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.

'--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.  It is not built by default because

     systems using the Linux kernel are commonly built with the 'devpts'

     filesystem enabled and mounted at '/dev/pts', which manages

     pseudo-terminal ownership automatically.  By using

     '--enable-pt_chown', you may build 'pt_chown' and install it setuid

     and owned by 'root'.  The use of 'pt_chown' introduces additional

     security risks to the system and you should enable it only if you

     understand and accept those risks.

'--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'.

'--enable-obsolete-nsl'

     By default, libnsl is only built as shared library for backward

     compatibility and the NSS modules libnss_compat, libnss_nis and

     libnss_nisplus are not built at all.  Use this option to enable

     libnsl with all depending NSS modules and header files.

'--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.

   The GNU C Library pretty printers come with their own set of scripts

for testing, which run together with the rest of the testsuite through

'make check'.  These scripts require the following tools to run

successfully:

   * Python 2.7/3.4 or later

     Python is required for running the printers' test scripts.  As of

     release time, Python 3.6 is the newest verified to work to test the

     pretty printers.

   * PExpect 4.0

     The 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' 8.0.1 is the newest verified to work to test

     the pretty printers.

If these tools are absent, 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.

   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 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 the GNU C Library, 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'.

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 4.9 or newer

     GCC 4.9 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 8.1.1 is the newest compiler

     verified to work to build the GNU C Library.

     For PowerPC 64-bits little-endian (powerpc64le), GCC 6.2 or higher

     is required.  This compiler version is the first to provide the

     features required for building the GNU C Library with support for

     '_Float128'.

     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' 2.31.1 is the newest

     verified to work to build the GNU C Library.

   * 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' 6.5 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 4.2.1 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 3.0.4 is the

     newest verified to work to build the GNU C Library.

   * Perl 5

     Perl is not required, but it is used if present to test the

     installation.  We may decide to use it elsewhere in the future.  As

     of release time 'perl' version 5.28.0 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.

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 <http://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.