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 ? Compiling glibc ?? 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 powerpc-*-linux-gnu Linux and MkLinux on PowerPC systems sparc-*-linux-gnu Linux-2.x on SPARC sparc64-*-linux-gnu Linux-2.x on UltraSPARC arm-*-none ARM standalone systems arm-*-linux Linux-2.x on ARM arm-*-linuxaout Linux-2.x on ARM using a.out binaries 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 ?? 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 should always try to use the latest official release. Older versions may not have all the features GNU libc requires. The current releases of egcs (1.0.3 and 1.1) and GNU CC (2.8.1) should work with the GNU C library (for powerpc see ?powerpc; for ARM see ?arm). {ZW} You may have problems if you try to mix code compiled with EGCS and with GCC 2.8.1. See ?exception for details. ?? 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 or 3.77. Versions before 3.75 have bugs and/or are missing features. Version 3.76 has bugs which appear when building big projects like GNU libc. 3.76.1 appears to work but some people have reported problems. If you build GNU make 3.77 from source, please read ?make first. ?? Do I need a special linker or assembler? {ZW} If you want a shared library, you need a linker and assembler that understand all the features of ELF, including weak and versioned symbols. The static library can be compiled with less featureful tools, but lacks key features such as NSS. For Linux or Hurd, you want binutils 2.8.1.0.23, 2.9.1, or 2.9.1.0.15 or higher. These are the only versions we've tested and found reliable. Other versions after 2.8.1.0.23 may work but we don't recommend them, especially not when C++ is involved. Earlier versions do not work at all. Other operating systems may come with system tools that have all the necessary features, but this is moot because glibc hasn't been ported to them. ??powerpc Which compiler should I use for powerpc? {GK} You want to use egcs 1.1 or later (together with the right versions of all the other tools, of course). In fact, egcs 1.1 has a bug that causes linuxthreads to be miscompiled, resulting in segmentation faults when using condition variables. There is a temporary patch at: Later versions of egcs may fix this problem. ??arm Which tools should I use for ARM? {PB} You should use egcs 1.1 or a later version. For ELF systems some changes are needed to the compiler; a patch against egcs-1.1.x can be found at: Binutils 2.9.1.0.16 or later is also required. ?? Do I need some more things to compile the 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 are built with special tools. E.g., files ending in .gperf need a `gperf' program. The GNU version (now available in a separate package, formerly only as 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. * Perl 5 is needed if you wish to test an installation of GNU libc as the primary C library. * When compiling for Linux, the header files of the Linux kernel must be available to the compiler as and . * lots of disk space (~170MB for i?86-linux; more for RISC platforms, as much as 400MB). * 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 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 reports 22h48m on Atari TT030 (Motorola 68030 @ 32 Mhz, 34 Mb memory) If you have some more measurements let me know. ?? What version of the Linux kernel headers should be used? {AJ,UD} The headers from the most recent Linux kernel should be used. The headers used while compiling the GNU C library and the kernel binary used when using the library do not need to match. The GNU C library runs without problems on kernels that are older than the kernel headers used. The other way round (compiling the GNU C library with old kernel headers and running on a recent kernel) does not necessarily work. For example you can't use new kernel features when using old kernel headers for compiling the GNU C library. {ZW} Even if you are using a 2.0 kernel on your machine, we recommend you compile GNU libc with 2.1 kernel headers. That way you won't have to recompile libc if you ever upgrade to kernel 2.1 or 2.2. To tell libc which headers to use, give configure the --with-headers switch (e.g. --with-headers=/usr/src/linux-2.1.107/include). Note that you must configure the 2.1 kernel if you do this, otherwise libc will be unable to find . Just change the current directory to the root of the 2.1 tree and do `make include/linux/version.h'. ?? The compiler hangs while building iconvdata modules. What's wrong? {ZW} This is a problem with old versions of GCC. Initialization of large static arrays is very slow. The compiler will eventually finish; give it time. The problem is fixed in egcs 1.1 but not in earlier releases. ?? 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 * 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. ??addon 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 ?crypt). 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. ?? 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. ?? 