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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.
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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 Can I use it on my desktop i386 system?
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88 Sure! In fact, this can be very nice during development. By
89 installing uClibc on your development system, you can be sure that
90 the code you are working on will actually run when you deploy it
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98 Does uClibc support shared libraries?
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103 Yes. uClibc has native shared library support on i386, ARM, mips/mipsel,
104 SH, and PowerPC processors. Other architectures can use shared libraries
105 but will need to use the GNU libc shared library loader.
107 Shared Libraries are not currently supported by uClibc on MMU-less systems.
108 <a href="http://www.snapgear.com/">SnapGear</a> has implemented
109 shared library support for MMU-less systems, however, so if you need MMU-less
110 shared library support they may be able to help.
114 <TR><TD BGCOLOR="#ccccc0" ALIGN=left>
116 Why are you doing this? What's wrong with glibc?
119 <TR><TD BGCOLOR="#eeeee0">
121 Initially, the project began since the GNU C library lacks support for
122 MMU-less systems, and because glibc is very large. The GNU C library is
123 designed with a very different set of goals then uClibc. The GNU C library
124 is a great piece of software, make no mistake. It is compliant with just
125 about every standard ever created, and runs on just about every operating
126 system and architecture -- no small task! But there is a price to be paid
127 for that. It is quite a large library, and keeps getting larger with each
128 release. It does not even pretend to target embedded systems. To quote
129 from Ulrich Drepper, the maintainer of GNU libc: "...glibc is not the right
130 thing for [an embedded OS]. It is designed as a native library (as opposed
131 to embedded). Many functions (e.g., printf) contain functionality which is
132 not wanted in embedded systems." 24 May 1999
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139 So uClibc is smaller then glibc? Doesn't that mean it completely sucks?
140 How could it be smaller and not suck?
143 <TR><TD BGCOLOR="#eeeee0">
145 uClibc has been designed from the ground up to be a C library for
146 embedded Linux. We don't need to worry about things like MS-DOS
147 support, or BeOS, or AmigaOs any other system. This lets us cut out
148 a lot of complexity and very carefully optimize for Linux. By very
149 careful design, we can also take a few shortcuts. For example, glibc
150 contains an implementation of the wordexp() function, in compliance
151 with the Single Unix Specification, version 3. Well, standards are
152 important. But so is pragmatism. The wordexp function is huge, yet I
153 am not aware of even one Linux application that uses it! So uClibc
154 doesn't provide wordexp(). There are many similar examples. In other
155 cases, uClibc leaves certain features (such as full C99 Math library
156 support, IPV6, and RPC support) disabled by default. Those features
157 can be enabled for people that need them, but are otherwise disabled to
162 Glibc is a general purpose C library, and so as policy things are optimized
163 for speed. Most of uClibc's routines have been very carefully written to
164 optimize them for size instead.
168 The end result is a C library that will compile just about everything you
169 throw at it, that looks like glibc to application programs when you
170 compile, but is many times smaller.
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177 Why should I use uClibc?
180 <TR><TD BGCOLOR="#eeeee0">
182 I don't know if you should use uClibc or not. It depends on your needs.
183 If you are building an embedded Linux system and you are tight on space, then
184 using uClibc instead if glibc may be a very good idea.
186 If you are trying to build a huge fileserver for your company that will
187 have 12 Terabytes of storage, then using glibc may make more sense.
188 Unless, for example, that 12 Terabytes will be Network Attached Storage
189 and you plan to burn Linux into the system's firmware...
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196 If I use uClibc, do I have to release all my source code to the world for
197 free? I want to create a closed source commercial application and I want
198 to protect my intellectual property.
