path: root/FAQ

		Frequently Asked Question on GNU C Library

As every FAQ this one also tries to answer questions the user might have
when using the package.  Please make sure you read this before sending
questions or bug reports to the maintainers.

The GNU C Library is very complex.  The building process exploits the
features available in tools generally available.  But many things can
only be done using GNU tools.  Also the code is sometimes hard to
understand because it has to be portable but on the other hand must be
fast.  But you need not understand the details to use GNU C Library.
This will only be necessary if you intend to contribute or change it.

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

						  --drepper@cygnus.com

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[Q1]	``What systems does the GNU C Library run on?''

[Q2]	``What compiler do I need to build GNU libc?''

[Q3]	``When starting make I get only error messages.
	  What's wrong?''

[Q4]	``After I changed configure.in I get `Autoconf version X.Y.
	  or higher is required for this script'.  What can I do?''

[Q5]	``Do I need a special linker or archiver?''

[Q6]	``Do I need some more things to compile GNU C Library?''

[Q7]	``When I run `nm -u libc.so' on the produced library I still
	  find unresolved symbols?  Can this be ok?''

[Q8]	``Can I replace the libc on my Linux system with GNU libc?''

[Q9]	``I expect GNU libc to be 100% source code compatible with
	  the old Linux based GNU libc.  Why isn't it like this?''

[Q10]	``Why does getlogin() always return NULL on my Linux box?''

[Q11]	``Where are the DST_* constants found in <sys/time.h> on many
	  systems?''

[Q12]	``The `gencat' utility cannot process the input which are
	  successfully used on my Linux libc based system.  Why?''

[Q13]	``How do I configure GNU libc so that the essential libraries
	  like libc.so go into /lib and the other into /usr/lib?''

[Q14]	``When linking with the new libc I get unresolved symbols
	  `crypt' and `setkey'.  Why aren't these functions in the
	  libc anymore?''

[Q15]	``What are these `add-ons'?''

[Q16]	``When I use GNU libc on my Linux system by linking against
	  to libc.so which comes with glibc all I get is a core dump.''

[Q17]	``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?''

[Q18]	``The prototypes for `connect', `accept', `getsockopt',
	  `setsockopt', `getsockname', `getpeername', `send',
	  `sendto', and `recvfrom' are different in GNU libc than
	  on any other system I saw.  This is a bug, isn't it?''

[Q19]	``My XXX kernel emulates a floating-point coprocessor for me.
	  Should I enable --with-fp?''

[Q20]	``How can I compile gcc 2.7.2.1 from the gcc source code using
	  glibc 2.x?

[Q21]    ``On Linux I've got problems with the declarations in Linux
           kernel headers.''

[Q22]	``When I try to compile code which uses IPv6 header and
	  definitions on my Linux 2.x.y system I am in trouble.
	  Nothing seems to work.''

[Q23]	``When compiling GNU libc I get lots of errors saying functions
	  in glibc are duplicated in libgcc.''

[Q24]	``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.''

[Q25]	``After installing glibc name resolving doesn't work properly.''


[Q26]	``I have /usr/include/net and /usr/include/scsi as symlinks
	  into my Linux source tree.  Is that wrong?''

[Q27]	``Programs like `logname', `top', `uptime' `users', `w' and
	  `who', show incorrect information about the (number of)
	  users on my system.  Why?''

[Q28]	``After upgrading to a glibc 2.1 with symbol versioning I get
	  errors about undefined symbols.  What went wrong?''

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[Q1]	``What systems does the GNU C Library run on?''

[A1] {UD} This is difficult to answer.  The file `README' lists the
architectures GNU libc is known to run *at some time*.  This does not
mean that it still can be compiled and run on them in the moment.

The systems glibc is known to work on in the moment and most probably
in the future are:

	*-*-gnu			GNU Hurd
	i[3456]86-*-linux-gnu	Linux-2.0 on Intel
	m68k-*-linux-gnu	Linux-2.0 on Motorola 680x0
	alpha-*-linux-gnu	Linux-2.0 on DEC Alpha
	powerpc-*-linux-gnu     Linux and MkLinux on PowerPC systems

Other Linux platforms are also on the way to be supported but I need
some success reports first.