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. ?? Why do I get messages about missing thread functions when I use librt? I don't even use threads. {UD} In this case you probably mixed up your installation. librt uses threads internally and has implicit references to the thread library. Normally these references are satisfied automatically but if the thread library is not in the expected place you must tell the linker where it is. When using GNU ld it works like this: gcc -o foo foo.c -Wl,-rpath-link=/some/other/dir -lrt The `/some/other/dir' should contain the thread library. `ld' will use the given path to find the implicitly referenced library while not disturbing any other link path. ?? 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. ?? I get failures during `make check'. What shall I do? {AJ} The testsuite should compile and run cleanly on your system, every failure should be looked into. Depending on the failure I wouldn't advise installing the library at all. You should consider using the `glibcbug' script to report the failure, providing as much detail as possible. If you run a test directly, please remember to set up the environment correctly. You want to test the compiled library - and not your installed one. The best way is to copy the exact command line which failed and run the test from the subdirectory for this test in the sources. There are some failures which are not directly related to the GNU libc: - Some compiler produce buggy code. The egcs 1.1 release should be ok. gcc 2.8.1 might cause some failures, gcc 2.7.2.x is so buggy, that explicit checks have been used so that you can't build with it. - The kernel might have bugs. For example on Linux/Alpha 2.0.34 the floating point handling has quite a number of bugs and therefore most of the test cases in the math subdirectory will fail. The current Linux 2.1 development kernels have fixes for the floating point support on Alpha. ?? What is symbol versioning good for? Do I need it? {AJ} Symbol versioning solves problems that are related to interface changes. One version of an interface might have been introduced in a previous version of the GNU C library but the interface or the semantics of the function has been changed in the meantime. For binary compatibility with the old library, a newer library needs to still have the old interface for old programs. On the other hand new programs should use the new interface. Symbol versioning is the solution for this problem. The GNU libc version 2.1 uses by default symbol versioning if the binutils support it. We don't advise to build without symbol versioning since you lose binary compatibility if you do - for ever! The binary compatibility you lose is not only against the previous version of the GNU libc (version 2.0) but also against future versions. ? Installation and configuration issues ?? 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. ?? 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 ). Note that this can damage your system; see ?safety 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. ??safety 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. ?? 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 string functions. These can only be used with GCC. See ?string for details. ??crypt 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 ?addon). 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. ?? 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 to the linker --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. When linking via gcc, you've got to add -Wl,--dynamic-linker=/lib/ld-linux.so.2 to the gcc command line. 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///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. ?? 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 libc_nonshared.a ) ??exception When I run an executable on one system which I compiled on another, I get dynamic linker errors. Both systems have the same version of glibc installed. What's wrong? {ZW} Glibc on one of these systems was compiled with gcc 2.7 or 2.8, the other with egcs (any version). Egcs has functions in its internal `libgcc.a' to support exception handling with C++. They are linked into any program or dynamic library compiled with egcs, whether it needs them or not. Dynamic libraries then turn around and export those functions again unless special steps are taken to prevent them. When you link your program, it resolves its references to the exception functions to the ones exported accidentally by libc.so. That works fine as long as libc has those functions. On the other system, libc doesn't have those functions because it was compiled by gcc 2.8, and you get undefined symbol errors. The symbols in question are named things like `__register_frame_info'. For glibc 2.0, the workaround is to not compile libc with egcs. We've also incorporated a patch which should prevent the EH functions sneaking into libc. It doesn't matter what compiler you use to compile your program. For glibc 2.1, we've chosen to do it the other way around: libc.so explicitly provides the EH functions. This is to prevent other shared libraries from doing it. You must therefore compile glibc 2.1 with EGCS unless you don't care about ever importing binaries from other systems. Again, it doesn't matter what compiler you use for your programs. ?? 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. But you should get at least gcc 2.8.1 or egcs 1.0.2 (or later versions) instead. ?? 