[ < ] [ > ]   [ << ] [ Up ] [ >> ]         [Top] [Contents] [Index] [ ? ]

C. Installing the GNU C Library

Before you do anything else, you should read the file ‘FAQ’ located at the top level of the source tree. This file answers common questions and describes problems you may experience with compilation and installation. It is updated more frequently than this manual.

Features can be added to GNU Libc via add-on bundles. These are separate tar files, which you unpack into the top level of the source tree. Then you give configure the ‘--enable-add-ons’ option to activate them, and they will be compiled into the library.

You will need recent versions of several GNU tools: definitely GCC and GNU Make, and possibly others. See section Recommended Tools for Compilation, below.

[ < ] [ > ]   [ << ] [ Up ] [ >> ]         [Top] [Contents] [Index] [ ? ]

C.1 Configuring and compiling GNU Libc

GNU libc cannot be compiled in the source directory. You must build it in a separate build directory. For example, if you have unpacked the glibc sources in ‘/src/gnu/glibc-2.4’, create a directory ‘/src/gnu/glibc-build’ to put the object files in. This allows removing the whole build directory in case an error occurs, which is the safest way to get a fresh start and should always be done.

From your object directory, run the shell script ‘configure’ located at the top level of the source tree. In the scenario above, you'd type

$ ../glibc-2.4/configure args…

Please note that even though you're building in a separate build directory, the compilation needs to modify a few files in the source directory, especially some files in the manual subdirectory.

configure takes many options, but the only one that is usually mandatory is ‘--prefix’. This option tells configure where you want glibc installed. This defaults to ‘/usr/local’, but the normal setting to install as the standard system library is ‘--prefix=/usr’ for GNU/Linux systems and ‘--prefix=’ (an empty prefix) for GNU/Hurd systems.

It may also be useful to set the CC and CFLAGS variables in the environment when running configure. CC selects the C compiler that will be used, and CFLAGS sets optimization options for the compiler.

The following list describes all of the available options for configure:


Install machine-independent data files in subdirectories of ‘directory’. The default is to install in ‘/usr/local’.


Install the library and other machine-dependent files in subdirectories of ‘directory’. The default is to the ‘--prefix’ directory if that option is specified, or ‘/usr/local’ otherwise.


Look for kernel header files in directory, not ‘/usr/include’. Glibc needs information from the kernel's private header files. Glibc will normally look in ‘/usr/include’ for them, but if you specify this option, it will look in DIRECTORY instead.

This option is primarily of use on a system where the headers in ‘/usr/include’ come from an older version of glibc. Conflicts can occasionally happen in this case. Note that Linux libc5 qualifies as an older version of glibc. You can also use this option if you want to compile glibc with a newer set of kernel headers than the ones found in ‘/usr/include’.


Specify add-on packages to include in the build. If this option is specified with no list, it enables all the add-on packages it finds in the main source directory; this is the default behavior. You may specify an explicit list of add-ons to use in list, separated by spaces or commas (if you use spaces, remember to quote them from the shell). Each add-on in list can be an absolute directory name or can be a directory name relative to the main source directory, or relative to the build directory (that is, the current working directory). For example, ‘--enable-add-ons=nptl,../glibc-libidn-2.4’.


This option is currently only useful on GNU/Linux systems. The version parameter should have the form X.Y.Z and describes the smallest version of the Linux kernel the generated library is expected to support. The higher the version number is, the less compatibility code is added, and the faster the code gets.


Use the binutils (assembler and linker) in ‘directory’, not the ones the C compiler would default to. You can use this option if the default binutils on your system cannot deal with all the constructs in the GNU C library. In that case, configure will detect the problem and suppress these constructs, so that the library will still be usable, but functionality may be lost—for example, you can't build a shared libc with old binutils.


Use this option if your computer lacks hardware floating-point support and your operating system does not emulate an FPU.



Don't build shared libraries even if it is possible. Not all systems support shared libraries; you need ELF support and (currently) the GNU linker.


