malloc: Use __libc_initial to detect an inner libc

[glibc.git] / INSTALL

Installing the GNU C Library
****************************

Before you do anything else, you should read the FAQ at
<https://sourceware.org/glibc/wiki/FAQ>.  It answers common questions
and describes problems you may experience with compilation and
installation.

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

Configuring and compiling the GNU C Library
===========================================

The GNU C Library cannot be compiled in the source directory.  You must
build it in a separate build directory.  For example, if you have
unpacked the GNU C Library sources in '/src/gnu/glibc-VERSION', 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-VERSION/configure ARGS...

   Please note that even though you're building in a separate build
directory, the compilation may need to create or modify files and
directories in the source directory.

'configure' takes many options, but the only one that is usually
mandatory is '--prefix'.  This option tells 'configure' where you want
the GNU C Library 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 pass 'CC=COMPILER' and 'CFLAGS=FLAGS'
arguments to 'configure'.  'CC' selects the C compiler that will be
used, and 'CFLAGS' sets optimization options for the compiler.  Any
compiler options required for all compilations, such as options
selecting an ABI or a processor for which to generate code, should be
included in 'CC'.  Options that may be overridden by the GNU C Library
build system for particular files, such as for optimization and
debugging, should go in 'CFLAGS'.  The default value of 'CFLAGS' is '-g
-O2', and the GNU C Library cannot be compiled without optimization, so
if 'CFLAGS' is specified it must enable optimization.  For example:

     $ ../glibc-VERSION/configure CC="gcc -m32" CFLAGS="-O3"

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

'--prefix=DIRECTORY'
     Install machine-independent data files in subdirectories of
     'DIRECTORY'.  The default is to install in '/usr/local'.

'--exec-prefix=DIRECTORY'
     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.

'--with-headers=DIRECTORY'
     Look for kernel header files in DIRECTORY, not '/usr/include'.  The
     GNU C Library needs information from the kernel's header files
     describing the interface to the kernel.  The GNU C Library 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 the GNU C Library.
     Conflicts can occasionally happen in this case.  You can also use
     this option if you want to compile the GNU C Library with a newer
     set of kernel headers than the ones found in '/usr/include'.

'--enable-kernel=VERSION'
     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.

'--with-binutils=DIRECTORY'
     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.

'--with-nonshared-cflags=CFLAGS'
     Use additional compiler flags CFLAGS to build the parts of the
     library which are always statically linked into applications and
     libraries even with shared linking (that is, the object files
     contained in 'lib*_nonshared.a' libraries).  The build process will
     automatically use the appropriate flags, but this option can be
     used to set additional flags required for building applications and
     libraries, to match local policy.  For example, if such a policy
     requires that all code linked into applications must be built with
     source fortification,
     '--with-nonshared-cflags=-Wp,-D_FORTIFY_SOURCE=2' will make sure
     that the objects in 'libc_nonshared.a' are compiled with this flag
     (although this will not affect the generated code in this
     particular case and potentially change debugging information and
     metadata only).

'--disable-shared'
     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.

'--enable-static-pie'
     Enable static position independent executable (static PIE) support.
     Static PIE is similar to static executable, but can be loaded at
     any address without help from a dynamic linker.  All static
     programs as well as static tests are built as static PIE, except
     for those marked with no-pie.  The resulting glibc can be used with
     the GCC option, -static-pie, which is available with GCC 8 or
     above, to create static PIE. This option also implies that glibc
     programs and tests are created as dynamic position independent
     executables (PIE) by default.

'--enable-cet'
'--enable-cet=permissive'
     Enable Intel Control-flow Enforcement Technology (CET) support.
     When the GNU C Library is built with '--enable-cet' or
     '--enable-cet=permissive', the resulting library is protected with
     indirect branch tracking (IBT) and shadow stack (SHSTK).  When CET
     is enabled, the GNU C Library is compatible with all existing
     executables and shared libraries.  This feature is currently
     supported on i386, x86_64 and x32 with GCC 8 and binutils 2.29 or
     later.  Note that when CET is enabled, the GNU C Library requires
     CPUs capable of multi-byte NOPs, like x86-64 processors as well as
     Intel Pentium Pro or newer.  With '--enable-cet', it is an error to
     dlopen a non CET enabled shared library in CET enabled application.
     With '--enable-cet=permissive', CET is disabled when dlopening a
     non CET enabled shared library in CET enabled application.