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 , 1996. # /^\$ #/ { h s/\$ #\([^ ]*\).*/\1/ x s/\$ #[^ ]* *\(.*\)/\$ \1/ } /^# / { s/^# \(.*\)/\1/ G s/\(.*\)\n\(.*\)/\2 \1/ } ----------------------------------------------------------------------- ?? Programs using libc have their messages translated, but other behavior is not localized (e.g. collating order); why? {ZW} Translated messages are automatically installed, but the locale database that controls other behaviors is not. You need to run localedef to install this database, after you have run `make install'. For example, to set up the French Canadian locale, simply issue the command localedef -i fr_CA -f ISO-8859-1 fr_CA Please see localedata/README in the source tree for further details. ?? 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 ?? I have killed ypbind to stop using NIS, but glibc continues 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 continue to use them. Other BSD versions seem to work correctly. Until ypbind 3.4 is released, you can find a patch at ftp://ftp.kernel.org/pub/linux/utils/net/NIS/ypbind-3.3-glibc4.diff.gz ?? Under Linux/Alpha, I always get "do_ypcall: clnt_call: RPC: Unable to receive; errno = Connection refused" when using NIS. {TK} You need a ypbind version which is 64bit clean. Some versions are not 64bit clean. A 64bit clean implementation is ypbind-mt. For ypbind 3.3, you need the patch from ftp.kernel.org (See the previous question). I don't know about other versions. ?? 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. ?? How do I create the databases for NSS? {AJ} If you have an entry "db" in /etc/nsswitch.conf you should also create the database files. The glibc sources contain a Makefile which does the necessary conversion and calls to create those files. The file is `db-Makefile' in the subdirectory `nss' and you can call it with `make -f db-Makefile'. Please note that not all services are capable of using a database. Currently passwd, group, ethers, protocol, rpc, services shadow and netgroup are implemented. ?? 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 ?kerhdr). glibc provides its own and 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. ?? Programs like `logname', `top', `uptime' `users', `w' and `who', show incorrect information about the (number of) users on my system. Why? {MK} See ?getlog. ?? After upgrading to glibc 2.1 with symbol versioning I get errors about undefined symbols. What went wrong? {AJ} The problem is caused either by wrong program code or tools. In the versioned libc a lot of symbols are now local that were global symbols in previous versions. It seems that programs linked against older versions often accidentally used libc global variables -- something that should not happen. The only way to fix this is to recompile your program. Sorry, that's the price you might have to pay once for quite a number of advantages with symbol versioning. ?? 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. ?? What do I need for C++ development? {HJ,AJ} You need either egcs 1.1 which comes directly with libstdc++ or gcc-2.8.1 together with libstdc++ 2.8.1.1. egcs 1.1 has the better C++ support and works directly with glibc 2.1. If you use gcc-2.8.1 with libstdc++ 2.8.1.1, you need to modify libstdc++ a bit. A patch is available as: ftp://alpha.gnu.org/gnu/libstdc++-2.8.1.1-glibc2.1-diff.gz Please note that 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 libstdc++ since the library compiled for 2.0 is not compatible due to the new Large File Support (LFS) in version 2.1. {UD} But since in the case of a shared libstdc++ the version numbers should be different existing programs will continue to work. ?? Even statically linked programs need some shared libraries which is not acceptable for me. What can I do? {AJ} NSS (for details just type `info libc "Name Service Switch"') won't work properly without shared libraries. NSS allows using different services (e.g. NIS, files, db, hesiod) by just changing one configuration file (/etc/nsswitch.conf) without relinking any programs. The only disadvantage is that now static libraries need to access shared libraries. This is handled transparently by the GNU C library. A solution is to configure glibc with --enable-static-nss. In this case you can create a static binary that will use only the services dns and files (change /etc/nsswitch.conf for this). You need to link explicitly against all these services. For example: gcc -static test-netdb.c -o test-netdb.c \ -lc -lnss_files -lnss_dns -lresolv The problem with this approach is that you've got to link every static program that uses NSS routines with all those libraries. {UD} In fact, one cannot say anymore that a libc compiled with this option is using NSS. There is no switch anymore. Therefore it is *highly* recommended *not* to use --enable-static-nss since this makes the behaviour of the programs on the system inconsistent. ?? I just upgraded my Linux system to glibc and now I get errors whenever I try to link any program. {ZW} This happens when you have installed glibc as the primary C library but have stray symbolic links pointing at your old C library. If the first `libc.so' the linker finds is libc 5, it will use that. Your program expects to be linked with glibc, so the link fails. The most common case is that glibc put its `libc.so' in /usr/lib, but there was a `libc.so' from libc 5 in /lib, which gets searched first. To fix the problem, just delete /lib/libc.so. You may also need to delete other symbolic links in /lib, such as /lib/libm.so if it points to libm.so.5. {AJ} The perl script test-installation.pl which is run as last step during an installation of glibc that is configured with --prefix=/usr should help detect these situations. If the script reports problems, something is really screwed up. ?? When I use nscd the machine freezes. {UD} You cannot use nscd with Linux 2.0.