diff options
| -rw-r--r-- | docs/busybox.net/programming.html | 114 |
1 files changed, 114 insertions, 0 deletions
diff --git a/docs/busybox.net/programming.html b/docs/busybox.net/programming.html index 61777af..b73e6ef 100644 --- a/docs/busybox.net/programming.html +++ b/docs/busybox.net/programming.html @@ -12,12 +12,14 @@ </ul> <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> @@ -180,6 +182,82 @@ 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 @@ -411,6 +489,42 @@ 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> _two years_ 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>We should never directly include kernel headers when there's a better +way to do it, but 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 |