     NOTE: '--enable-cet' has been tested for i686, x86_64 and x32 on
     non-CET processors.  '--enable-cet' has been tested for i686,
     x86_64 and x32 on CET processors.

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

'--enable-static-nss'
     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.

'--enable-hardcoded-path-in-tests'
     By default, dynamic tests are linked to run with the installed C
     library.  This option hardcodes the newly built C library path in
     dynamic tests so that they can be invoked directly.

'--disable-timezone-tools'
     By default, timezone related utilities ('zic', 'zdump', and
     'tzselect') are installed with the GNU C Library.  If you are
     building these independently (e.g.  by using the 'tzcode' package),
     then this option will allow disabling the install of these.

     Note that you need to make sure the external tools are kept in sync
     with the versions that the GNU C Library expects as the data
     formats may change over time.  Consult the 'timezone' subdirectory
     for more details.

'--enable-stack-protector'
'--enable-stack-protector=strong'
'--enable-stack-protector=all'
     Compile the C library and all other parts of the glibc package
     (including the threading and math libraries, NSS modules, and
     transliteration modules) using the GCC '-fstack-protector',
     '-fstack-protector-strong' or '-fstack-protector-all' options to
     detect stack overruns.  Only the dynamic linker and a small number
     of routines called directly from assembler are excluded from this
     protection.

'--enable-bind-now'
     Disable lazy binding for installed shared objects and programs.
     This provides additional security hardening because it enables full
     RELRO and a read-only global offset table (GOT), at the cost of
     slightly increased program load times.

'--enable-pt_chown'
     The file 'pt_chown' is a helper binary for 'grantpt' (*note
     Pseudo-Terminals: Allocation.) that is installed setuid root to fix
     up pseudo-terminal ownership on GNU/Hurd.  It is not required on
     GNU/Linux, and the GNU C Library will not use the installed
     'pt_chown' program when configured with '--enable-pt_chown'.

'--disable-werror'
     By default, the GNU C Library is built with '-Werror'.  If you wish
     to build without this option (for example, if building with a newer
     version of GCC than this version of the GNU C Library was tested
     with, so new warnings cause the build with '-Werror' to fail), you
     can configure with '--disable-werror'.

'--disable-mathvec'
     By default for x86_64, the GNU C Library is built with the vector
     math library.  Use this option to disable the vector math library.

'--enable-tunables'
     Tunables support allows additional library parameters to be
     customized at runtime.  This feature is enabled by default.  This
     option can take the following values:

     'yes'
          This is the default if no option is passed to configure.  This
          enables tunables and selects the default frontend (currently
          'valstring').

     'no'
          This option disables tunables.

     'valstring'
          This enables tunables and selects the 'valstring' frontend for
          tunables.  This frontend allows users to specify tunables as a
          colon-separated list in a single environment variable
          'GLIBC_TUNABLES'.

'--disable-crypt'
     Do not install the passphrase-hashing library 'libcrypt' or the
     header file 'crypt.h'.  'unistd.h' will still declare the function
     'crypt'.  Using this option does not change the set of programs
     that may need to be linked with '-lcrypt'; it only means that the
     GNU C Library will not provide that library.

     This option is for hackers and distributions experimenting with
     independently-maintained implementations of libcrypt.  It may
     become the default in a future release.

'--disable-experimental-malloc'
     By default, a per-thread cache is enabled in 'malloc'.  While this
     cache can be disabled on a per-application basis using tunables
     (set glibc.malloc.tcache_count to zero), this option can be used to
     remove it from the build completely.

'--build=BUILD-SYSTEM'
'--host=HOST-SYSTEM'
     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 the GNU C Library 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 'i686-pc-linux-gnu'
     but you want to compile a library for 586es, give
     '--host=i586-pc-linux-gnu' or just '--host=i586-linux' and add the
     appropriate compiler flags ('-mcpu=i586' will do the trick) to
     'CC'.

