1 Frequently Asked Questions about the GNU C Library
3 This document tries to answer questions a user might have when installing
4 and using glibc. Please make sure you read this before sending questions or
5 bug reports to the maintainers.
7 The GNU C library is very complex. The installation process has not been
8 completely automated; there are too many variables. You can do substantial
9 damage to your system by installing the library incorrectly. Make sure you
10 understand what you are undertaking before you begin.
12 If you have any questions you think should be answered in this document,
19 ?? What systems does the GNU C Library run on?
21 {UD} This is difficult to answer. The file `README' lists the architectures
22 GNU libc was known to run on *at some time*. This does not mean that it
23 still can be compiled and run on them now.
25 The systems glibc is known to work on as of this release, and most probably
29 i[3456]86-*-linux-gnu Linux-2.x on Intel
30 m68k-*-linux-gnu Linux-2.x on Motorola 680x0
31 alpha-*-linux-gnu Linux-2.x on DEC Alpha
32 powerpc-*-linux-gnu Linux and MkLinux on PowerPC systems
33 sparc-*-linux-gnu Linux-2.x on SPARC
34 sparc64-*-linux-gnu Linux-2.x on UltraSPARC
35 arm-*-none ARM standalone systems
36 arm-*-linuxaout Linux-2.x on ARM using a.out binaries
38 Ports to other Linux platforms are in development, and may in fact work
39 already, but no one has sent us success reports for them. Currently no
40 ports to other operating systems are underway, although a few people have
43 If you have a system not listed above (or in the `README' file) and you are
44 really interested in porting it, contact
48 ?? What compiler do I need to build GNU libc?
50 {UD} You must use GNU CC to compile GNU libc. A lot of extensions of GNU CC
51 are used to increase portability and speed.
53 GNU CC is found, like all other GNU packages, on
55 ftp://ftp.gnu.org/pub/gnu
57 and the many mirror sites. ftp.gnu.org is always overloaded, so try to find
60 You should always try to use the latest official release. Older versions
61 may not have all the features GNU libc requires. The current releases of
62 egcs (1.0.2) and GNU CC (2.8.1) should work with the GNU C library (for
63 powerpc see question ?powerpc).
65 ?? When I try to compile glibc I get only error messages.
68 {UD} You definitely need GNU make to translate GNU libc. No other make
69 program has the needed functionality.
71 We recommend version GNU make version 3.75. Versions 3.76 and 3.76.1 have
72 bugs which appear when building big projects like GNU libc. Versions before
73 3.74 have bugs and/or are missing features.
75 ?? Do I need a special linker or archiver?
77 {UD} You may be able to use your system linker, but GNU libc works best with
80 On systems where the native linker does not support weak symbols you will
81 not get a fully ISO C compliant C library. Generally speaking you should
82 use the GNU binutils if they provide at least the same functionality as your
85 Always get the newest release of GNU binutils available. Older releases are
86 known to have bugs that prevent a successful compilation.
88 {ZW} As of release 2.1 a linker supporting symbol versions is required. For
89 Linux, get binutils-2.8.1.0.23 or later. Other systems may have native
90 linker support, but it's moot right now, because glibc has not been ported
93 ??powerpc Which compiler should I use for powerpc?
95 {GK} You want to use egcs 1.0.1 or later (together with the right versions
96 of all the other tools, of course).
98 In fact, egcs 1.0.1 has a serious bug that prevents a clean make, relating
99 to switch statement folding. It also causes the resulting shared libraries
100 to use more memory than they should. There is a patch at:
102 <http://discus.anu.edu.au/~geoffk/egcs-1.0.1-geoffk.diff>
104 Later versions of egcs may fix these problems.
107 ?? Do I need some more things to compile GNU C Library?
109 {UD} Yes, there are some more :-).
111 * GNU gettext. This package contains the tools needed to construct
112 `message catalog' files containing translated versions of system
113 messages. See ftp://ftp.gnu.org/pub/gnu or better any mirror
114 site. (We distribute compiled message catalogs, but they may not be
117 * Some files depend on special tools. E.g., files ending in .gperf
118 need a `gperf' program. The GNU version (part of libg++) is known
119 to work while some vendor versions do not.
