(Reading/Closing Directory): Document readdir_r behaviour on EOD.

[glibc.git] / FAQ

            Frequently Asked Questions about the GNU C Library

This document tries to answer questions a user might have when
installing and using glibc.  Please make sure you read this before
sending questions or bug reports to the maintainers.

The GNU C library is very complex.  The installation process has not
been completely automated; there are too many variables. You can do
substantial damage to your system by installing the library
incorrectly.  Make sure you understand what you are undertaking before
you begin.

If you have any questions you think should be answered in this document,
please let me know.

                                                  --drepper@cygnus.com
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1. Compiling glibc

1.1.    What systems does the GNU C Library run on?
1.2.    What compiler do I need to build GNU libc?
1.3.    When I try to compile glibc I get only error messages.
        What's wrong?
1.4.    Do I need a special linker or archiver?
1.5.    Do I need some more things to compile GNU C Library?
1.6.    When I run `nm -u libc.so' on the produced library I still
        find unresolved symbols.  Can this be ok?
1.7.    What are these `add-ons'?
1.8.    My XXX kernel emulates a floating-point coprocessor for me.
        Should I enable --with-fp?
1.9.    When compiling GNU libc I get lots of errors saying functions
        in glibc are duplicated in libgcc.
1.10.   What's the problem with configure --enable-omitfp?

2. Installation and configuration issues

2.1.    Can I replace the libc on my Linux system with GNU libc?
2.2.    How do I configure GNU libc so that the essential libraries
        like libc.so go into /lib and the other into /usr/lib?
2.3.    How should I avoid damaging my system when I install GNU libc?
2.4.    Do I need to use GNU CC to compile programs that will use the
        GNU C Library?
2.5.    When linking with the new libc I get unresolved symbols
        `crypt' and `setkey'.  Why aren't these functions in the
        libc anymore?
2.6.    When I use GNU libc on my Linux system by linking against
        the libc.so which comes with glibc all I get is a core dump.
2.7.    Looking through the shared libc file I haven't found the
        functions `stat', `lstat', `fstat', and `mknod' and while
        linking on my Linux system I get error messages.  How is
        this supposed to work?
2.8.    How can I compile gcc 2.7.2.1 from the gcc source code using
        glibc 2.x?
2.9.    The `gencat' utility cannot process the catalog sources which
        were used on my Linux libc5 based system.  Why?
2.10.   I have set up /etc/nis.conf, and the Linux libc 5 with NYS
        works great.  But the glibc NIS+ doesn't seem to work.
2.11.   I have killed ypbind to stop using NIS, but glibc will
        continue using NIS.
2.12.   After installing glibc name resolving doesn't work properly.
2.13.   I have /usr/include/net and /usr/include/scsi as symlinks
        into my Linux source tree.  Is that wrong?
2.14.   Programs like `logname', `top', `uptime' `users', `w' and
        `who', show incorrect information about the (number of)
        users on my system.  Why?
2.15.   When I start the program XXX after upgrading the library
        I get
          XXX: Symbol `_sys_errlist' has different size in shared
          object, consider re-linking
        Why?  What should I do?
2.16    What do I need for C++ development?

3. Source and binary incompatibilities, and what to do about them

3.1.    I expect GNU libc to be 100% source code compatible with
        the old Linux based GNU libc.  Why isn't it like this?
3.2.    Why does getlogin() always return NULL on my Linux box?
3.3.    Where are the DST_* constants found in <sys/time.h> on many
        systems?
3.4.    The prototypes for `connect', `accept', `getsockopt',
        `setsockopt', `getsockname', `getpeername', `send',
        `sendto', and `recvfrom' are different in GNU libc from
        any other system I saw.  This is a bug, isn't it?
3.5.    On Linux I've got problems with the declarations in Linux
        kernel headers.
3.6.    I don't include any kernel headers myself but the compiler
        still complains about redeclarations of types in the kernel
        headers.
3.7.    Why don't signals interrupt system calls anymore?