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

'--with-pkgversion=VERSION'
     Specify a description, possibly including a build number or build
     date, of the binaries being built, to be included in '--version'
     output from programs installed with the GNU C Library.  For
     example, '--with-pkgversion='FooBar GNU/Linux glibc build 123''.
     The default value is 'GNU libc'.

'--with-bugurl=URL'
     Specify the URL that users should visit if they wish to report a
     bug, to be included in '--help' output from programs installed with
     the GNU C Library.  The default value refers to the main
     bug-reporting information for the GNU C Library.

   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 aren'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.  *Note 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 the GNU C
Library 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.

   Normally, 'make check' will run all the tests before reporting all
problems found and exiting with error status if any problems occurred.
You can specify 'stop-on-test-failure=y' when running 'make check' to
make the test run stop and exit with an error status immediately when a
failure occurs.

   To format the 'GNU C Library Reference Manual' for printing, type
'make dvi'.  You need a working TeX installation to do this.  The
distribution builds the on-line formatted version of the manual, as Info
files, as part of the build process.  You can build them manually with
'make info'.

   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: 'configure TARGET CC=TARGET-gcc'.  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' to cross-compiling versions of 'ar'
if the native tools are not configured to work with object files for the
target you configured for.  When cross-compiling the GNU C Library, it
may be tested using 'make check
test-wrapper="SRCDIR/scripts/cross-test-ssh.sh HOSTNAME"', where SRCDIR
is the absolute directory name for the main source directory and
HOSTNAME is the host name of a system that can run the newly built
binaries of the GNU C Library.  The source and build directories must be
visible at the same locations on both the build system and HOSTNAME.

   In general, when testing the GNU C Library, 'test-wrapper' may be set
to the name and arguments of any program to run newly built binaries.
This program must preserve the arguments to the binary being run, its
working directory and the standard input, output and error file
descriptors.  If 'TEST-WRAPPER env' will not work to run a program with
environment variables set, then 'test-wrapper-env' must be set to a
program that runs a newly built program with environment variable
assignments in effect, those assignments being specified as 'VAR=VALUE'
before the name of the program to be run.  If multiple assignments to
the same variable are specified, the last assignment specified must take
precedence.  Similarly, if 'TEST-WRAPPER env -i' will not work to run a
program with an environment completely empty of variables except those
directly assigned, then 'test-wrapper-env-only' must be set; its use has
the same syntax as 'test-wrapper-env', the only difference in its
semantics being starting with an empty set of environment variables
rather than the ambient set.

Installing the C Library
========================

To install the library and its header files, and the Info files of the
manual, type 'make install'.  This will build things, if necessary,
before installing them; however, you should still compile everything
first.  If you are installing the GNU C Library 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.

   'make install' will do the entire job of upgrading from a previous
installation of the GNU C Library version 2.x.  There may sometimes be
headers left behind from the previous installation, but those are
generally harmless.  If you want to avoid leaving headers behind you can
do things in the following order.

   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.  The new '/usr/include', after switching the include
directories and before installing the library should contain the Linux
headers, but nothing else.  If you do this, you will need to restore any
headers from libraries other than the GNU C Library yourself after
installing the library.

   You can install the GNU C Library somewhere other than where you
configured it to go by setting the 'DESTDIR' GNU standard make 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.  Installing
with the 'prefix' and 'exec_prefix' GNU standard make variables set is
not supported.

   The GNU C Library 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' if the '--enable-pt_chown' configuration option is used.  This
program is invoked by the 'grantpt' function; it sets the permissions on
a pseudoterminal so it can be used by the calling process.  If you are
using a Linux kernel with the 'devpts' filesystem enabled and mounted at
'/dev/pts', you don't need this program.

   After installation you should configure the timezone and install
locales for your system.  The time zone configuration ensures that your
system time matches the time for your current timezone.  The locales
ensure that the display of information on your system matches the
expectations of your language and geographic region.