121 You should not need these tools unless you change the source files.
123 * Some scripts need perl5 - but at the moment those scripts are not
124 vital for building and installing GNU libc (some data files will not
127 * When compiling for Linux, the header files of the Linux kernel must
128 be available to the compiler as <linux/*.h> and <asm/*.h>.
130 * lots of disk space (~170MB for i?86-linux; more for RISC platforms,
133 * plenty of time. Compiling just the shared and static libraries for
134 i?86-linux takes approximately 1h on an i586@133, or 2.5h on
135 i486@66, or 4.5h on i486@33. Multiply this by 1.5 or 2.0 if you
136 build profiling and/or the highly optimized version as well. For
137 Hurd systems times are much higher.
139 You should avoid compiling in a NFS mounted filesystem. This is
142 James Troup <J.J.Troup@comp.brad.ac.uk> reports a compile time of
143 45h34m for a full build (shared, static, and profiled) on Atari
144 Falcon (Motorola 68030 @ 16 Mhz, 14 Mb memory) and Jan Barte
145 <yann@plato.uni-paderborn.de> reports 22h48m on Atari TT030
146 (Motorola 68030 @ 32 Mhz, 34 Mb memory)
148 If you have some more measurements let me know.
150 ?? What version of the Linux kernel headers should be used?
152 {AJ,UD} The headers from the most recent Linux kernel should be used. The
153 headers used while compiling the GNU C library and the kernel binary used
154 when using the library do not need to match. The GNU C library runs without
155 problems on kernels that are older than the kernel headers used. The other
156 way round (compiling the GNU C library with old kernel headers and running
157 on a recent kernel) does not necessarily work. For example you can't use
158 new kernel features when using old kernel headers for compiling the GNU C
161 {ZW} Even if you are using a 2.0 kernel on your machine, we recommend you
162 compile GNU libc with 2.1 kernel headers. That way you won't have to
163 recompile libc if you ever upgrade to kernel 2.1 or 2.2. To tell libc which
164 headers to use, give configure the --with-headers switch
165 (e.g. --with-headers=/usr/src/linux-2.1.107/include).
167 Note that you must configure the 2.1 kernel if you do this; otherwise libc
168 will be unable to find <linux/version.h>. Just copy .config from your 2.0
169 kernel sources to the 2.1 tree, do `make oldconfig', and say no to all the
172 ?? The compiler hangs while building iconvdata modules. What's
175 {ZW} This is a problem with all current releases of GCC. Initialization of
176 large static arrays is very slow. The compiler will eventually finish; give
179 The problem will be fixed in egcs 1.1 but probably not before then.
181 ?? When I run `nm -u libc.so' on the produced library I still
182 find unresolved symbols. Can this be ok?
184 {UD} Yes, this is ok. There can be several kinds of unresolved symbols:
186 * magic symbols automatically generated by the linker. These have names
187 like __start_* and __stop_*
189 * symbols starting with _dl_* come from the dynamic linker
191 * weak symbols, which need not be resolved at all (fabs for example)
193 Generally, you should make sure you find a real program which produces
194 errors while linking before deciding there is a problem.
196 ??addon What are these `add-ons'?
198 {UD} To avoid complications with export rules or external source code some
199 optional parts of the libc are distributed as separate packages (e.g., the
200 crypt package, see ?crypt).
202 To use these packages as part of GNU libc, just unpack the tarfiles in the
203 libc source directory and tell the configuration script about them using the
204 --enable-add-ons option. If you give just --enable-add-ons configure tries
205 to find all the add-on packages in your source tree. This may not work. If
206 it doesn't, or if you want to select only a subset of the add-ons, give a
207 comma-separated list of the add-ons to enable:
209 configure --enable-add-ons=crypt,linuxthreads
213 Add-ons can add features (including entirely new shared libraries), override
214 files, provide support for additional architectures, and just about anything
215 else. The existing makefiles do most of the work; only some few stub rules
216 must be written to get everything running.
218 ?? My XXX kernel emulates a floating-point coprocessor for me.
219 Should I enable --with-fp?
221 {ZW} An emulated FPU is just as good as a real one, as far as the C library
222 is concerned. You only need to say --without-fp if your machine has no way
223 to execute floating-point instructions.
225 People who are interested in squeezing the last drop of performance
226 out of their machine may wish to avoid the trap overhead, but this is
227 far more trouble than it's worth: you then have to compile
228 *everything* this way, including the compiler's internal libraries
229 (libgcc.a for GNU C), because the calling conventions change.