Don't build libraries with profiling information. You may want to use this option if you don't plan to do profiling.


Use maximum optimization for the normal (static and shared) libraries, and compile separate static libraries with debugging information and no optimization. We recommend not doing this. The extra optimization doesn't gain you much, it may provoke compiler bugs, and you won't be able to trace bugs through the C library.


Don't compile the shared libraries with symbol version information. Doing this will make the resulting library incompatible with old binaries, so it's not recommended.


Compile static versions of the NSS (Name Service Switch) libraries. This is not recommended because it defeats the purpose of NSS; a program linked statically with the NSS libraries cannot be dynamically reconfigured to use a different name database.


By default the C library is built with support for thread-local storage if the used tools support it. By using ‘--without-tls’ this can be prevented though there generally is no reason since it creates compatibility problems.


These options are for cross-compiling. If you specify both options and build-system is different from host-system, configure will prepare to cross-compile glibc from build-system to be used on host-system. You'll probably need the ‘--with-headers’ option too, and you may have to override configure's selection of the compiler and/or binutils.

If you only specify ‘--host’, configure will prepare for a native compile but use what you specify instead of guessing what your system is. This is most useful to change the CPU submodel. For example, if configure guesses your machine as i586-pc-linux-gnu but you want to compile a library for 386es, give ‘--host=i386-pc-linux-gnu’ or just ‘--host=i386-linux’ and add the appropriate compiler flags (‘-mcpu=i386’ will do the trick) to CFLAGS.

If you specify just ‘--build’, configure will get confused.

To build the library and related programs, type make. This will produce a lot of output, some of which may look like errors from make but isn't. Look for error messages from make containing ‘***’. Those indicate that something is seriously wrong.

The compilation process can take a long time, depending on the configuration and the speed of your machine. Some complex modules may take a very long time to compile, as much as several minutes on slower machines. Do not panic if the compiler appears to hang.

If you want to run a parallel make, simply pass the ‘-j’ option with an appropriate numeric parameter to make. You need a recent GNU make version, though.

To build and run test programs which exercise some of the library facilities, type make check. If it does not complete successfully, do not use the built library, and report a bug after verifying that the problem is not already known. See section Reporting Bugs, for instructions on reporting bugs. Note that some of the tests assume they are not being run by root. We recommend you compile and test glibc as an unprivileged user.

Before reporting bugs make sure there is no problem with your system. The tests (and later installation) use some pre-existing files of the system such as ‘/etc/passwd’, ‘/etc/nsswitch.conf’ and others. These files must all contain correct and sensible content.

To format the GNU C Library Reference Manual for printing, type make dvi. You need a working TeX installation to do this. The distribution already includes the on-line formatted version of the manual, as Info files. You can regenerate those with make info, but it shouldn't be necessary.

The library has a number of special-purpose configuration parameters which you can find in ‘Makeconfig’. These can be overwritten with the file ‘configparms’. To change them, create a ‘configparms’ in your build directory and add values as appropriate for your system. The file is included and parsed by make and has to follow the conventions for makefiles.

It is easy to configure the GNU C library for cross-compilation by setting a few variables in ‘configparms’. Set CC to the cross-compiler for the target you configured the library for; it is important to use this same CC value when running configure, like this: ‘CC=target-gcc configure target’. Set BUILD_CC to the compiler to use for programs run on the build system as part of compiling the library. You may need to set AR and RANLIB to cross-compiling versions of ar and ranlib if the native tools are not configured to work with object files for the target you configured for.

[ < ] [ > ]   [ << ] [ Up ] [ >> ]         [Top] [Contents] [Index] [ ? ]

C.2 Installing the C Library

To install the library and its header files, and the Info files of the manual, type env LANGUAGE=C LC_ALL=C make install. This will build things, if necessary, before installing them; however, you should still compile everything first. If you are installing glibc as your primary C library, we recommend that you shut the system down to single-user mode first, and reboot afterward. This minimizes the risk of breaking things when the library changes out from underneath.