   The GNU C Library is able to use two kinds of localization
information sources, the first is a locale database named
'locale-archive' which is generally installed as
'/usr/lib/locale/locale-archive'.  The locale archive has the benefit of
taking up less space and being very fast to load, but only if you plan
to install sixty or more locales.  If you plan to install one or two
locales you can instead install individual locales into their self-named
directories e.g. '/usr/lib/locale/en_US.utf8'.  For example to install
the German locale using the character set for UTF-8 with name 'de_DE'
into the locale archive issue the command 'localedef -i de_DE -f UTF-8
de_DE', and to install just the one locale issue the command 'localedef
--no-archive -i de_DE -f UTF-8 de_DE'.  To configure all locales that
are supported by the GNU C Library, you can issue from your build
directory the command 'make localedata/install-locales' to install all
locales into the locale archive or 'make
localedata/install-locale-files' to install all locales as files in the
default configured locale installation directory (derived from
'--prefix' or '--localedir').  To install into an alternative system
root use 'DESTDIR' e.g. 'make localedata/install-locale-files
DESTDIR=/opt/glibc', but note that this does not change the configured
prefix.

   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'.

Recommended Tools for Compilation
=================================

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

   * GNU 'make' 4.0 or newer

     As of relase time, GNU 'make' 4.2.1 is the newest verified to work
     to build the GNU C Library.

   * GCC 6.2 or newer

     GCC 6.2 or higher is required.  In general it is recommended to use
     the newest version of the compiler that is known to work for
     building the GNU C Library, as newer compilers usually produce
     better code.  As of release time, GCC 9.2.1 is the newest compiler
     verified to work to build the GNU C Library.

     For PowerPC 64-bits little-endian (powerpc64le), a GCC version with
     support for '-mno-gnu-attribute', '-mabi=ieeelongdouble', and
     '-mabi=ibmlondouble' is required.  Likewise, the compiler must also
     support passing '-mlong-double-128' with the preceding options.  As
     of release, this implies GCC 7.4 and newer (excepting GCC 7.5.0,
     see GCC PR94200).  These additional features are required for
     building the GNU C Library with support for IEEE long double.

     For ARC architecture builds, GCC 8.3 or higher is needed.

     For multi-arch support it is recommended to use a GCC which has
     been built with support for GNU indirect functions.  This ensures
     that correct debugging information is generated for functions
     selected by IFUNC resolvers.  This support can either be enabled by
     configuring GCC with '--enable-gnu-indirect-function', or by
     enabling it by default by setting 'default_gnu_indirect_function'
     variable for a particular architecture in the GCC source file
     'gcc/config.gcc'.

     You can use whatever compiler you like to compile programs that use
     the GNU C Library.

     Check the FAQ for any special compiler issues on particular
     platforms.

   * GNU 'binutils' 2.25 or later

     You must use GNU 'binutils' (as and ld) to build the GNU C Library.
     No other assembler or linker has the necessary functionality at the
     moment.  As of release time, GNU 'binutils' 2.32 is the newest
     verified to work to build the GNU C Library.

     For PowerPC 64-bits little-endian (powerpc64le), 'objcopy' is
     required to support '--update-section'.  This option requires
     binutils 2.26 or newer.

     ARC architecture needs 'binutils' 2.32 or higher for TLS related
     fixes.

   * GNU 'texinfo' 4.7 or later

     To correctly translate and install the Texinfo documentation you
     need this version of the 'texinfo' package.  Earlier versions do
     not understand all the tags used in the document, and the
     installation mechanism for the info files is not present or works
     differently.  As of release time, 'texinfo' 6.6 is the newest
     verified to work to build the GNU C Library.

   * GNU 'awk' 3.1.2, or higher

     'awk' is used in several places to generate files.  Some 'gawk'
     extensions are used, including the 'asorti' function, which was
     introduced in version 3.1.2 of 'gawk'.  As of release time, 'gawk'
     version 5.0.1 is the newest verified to work to build the GNU C
     Library.

   * GNU 'bison' 2.7 or later

     'bison' is used to generate the 'yacc' parser code in the 'intl'
     subdirectory.  As of release time, 'bison' version 3.4.1 is the
     newest verified to work to build the GNU C Library.