231 ?? When compiling GNU libc I get lots of errors saying functions
232 in glibc are duplicated in libgcc.
234 {EY} This is *exactly* the same problem that I was having. The problem was
235 due to the fact that configure didn't correctly detect that the linker flag
236 --no-whole-archive was supported in my linker. In my case it was because I
237 had run ./configure with bogus CFLAGS, and the test failed.
239 One thing that is particularly annoying about this problem is that once this
240 is misdetected, running configure again won't fix it unless you first delete
243 {UD} Starting with glibc-2.0.3 there should be a better test to avoid some
244 problems of this kind. The setting of CFLAGS is checked at the very
245 beginning and if it is not usable `configure' will bark.
247 ?? Why do I get messages about missing thread functions when I use
248 librt? I don't even use threads.
250 {UD} In this case you probably mixed up your installation. librt uses
251 threads internally and has implicit references to the thread library.
252 Normally these references are satisfied automatically but if the thread
253 library is not in the expected place you must tell the linker where it is.
254 When using GNU ld it works like this:
256 gcc -o foo foo.c -Wl,-rpath-link=/some/other/dir -lrt
258 The `/some/other/dir' should contain the thread library. `ld' will use the
259 given path to find the implicitly referenced library while not disturbing
262 ?? What's the problem with configure --enable-omitfp?
264 {AJ} When --enable-omitfp is set the libraries are built without frame
265 pointers. Some compilers produce buggy code for this model and therefore we
266 don't advise using it at the moment.
268 If you use --enable-omitfp, you're on your own. If you encounter problems
269 with a library that was build this way, we advise you to rebuild the library
270 without --enable-omitfp. If the problem vanishes consider tracking the
271 problem down and report it as compiler failure.
273 Since a library build with --enable-omitfp is undebuggable on most systems,
274 debuggable libraries are also built - you can use it by appending "_g" to
277 The compilation of these extra libraries and the compiler optimizations slow
278 down the build process and need more disk space.
280 ? Installation and configuration issues
282 ?? Can I replace the libc on my Linux system with GNU libc?
284 {UD} You cannot replace any existing libc for Linux with GNU libc. It is
285 binary incompatible and therefore has a different major version. You can,
286 however, install it alongside your existing libc.
288 For Linux there are three major libc versions:
290 libc-5 original ELF libc
293 You can have any combination of these three installed. For more information
294 consult documentation for shared library handling. The Makefiles of GNU
295 libc will automatically generate the needed symbolic links which the linker
298 ?? How do I configure GNU libc so that the essential libraries
299 like libc.so go into /lib and the other into /usr/lib?
301 {UD,AJ} Like all other GNU packages GNU libc is designed to use a base
302 directory and install all files relative to this. The default is
303 /usr/local, because this is safe (it will not damage the system if installed
304 there). If you wish to install GNU libc as the primary C library on your
305 system, set the base directory to /usr (i.e. run configure --prefix=/usr
306 <other_options>). Note that this can damage your system; see ?safety for
309 Some systems like Linux have a filesystem standard which makes a difference
310 between essential libraries and others. Essential libraries are placed in
311 /lib because this directory is required to be located on the same disk
312 partition as /. The /usr subtree might be found on another
313 partition/disk. If you configure for Linux with --prefix=/usr, then this
314 will be done automatically.
316 To install the essential libraries which come with GNU libc in /lib on
317 systems other than Linux one must explicitly request it. Autoconf has no
318 option for this so you have to use a `configparms' file (see the `INSTALL'
319 file for details). It should contain:
324 The first line specifies the directory for the essential libraries, the
325 second line the directory for system configuration files.
327 ??safety How should I avoid damaging my system when I install GNU libc?
329 {ZW} If you wish to be cautious, do not configure with --prefix=/usr. If
330 you don't specify a prefix, glibc will be installed in /usr/local, where it
331 will probably not break anything. (If you wish to be certain, set the
332 prefix to something like /usr/local/glibc2 which is not used for anything.)
334 The dangers when installing glibc in /usr are twofold:
336 * glibc will overwrite the headers in /usr/include. Other C libraries
337 install a different but overlapping set of headers there, so the
338 effect will probably be that you can't compile anything. You need to
339 rename /usr/include out of the way first. (Do not throw it away; you
340 will then lose the ability to compile programs against your old libc.)
