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20 <B>µ C l i b c</B>
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34 uClibc Frequently Asked Questions (FAQ)
38 <TR><TD BGCOLOR="#eeeee0">
41 This is a collection of some of the frequently asked questions
42 about uClibc. Some of the questions even have answers. If you
43 have additions to this FAQ document, we would love to add them,
45 When you are done, <a href="http://uclibc.org/">you can click here to return
46 to the uClibc home page.</a>
49 <TR><TD BGCOLOR="#ccccc0" ALIGN=left>
51 What platforms does uClibc run on?
54 <TR><TD BGCOLOR="#eeeee0">
56 Currently uClibc runs on alpha, ARM, i386, i960, h8300, m68k, mips/mipsel,
57 PowerPC, SH, SPARC, and v850 processors.
61 <TR><TD BGCOLOR="#ccccc0" ALIGN=left>
63 Why is it called uClibc?
66 <TR><TD BGCOLOR="#eeeee0">
68 The letter 'u' is short for µ (the greek letter "mu"). µ is commonly used
69 as the abbreviation for the word "micro". The capital "C" is short for
70 "controller". So the name uClibc is sortof an abbreviation for "the
71 microcontroller C library". For simplicity, uClibc is pronounced
74 The name is partly historical, since uClibc was originally
75 created to support <a href="http://www.uclinux.org">µClinux</a>, a port of
76 Linux for MMU-less microcontrollers such as the Dragonball, Coldfire, and
77 ARM7TDMI. These days, uClibc also works just fine on normal Linux systems
78 (such as i386, ARM, and PowerPC), but we couldn't think of a better name.
81 <TR><TD BGCOLOR="#ccccc0" ALIGN=left>
83 Why are you doing this? What's wrong with glibc?
86 <TR><TD BGCOLOR="#eeeee0">
88 Initially, the project began since the GNU C library lacks support for
89 MMU-less systems, and because glibc is very large. The GNU C library is
90 designed with a very different set of goals then uClibc. The GNU C library
91 is a great piece of software, make no mistake. It is compliant with just
92 about every standard ever created, and runs on just about every operating
93 system and architecture -- no small task! But there is a price to be paid
94 for that. It is quite a large library, and keeps getting larger with each
95 release. It does not even pretend to target embedded systems. To quote
96 from Ulrich Drepper, the maintainer of GNU libc: "...glibc is not the right
97 thing for [an embedded OS]. It is designed as a native library (as opposed
98 to embedded). Many functions (e.g., printf) contain functionality which is
99 not wanted in embedded systems." 24 May 1999
104 <TR><TD BGCOLOR="#ccccc0" ALIGN=left>
106 So uClibc is smaller then glibc? Doesn't that mean it completely sucks?
107 How could it be smaller and not suck?
110 <TR><TD BGCOLOR="#eeeee0">
112 uClibc has been designed from the ground up to be a C library for embedded
113 Linux. We don't need to worry about things like MS-DOS support, or BeOS,
114 or AmigaOs any other system. This lets us cut out a lot of complexity and
115 very carefully optimize for Linux. By very careful design, we can also
116 take a few shortcuts.
118 For example, glibc's stdio code (handling things
119 like printf, scanf, fopen, etc) has been evolved over many years by
120 patching various bits of additional functionality as needed. uClibc's
121 stdio code was written by just one person, and was carefully designed from
122 the outset to comply with the latest standards while carefully reusing code
123 and being as small and configurable as possible, In this way, uClibc's
126 There are many similar examples.
128 In other cases, uClibc
129 leaves certain features (such as full C99 Math library support, IPV6, and
130 RPC support) disabled by default. Those features can be enabled for people
131 that need them, but are otherwise disabled to save space.
135 Glibc is a general purpose C library, and so as policy things are optimized
136 for speed. Most of uClibc's routines have been very carefully written to
137 optimize them for size instead.
141 The end result is a C library that will compile just about everything you
142 throw at it, that looks like glibc to application programs when you
143 compile, but is many times smaller.
148 <TR><TD BGCOLOR="#ccccc0" ALIGN=left>
150 Why should I use uClibc?
153 <TR><TD BGCOLOR="#eeeee0">
155 I don't know if you should use uClibc or not. It depends on your needs.
156 If you are building an embedded Linux system and you are tight on space, then
157 using uClibc instead if glibc may be a very good idea.