4. Miscellaneous

4.1.    After I changed configure.in I get `Autoconf version X.Y.
        or higher is required for this script'.  What can I do?
4.2.    When I try to compile code which uses IPv6 headers and
        definitions on my Linux 2.x.y system I am in trouble.
        Nothing seems to work.

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1. Compiling glibc

1.1.    What systems does the GNU C Library run on?

{UD} This is difficult to answer.  The file `README' lists the
architectures GNU libc was known to run on *at some time*.  This does
not mean that it still can be compiled and run on them now.

The systems glibc is known to work on as of this release, and most
probably in the future, are:

        *-*-gnu                 GNU Hurd
        i[3456]86-*-linux-gnu   Linux-2.x on Intel
        m68k-*-linux-gnu        Linux-2.x on Motorola 680x0
        alpha-*-linux-gnu       Linux-2.x on DEC Alpha

Ports to other Linux platforms are in development, and may in fact
work already, but no one has sent us success reports for them.
Currently no ports to other operating systems are underway, although a
few people have expressed interest.

If you have a system not listed above (or in the `README' file) and
you are really interested in porting it, contact

        <bug-glibc@gnu.org>


1.2.    What compiler do I need to build GNU libc?

{UD} You must use GNU CC to compile GNU libc.  A lot of extensions of
GNU CC are used to increase portability and speed.

GNU CC is found, like all other GNU packages, on
        ftp://ftp.gnu.org/pub/gnu
and the many mirror sites.  ftp.gnu.org is always overloaded, so try to find
a local mirror first.

You always should try to use the latest official release.  Older
versions may not have all the features GNU libc requires.  On most
supported platforms, 2.7.2.3 is the earliest version that works at all.


1.3.    When I try to compile glibc I get only error messages.
        What's wrong?

{UD} You definitely need GNU make to translate GNU libc.  No
other make program has the needed functionality.

We recommend version GNU make version 3.75.  Versions 3.76 and 3.76.1
have bugs which appear when building big projects like GNU libc.
Versions before 3.74 have bugs and/or are missing features.


1.4.    Do I need a special linker or archiver?

{UD} You may be able to use your system linker, but GNU libc works
best with GNU binutils.

On systems where the native linker does not support weak symbols you
will not get a fully ISO C compliant C library.  Generally speaking
you should use the GNU binutils if they provide at least the same
functionality as your system's tools.

Always get the newest release of GNU binutils available.  Older
releases are known to have bugs that prevent a successful compilation.

For alpha-linux you need at least GNU binutils 2.8.1.0.18 available from
ftp://ftp.yggdrasil.com/private/hjl


1.5.    Do I need some more things to compile GNU C Library?

{UD} Yes, there are some more :-).

* GNU gettext.  This package contains the tools needed to construct
  `message catalog' files containing translated versions of system
  messages. See ftp://ftp.gnu.org/pub/gnu or better any mirror
  site.  (We distribute compiled message catalogs, but they may not be
  updated in patches.)

* Some files depend on special tools.  E.g., files ending in .gperf
  need a `gperf' program.  The GNU version (part of libg++) is known
  to work while some vendor versions do not.

  You should not need these tools unless you change the source files.

* Some scripts need perl5 - but at the moment those scripts are not
  vital for building and installing GNU libc (some data files will not
  be created).

* When compiling for Linux, the header files of the Linux kernel must
  be available to the compiler as <linux/*.h> and <asm/*.h>.

* lots of disk space (~170MB for i?86-linux; more for RISC platforms).

* plenty of time.  Compiling just the shared and static libraries for
  i?86-linux takes approximately 1h on an i586@133, or 2.5h on
  i486@66, or 4.5h on i486@33.  Multiply this by 1.5 or 2.0 if you
  build profiling and/or the highly optimized version as well.  For
  Hurd systems times are much higher.

  You should avoid compiling in a NFS mounted filesystem.  This is
  very slow.

  James Troup <J.J.Troup@comp.brad.ac.uk> reports a compile time of
  45h34m for a full build (shared, static, and profiled) on Atari
  Falcon (Motorola 68030 @ 16 Mhz, 14 Mb memory) and Jan Barte
  <yann@plato.uni-paderborn.de> reports 22h48m on Atari TT030
  (Motorola 68030 @ 32 Mhz, 34 Mb memory)

  If you have some more measurements let me know.