342 * None of your old libraries, static or shared, can be used with a
343 different C library major version. For shared libraries this is not a
344 problem, because the filenames are different and the dynamic linker
345 will enforce the restriction. But static libraries have no version
346 information. You have to evacuate all the static libraries in
347 /usr/lib to a safe location.
349 The situation is rather similar to the move from a.out to ELF which
350 long-time Linux users will remember.
352 ?? Do I need to use GNU CC to compile programs that will use the
355 {ZW} In theory, no; the linker does not care, and the headers are supposed
356 to check for GNU CC before using its extensions to the C language.
358 However, there are currently no ports of glibc to systems where another
359 compiler is the default, so no one has tested the headers extensively
360 against another compiler. You may therefore encounter difficulties. If you
361 do, please report them as bugs.
363 Also, in several places GNU extensions provide large benefits in code
364 quality. For example, the library has hand-optimized, inline assembly
365 versions of some string functions. These can only be used with GCC. See
368 ??crypt When linking with the new libc I get unresolved symbols
369 `crypt' and `setkey'. Why aren't these functions in the
372 {UD} The US places restrictions on exporting cryptographic programs and
373 source code. Until this law gets abolished we cannot ship the cryptographic
374 functions together with glibc.
376 The functions are available, as an add-on (see ?addon). People in the US
377 may get it from the same place they got GNU libc from. People outside the
378 US should get the code from ftp://ftp.ifi.uio.no/pub/gnu, or another archive
379 site outside the USA. The README explains how to install the sources.
381 If you already have the crypt code on your system the reason for the failure
382 is probably that you did not link with -lcrypt. The crypto functions are in
383 a separate library to make it possible to export GNU libc binaries from the
386 ?? When I use GNU libc on my Linux system by linking against
387 the libc.so which comes with glibc all I get is a core dump.
389 {UD} On Linux, gcc sets the dynamic linker to /lib/ld-linux.so.1 unless the
390 user specifies a -dynamic-linker argument. This is the name of the libc5
391 dynamic linker, which does not work with glibc.
393 For casual use of GNU libc you can just specify
394 -dynamic-linker=/lib/ld-linux.so.2
396 which is the glibc dynamic linker, on Linux systems. On other systems the
397 name is /lib/ld.so.1.
399 To change your environment to use GNU libc for compiling you need to change
400 the `specs' file of your gcc. This file is normally found at
402 /usr/lib/gcc-lib/<arch>/<version>/specs
404 In this file you have to change a few things:
406 - change `ld-linux.so.1' to `ld-linux.so.2'
408 - remove all expression `%{...:-lgmon}'; there is no libgmon in glibc
410 - fix a minor bug by changing %{pipe:-} to %|
412 Here is what the gcc-2.7.2 specs file should look like when GNU libc is
415 -----------------------------------------------------------------------
417 %{V} %{v:%{!V:-V}} %{Qy:} %{!Qn:-Qy} %{n} %{T} %{Ym,*} %{Yd,*} %{Wa,*:%*}
423 %{fPIC:-D__PIC__ -D__pic__} %{fpic:-D__PIC__ -D__pic__} %{!m386:-D__i486__} %{posix:-D_POSIX_SOURCE} %{pthread:-D_REENTRANT}
432 %{!shared:crtend.o%s} %{shared:crtendS.o%s} crtn.o%s
435 -m elf_i386 %{shared:-shared} %{!shared: %{!ibcs: %{!static: %{rdynamic:-export-dynamic} %{!dynamic-linker:-dynamic-linker /lib/ld-linux.so.2}} %{static:-static}}}
438 %{!shared: %{pthread:-lpthread} %{profile:-lc_p} %{!profile: -lc}}
444 %{!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}
446 *switches_need_spaces:
450 %{funsigned-char:-D__CHAR_UNSIGNED__}
453 -D__ELF__ -Dunix -Di386 -Dlinux -Asystem(unix) -Asystem(posix) -Acpu(i386) -Amachine(i386)
461 -----------------------------------------------------------------------
463 Things get a bit more complicated if you have GNU libc installed in some
464 other place than /usr, i.e., if you do not want to use it instead of the old
465 libc. In this case the needed startup files and libraries are not found in
466 the regular places. So the specs file must tell the compiler and linker
469 Version 2.7.2.3 does and future versions of GCC will automatically
470 provide the correct specs.