159 If you are trying to build a huge fileserver for your company that will
160 have 12 Terabytes of storage, then using glibc may make more sense.
161 Unless, for example, that 12 Terabytes will be Network Attached Storage
162 and you plan to burn Linux into the system's firmware...
167 <TR><TD BGCOLOR="#ccccc0" ALIGN=left>
169 If I use uClibc, do I have to release all my source code to the world for
170 free? I want to create a closed source commercial application and I want
171 to protect my intellectual property.
174 <TR><TD BGCOLOR="#eeeee0">
176 No, you do not need to give away your application source code just because
177 you use uClibc and/or run on Linux. uClibc is licensed under the <a
178 href="http://www.gnu.org/copyleft/lesser.html">Lesser GPL</a> licence, just
179 like the GNU C library (glibc). Please read this licence, or have a lawyer
180 read this licence if you have any questions. Here is my brief summary...
181 Using shared libraries makes complying with the license easy. You can
182 distribute a closed source application which is linked with an unmodified
183 uClibc shared library. In this case, you do not need to give away any
184 source code for your application or for the uClibc library. Please
185 consider sharing some of the money you make with us! :-)
188 If you make any changes to uClibc, and distribute uClibc or distribute any
189 applications using your modified version, you must also distribute the
190 source code for uClibc containing all of your changes.
193 If you distribute an application which has uClibc statically linked, you
194 must also make your application available as an object file which can later
195 be re-linked against updated versions of uClibc. This will (in theory)
196 allow your customers to apply uClibc bug fixes to your application. You do
197 not need to make the application object file available to everyone, just to
198 those you gave the fully linked application.
202 <TR><TD BGCOLOR="#ccccc0" ALIGN=left>
204 Can I use it on my desktop i386 system?
207 <TR><TD BGCOLOR="#eeeee0">
209 Sure! In fact, this can be very nice during development. By
210 installing uClibc on your development system, you can be sure that
211 the code you are working on will actually run when you deploy it
217 <TR><TD BGCOLOR="#ccccc0" ALIGN=left>
219 Does uClibc support shared libraries?
222 <TR><TD BGCOLOR="#eeeee0">
224 Yes. uClibc has native shared library support on i386, ARM, mips/mipsel,
225 SH, CRIS, and PowerPC processors. Other architectures can use shared libraries
226 but will need to use the GNU libc shared library loader.
228 Shared Libraries are not currently supported by uClibc on MMU-less systems.
229 <a href="http://www.snapgear.com/">SnapGear</a> has implemented
230 shared library support for MMU-less systems, however, so if you need MMU-less
231 shared library support they may be able to help.
235 <TR><TD BGCOLOR="#ccccc0" ALIGN=left>
237 How do I compile programs with uClibc?
240 <TR><TD BGCOLOR="#eeeee0">
242 The easiest way is to use the compiler wrapper built by uClibc. Instead of
243 using your usual compiler or cross compiler, you can use i386-uclibc-gcc,
244 (or whatever is appropriate for your target architecture) and your
245 applications will auto-magically link against uClibc. You can also
246 build your own native uClibc toolchain. Just download the uClibc toolchain
248 <a href="/cgi-bin/cvsweb/toolchain/">uClibc toolchain builder</a>,
249 or the uClibc buildroot system from
250 <a href="/cgi-bin/cvsweb/buildroot/">uClibc buildroot system</a>,
251 adjust the Makefile settings to match your target system, and then run 'make'.
253 If you want to be <em>really</em> lazy and start using uClibc right
254 away without needing to compile your own toolchain or anything, you can
255 grab a copy of the uClibc development systems, currently available for
256 <a href="http://www.uclibc.org/downloads/root_fs-i386.bz2">i386</a>,
257 <a href="http://www.uclibc.org/downloads/root_fs-powerpc.bz2">powerpc</a>,
259 <a href="http://www.uclibc.org/downloads/root_fs-arm.bz2">arm</a>.
