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uClibc Frequently Asked Questions (FAQ)

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, When you are done, you can click here to return to the uClibc home page.

What platforms does uClibc run on?

Currently uClibc runs on alpha, ARM, i386, i960, h8300, m68k, mips/mipsel, PowerPC, SH, SPARC, and v850 processors.

Why is it called uClibc?

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 the name uClibc is sortof an abbreviation for "the microcontroller C library". For simplicity, uClibc is pronounced "yew-see-lib-see". The name 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 also works just fine on normal Linux systems (such as i386, ARM, and PowerPC), but we couldn't think of a better name.

Can I use it on my desktop i386 system?

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.

Does uClibc support shared libraries?

Yes. uClibc has native shared library support on i386, ARM, mips/mipsel, SH, and PowerPC processors. Other architectures can use shared libraries but will need to use the GNU libc shared library loader. Shared Libraries are not currently supported by uClibc on MMU-less systems. SnapGear has implemented shared library support for MMU-less systems, however, so if you need MMU-less shared library support they may be able to help.

Why are you doing this? What's wrong with glibc?

Initially, the project began since the GNU C library lacks support for MMU-less systems, and because glibc is very large. 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 with 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

So uClibc is smaller then glibc? Doesn't that mean it completely sucks? How could it be smaller and not suck?

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 BeOS, 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 3. 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 them, 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.

Why should I use uClibc?

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. Unless, for example, that 12 Terabytes will be Network Attached Storage and you plan to burn Linux into the system's firmware...

If I use uClibc, do I have to release all my source code to the world for free? I want to create a closed source commercial application and I want to protect my intellectual property.

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. Using shared libraries makes complying with the license easy. 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.

How do I compile programs with uClibc?

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. You can also build your own native uClibc toolchain. Just download the uClibc toolchain builder from http://www.uclibc.org/downloads/toolchain/, ajust the Makefile settings to match your target system, and then run 'make'.

I have code that uses constructors and destructors. Why is it when I use uClibc, the ctors/dtors do not run?

The uClibc compiler wrapper toolchain by default, does not enable constructor and destructor support for C code. It only enables ctors/dtors support by default for C++ code. If you have C code that uses ctors/dtors and you wish to use the uClibc compiler wrapper toolchain, you will need to add the --uclibc-ctors option to the gcc command line. i.e. $ cat test.c #include void __attribute__((constructor)) my_ctor(void) { char msg[]="I am a constructor!\n"; write(2, msg, sizeof(msg)); } int main(void) { _exit(42); } $ /usr/i386-linux-uclibc/bin/i386-uclibc-gcc --uclibc-ctors ./test.c -o test $ ./test I am a constructor! Another option is to build a native uClibc toolchain. Native toolchains always enable ctors/dtors support, even for C code.

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_.

When I run 'ldd' to get a list of the library dependencies for a uClibc binary, ldd segfaults! What should I do?

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. It generally will not 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.

Why does localtime() return times in UTC even when I have my timezone set?

The uClibc time functions get timezone information from the TZ environment variable, as described in the Single Unix Specification Version 3. See http://www.opengroup.org/onlinepubs/007904975/basedefs/xbd_chap08.html for details on valid settings of TZ. For some additional examples, read http://www.uclibc.org/lists/uclibc/2002-August/006261.html in the uClibc mailing list archive.

What is the history of uClibc? Where did it come from?

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 i386. 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. These days, uClibc is being developed and enhanced by Erik Andersen of CodePoet Consulting along with the rest of the embedded Linux community.

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!

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.

I need you to add <favorite feature>! Are the uClibc developers willing to be paid in order to fix bugs or add in <favorite feature>? Are you willing to provide support contracts?

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.

I think you guys are great and I want to help support your work!

Wow, that would be great! You can click here to help support uClibc and/or request features. If you prefer to contact us directly for payments (Erik has a credit card machine so you can avoid making payments online), hardware donations, support requests, etc., you can contact CodePoet Consulting here.

Ok, I'm done reading all these questions.

Well then, click here to return to the uClibc home page.

Mail all comments, insults, suggestions and bribes to Erik Andersen