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203 No, you do not need to give away your source code just because you use
204 uClibc and/or run on Linux. uClibc is licensed under the LGPL, just like
205 GNU libc. Using shared libraries makes complying with the license easy.
206 If you are using uClibc as a shared library, then your closed source
207 application is 100% legal. Please consider sharing some of the money you
212 If you are statically linking your closed source application with
213 uClibc, then you must take additional steps to comply with the uClibc
214 license. You may sell your statically linked application as usual, but
215 you must also make your application available to your customers as an
216 object file which can later be re-linked against updated versions of
217 uClibc. This will (in theory) allow your customers to apply uClibc bug
218 fixes to your application. You do not need to make the application
219 object file available to everyone, just to those you gave the fully
224 <TR><TD BGCOLOR="#ccccc0" ALIGN=left>
226 How do I compile programs with uClibc?
229 <TR><TD BGCOLOR="#eeeee0">
231 The easiest way is to use the compiler wrapper built by uClibc. Instead of
232 using your usual compiler or cross compiler, you can use i386-uclibc-gcc,
233 (or whatever is appropriate for your target architecture) and your
234 applications will auto-magically link against uClibc. You can also
235 build your own native uClibc toolchain. Just download the uClibc toolchain
236 builder from <a href="http://www.uclibc.org/downloads/toolchain/">
237 http://www.uclibc.org/downloads/toolchain/</a>, ajust the Makefile settings
238 to match your target system, and then run 'make'.
242 <TR><TD BGCOLOR="#ccccc0" ALIGN=left>
244 I have code that uses constructors and destructors. Why is it
245 when I use uClibc, the ctors/dtors do not run?
248 <TR><TD BGCOLOR="#eeeee0">
250 The uClibc compiler wrapper toolchain by default, does not
251 enable constructor and destructor support for C code. It
252 only enables ctors/dtors support by default for C++ code.
253 If you have C code that uses ctors/dtors and you wish to use
254 the uClibc compiler wrapper toolchain, you will need to add
255 the <b>--uclibc-ctors</b> option to the gcc command line. i.e.
261 void __attribute__((constructor)) my_ctor(void)
263 char msg[]="I am a constructor!\n";
264 write(2, msg, sizeof(msg));
272 $ /usr/i386-linux-uclibc/bin/i386-uclibc-gcc --uclibc-ctors ./test.c -o test
277 Another option is to build a native uClibc toolchain. Native toolchains
278 always enable ctors/dtors support, even for C code.
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285 How do I make autoconf and automake behave?
288 <TR><TD BGCOLOR="#eeeee0">
291 <pre>export PATH=/usr/i386-linux-uclibc/bin:$PATH</pre>
292 (or similar adjusted for your target architecture) then run you can simply
293 run autoconf/automake and it should _just work_.
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300 When I run 'ldd' to get a list of the library dependencies for a uClibc
301 binary, ldd segfaults! What should I do?
304 <TR><TD BGCOLOR="#eeeee0">
306 Use the ldd that is built by uClibc, not your system's one. When your
307 system's ldd looks for library dependencies, it actually _runs_ that
308 program. This works fine -- usually. It generally will not work at all
309 when you have been cross compiling (which is why ldd segfaults). The ldd
310 program created by uClibc is cross platform and doesn't even try to run
311 the target program (like your system one does). So use the uClibc one
312 and it will do the right thing, and it won't segfault even when you are
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319 Why does localtime() return times in UTC even when I have my timezone set?
322 <TR><TD BGCOLOR="#eeeee0">
325 The uClibc time functions get timezone information from the TZ environment
326 variable, as described in the Single Unix Specification Version 3. See
327 <a href="http://www.opengroup.org/onlinepubs/007904975/basedefs/xbd_chap08.html">
328 http://www.opengroup.org/onlinepubs/007904975/basedefs/xbd_chap08.html</a>
329 for details on valid settings of TZ. For some additional examples, read
330 <a href="http://www.uclibc.org/lists/uclibc/2002-August/006261.html">
331 http://www.uclibc.org/lists/uclibc/2002-August/006261.html</a> in the uClibc
332 mailing list archive.
336 <TR><TD BGCOLOR="#ccccc0" ALIGN=left>
338 What is the history of uClibc? Where did it come from?
341 <TR><TD BGCOLOR="#eeeee0">
343 The history and origin of uClibc is long and twisty.
344 In the beginning, there was <a href="http://www.gnu.org/software/libc/libc.html">GNU libc</a>. Then, libc4
345 (which later became linux libc 5) forked from GNU libc version 1.07.4, with
346 additions from 4.4BSD, in order to support Linux. Later, the <a
347 href="http://www.cix.co.uk/~mayday/">Linux-8086 C library</a>, which is part of
348 the <a href="http://www.elks.ecs.soton.ac.uk/">elks project</a>, was created,
349 which was, apparently, largely written from scratch but also borrowed code from
350 libc4, glibc, some Atari library code, with bits and pieces from about 20 other
351 places. Then uClibc forked off from the Linux-8086 C library in order to run
352 on <a href="http://www.uclinux.org">µClinux</a>.