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

	<bug-glibc@prep.ai.mit.edu>


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[Q2]	``What compiler do I need to build GNU libc?''

[A2] {UD} It is (almost) impossible to compile GNU C Library using a
different compiler than GNU CC.  A lot of extensions of GNU CC are
used to increase the portability and speed.

But this does not mean you have to use GNU CC for using the GNU C
Library.  In fact you should be able to use the native C compiler
because the success only depends on the binutils: the linker and
archiver.

The GNU CC is found like all other GNU packages on
	ftp://prep.ai.mit.edu/pub/gnu
or better one of the many mirror sites.

You always should try to use the latest official release.  Older
versions might not have all the features GNU libc could use.  It is
known that on most platforms compilers earlier than 2.7.2.3 fail so
at least use this version.


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[Q3]	``When starting `make' I get only errors messages.
	  What's wrong?''

[A3] {UD} You definitely need GNU make to translate GNU libc.  No
other make program has the needed functionality.

Versions before 3.74 have bugs which prevent correct execution so you
should upgrade to the latest version before starting the compilation.


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[Q4]	``After I changed configure.in I get `Autoconf version X.Y.
	  or higher is required for this script'.  What can I do?''

[A4] {UD} You have to get the specified autoconf version (or a later)
from your favourite mirror of prep.ai.mit.edu.


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[Q5]	``Do I need a special linker or archiver?''

[A5] {UD} If your native versions are not too buggy you can probably
work with them.  But GNU libc works best with GNU binutils.

On systems where the native linker does not support weak symbols you
will not get a really 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 affect building the GNU C
Library.


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[Q6]	``Do I need some more things to compile GNU C Library?''

[A6] {UD} Yes, there are some more :-).

* GNU gettext; the GNU libc is internationalized and partly localized.
  For bringing the messages for the different languages in the needed
  form the tools from the GNU gettext package are necessary.  See
  ftp://prep.ai.mit.edu/pub/gnu or better any mirror site.

* lots of diskspace (for i?86-linux this means, e.g., ~170MB; for ppc-linux
  even ~200MB).

  You should avoid compiling on a NFS mounted device.  This is very
  slow.

* plenty of time (approx 1h for i?86-linux on i586@133 or 2.5h on
  i486@66 or 4.5h on i486@33), both for shared and static only).
  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.

  For Atari Falcon (Motorola 68030 @ 16 Mhz, 14 Mb memory) James Troup
  <J.J.Troup@comp.brad.ac.uk> reports for a full build (shared, static,
  and profiled) a compile time of 45h34m.

  For Atari TT030 (Motorola 68030 @ 32 Mhz, 34 Mb memory) (full build)
  a compile time of 22h48m.

  If you have some more measurements let me know.

* When compiling for Linux:

  + the header files of the Linux kernel must be available in the
    search path of the CPP as <linux/*.h> and <asm/*.h>.

* 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.

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[Q7]	``When I run `nm -u libc.so' on the produced library I still
	  find unresolved symbols?  Can this be ok?''

[A7] {UD} Yes, this is ok.  There can be several kinds of unresolved
symbols:

* magic symbols automatically generated by the linker.  Names are
  often 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
  (currently fabs among others; this gets resolved if the program
   is linked against libm, too.)

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


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[Q8]	``Can I replace the libc on my Linux system with GNU libc?''

[A8] {UD} You cannot replace any existing libc for Linux with GNU
libc.  There are different versions of C libraries and you can run
libcs with different major version independently.

For Linux there are today two libc versions:
	libc-4		old a.out libc
	libc-5		current ELF libc

GNU libc will have the major number 6 and therefore you can have this
additionally 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.


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[Q9]	``I expect GNU libc to be 100% source code compatible with
	  the old Linux based GNU libc.  Why isn't it like this?''

[A9] {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 now can be corrected.  Here is a list of the known source code
incompatibilities:

* _GNU_SOURCE: glibc does not automatically define _GNU_SOURCE.  Thus,
  if a program depends on GNU extensions or some other non-standard
  functionality, it is necessary to compile it with C compiler option
  -D_GNU_SOURCE, or better, to put `#define _GNU_SOURCE' at the beginning
  of your source files, before a