1.6.    When I run `nm -u libc.so' on the produced library I still
        find unresolved symbols.  Can this be ok?

{UD} Yes, this is ok.  There can be several kinds of unresolved
symbols:

* magic symbols automatically generated by the linker.  These have names
  like __start_* and __stop_*

* symbols starting with _dl_* come from the dynamic linker

* symbols resolved by using libgcc.a
  (__udivdi3, __umoddi3, or similar)

* weak symbols, which need not be resolved at all (fabs for example)

Generally, you should make sure you find a real program which produces
errors while linking before deciding there is a problem.


1.7.    What are these `add-ons'?

{UD} To avoid complications with export rules or external source
code some optional parts of the libc are distributed as separate
packages (e.g., the crypt package, see question 2.5).

To use these packages as part of GNU libc, just unpack the tarfiles in
the libc source directory and tell the configuration script about them
using the --enable-add-ons option.  If you give just --enable-add-ons
configure tries to find all the add-on packages in your source tree.
This may not work.  If it doesn't, or if you want to select only a
subset of the add-ons, give a comma-separated list of the add-ons to
enable:

        configure --enable-add-ons=crypt,linuxthreads

for example.

Add-ons can add features (including entirely new shared libraries),
override files, provide support for additional architectures, and
just about anything else.  The existing makefiles do most of the work;
only some few stub rules must be written to get everything running.


1.8.    My XXX kernel emulates a floating-point coprocessor for me.
        Should I enable --with-fp?

{ZW} An emulated FPU is just as good as a real one, as far as the C
library is concerned.  You only need to say --without-fp if your
machine has no way to execute floating-point instructions.

People who are interested in squeezing the last drop of performance
out of their machine may wish to avoid the trap overhead, but this is
far more trouble than it's worth: you then have to compile
*everything* this way, including the compiler's internal libraries
(libgcc.a for GNU C), because the calling conventions change.


1.9.    When compiling GNU libc I get lots of errors saying functions
        in glibc are duplicated in libgcc.

{EY} This is *exactly* the same problem that I was having.  The
problem was due to the fact that configure didn't correctly detect
that the linker flag --no-whole-archive was supported in my linker.
In my case it was because I had run ./configure with bogus CFLAGS, and
the test failed.

One thing that is particularly annoying about this problem is that
once this is misdetected, running configure again won't fix it unless
you first delete config.cache.

{UD} Starting with glibc-2.0.3 there should be a better test to avoid
some problems of this kind.  The setting of CFLAGS is checked at the
very beginning and if it is not usable `configure' will bark.


1.10.   What's the problem with configure --enable-omitfp?

{AJ} When --enable-omitfp is set the libraries are built without frame
pointers. Some compilers produce buggy code for this model and
therefore we don't advise using it at the moment.

If you use --enable-omitfp, you're on your own. If you encounter
problems with a library that was build this way, we advise you to
rebuild the library without --enable-omitfp.  If the problem vanishes
consider tracking the problem down and report it as compiler failure.

Since a library build with --enable-omitfp is undebuggable on most
systems, debuggable libraries are also built - you can use it by
appending "_g" to the library names.

The compilation of these extra libraries and the compiler optimizations
slow down the build process and need more disk space.

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2. Installation and configuration issues

2.1.    Can I replace the libc on my Linux system with GNU libc?

{UD} You cannot replace any existing libc for Linux with GNU
libc.  It is binary incompatible and therefore has a different major
version.  You can, however, install it alongside your existing libc.

For Linux there are three major libc versions:
        libc-4          a.out libc
        libc-5          original ELF libc
        libc-6          GNU libc

You can have any combination of these three installed.  For more
information consult documentation for shared library handling.  The
Makefiles of GNU libc will automatically generate the needed symbolic
links which the linker will use.


2.2.    How do I configure GNU libc so that the essential libraries
        like libc.so go into /lib and the other into /usr/lib?