472 ?? Looking through the shared libc file I haven't found the
473 functions `stat', `lstat', `fstat', and `mknod' and while
474 linking on my Linux system I get error messages. How is
475 this supposed to work?
477 {RM} Believe it or not, stat and lstat (and fstat, and mknod) are supposed
478 to be undefined references in libc.so.6! Your problem is probably a missing
479 or incorrect /usr/lib/libc.so file; note that this is a small text file now,
480 not a symlink to libc.so.6. It should look something like this:
482 GROUP ( libc.so.6 libc_nonshared.a )
484 ?? How can I compile gcc 2.7.2.1 from the gcc source code using
487 {AJ} There's only correct support for glibc 2.0.x in gcc 2.7.2.3 or later.
488 But you should get at least gcc 2.8.1 or egcs 1.0.2 (or later versions)
491 ?? The `gencat' utility cannot process the catalog sources which
492 were used on my Linux libc5 based system. Why?
494 {UD} The `gencat' utility provided with glibc complies to the XPG standard.
495 The older Linux version did not obey the standard, so they are not
498 To ease the transition from the Linux version some of the non-standard
499 features are also present in the `gencat' program of GNU libc. This mainly
500 includes the use of symbols for the message number and the automatic
501 generation of header files which contain the needed #defines to map the
504 Here is a simple SED script to convert at least some Linux specific catalog
505 files to the XPG4 form:
507 -----------------------------------------------------------------------
508 # Change catalog source in Linux specific format to standard XPG format.
509 # Ulrich Drepper <drepper@cygnus.com>, 1996.
513 s/\$ #\([^ ]*\).*/\1/
515 s/\$ #[^ ]* *\(.*\)/\$ \1/
521 s/\(.*\)\n\(.*\)/\2 \1/
523 -----------------------------------------------------------------------
525 ?? Programs using libc have their messages translated, but other
526 behavior is not localized (e.g. collating order); why?
528 {ZW} Translated messages are automatically installed, but the locale
529 database that controls other behaviors is not. You need to run localedef to
530 install this database, after you have run `make install'. For example, to
531 set up the French Canadian locale, simply issue the command
533 localedef -i fr_CA -f ISO-8859-1 fr_CA
535 Please see localedata/README in the source tree for further details.
537 ?? I have set up /etc/nis.conf, and the Linux libc 5 with NYS
538 works great. But the glibc NIS+ doesn't seem to work.
540 {TK} The glibc NIS+ implementation uses a /var/nis/NIS_COLD_START file for
541 storing information about the NIS+ server and their public keys, because the
542 nis.conf file does not contain all the necessary information. You have to
543 copy a NIS_COLD_START file from a Solaris client (the NIS_COLD_START file is
544 byte order independent) or generate it with nisinit from the nis-tools
545 package; available at
547 http://www-vt.uni-paderborn.de/~kukuk/linux/nisplus.html
549 ?? I have killed ypbind to stop using NIS, but glibc
552 {TK} For faster NIS lookups, glibc uses the /var/yp/binding/ files from
553 ypbind. ypbind 3.3 and older versions don't always remove these files, so
554 glibc will continue to use them. Other BSD versions seem to work correctly.
555 Until ypbind 3.4 is released, you can find a patch at
557 ftp://ftp.kernel.org/pub/linux/utils/net/NIS/ypbind-3.3-glibc3.diff.
559 ?? Under Linux/Alpha, I always get "do_ypcall: clnt_call:
560 RPC: Unable to receive; errno = Connection refused" when using NIS.
562 {TK} You need a ypbind version which is 64bit clean. Some versions are not
563 64bit clean. A 64bit clean implementation is ypbind-mt. For ypbind 3.3,
564 you need the patch from ftp.kernel.org (See the previous question). I don't
565 know about other versions.
568 ?? After installing glibc name resolving doesn't work properly.
570 {AJ} You probably should read the manual section describing nsswitch.conf
571 (just type `info libc "NSS Configuration File"'). The NSS configuration
572 file is usually the culprit.
575 ?? How do I create the databases for NSS?