*. There is functionality missing in the kernel and work-arounds are not suitable. Besides, some parts of the kernel are too buggy when it comes to using threads. If you need nscd, you have to use a 2.1 kernel. Note that I have at this point no information about any other platform. ?? I need lots of open files. What do I have to do? {AJ} This is at first a kernel issue. The kernel defines limits with OPEN_MAX the number of simultaneous open files and with FD_SETSIZE the number of used file descriptors. You need to change these values in your kernel and recompile the kernel so that the kernel allows to use more open files. You don't necessarily need to recompile the GNU C library since the only place where OPEN_MAX and FD_SETSIZE is really needed in the library itself is the size of fd_set which is used by select. The GNU C library is now (nearly) select free. This means it internally has no limits imposed by the `fd_set' type. Instead almost all places where the functionality is needed the `poll' function is used. If you increase the number of file descriptors in the kernel you don't need to recompile the C library. The remaining select calls are in the RPC code. If your RPC daemons don't need more than FD_SETSIZE file descriptors, you don't need to change anything at all. {UD} You can always get the maximum number of file descriptors a process is allowed to have open at any time using number = sysconf (_SC_OPEN_MAX); This will work even if the kernel limits change. ?? How do I get the same behavior on parsing /etc/passwd and /etc/group as I have with libc5 ? {TK} The name switch setup in /etc/nsswitch.conf selected by most Linux distributions does not support +/- and netgroup entries in the files like /etc/passwd. Though this is the preferred setup some people might have setups coming over from the libc5 days where it was the default to recognize lines like this. To get back to the old behaviour one simply has to change the rules for passwd, group, and shadow in the nsswitch.conf file as follows: passwd: compat group: compat shadow: compat passwd_compat: nis group_compat: nis shadow_compat: nis ?? What needs to be recompiled when upgrading from glibc 2.0 to glibc 2.1? {AJ,CG} If you just upgrade the glibc from 2.0.x (x <= 7) to 2.1, binaries that have been linked against glibc 2.0 will continue to work. If you compile your own binaries against glibc 2.1, you also need to recompile some other libraries. The problem is that libio had to be changed and therefore libraries that are based or depend on the libio of glibc, e.g. ncurses or slang, need to be recompiled. If you experience strange segmentation faults in your programs linked against glibc 2.1, you might need to recompile your libraries. Another problem is that older binaries that were linked statically against glibc 2.0 will reference the older nss modules (libnss_files.so.1 instead of libnss_files.so.2), so don't remove them. Also, the old glibc-2.0 compiled static libraries (libfoo.a) which happen to depend on the older libio behavior will be broken by the glibc 2.1 upgrade. The idea is to produce a libcompat.a that people will be able to use to link in if they want to compile a static library generated against glibc 2.0 into a program on a glibc 2.1 system. You should just add -lcompat and you should be fine. The glibc-compat add-on will provide the libnss_* files and other stuff so that it is possible to use static library compiled against glibc 2.0 for development on a glibc 2.1 based system. Please note that this is currently just an experiment, although the nss modules part should work. ? Source and binary incompatibilities, and what to do about them ?? 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 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 . This header file also provides the SWAP_* constants defined by ; you should use them for the second argument to swapon(). * errno: If a program uses the variable "errno", then it _must_ include . 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 syslog ksyslog_ctl * 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 ?signal for details. ??getlog 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. {MK} There is however a (partial) solution for this problem. Please take a look at the file `login/README.utmpd'. ?? Where are the DST_* constants found in 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. For former is very much preferred (see ?tzdb). ?? 