   * Perl 5

     Perl is not required, but if present it is used in some tests and
     the 'mtrace' program, to build the GNU C Library manual.  As of
     release time 'perl' version 5.30.1 is the newest verified to work
     to build the GNU C Library.

   * GNU 'sed' 3.02 or newer

     'Sed' is used in several places to generate files.  Most scripts
     work with any version of 'sed'.  As of release time, 'sed' version
     4.5 is the newest verified to work to build the GNU C Library.

   * Python 3.4 or later

     Python is required to build the GNU C Library.  As of release time,
     Python 3.7.4 is the newest verified to work for building and
     testing the GNU C Library.

   * PExpect 4.0

     The pretty printer tests drive GDB through test programs and
     compare its output to the printers'.  PExpect is used to capture
     the output of GDB, and should be compatible with the Python version
     in your system.  As of release time PExpect 4.3 is the newest
     verified to work to test the pretty printers.

   * GDB 7.8 or later with support for Python 2.7/3.4 or later

     GDB itself needs to be configured with Python support in order to
     use the pretty printers.  Notice that your system having Python
     available doesn't imply that GDB supports it, nor that your
     system's Python and GDB's have the same version.  As of release
     time GNU 'debugger' 8.3 is the newest verified to work to test the
     pretty printers.

     Unless Python, PExpect and GDB with Python support are present, the
     printer tests will report themselves as 'UNSUPPORTED'.  Notice that
     some of the printer tests require the GNU C Library to be compiled
     with debugging symbols.

If you change any of the 'configure.ac' files you will also need

   * GNU 'autoconf' 2.69 (exactly)

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

   * GNU 'gettext' 0.10.36 or later

     As of release time, GNU 'gettext' version 0.19.8.1 is the newest
     version verified to work to build the GNU C Library.

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

Specific advice for GNU/Linux systems
=====================================

If you are installing the GNU C Library on GNU/Linux systems, you need
to have the header files from a 3.2 or newer kernel around for
reference.  (For the ia64 architecture, you need version 3.2.18 or newer
because this is the first version with support for the 'accept4' system
call.)  These headers must be installed using 'make headers_install';
the headers present in the kernel source directory are not suitable for
direct use by the GNU C Library.  You do not need to use that kernel,
just have its headers installed where the GNU C Library can access them,
referred to here as INSTALL-DIRECTORY.  The easiest way to do this is to
unpack it in a directory such as '/usr/src/linux-VERSION'.  In that
directory, run 'make headers_install
INSTALL_HDR_PATH=INSTALL-DIRECTORY'.  Finally, configure the GNU C
Library with the option '--with-headers=INSTALL-DIRECTORY/include'.  Use
the most recent kernel you can get your hands on.  (If you are
cross-compiling the GNU C Library, you need to specify
'ARCH=ARCHITECTURE' in the 'make headers_install' command, where
ARCHITECTURE is the architecture name used by the Linux kernel, such as
'x86' or 'powerpc'.)

   After installing the GNU C Library, you may need to remove or rename
directories such as '/usr/include/linux' and '/usr/include/asm', and
replace them with copies of directories such as 'linux' and 'asm' from
'INSTALL-DIRECTORY/include'.  All directories present in
'INSTALL-DIRECTORY/include' should be copied, except that the GNU C
Library provides its own version of '/usr/include/scsi'; the files
provided by the kernel should be copied without replacing those provided
by the GNU C Library.  The 'linux', 'asm' and 'asm-generic' directories
are required to compile programs using the GNU C Library; the other
directories describe interfaces to the kernel but are not required if
not compiling programs using those interfaces.  You do not need to copy
kernel headers if you did not specify an alternate kernel header source
using '--with-headers'.

   The Filesystem Hierarchy Standard for GNU/Linux systems expects some
components of the GNU C Library installation to be in '/lib' and some in
'/usr/lib'.  This is handled automatically if you configure the GNU C
Library with '--prefix=/usr'.  If you set some other prefix or allow it
to default to '/usr/local', then all the components are installed there.

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 central GNU C Library bug tracking
system has a WWW interface at <https://sourceware.org/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 C 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 (*note 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 at <https://www.gnu.org/software/libc/bugs.html>.

   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.