577 {AJ} If you have an entry "db" in /etc/nsswitch.conf you should also create
578 the database files. The glibc sources contain a Makefile which does the
579 neccessary conversion and calls to create those files. The file is
580 `db-Makefile' in the subdirectory `nss' and you can call it with `make -f
581 db-Makefile'. Please note that not all services are capable of using a
582 database. Currently passwd, group, ethers, protocol, rpc, services shadow
583 and netgroup are implemented.
586 ?? I have /usr/include/net and /usr/include/scsi as symlinks
587 into my Linux source tree. Is that wrong?
589 {PB} This was necessary for libc5, but is not correct when using glibc.
590 Including the kernel header files directly in user programs usually does not
591 work (see ?kerhdr). glibc provides its own <net/*> and <scsi/*> header
592 files to replace them, and you may have to remove any symlink that you have
593 in place before you install glibc. However, /usr/include/asm and
594 /usr/include/linux should remain as they were.
596 ?? Programs like `logname', `top', `uptime' `users', `w' and
597 `who', show incorrect information about the (number of)
598 users on my system. Why?
602 ?? After upgrading to glibc 2.1 with symbol versioning I get
603 errors about undefined symbols. What went wrong?
605 {AJ} The problem is caused either by wrong program code or tools. In the
606 versioned libc a lot of symbols are now local that were global symbols in
607 previous versions. It seems that programs linked against older versions
608 often accidentally used libc global variables -- something that should not
611 The only way to fix this is to recompile your program. Sorry, that's the
612 price you might have to pay once for quite a number of advantages with
615 ?? When I start the program XXX after upgrading the library
617 XXX: Symbol `_sys_errlist' has different size in shared
618 object, consider re-linking
619 Why? What should I do?
621 {UD} As the message says, relink the binary. The problem is that a few
622 symbols from the library can change in size and there is no way to avoid
623 this. _sys_errlist is a good example. Occasionally there are new error
624 numbers added to the kernel and this must be reflected at user level,
625 breaking programs that refer to them directly.
627 Such symbols should normally not be used at all. There are mechanisms to
628 avoid using them. In the case of _sys_errlist, there is the strerror()
629 function which should _always_ be used instead. So the correct fix is to
630 rewrite that part of the application.
632 In some situations (especially when testing a new library release) it might
633 be possible that a symbol changed size when that should not have happened.
634 So in case of doubt report such a warning message as a problem.
636 ?? What do I need for C++ development?
638 {HJ,AJ} You need either egcs 1.0.2 or gcc-2.8.1 with libstdc++ 2.8.1 (or
639 more recent versions). libg++ 2.7.2 (and the Linux Versions 2.7.2.x) doesn't
640 work very well with the GNU C library due to vtable thunks. If you're
641 upgrading from glibc 2.0.x to 2.1 you have to recompile libstdc++ since the
642 library compiled for 2.0 is not compatible due to the new Large File Support
643 (LFS) in version 2.1.
645 ?? Even statically linked programs need some shared libraries
646 which is not acceptable for me. What can I do?
648 {AJ} NSS (for details just type `info libc "Name Service Switch"') won't
649 work properly without shared libraries. NSS allows using different services
650 (e.g. NIS, files, db, hesiod) by just changing one configuration file
651 (/etc/nsswitch.conf) without relinking any programs. The only disadvantage
652 is that now static libraries need to access shared libraries. This is
653 handled transparently by the GNU C library.
655 A solution is to configure glibc with --enable-static-nss. In this case you
656 can create a static binary that will use only the services dns and files
657 (change /etc/nsswitch.conf for this). You need to link explicitly against
658 all these services. For example:
660 gcc -static test-netdb.c -o test-netdb.c \
661 -lc -lnss_files -lnss_dns -lresolv
663 The problem with this approach is that you've got to link every static
664 program that uses NSS routines with all those libraries.
666 {UD} In fact, one cannot say anymore that a libc compiled with this
667 option is using NSS. There is no switch anymore. Therefore it is
668 *highly* recommended *not* to use --enable-static-nss since this makes
669 the behaviour of the programs on the system inconsistent.
671 ?? I just upgraded my Linux system to glibc and now I get
672 errors whenever I try to link any program.
674 {ZW} This happens when you have installed glibc as the primary C library but
675 have stray symbolic links pointing at your old C library. If the first
676 `libc.so' the linker finds is libc 5, it will use that. Your program
677 expects to be linked with glibc, so the link fails.
