| µ C l i b c |
| uClibc Frequently Asked Questions (FAQ) |
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This is a collection of some of the frequently asked questions
about uClibc. Some of the questions even have answers. If you
have additions to this FAQ document, we would love to add them,
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| What platforms does uClibc run on? |
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Currently uClibc runs on arm, i386, m68k, mipsel, powerpc, sh,
sparc, and v850.
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| Does uClibc support shared libraries? |
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Yes. uClibc has shared library support on x86, arm, and powerpc.
Shared Libraries are _not_ currently supported on MMU-less systems.
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| Why is it called uClibc? |
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For simplicity, uClibc is pronounced "yew-see-lib-see". The letter
'u' is short for µ (the greek letter "mu"). µ is commonly used as
the abbreviation for the word "micro". The capital "C" is short
for "controller". So uClibc is sortof an abbreviation for "the
microcontroller C library". This is partly historical, since
uClibc was originally created to support µClinux, a port of Linux
for MMU-less microcontrollers such as the Dragonball, Coldfire, and
ARM7TDMI. These days, uClibc works just fine with normal Linux
system (like on x86, strongArm, and powerpc).
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| Can I use it on my desktop x86 system? |
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Sure! In fact, this can be very nice during development. By
installing uClibc on your development system, you can be sure that
the code you are working on will actually run when you deploy it
your target system.
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| Why are you doing this? What's wrong with glibc? |
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Initially, the project began because glibc does not support
MMU-less systems. But uClibc is also very useful because it is so
much smaller then the GNU C library. The GNU C library is designed
with a very different set of goals then uClibc. The GNU C library
is a great piece of software, make no mistake. It is compliant to
just about every standard ever created, and runs on just about
every operating system and architecture -- no small task! But
there is a price to be paid for that. It is quite a large library,
and keeps getting larger with each release. It does not even
pretend to target embedded systems. To quote from Ulrich Drepper,
the maintainer of GNU libc: "...glibc is not the right thing for
[an embedded OS]. It is designed as a native library (as opposed to
embedded). Many functions (e.g., printf) contain functionality
which is not wanted in embedded systems." 24 May 1999
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| So uClibc is smaller then glibc? Doesn't that mean it completely sucks? How could it be smaller and not suck? |
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uClibc has been designed from the ground up to be a C library for
embedded Linux. We don't need to worry about things like MS-DOS
support, or Cygwin, or AmigaOs any other system. This lets us cut out
a lot of complexity and very carefully optimize for Linux. By very
careful design, we can also take a few shortcuts. For example, glibc
contains an implementation of the wordexp() function, in compliance
with the Single Unix Specification, version 2. Well, standards are
important. But so is pragmatism. The wordexp function is huge, yet I
am not aware of even one Linux application that uses it! So uClibc
doesn't provide wordexp(). There are many similar examples. In other
cases, uClibc leaves certain features (such as full C99 Math library
support, IPV6, and RPC support) disabled by default. Those features
can be enabled for people that need then, but are otherwise disabled to
save space.
Glibc is a general purpose C library, and so as policy things are optimized for speed. Most of uClibc's routines have been very carefully written to optimize them for size instead. The end result is a C library that will compile just about everything you throw at it, that looks like glibc to application programs when you compile, but is many times smaller.
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| Why should I use uClibc? |
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I don't know if you should use uClibc or not. It depends on your needs.
If you are building an embedded Linux system and you are tight on space, then
using uClibc instead if glibc may be a very good idea.
If you are trying to build a huge fileserver for your company that will
have 12 Terabytes of storage, then using glibc may make more sense...
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| I want to create a closed source commercial application and I want to protect my intellectual property. If I use uClibc, don't I have to release all my source code for free? Is that legal? |
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No, you do not need to give away your source code just because you use
uClibc and/or run on Linux. uClibc is licensed under the LGPL, just
like GNU libc. If you are using uClibc as a shared library, then your
closed source application is 100% legal. Please consider sharing some
of the money you make with us! :-)
If you are statically linking your closed source application with uClibc, then you must take additional steps to comply with the uClibc license. You may sell your statically linked application as usual, but you must also make your application available to your customers as an object file which can later be re-linked against updated versions of uClibc. This will (in theory) allow your customers to apply uClibc bug fixes to your application. You do not need to make the application object file available to everyone, just to those you gave the fully linked application.