{UD,AJ} Like all other GNU packages GNU libc is designed to use a base
directory and install all files relative to this.  The default is
/usr/local, because this is safe (it will not damage the system if
installed there).  If you wish to install GNU libc as the primary C
library on your system, set the base directory to /usr (i.e. run
configure --prefix=/usr <other_options>).  Note that this can damage
your system; see question 2.3 for details.

Some systems like Linux have a filesystem standard which makes a
difference between essential libraries and others.  Essential
libraries are placed in /lib because this directory is required to be
located on the same disk partition as /.  The /usr subtree might be
found on another partition/disk. If you configure for Linux with
--prefix=/usr, then this will be done automatically.

To install the essential libraries which come with GNU libc in /lib on
systems other than Linux one must explicitly request it.  Autoconf has
no option for this so you have to use a `configparms' file (see the
`INSTALL' file for details).  It should contain:

slibdir=/lib
sysconfdir=/etc

The first line specifies the directory for the essential libraries,
the second line the directory for system configuration files.


2.3.    How should I avoid damaging my system when I install GNU libc?

{ZW} If you wish to be cautious, do not configure with --prefix=/usr.
If you don't specify a prefix, glibc will be installed in /usr/local,
where it will probably not break anything.  (If you wish to be
certain, set the prefix to something like /usr/local/glibc2 which is
not used for anything.)

The dangers when installing glibc in /usr are twofold:

* glibc will overwrite the headers in /usr/include.  Other C libraries
  install a different but overlapping set of headers there, so the
  effect will probably be that you can't compile anything.  You need to
  rename /usr/include out of the way first.  (Do not throw it away; you
  will then lose the ability to compile programs against your old libc.)

* None of your old libraries, static or shared, can be used with a
  different C library major version.  For shared libraries this is not a
  problem, because the filenames are different and the dynamic linker
  will enforce the restriction.  But static libraries have no version
  information.  You have to evacuate all the static libraries in
  /usr/lib to a safe location.

The situation is rather similar to the move from a.out to ELF which
long-time Linux users will remember.


2.4.    Do I need to use GNU CC to compile programs that will use the
        GNU C Library?

{ZW} In theory, no; the linker does not care, and the headers are
supposed to check for GNU CC before using its extensions to the C
language.

However, there are currently no ports of glibc to systems where
another compiler is the default, so no one has tested the headers
extensively against another compiler.  You may therefore encounter
difficulties.  If you do, please report them as bugs.

Also, in several places GNU extensions provide large benefits in code
quality.  For example, the library has hand-optimized, inline assembly
versions of some functions.  These can only be used with GCC.


2.5.    When linking with the new libc I get unresolved symbols
        `crypt' and `setkey'.  Why aren't these functions in the
        libc anymore?

{UD} The US places restrictions on exporting cryptographic programs
and source code.  Until this law gets abolished we cannot ship the
cryptographic functions together with glibc.

The functions are available, as an add-on (see question 1.7).  People in the
US may get it from the same place they got GNU libc from.  People
outside the US should get the code from ftp://ftp.ifi.uio.no/pub/gnu,
or another archive site outside the USA.  The README explains how to
install the sources.

If you already have the crypt code on your system the reason for the
failure is probably that you did not link with -lcrypt.  The crypto
functions are in a separate library to make it possible to export GNU
libc binaries from the US.


2.6.    When I use GNU libc on my Linux system by linking against
        the libc.so which comes with glibc all I get is a core dump.

{UD} On Linux, gcc sets the dynamic linker to /lib/ld-linux.so.1
unless the user specifies a -dynamic-linker argument.  This is the
name of the libc5 dynamic linker, which does not work with glibc.

For casual use of GNU libc you can just specify
    -dynamic-linker=/lib/ld-linux.so.2

which is the glibc dynamic linker, on Linux systems.  On other systems
the name is /lib/ld.so.1.