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. ??kerhdr 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. ?? 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. ??signal 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 . 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(). ??string I've got errors compiling code that uses certain string functions. Why? {AJ} glibc 2.1 has special string functions that are faster than the normal library functions. Some of the functions are additionally implemented as inline functions and others as macros. The optimized string functions are only used when compiling with optimizations (-O1 or higher). The behavior can be changed with two feature macros: * __NO_STRING_INLINES: Don't do any string optimizations. * __USE_STRING_INLINES: Use assembly language inline functions (might increase code size dramatically). Since some of these string functions are now additionally defined as macros, code like "char *strncpy();" doesn't work anymore (and is unnecessary, since has the necessary declarations). Either change your code or define __NO_STRING_INLINES. {UD} Another problem in this area is that gcc still has problems on machines with very few registers (e.g., ix86). The inline assembler code can require almost all the registers and the register allocator cannot always handle this situation. One can disable the string optimizations selectively. Instead of writing cp = strcpy (foo, "lkj"); one can write cp = (strcpy) (foo, "lkj"); This disables the optimization for that specific call. ?? I get compiler messages "Initializer element not constant" with stdin/stdout/stderr. Why? {RM,AJ} Constructs like: static FILE *InPtr = stdin; lead to this message. This is correct behaviour with glibc since stdin is not a constant expression. Please note that a strict reading of ISO C does not allow above constructs. One of the advantages of this is that you can assign to stdin, stdout, and stderr just like any other global variable (e.g. `stdout = my_stream;'), which can be very useful with custom streams that you can write with libio (but beware this is not necessarily portable). The reason to implement it this way were versioning problems with the size of the FILE structure. To fix those programs you've got to initialize the variable at run time. This can be done, e.g. in main, like: static FILE *InPtr; int main(void) { InPtr = stdin; } or by constructors (beware this is gcc specific): static FILE *InPtr; static void inPtr_construct (void) __attribute__((constructor)); static void inPtr_construct (void) { InPtr = stdin; } ?? I can't compile with gcc -traditional (or -traditional-cpp). Why? {AJ} glibc2 does break -traditional and -traditonal-cpp - and will continue to do so. For example constructs of the form: enum {foo #define foo foo } are useful for debugging purposes (you can use foo with your debugger that's why we need the enum) and for compatibility (other systems use defines and check with #ifdef). ?? I get some errors with `gcc -ansi'. Isn't glibc ANSI compatible? {AJ} The GNU C library is compatible with the ANSI/ISO C standard. If you're using `gcc -ansi', the glibc includes which are specified in the standard follow the standard. The ANSI/ISO C standard defines what has to be in the include files - and also states that nothing else should be in the include files (btw. you can still enable additional standards with feature flags). The GNU C library is conforming to ANSI/ISO C - if and only if you're only using the headers and library functions defined in the standard. ?? I can't access some functions anymore. nm shows that they do exist but linking fails nevertheless. {AJ} With the introduction of versioning in glibc 2.1 it is possible to export only those identifiers (functions, variables) that are really needed by application programs and by other parts of glibc. This way a lot of internal interfaces are now hidden. nm will still show those identifiers but marking them as internal. ISO C states that identifiers beginning with an underscore are internal to the libc. An application program normally shouldn't use those internal interfaces (there are exceptions, e.g. __ivaliduser). If a program uses these interfaces, it's broken. These internal interfaces might change between glibc releases or dropped completely. ?? When using the db-2 library which comes with glibc is used in the Perl db modules the testsuite is not passed. This did not happen with db-1, gdbm, or ndbm. {UD} You are using an outdated copy of the DB_File Perl module. In fact db-2 finally removed the handling of zero-sized keys which was one of the features tested by the old Perl testsuite and therefore you see an error. But this never was documented and guaranteed, only broken programs used this feature. Consequently db-2 does not need to support this feature and instead signals an error which leads to easier debugging. The DB_File module maintainer Paul Marquess acknowledged this change and fixed the testsuite so that if you use DB_File v1.60 or later you should not have any more problems with db-2. ?? The pow() inline function I get when including is broken. I get segmentation faults when I run the program. {UD} Nope, the implementation is correct. The problem is with egcs version prior to 