non-directory entries. The remaining flags only take effect when
recursing into the children of a directory.</p></li>
<li><p><b>DIRTREE_COMEAGAIN</b> - Call the callback a second time after
examining all directory contents, allowing depth-first traversal.
On the second call, dirtree->data = -1.</p></li>
<li><p><b>DIRTREE_SYMFOLLOW</b> - follow symlinks when populating children's
<b>struct stat st</b> (by feeding a nonzero value to the symfollow argument of
dirtree_add_node()), which means DIRTREE_RECURSE treats symlinks to
directories as directories. (Avoiding infinite recursion is the callback's
problem: the non-NULL dirtree->symlink can still distinguish between
them.)</p></li>
</ul>

<p>Each struct dirtree contains three pointers (next, parent, and child)
to other struct dirtree.</p>

<p>The <b>parent</b> pointer indicates the directory
containing this entry; even when not assembling a persistent tree of
nodes the parent entries remain live up to the root of the tree while
child nodes are active. At the top of the tree the parent pointer is
NULL, meaning the node's name[] is either an absolute path or relative
to cwd. The function dirtree_parentfd() gets the directory file descriptor
for use with openat() and friends, returning AT_FDCWD at the top of tree.</p>

<p>The <b>child</b> pointer points to the first node of the list of contents of
this directory. If the directory contains no files, or the entry isn't
a directory, child is NULL.</p>

<p>The <b>next</b> pointer indicates sibling nodes in the same directory as this
node, and since it's the first entry in the struct the llist.c traversal
mechanisms work to iterate over sibling nodes. Each dirtree node is a
single malloc() (even char *symlink points to memory at the end of the node),
so llist_free() works but its callback must descend into child nodes (freeing
a tree, not just a linked list), plus whatever the user stored in extra.</p>

<p>The <b>dirtree_read</b>() function is a simple wrapper, calling <b>dirtree_add_node</b>()
to create a root node relative to the current directory, then calling
<b>handle_callback</b>() on that node (which recurses as instructed by the callback
return flags). Some commands (such as chgrp) bypass this wrapper, for example
to control whether or not to follow symlinks to the root node; symlinks
listed on the command line are often treated differently than symlinks
encountered during recursive directory traversal).

<p>The ls command not only bypasses the wrapper, but never returns
<b>DIRTREE_RECURSE</b> from the callback, instead calling <b>dirtree_recurse</b>() manually
from elsewhere in the program. This gives ls -lR manual control
of traversal order, which is neither depth first nor breadth first but
instead a sort of FIFO order requried by the ls standard.</p>

<a name="#toys">
<h2>Directory toys/</h2>

<p>This directory contains command implementations. Each command is a single
self-contained file. Adding a new command involves adding a single
file, and removing a command involves removing that file. Commands use
shared infrastructure from the lib/ and generated/ directories.</p>

<p>Currently there are three subdirectories under "toys/" containing commands
described in POSIX-2008, the Linux Standard Base 4.1, or "other". The only
difference this makes is which menu the command shows up in during "make
menuconfig", the directories are otherwise identical. Note that they commands
exist within a single namespace at runtime, so you can't have the same
command in multiple subdirectories.</p>

<p>(There are actually four sub-menus in "make menuconfig", the fourth
contains global configuration options for toybox, and lives in Config.in at
the top level.)</p>

<p>See <a href="#adding">adding a new command</a> for details on the
layout of a command file.</p>

<h2>Directory scripts/</h2>

<p>Build infrastructure. The makefile calls scripts/make.sh for "make"
and scripts/install.sh for "make install".</p>

<p>There's also a test suite, "make test" calls make/test.sh, which runs all
the tests in make/test/*. You can run individual tests via
"scripts/test.sh command", or "TEST_HOST=1 scripts/test.sh command" to run
that test against the host implementation instead of the toybox one.</p>

<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>

<a name="generated">
<h2>Directory generated/</h2>

<p>All the files in this directory except the README are generated by the
build.  (See scripts/make.sh)</p>

<ul>
<li><p><b>config.h</b> - CFG_COMMAND and USE_COMMAND() macros set by menuconfig via .config.</p></li>

<li><p><b>Config.in</b> - Kconfig entries for each command.  Included by top level Config.in.  The help text in here is used to generated help.h</p></li>

<li><p><b>help.h</b> - Help text strings for use by "help" command.  Building
this file requires python on the host system, so the prebuilt file is shipped
in the build tarball to avoid requiring python to build toybox.</p></li>

<li><p><b>newtoys.h</b> - List of NEWTOY() or OLDTOY() macros for all available
commands.  Associates command_main() functions with command names, provides
option string for command line parsing (<a href="#lib_args">see lib/args.c</a>),
specifies where to install each command and whether toysh should fork before
calling it.</p></li>
</ul>

<p>Everything in this directory is a derivative file produced from something
else.  The entire directory is deleted by "make distclean".</p>
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