To change your environment to use GNU libc for compiling you need to
change the `specs' file of your gcc.  This file is normally found at

        /usr/lib/gcc-lib/<arch>/<version>/specs

In this file you have to change a few things:

- change `ld-linux.so.1' to `ld-linux.so.2'

- remove all expression `%{...:-lgmon}';  there is no libgmon in glibc

- fix a minor bug by changing %{pipe:-} to %|

Here is what the gcc-2.7.2 specs file should look like when GNU libc
is installed at /usr:

-----------------------------------------------------------------------
*asm:
%{V} %{v:%{!V:-V}} %{Qy:} %{!Qn:-Qy} %{n} %{T} %{Ym,*} %{Yd,*} %{Wa,*:%*}

*asm_final:
%|

*cpp:
%{fPIC:-D__PIC__ -D__pic__} %{fpic:-D__PIC__ -D__pic__} %{!m386:-D__i486__} %{posix:-D_POSIX_SOURCE} %{pthread:-D_REENTRANT}

*cc1:
%{profile:-p}

*cc1plus:


*endfile:
%{!shared:crtend.o%s} %{shared:crtendS.o%s} crtn.o%s

*link:
-m elf_i386 %{shared:-shared}   %{!shared:     %{!ibcs:       %{!static:        %{rdynamic:-export-dynamic}     %{!dynamic-linker:-dynamic-linker /lib/ld-linux.so.2}}  %{static:-static}}}

*lib:
%{!shared: %{pthread:-lpthread}         %{profile:-lc_p} %{!profile: -lc}}

*libgcc:
-lgcc

*startfile:
%{!shared:      %{pg:gcrt1.o%s} %{!pg:%{p:gcrt1.o%s}                 %{!p:%{profile:gcrt1.o%s}                   %{!profile:crt1.o%s}}}}    crti.o%s %{!shared:crtbegin.o%s} %{shared:crtbeginS.o%s}

*switches_need_spaces:


*signed_char:
%{funsigned-char:-D__CHAR_UNSIGNED__}

*predefines:
-D__ELF__ -Dunix -Di386 -Dlinux -Asystem(unix) -Asystem(posix) -Acpu(i386) -Amachine(i386)

*cross_compile:
0

*multilib:
. ;

-----------------------------------------------------------------------

Things get a bit more complicated if you have GNU libc installed in
some other place than /usr, i.e., if you do not want to use it instead
of the old libc.  In this case the needed startup files and libraries
are not found in the regular places.  So the specs file must tell the
compiler and linker exactly what to use.

Version 2.7.2.3 does and future versions of GCC will automatically
provide the correct specs.


2.7.    Looking through the shared libc file I haven't found the
        functions `stat', `lstat', `fstat', and `mknod' and while
        linking on my Linux system I get error messages.  How is
        this supposed to work?

{RM} Believe it or not, stat and lstat (and fstat, and mknod)
are supposed to be undefined references in libc.so.6!  Your problem is
probably a missing or incorrect /usr/lib/libc.so file; note that this
is a small text file now, not a symlink to libc.so.6.  It should look
something like this:

GROUP ( libc.so.6 ld.so.1 libc.a )

or in ix86/Linux and alpha/Linux:

GROUP ( libc.so.6 ld-linux.so.2 libc.a )


2.8.    How can I compile gcc 2.7.2.1 from the gcc source code using
        glibc 2.x?

{AJ} There's only correct support for glibc 2.0.x in gcc 2.7.2.3
or later.  You should get at least gcc 2.7.2.3.  All previous versions
had problems with glibc support.


2.9.    The `gencat' utility cannot process the catalog sources which
        were used on my Linux libc5 based system.  Why?

{UD} The `gencat' utility provided with glibc complies to the XPG
standard.  The older Linux version did not obey the standard, so they
are not compatible.

To ease the transition from the Linux version some of the non-standard
features are also present in the `gencat' program of GNU libc.  This
mainly includes the use of symbols for the message number and the automatic
generation of header files which contain the needed #defines to map the
symbols to integers.

Here is a simple SED script to convert at least some Linux specific
catalog files to the XPG4 form:

-----------------------------------------------------------------------
# Change catalog source in Linux specific format to standard XPG format.
# Ulrich Drepper <drepper@cygnus.com>, 1996.
#
/^\$ #/ {
  h
  s/\$ #\([^ ]*\).*/\1/
  x
  s/\$ #[^ ]* *\(.*\)/\$ \1/
}

/^# / {
  s/^# \(.*\)/\1/
  G
  s/\(.*\)\n\(.*\)/\2 \1/
}
-----------------------------------------------------------------------


2.10.   I have set up /etc/nis.conf, and the Linux libc 5 with NYS
        works great.  But the glibc NIS+ doesn't seem to work.

{TK} The glibc NIS+ implementation uses a /var/nis/NIS_COLD_START
file for storing information about the NIS+ server and their public
keys, because the nis.conf file does not contain all the necessary
information.  You have to copy a NIS_COLD_START file from a Solaris
client (the NIS_COLD_START file is byte order independent) or generate
it with nisinit from the nis-tools package (available at
http://www-vt.uni-paderborn.de/~kukuk/linux/nisplus.html).


+2.11.  I have killed ypbind to stop using NIS, but glibc will
        continue using NIS.

{TK} For faster NIS lookups, glibc uses the /var/yp/binding/ files
from ypbind.  ypbind 3.3 and older versions don't always remove these
files, so glibc will use them furthermore.  Other BSD versions seem to
work correct.  Until ypbind 3.4 is released, you can find a patch at
ftp://ftp.kernel.org/pub/linux/utils/net/NIS/ypbind-3.3-glibc2.diff.

2.12.   After installing glibc name resolving doesn't work properly.

{AJ} You probably should read the manual section describing
nsswitch.conf (just type `info libc "NSS Configuration File"').
The NSS configuration file is usually the culprit.


2.13.   I have /usr/include/net and /usr/include/scsi as symlinks
        into my Linux source tree.  Is that wrong?

{PB} This was necessary for libc5, but is not correct when using
glibc.  Including the kernel header files directly in user programs
usually does not work (see question 3.5).  glibc provides its own <net/*>
and <scsi/*> header files to replace them, and you may have to remove
any symlink that you have in place before you install glibc.  However,
/usr/include/asm and /usr/include/linux should remain as they were.


2.14.   Programs like `logname', `top', `uptime' `users', `w' and
        `who', show incorrect information about the (number of)
        users on my system.  Why?

{MK} See question 3.2.


2.15.   When I start the program XXX after upgrading the library
        I get
          XXX: Symbol `_sys_errlist' has different size in shared
          object, consider re-linking
        Why?  What should I do?

{UD} As the message says, relink the binary.  The problem is that
a few symbols from the library can change in size and there is no way
to avoid this.  _sys_errlist is a good example.  Occasionally there are
new error numbers added to the kernel and this must be reflected at user
level, breaking programs that refer to them directly.

Such symbols should normally not be used at all.  There are mechanisms
to avoid using them.  In the case of _sys_errlist, there is the
strerror() function which should _always_ be used instead.  So the
correct fix is to rewrite that part of the application.

In some situations (especially when testing a new library release) it
might be possible that a symbol changed size when that should not have
happened.  So in case of doubt report such a warning message as a
problem.


2.16.   What do I need for C++ development?

{HJ,AJ} You need either egcs 1.0.1 or gcc-2.8.0 with libstdc++
2.8.0. libg++ 2.7.2 (and the Linux Versions 2.7.2.x) doesn't work very
well with the GNU C library due to vtable thunks.
If you're upgrading from glibc 2.0.x to 2.1 you have to recompile
libstc++ the library compiled for 2.0 is not compatible due to the new
Large File Support (LFS) in version 2.1.

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3. Source and binary incompatibilities, and what to do about them

3.1.    I expect GNU libc to be 100% source code compatible with
        the old Linux based GNU libc.  Why isn't it like this?

{DMT,UD} Not every extension in Linux libc's history was well
thought-out.  In fact it had a lot of problems with standards compliance
and with cleanliness.  With the introduction of a new version number these
errors can now be corrected.  Here is a list of the known source code
incompatibilities:

* _GNU_SOURCE: glibc does not make the GNU extensions available
  automatically.  If a program depends on GNU extensions or some
  other non-standard functionality, it is necessary to compile it
  with the C compiler option -D_GNU_SOURCE, or better, to put
  `#define _GNU_SOURCE' at the beginning of your source files, before
  any C library header files are included.  This difference normally
  manifests itself in the form of missing prototypes and/or data type
  definitions.  Thus, if you get such errors, the first thing you
  should do is try defining _GNU_SOURCE and see if that makes the
  problem go away.