355 I had for some time been despairing over the state of C libraries in Linux.
356 GNU libc, the standard, is very poorly suited to embedded systems and
357 has been getting bigger with every release. I spent quite a bit of time looking over the
358 available Open Source C libraries that I knew of (listed below), and none of them really
359 impressed me. I felt there was a real vacancy in the embedded Linux ecology.
360 The closest library to what I imagined an embedded C library should be was
361 uClibc. But it had a lot of problems too -- not the least of which was that,
362 traditionally, uClibc had a complete source tree fork in order to support each
363 and every new platform. This resulted in a big mess of twisty versions, all
364 different. I decided to fix it and the result is what you see here.
365 My source tree has now become the official uClibc source tree and it now lives
366 on cvs.uclinux.org and www.uclibc.org.
370 To start with, (with some initial help from <a
371 href="http://www.uclinux.org/developers/">D. Jeff Dionne</a>), I
372 ported it to run on i386. I then grafted in the header files from glibc 2.1.3
373 and cleaned up the resulting breakage. This (plus some additional work) has
374 made it almost completely independent of kernel headers, a large departure from
375 its traditional tightly-coupled-to-the-kernel origins. I have written and/or
376 rewritten a number of things that were missing or broken, and sometimes grafted
377 in bits of code from the current glibc and libc5. I have also built a proper
378 platform abstraction layer, so now you can simply edit the file "Config" and
379 use that to decide which architecture you will be compiling for, and whether or
380 not your target has an MMU, and FPU, etc. I have also added a test suite,
381 which, though incomplete, is a good start. Several people have helped by
382 contributing ports to new architectures, and a lot of work has been done on
383 adding support for missing features.
387 These days, uClibc is being developed and enhanced by Erik Andersen of
388 <a href="http://codepoet-consulting.com/">CodePoet Consulting</a> along
389 with the rest of the embedded Linux community.
394 <TR><TD BGCOLOR="#ccccc0" ALIGN=left>
396 I demand that you to add <favorite feature> right now! How come
397 you don't answer all my questions on the mailing list instantly? I demand
398 that you help me with all of my problems <em>Right Now</em>!
401 <TR><TD BGCOLOR="#eeeee0">
403 You have not paid us a single cent and yet you still have the
404 product of nearly two years of work from Erik and Manuel and
405 many other people. We are not your slaves! We work on uClibc
406 because we find it interesting. If you go off flaming us, we will
411 <TR><TD BGCOLOR="#ccccc0" ALIGN=left>
413 I need you to add <favorite feature>! Are the uClibc developers willing to
414 be paid in order to fix bugs or add in <favorite feature>? Are you willing to provide
418 <TR><TD BGCOLOR="#eeeee0">
420 Sure! Now you have our attention! What you should do is contact <a
421 href="mailto:andersen@codepoet.org">Erik Andersen</a> of <a
422 href="http://codepoet-consulting.com/">CodePoet Consulting</a> to bid
423 on your project. If Erik is too busy to personally add your feature, there
424 are several other active uClibc contributors who will almost certainly be able
425 to help you out. Erik can contact them and ask them about their availability.
429 <TR><TD BGCOLOR="#ccccc0" ALIGN=left>
431 I think you guys are great and I want to help support your work!
434 <TR><TD BGCOLOR="#eeeee0">
436 Wow, that would be great! You can click here to help support uClibc and/or request features.
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452 you can avoid making payments online), hardware donations, support requests, etc., you can
453 contact <a href="http://codepoet-consulting.com/">CodePoet Consulting</a> here.
456 <TR><TD BGCOLOR="#ccccc0" ALIGN=left>
458 Ok, I'm done reading all these questions.
461 <TR><TD BGCOLOR="#eeeee0">
463 <a href="http://uclibc.org/">Well then, click here to return to the uClibc home page.</a>
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481 Mail all comments, insults, suggestions and bribes to
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