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Installing the GNU C Library

Before you do anything else, you should read the file `FAQ' found 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 tarfiles 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. As of the 2.2 release, one important component of glibc is distributed as "official" add-ons: the linuxthreads add-on. Unless you are doing an unusual installation, you should get this.

Support for POSIX threads is maintained by someone else, so it's in a separate package. It is only available for Linux systems, but this will change in the future. Get it from the same place you got the main bundle; the file is `glibc-linuxthreads-VERSION.tar.gz'.

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

Configuring and compiling GNU Libc

GNU libc can be compiled in the source directory, but we strongly advise to build it in a separate build directory. For example, if you have unpacked the glibc sources in `/src/gnu/glibc-2.2.0', 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' found at the top level of the source tree. In the scenario above, you'd type

$ ../glibc-2.2.0/configure args...

Please note that even if 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 you can get away with knowing only two: `--prefix' and `--enable-add-ons'. The --prefix option tells configure where you want glibc installed. This defaults to `/usr/local'. The `--enable-add-ons' option tells configure to use all the add-on bundles it finds in the source directory. Since important functionality is provided in add-ons, you should always specify this option.

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. It 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'.
Enable add-on packages in your source tree. If this option is specified with no list, it enables all the add-on packages it finds. If you do not wish to use some add-on package that you have present in your source tree, give this option a list of the add-ons that you do want used, like this: `--enable-add-ons=linuxthreads'
This option is currently only useful on 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 could 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 optimisation. We recommend against 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.
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 really wrong.

The compilation process takes several hours even on fast hardware. Expect at least two hours for the default configuration on i586 for Linux. For Hurd times are much longer. Except for EGCS 1.1 and GCC 2.95 (and later versions of GCC), all supported versions of GCC have a problem which causes them to take several minutes to compile certain files in the iconvdata directory. Do not panic if the compiler appears to hang.

If you want to run a parallel make, you can't just give make the `-j' option, because it won't be passed down to the sub-makes. Instead, edit the generated `Makefile' and uncomment the line


You can change the `4' to some other number as appropriate for your system. Instead of changing the `Makefile', you could give this option directly to make and call it as, for example, make PARALLELMFLAGS=-j4. If you're building in the source directory, you must use the latter approach since in this case no new `Makefile' is generated for you to change.

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.

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 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.

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 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, you can either 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'.

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.

Supported Configurations

The GNU C Library currently supports configurations that match the following patterns:


Former releases of this library (version 2.1 and/or 2.0) used to run on the following configurations:


Very early releases (version 1.09.1 and perhaps earlier versions) used to run on the following configurations:


Since no one has volunteered to test and fix these configurations, they are not supported at the moment. They probably don't compile; they definitely don't work anymore. Porting the library is not hard. If you are interested in doing a port, please contact the glibc maintainers by sending electronic mail to @email{bug-glibc@gnu.org}.

Valid cases of `ix86' include `i386', `i486', `i586', and `i686'. All of those configurations produce a library that can run on this processor and newer processors. The GCC compiler by default generates code that's optimized for the machine it's configured for and will use the instructions available on that machine. For example if your GCC is configured for `i686', gcc will optimize for `i686' and might issue some `i686' specific instructions. To generate code for other models, you have to configure for that model and give GCC the appropriate `-march=' and `-mcpu=' compiler switches via CFLAGS.

Specific advice for Linux systems

If you are installing GNU libc on a Linux system, you need to have the header files from a 2.2 kernel around for reference. You do not need to use the 2.2 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 the usual symbolic links of `/usr/include/linux' and `/usr/include/asm' into the 2.2 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.

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.

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 @url{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.

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 @url{http://www-gnats.gnu.org:8080/cgi-bin/wwwgnats.pl}. The WWW interface gives you access to open and closed reports. The closed reports normally include a patch or a hint on solving the problem.

To report a bug, first you must find it. Hopefully, 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 glibcbug script. It is installed with libc, or if you haven't installed it, will be in your build directory. Send your test case, the results you got, the results you expected, and what you think the problem might be (if you've thought of anything). glibcbug will insert the configuration information we need to see, and ship the report off to @email{bugs@gnu.org}. Don't send a message there directly; it is fed to a program that expects mail to be formatted in a particular way. Use the script.

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 Internet address @email{bug-glibc-manual@gnu.org}. If you refer to specific sections of the manual, please include the section names for easier identification.

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