260 These are pre-built uClibc only development systems (created using
261 <a href="/cgi-bin/cvsweb/buildroot/">buildroot</a>), and provide a
262 really really easy way to get started. These are about 20 MB bzip2
263 compressed ext2 filesystems containing all the development software you
264 need to build your own uClibc applications. With bash, awk, make, gcc, g++,
265 autoconf, automake, ncurses, zlib, openssl, openssh, gdb, strace, busybox,
266 GNU coreutils, GNU tar, GNU grep, etc, these should have pretty much everything
267 you need to get started building your own applications linked against
268 uClibc. You can boot into them, loop mount them, dd them to a spare drive
269 and use resize2fs to make them fill a partition... Whatever works best
273 <TR><TD BGCOLOR="#ccccc0" ALIGN=left>
275 Why do I keep getting "sh: can't access tty; job control turned off" errors?
276 Why doesn't Control-C work within my shell?
279 <TR><TD BGCOLOR="#eeeee0">
281 This isn't really a uClibc question, but I'll answer it here anyways. Job
282 control will be turned off since your shell can not obtain a controlling
283 terminal. This typically happens when you run your shell on /dev/console.
284 The kernel will not provide a controlling terminal on the /dev/console
285 device. Your should run your shell on a normal tty such as tty1 or ttyS0
286 and everything will work perfectly. If you <em>REALLY</em> want your shell
287 to run on /dev/console, then you can hack your kernel (if you are into that
288 sortof thing) by changing drivers/char/tty_io.c to change the lines where
289 it sets "noctty = 1;" to instead set it to "0". I recommend you instead
290 run your shell on a real console...
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296 How do I make autoconf and automake behave?
299 <TR><TD BGCOLOR="#eeeee0">
302 <pre>export PATH=/usr/i386-linux-uclibc/bin:$PATH</pre>
303 (or similar adjusted for your target architecture) then run you can simply
304 run autoconf/automake and it should _just work_. Unfortunately, a large
305 number of configure scripts (such as the one from openssh) try to execute
306 test applications using your target C library, even if you are cross-
307 compiling. This is bad, since it will prevent these programs from compiling.
308 You need to complain to the authors of these programs and ask them to fix
309 their broken configure scripts.
315 <TR><TD BGCOLOR="#ccccc0" ALIGN=left>
317 When I run 'ldd' to get a list of the library dependencies for a uClibc
318 binary, ldd segfaults! What should I do?
321 <TR><TD BGCOLOR="#eeeee0">
323 Use the ldd that is built by uClibc, not your system's one. When your
324 system's ldd looks for library dependencies, it actually _runs_ that
325 program. This works fine -- usually. It generally will not work at all
326 when you have been cross compiling (which is why ldd segfaults). The ldd
327 program created by uClibc is cross platform and doesn't even try to run
328 the target program (like your system one does). So use the uClibc one
329 and it will do the right thing, and it won't segfault even when you are
334 <TR><TD BGCOLOR="#ccccc0" ALIGN=left>
336 Why does localtime() return times in UTC even when I have my timezone set?
339 <TR><TD BGCOLOR="#eeeee0">
342 The uClibc time functions get timezone information from the TZ environment
343 variable, as described in the Single Unix Specification Version 3. See
344 <a href="http://www.opengroup.org/onlinepubs/007904975/basedefs/xbd_chap08.html">
345 http://www.opengroup.org/onlinepubs/007904975/basedefs/xbd_chap08.html</a>
346 for details on valid settings of TZ. For some additional examples, read
347 <a href="http://www.uclibc.org/lists/uclibc/2002-August/006261.html">
348 http://www.uclibc.org/lists/uclibc/2002-August/006261.html</a> in the uClibc
349 mailing list archive.
350 You can store the value of TZ in the file '/etc/TZ' and uClibc will then
351 automagically use the specified setting.
355 <TR><TD BGCOLOR="#ccccc0" ALIGN=left>
357 What is the history of uClibc? Where did it come from?
360 <TR><TD BGCOLOR="#eeeee0">
362 The history and origin of uClibc is long and twisty.
363 In the beginning, there was <a href="http://www.gnu.org/software/libc/libc.html">GNU libc</a>. Then, libc4
364 (which later became linux libc 5) forked from GNU libc version 1.07.4, with
365 additions from 4.4BSD, in order to support Linux. Later, the <a
366 href="http://www.cix.co.uk/~mayday/">Linux-8086 C library</a>, which is part of
367 the <a href="http://www.elks.ecs.soton.ac.uk/">elks project</a>, was created,
368 which was, apparently, largely written from scratch but also borrowed code from
369 libc4, glibc, some Atari library code, with bits and pieces from about 20 other
370 places. Then uClibc forked off from the Linux-8086 C library in order to run
371 on <a href="http://www.uclinux.org">µClinux</a>.
374 I had for some time been despairing over the state of C libraries in Linux.
375 GNU libc, the standard, is very poorly suited to embedded systems and
376 has been getting bigger with every release. I spent quite a bit of time looking over the
377 available Open Source C libraries that I knew of (listed below), and none of them really
378 impressed me. I felt there was a real vacancy in the embedded Linux ecology.
379 The closest library to what I imagined an embedded C library should be was
380 uClibc. But it had a lot of problems too -- not the least of which was that,
381 traditionally, uClibc had a complete source tree fork in order to support each
382 and every new platform. This resulted in a big mess of twisty versions, all
383 different. I decided to fix it and the result is what you see here.
384 My source tree has now become the official uClibc source tree and it now lives
385 on cvs.uclinux.org and www.uclibc.org.
389 To start with, (with some initial help from <a
390 href="http://www.uclinux.org/developers/">D. Jeff Dionne</a>), I
391 ported it to run on i386. I then grafted in the header files from glibc 2.1.3
392 and cleaned up the resulting breakage. This (plus some additional work) has
393 made it almost completely independent of kernel headers, a large departure from
394 its traditional tightly-coupled-to-the-kernel origins. I have written and/or
395 rewritten a number of things that were missing or broken, and sometimes grafted
396 in bits of code from the current glibc and libc5. I have also built a proper
397 platform abstraction layer, so now you can simply edit the file "Config" and
398 use that to decide which architecture you will be compiling for, and whether or
399 not your target has an MMU, and FPU, etc. I have also added a test suite,
400 which, though incomplete, is a good start. Several people have helped by
401 contributing ports to new architectures, and a lot of work has been done on
402 adding support for missing features.
406 These days, uClibc is being developed and enhanced by Erik Andersen of
407 <a href="http://codepoet-consulting.com/">CodePoet Consulting</a> along
408 with the rest of the embedded Linux community.
413 <TR><TD BGCOLOR="#ccccc0" ALIGN=left>
415 I demand that you to add <favorite feature> right now! How come
416 you don't answer all my questions on the mailing list instantly? I demand
417 that you help me with all of my problems <em>Right Now</em>!
420 <TR><TD BGCOLOR="#eeeee0">
422 You have not paid us a single cent and yet you still have the
423 product of nearly two years of work from Erik and Manuel and
424 many other people. We are not your slaves! We work on uClibc
425 because we find it interesting. If you go off flaming us, we will
430 <TR><TD BGCOLOR="#ccccc0" ALIGN=left>
432 I need you to add <favorite feature>! Are the uClibc developers willing to
433 be paid in order to fix bugs or add in <favorite feature>? Are you willing to provide
437 <TR><TD BGCOLOR="#eeeee0">
439 Sure! Now you have our attention! What you should do is contact <a
440 href="mailto:andersen@codepoet.org">Erik Andersen</a> of <a
441 href="http://codepoet-consulting.com/">CodePoet Consulting</a> to bid
442 on your project. If Erik is too busy to personally add your feature, there
443 are several other active uClibc contributors who will almost certainly be able
444 to help you out. Erik can contact them and ask them about their availability.
448 <TR><TD BGCOLOR="#ccccc0" ALIGN=left>
450 I think you guys are great and I want to help support your work!
453 <TR><TD BGCOLOR="#eeeee0">
455 Wow, that would be great! You can click here to help support uClibc and/or request features.
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472 <a href="http://codepoet-consulting.com/">CodePoet Consulting</a> here.
475 <TR><TD BGCOLOR="#ccccc0" ALIGN=left>
477 Ok, I'm done reading all these questions.
480 <TR><TD BGCOLOR="#eeeee0">
482 <a href="http://uclibc.org/">Well then, click here to return to the uClibc home page.</a>
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500 Mail all comments, insults, suggestions and bribes to
501 <a href="mailto:andersen@codepoet.org">Erik Andersen</a><BR>
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