If you're upgrading from Linux libc5 or some other C library, you need to replace the ‘/usr/include’ with a fresh directory before installing it. The new ‘/usr/include’ should contain the Linux headers, but nothing else.

You must first build the library (‘make’), optionally check it (‘make check’), switch the include directories and then install (‘make install’). The steps must be done in this order. Not moving the directory before install will result in an unusable mixture of header files from both libraries, but configuring, building, and checking the library requires the ability to compile and run programs against the old library.

If you are upgrading from a previous installation of glibc 2.0 or 2.1, ‘make install’ will do the entire job. You do not need to remove the old includes – if you want to do so anyway you must then follow the order given above.

You may also need to reconfigure GCC to work with the new library. The easiest way to do that is to figure out the compiler switches to make it work again (‘-Wl,--dynamic-linker=/lib/ld-linux.so.2’ should work on GNU/Linux systems) and use them to recompile gcc. You can also edit the specs file (‘/usr/lib/gcc-lib/TARGET/VERSION/specs’), but that is a bit of a black art.

You can install glibc somewhere other than where you configured it to go by setting the install_root variable on the command line for ‘make install’. The value of this variable is prepended to all the paths for installation. This is useful when setting up a chroot environment or preparing a binary distribution. The directory should be specified with an absolute file name.

Glibc 2.2 includes a daemon called nscd, which you may or may not want to run. nscd caches name service lookups; it can dramatically improve performance with NIS+, and may help with DNS as well.

One auxiliary program, ‘/usr/libexec/pt_chown’, is installed setuid root. This program is invoked by the grantpt function; it sets the permissions on a pseudoterminal so it can be used by the calling process. This means programs like xterm and screen do not have to be setuid to get a pty. (There may be other reasons why they need privileges.) If you are using a 2.1 or newer Linux kernel with the devptsfs or devfs filesystems providing pty slaves, you don't need this program; otherwise you do. The source for ‘pt_chown’ is in ‘login/programs/pt_chown.c’.

After installation you might want to configure the timezone and locale installation of your system. The GNU C library comes with a locale database which gets configured with localedef. For example, to set up a German locale with name de_DE, simply issue the command ‘localedef -i de_DE -f ISO-8859-1 de_DE’. To configure all locales that are supported by glibc, you can issue from your build directory the command ‘make localedata/install-locales’.

To configure the locally used timezone, set the TZ environment variable. The script tzselect helps you to select the right value. As an example, for Germany, tzselect would tell you to use ‘TZ='Europe/Berlin'’. For a system wide installation (the given paths are for an installation with ‘--prefix=/usr’), link the timezone file which is in ‘/usr/share/zoneinfo’ to the file ‘/etc/localtime’. For Germany, you might execute ‘ln -s /usr/share/zoneinfo/Europe/Berlin /etc/localtime’.

[ < ] [ > ]   [ << ] [ Up ] [ >> ]         [Top] [Contents] [Index] [ ? ]

C.3 Recommended Tools for Compilation

We recommend installing the following GNU tools before attempting to build the GNU C library:

If you change any of the ‘configure.in’ files you will also need

and if you change any of the message translation files you will need

You may also need these packages if you upgrade your source tree using patches, although we try to avoid this.

[ < ] [ > ]   [ << ] [ Up ] [ >> ]         [Top] [Contents] [Index] [ ? ]

C.4 Specific advice for GNU/Linux systems

If you are installing GNU libc on a GNU/Linux system, you need to have the header files from a 2.2 or newer kernel around for reference. For some architectures, like ia64, sh and hppa, you need at least headers from kernel 2.3.99 (sh and hppa) or 2.4.0 (ia64). You do not need to use that kernel, just have its headers where glibc can access at them. The easiest way to do this is to unpack it in a directory such as ‘/usr/src/linux-2.2.1’. In that directory, run ‘make config’ and accept all the defaults. Then run ‘make include/linux/version.h’. Finally, configure glibc with the option ‘--with-headers=/usr/src/linux-2.2.1/include’. Use the most recent kernel you can get your hands on.

An alternate tactic is to unpack the 2.2 kernel and run ‘make config’ as above; then, rename or delete ‘/usr/include’, create a new ‘/usr/include’, and make symbolic links of ‘/usr/include/linux’ and ‘/usr/include/asm’ into the kernel sources. You can then configure glibc with no special options. This tactic is recommended if you are upgrading from libc5, since you need to get rid of the old header files anyway.

After installing GNU libc, you may need to remove or rename ‘/usr/include/linux’ and ‘/usr/include/asm’, and replace them with copies of ‘include/linux’ and ‘include/asm-$ARCHITECTURE’ taken from the Linux source package which supplied kernel headers for building the library. ARCHITECTURE will be the machine architecture for which the library was built, such as ‘i386’ or ‘alpha’. You do not need to do this if you did not specify an alternate kernel header source using ‘--with-headers’. The intent here is that these directories should be copies of, not symlinks to, the kernel headers used to build the library.

Note that ‘/usr/include/net’ and ‘/usr/include/scsi’ should not be symlinks into the kernel sources. GNU libc provides its own versions of these files.

GNU/Linux expects some components of the libc installation to be in ‘/lib’ and some in ‘/usr/lib’. This is handled automatically if you configure glibc with ‘--prefix=/usr’. If you set some other prefix or allow it to default to ‘/usr/local’, then all the components are installed there.

If you are upgrading from libc5, you need to recompile every shared library on your system against the new library for the sake of new code, but keep the old libraries around for old binaries to use. This is complicated and difficult. Consult the Glibc2 HOWTO at http://www.imaxx.net/~thrytis/glibc for details.

You cannot use nscd with 2.0 kernels, due to bugs in the kernel-side thread support. nscd happens to hit these bugs particularly hard, but you might have problems with any threaded program.

[ < ] [ > ]   [ << ] [ Up ] [ >> ]         [Top] [Contents] [Index] [ ? ]

C.5 Reporting Bugs

There are probably bugs in the GNU C library. There are certainly errors and omissions in this manual. If you report them, they will get fixed. If you don't, no one will ever know about them and they will remain unfixed for all eternity, if not longer.

It is a good idea to verify that the problem has not already been reported. Bugs are documented in two places: The file ‘BUGS’ describes a number of well known bugs and the bug tracking system has a WWW interface at http://sources.redhat.com/bugzilla/. The WWW interface gives you access to open and closed reports. A closed report normally includes a patch or a hint on solving the problem.

To report a bug, first you must find it. With any luck, this will be the hard part. Once you've found a bug, make sure it's really a bug. A good way to do this is to see if the GNU C library behaves the same way some other C library does. If so, probably you are wrong and the libraries are right (but not necessarily). If not, one of the libraries is probably wrong. It might not be the GNU library. Many historical Unix C libraries permit things that we don't, such as closing a file twice.

If you think you have found some way in which the GNU C library does not conform to the ISO and POSIX standards (see section Standards and Portability), that is definitely a bug. Report it!

Once you're sure you've found a bug, try to narrow it down to the smallest test case that reproduces the problem. In the case of a C library, you really only need to narrow it down to one library function call, if possible. This should not be too difficult.

The final step when you have a simple test case is to report the bug. Do this using the WWW interface to the bug database.

If you are not sure how a function should behave, and this manual doesn't tell you, that's a bug in the manual. Report that too! If the function's behavior disagrees with the manual, then either the library or the manual has a bug, so report the disagreement. If you find any errors or omissions in this manual, please report them to the bug database. If you refer to specific sections of the manual, please include the section names for easier identification.

[ << ] [ >> ]           [Top] [Contents] [Index] [ ? ]

This document was generated by root on May, 21 2010 using texi2html 1.78.