1.1. I.e., egcs 1.0 to 1.0.3 are all broken (at least on Intel). If you have to use this compiler you must define __NO_MATH_INLINES before including to prevent the inline functions from being used. egcs 1.1 fixes the problem. I don't know about gcc 2.8 and 2.8.1. ?? The sys/sem.h file lacks the definition of `union semun'. {UD} Nope. This union has to be provided by the user program. Former glibc versions defined this but it was an error since it does not make much sense when thinking about it. The standards describing the System V IPC functions define it this way and therefore programs must be adopted. ?? Why has disappeared? {AJ} The corresponding Linux kernel data structures and constants are totally different in Linux 2.0 and Linux 2.1. This situation has to be taken care in user programs using the firewall structures and therefore those programs (ipfw is AFAIK the only one) should deal with this problem themselves. ?? I get floods of warnings when I use -Wconversion and include or . {ZW} and intentionally use prototypes to override argument promotion. -Wconversion warns about all these. You can safely ignore the warnings. -Wconversion isn't really intended for production use, only for shakedown compiles after converting an old program to standard C. ? Miscellaneous ?? 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. ?? 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. {PB} The 2.1 release of GNU libc aims to comply with the current versions of all the relevant standards. The IPv6 support libraries for older Linux systems used a different naming convention and so code written to work with them may need to be modified. If the standards make incompatible changes in the future then the libc may need to change again. IPv6 will not work with a 2.0.x kernel. When kernel 2.2 is released it should contain all the necessary support; until then you should use the latest 2.1.x release you can find. As of 98/11/26 the currently recommended kernel for IPv6 is 2.1.129. Also, as of the 2.1 release the IPv6 API provided by GNU libc is not 100% complete. In particular the getipnodebyname and getipnodebyaddr functions are not implemented. ??tzdb When I set the timezone by setting the TZ environment variable to EST5EDT things go wrong since glibc computes the wrong time from this information. {UD} The problem is that people still use the braindamaged POSIX method to select the timezone using the TZ environment variable with a format EST5EDT or whatever. People, if you insist on using TZ instead of the timezone database (see below), read the POSIX standard, the implemented behaviour is correct! What you see is in fact the result of the decisions made while POSIX.1 was created. We've only implemented the handling of TZ this way to be POSIX compliant. It is not really meant to be used. The alternative approach to handle timezones which is implemented is the correct one to use: use the timezone database. This avoids all the problems the POSIX method has plus it is much easier to use. Simply run the tzselect shell script, answer the question and use the name printed in the end by making a symlink /etc/localtime pointing to /usr/share/zoneinfo/NAME (NAME is the returned value from tzselect). That's all. You never again have to worry. So, please avoid sending bug reports about time related problems if you use the POSIX method and you have not verified something is really broken by reading the POSIX standards. ?? What other sources of documentation about glibc are available? {AJ} The FSF has a page about the GNU C library at . The problem data base of open and solved bugs in GNU libc is available at . Eric Green has written a HowTo for converting from Linux libc5 to glibc2. The HowTo is accessable via the FSF page and at . Frodo Looijaard describes a different way installing glibc2 as secondary libc at . Please note that this is not a complete list. ?? The timezone string for Sydney/Australia is wrong since even when daylight saving time is in effect the timezone string is EST. {UD} The problem for some timezones is that the local authorities decided to use the term "summer time" instead of "daylight saving time". In this case the abbreviation character `S' is the same as the standard one. So, for Sydney we have Eastern Standard Time = EST Eastern Summer Time = EST Great! To get this bug fixed convince the authorities to change the laws and regulations of the country this effects. glibc behaves correctly. ??make I've build make 3.77 against glibc 2.1 and now make gets segmentation faults. {AJ} GNU make 3.77 has support for 64 bit filesystems which is slightly broken (and one of the new features in the GNU C library 2.1 is 64 bit filesystem support :-( ). To get a working make you can use either make 3.75 or patch 3.77. A working patch is available via RedHat's Rawhide server (ftp://rawhide.redhat.com/SRPMS/SRPMS/make-3.77-*src.rpm). Answers were given by: {UD} Ulrich Drepper, {DMT} David Mosberger-Tang, {RM} Roland McGrath, {AJ} Andreas Jaeger, {EY} Eric Youngdale, {PB} Phil Blundell, {MK} Mark Kettenis, {ZW} Zack Weinberg, {TK} Thorsten Kukuk, {GK} Geoffrey Keating, {HJ} H.J. Lu, {CG} Cristian Gafton, Local Variables: mode:outline outline-regexp:"\\?" fill-column:76 End: