Frequently Asked Question on 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


  1. Compiling glibc

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

  2. Installation and configuration issues

    1. Can I replace the libc on my Linux system with GNU libc?
    2. How do I configure GNU libc so that the essential libraries like libc.so go into /lib and the other into /usr/lib?
    3. How should I avoid damaging my system when I install GNU libc?
    4. Do I need to use GNU CC to compile programs that will use the GNU C Library?
    5. When linking with the new libc I get unresolved symbols `crypt' and `setkey'. Why aren't these functions in the libc anymore?
    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.
    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?
    8. How can I compile gcc 2.7.2.1 from the gcc source code using glibc 2.x?
    9. The `gencat' utility cannot process the catalog sources which were used on my Linux libc5 based system. Why?
    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.
    11. After installing glibc name resolving doesn't work properly.
    12. I have /usr/include/net and /usr/include/scsi as symlinks into my Linux source tree. Is that wrong?
    13. Programs like `logname', `top', `uptime' `users', `w' and `who', show incorrect information about the (number of) users on my system. Why?
    14. 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?

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

    1. I expect GNU libc to be 100% source code compatible with the old Linux based GNU libc. Why isn't it like this?
    2. Why does getlogin() always return NULL on my Linux box?
    3. Where are the DST_* constants found in <sys/time.h> on many systems?
    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?
    5. On Linux I've got problems with the declarations in Linux kernel headers.
    6. I don't include any kernel headers myself but the compiler still complains about redeclarations of types in the kernel headers.
    7. Why don't signals interrupt system calls anymore?

  4. Miscellaneous

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


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://prep.ai.mit.edu/pub/gnu and the many mirror sites. prep 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.

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

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

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:

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.


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:

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:

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. 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.

2.12. 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.

2.13. 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.

2.14. 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.


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:

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 fromany 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:

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().


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 prep.ai.mit.edu.

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.


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>