  For more information consult the file `NOTES' in the GNU C library
  sources.

* reboot(): GNU libc sanitizes the interface of reboot() to be more
  compatible with the interface used on other OSes.  reboot() as
  implemented in glibc takes just one argument.  This argument
  corresponds to the third argument of the Linux reboot system call.
  That is, a call of the form reboot(a, b, c) needs to be changed into
  reboot(c).  Beside this the header <sys/reboot.h> defines the needed
  constants for the argument.  These RB_* constants should be used
  instead of the cryptic magic numbers.

* swapon(): the interface of this function didn't change, but the
  prototype is in a separate header file <sys/swap.h>.  This header
  file also provides the SWAP_* constants defined by <linux/swap.h>;
  you should use them for the second argument to swapon().

* errno: If a program uses the variable "errno", then it _must_
  include <errno.h>.  The old libc often (erroneously) declared this
  variable implicitly as a side-effect of including other libc header
  files.  glibc is careful to avoid such namespace pollution, which,
  in turn, means that you really need to include the header files that
  you depend on.  This difference normally manifests itself in the
  form of the compiler complaining about references to an undeclared
  symbol "errno".

* Linux-specific syscalls: All Linux system calls now have appropriate
  library wrappers and corresponding declarations in various header files.
  This is because the syscall() macro that was traditionally used to
  work around missing syscall wrappers are inherently non-portable and
  error-prone.  The following table lists all the new syscall stubs,
  the header-file declaring their interface and the system call name.

       syscall name:    wrapper name:   declaring header file:
       -------------    -------------   ----------------------
       bdflush          bdflush         <sys/kdaemon.h>
       syslog           ksyslog_ctl     <sys/klog.h>

* lpd: Older versions of lpd depend on a routine called _validuser().
  The library does not provide this function, but instead provides
  __ivaliduser() which has a slightly different interface.  Simply
  upgrading to a newer lpd should fix this problem (e.g., the 4.4BSD
  lpd is known to be working).

* resolver functions/BIND: like on many other systems the functions of
  the resolver library are not included in libc itself.  There is a
  separate library libresolv.  If you get undefined symbol errors for
  symbols starting with `res_*' simply add -lresolv to your linker
  command line.

* the `signal' function's behavior corresponds to the BSD semantic and
  not the SysV semantic as it was in libc-5.  The interface on all GNU
  systems shall be the same and BSD is the semantic of choice.  To use
  the SysV behavior simply use `sysv_signal', or define _XOPEN_SOURCE.
  See question 3.7 for details.


3.2.    Why does getlogin() always return NULL on my Linux box?

{UD} The GNU C library has a format for the UTMP and WTMP file which
differs from what your system currently has.  It was extended to
fulfill the needs of the next years when IPv6 is introduced.  The
record size is different and some fields have different positions.
The files written by functions from the one library cannot be read by
functions from the other library.  Sorry, but this is what a major
release is for.  It's better to have a cut now than having no means to
support the new techniques later.


3.3.    Where are the DST_* constants found in <sys/time.h> on many
        systems?

{UD} These constants come from the old BSD days and are not used
anymore (libc5 does not actually implement the handling although the
constants are defined).

Instead GNU libc contains zone database support and compatibility code
for POSIX TZ environment variable handling.


3.4.    The prototypes for `connect', `accept', `getsockopt',
        `setsockopt', `getsockname', `getpeername', `send',
        `sendto', and `recvfrom' are different in GNU libc from
        any other system I saw.  This is a bug, isn't it?

{UD} No, this is no bug.  This version of GNU libc already follows the
new Single Unix specifications (and I think the POSIX.1g draft which
adopted the solution).  The type for a parameter describing a size is
now `socklen_t', a new type.