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| How do I compile stuff? |
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The easiest way is to use the compiler wrapper built by uClibc. Instead of
using your usual compiler or cross compiler, you can use i386-uclibc-gcc,
(or whatever is appropriate for your target architecture) and your
applications will auto-magically link against uClibc.
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| How do I make autoconf and automake behave? |
First run
export PATH=/usr/i386-linux-uclibc/bin:$PATH(or similar adjusted for your target architecture) then run you can simply run autoconf/automake and it should _just work_.
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| When I run 'ldd' to get a list of the library dependencies for a uClibc binary, ldd segfaults! Or it runs my application! Anyways, it doesn't work! What should I do? |
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Use the ldd that is built by uClibc, not your system's one. When your
system's ldd looks for library dependencies, it actually _runs_ that
program. This works fine -- usually. I doesn't work at all when you
have been cross compiling (which is why ldd segfaults). The ldd
program created by uClibc is cross platform and doesn't even try to run
the target program (like your system one does). So use the uClibc one
and it will do the right thing, and it won't segfault even when you are
cross compiling.
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| What is the history of uClibc? Where did it come from? |
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The history and origin of uClibc is long and twisty.
In the beginning, there was GNU libc. Then, libc4
(which later became linux libc 5) forked from GNU libc version 1.07.4, with
additions from 4.4BSD, in order to support Linux. Later, the Linux-8086 C library, which is part of
the elks project, was created,
which was, apparently, largely written from scratch but also borrowed code from
libc4, glibc, some Atari library code, with bits and pieces from about 20 other
places. Then uClibc forked off from the Linux-8086 C library in order to run
on µClinux.
I had for some time been despairing over the state of C libraries in Linux. GNU libc, the standard, is very poorly suited to embedded systems and has been getting bigger with every release. I spent quite a bit of time looking over the available Open Source C libraries that I knew of (listed below), and none of them really impressed me. I felt there was a real vacancy in the embedded Linux ecology. The closest library to what I imagined an embedded C library should be was uClibc. But it had a lot of problems too -- not the least of which was that, traditionally, uClibc had a complete source tree fork in order to support each and every new platform. This resulted in a big mess of twisty versions, all different. I decided to fix it and the result is what you see here. My source tree has now become the official uClibc source tree and it now lives on cvs.uclinux.org and www.uclibc.org. To start with, (with some initial help from D. Jeff Dionne), I ported it to run on x86. I then grafted in the header files from glibc 2.1.3 and cleaned up the resulting breakage. This (plus some additional work) has made it almost completely independent of kernel headers, a large departure from its traditional tightly-coupled-to-the-kernel origins. I have written and/or rewritten a number of things that were missing or broken, and sometimes grafted in bits of code from the current glibc and libc5. I have also built a proper platform abstraction layer, so now you can simply edit the file "Config" and use that to decide which architecture you will be compiling for, and whether or not your target has an MMU, and FPU, etc. I have also added a test suite, which, though incomplete, is a good start. Several people have helped by contributing ports to new architectures, and a lot of work has been done on adding support for missing features.
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| I demand that you to add <favorite feature> right now! How come you don't answer all my questions on the mailing list instantly? I demand that you help me with all of my problems Right Now! |
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You have not paid us a single cent and yet you still have the
product of nearly two years of work from Erik and Manuel and
many other people. We are not your slaves! We work on uClibc
because we find it interesting. If you go off flaming us, we will
ignore you.
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| I need you to add <favorite feature>! Are the uClibc developers willing to be paid in order to add in <favorite feature>? Are you willing to provide support contracts? |
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Sure! Now you have our attention! What you should do is contact Erik Andersen of CodePoet Consulting to bid
on your project. If Erik is too busy to personally add your feature, there
are several other active uClibc contributors who will almost certainly be able
to help you out. Erik can contact them and ask them about their availability.
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| I think you guys are great and I want to help support your work! |
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Wow, that would be great! You can click here to help support uClibc and/or request features.
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| Ok, I'm done reading all these questions. |
| Well then, click here to return to the uClibc home page. |
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Mail all comments, insults, suggestions and bribes to
Erik Andersen |
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