3.5.    On Linux I've got problems with the declarations in Linux
        kernel headers.

{UD,AJ} On Linux, the use of kernel headers is reduced to the minimum.
This gives Linus the ability to change the headers more freely.  Also,
user programs are now insulated from changes in the size of kernel
data structures.

For example, the sigset_t type is 32 or 64 bits wide in the kernel.
In glibc it is 1024 bits wide.  This guarantees that when the kernel
gets a bigger sigset_t (for POSIX.1e realtime support, say) user
programs will not have to be recompiled.  Consult the header files for
more information about the changes.

Therefore you shouldn't include Linux kernel header files directly if
glibc has defined a replacement. Otherwise you might get undefined
results because of type conflicts.


3.6.    I don't include any kernel headers myself but the compiler
        still complains about redeclarations of types in the kernel
        headers.

{UD} The kernel headers before Linux 2.1.61 and 2.0.32 don't work
correctly with glibc.  Compiling C programs is possible in most cases
but C++ programs have (due to the change of the name lookups for
`struct's) problems.  One prominent example is `struct fd_set'.

There might be some problems left but 2.1.61/2.0.32 fix most of the
known ones.  See the BUGS file for other known problems.


3.7.    Why don't signals interrupt system calls anymore?

{ZW} By default GNU libc uses the BSD semantics for signal(),
unlike Linux libc 5 which used System V semantics.  This is partially
for compatibility with other systems and partially because the BSD
semantics tend to make programming with signals easier.

There are three differences:

* BSD-style signals that occur in the middle of a system call do not
  affect the system call; System V signals cause the system call to
  fail and set errno to EINTR.

* BSD signal handlers remain installed once triggered.  System V signal
  handlers work only once, so one must reinstall them each time.

* A BSD signal is blocked during the execution of its handler.  In other
  words, a handler for SIGCHLD (for example) does not need to worry about
  being interrupted by another SIGCHLD.  It may, however, be interrupted
  by other signals.

There is general consensus that for `casual' programming with signals, the
BSD semantics are preferable.  You don't need to worry about system calls
returning EINTR, and you don't need to worry about the race conditions
associated with one-shot signal handlers.

If you are porting an old program that relies on the old semantics, you can
quickly fix the problem by changing signal() to sysv_signal() throughout.
Alternatively, define _XOPEN_SOURCE before including <signal.h>.

For new programs, the sigaction() function allows you to specify precisely
how you want your signals to behave.  All three differences listed above are
individually switchable on a per-signal basis with this function.

If all you want is for one specific signal to cause system calls to fail
and return EINTR (for example, to implement a timeout) you can do this with
siginterrupt().

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4. Miscellaneous

4.1.    After I changed configure.in I get `Autoconf version X.Y.
        or higher is required for this script'.  What can I do?

{UD} You have to get the specified autoconf version (or a later one)
from your favorite mirror of ftp.gnu.org.


4.2.    When I try to compile code which uses IPv6 headers and
        definitions on my Linux 2.x.y system I am in trouble.
        Nothing seems to work.

{UD} The problem is that IPv6 development still has not reached a
point where the headers are stable.  There are still lots of
incompatible changes made and the libc headers have to follow.

Also, make sure you have a suitably recent kernel.  As of the 970401
snapshot, according to Philip Blundell <Philip.Blundell@pobox.com>, the
required kernel version is at least 2.1.30.

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~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Answers were given by:
{UD} Ulrich Drepper, <drepper@cygnus.com>
{DMT} David Mosberger-Tang, <davidm@AZStarNet.com>
{RM} Roland McGrath, <roland@gnu.org>
{AJ} Andreas Jaeger, <aj@arthur.rhein-neckar.de>
{EY} Eric Youngdale, <eric@andante.jic.com>
{PB} Phil Blundell, <Philip.Blundell@pobox.com>
{MK} Mark Kettenis, <kettenis@phys.uva.nl>
{ZW} Zack Weinberg, <zack@rabi.phys.columbia.edu>
{TK} Thorsten Kukuk, <kukuk@vt.uni-paderborn.de>
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