679 The most common case is that glibc put its `libc.so' in /usr/lib, but there
680 was a `libc.so' from libc 5 in /lib, which gets searched first. To fix the
681 problem, just delete /lib/libc.so. You may also need to delete other
682 symbolic links in /lib, such as /lib/libm.so if it points to libm.so.5.
684 {AJ} The perl script test-installation.pl which is run as last step during
685 an installation of glibc that is configured with --prefix=/usr should help
686 detect these situations. If the script reports problems, something is
689 ? Source and binary incompatibilities, and what to do about them
691 ?? I expect GNU libc to be 100% source code compatible with
692 the old Linux based GNU libc. Why isn't it like this?
694 {DMT,UD} Not every extension in Linux libc's history was well thought-out.
695 In fact it had a lot of problems with standards compliance and with
696 cleanliness. With the introduction of a new version number these errors can
697 now be corrected. Here is a list of the known source code
700 * _GNU_SOURCE: glibc does not make the GNU extensions available
701 automatically. If a program depends on GNU extensions or some
702 other non-standard functionality, it is necessary to compile it
703 with the C compiler option -D_GNU_SOURCE, or better, to put
704 `#define _GNU_SOURCE' at the beginning of your source files, before
705 any C library header files are included. This difference normally
706 manifests itself in the form of missing prototypes and/or data type
707 definitions. Thus, if you get such errors, the first thing you
708 should do is try defining _GNU_SOURCE and see if that makes the
711 For more information consult the file `NOTES' in the GNU C library
714 * reboot(): GNU libc sanitizes the interface of reboot() to be more
715 compatible with the interface used on other OSes. reboot() as
716 implemented in glibc takes just one argument. This argument
717 corresponds to the third argument of the Linux reboot system call.
718 That is, a call of the form reboot(a, b, c) needs to be changed into
719 reboot(c). Beside this the header <sys/reboot.h> defines the needed
720 constants for the argument. These RB_* constants should be used
721 instead of the cryptic magic numbers.
723 * swapon(): the interface of this function didn't change, but the
724 prototype is in a separate header file <sys/swap.h>. This header
725 file also provides the SWAP_* constants defined by <linux/swap.h>;
726 you should use them for the second argument to swapon().
728 * errno: If a program uses the variable "errno", then it _must_
729 include <errno.h>. The old libc often (erroneously) declared this
730 variable implicitly as a side-effect of including other libc header
731 files. glibc is careful to avoid such namespace pollution, which,
732 in turn, means that you really need to include the header files that
733 you depend on. This difference normally manifests itself in the
734 form of the compiler complaining about references to an undeclared
737 * Linux-specific syscalls: All Linux system calls now have appropriate
738 library wrappers and corresponding declarations in various header files.
739 This is because the syscall() macro that was traditionally used to
740 work around missing syscall wrappers are inherently non-portable and
741 error-prone. The following table lists all the new syscall stubs,
742 the header-file declaring their interface and the system call name.
744 syscall name: wrapper name: declaring header file:
745 ------------- ------------- ----------------------
746 bdflush bdflush <sys/kdaemon.h>
747 syslog ksyslog_ctl <sys/klog.h>
749 * lpd: Older versions of lpd depend on a routine called _validuser().
750 The library does not provide this function, but instead provides
751 __ivaliduser() which has a slightly different interface. Simply
752 upgrading to a newer lpd should fix this problem (e.g., the 4.4BSD
753 lpd is known to be working).
755 * resolver functions/BIND: like on many other systems the functions of
756 the resolver library are not included in libc itself. There is a
757 separate library libresolv. If you get undefined symbol errors for
758 symbols starting with `res_*' simply add -lresolv to your linker
761 * the `signal' function's behavior corresponds to the BSD semantic and
762 not the SysV semantic as it was in libc-5. The interface on all GNU
763 systems shall be the same and BSD is the semantic of choice. To use
764 the SysV behavior simply use `sysv_signal', or define _XOPEN_SOURCE.
765 See ?signal for details.
767 ??getlog Why does getlogin() always return NULL on my Linux box?
769 {UD} The GNU C library has a format for the UTMP and WTMP file which differs
770 from what your system currently has. It was extended to fulfill the needs
771 of the next years when IPv6 is introduced. The record size is different and
772 some fields have different positions. The files written by functions from
773 the one library cannot be read by functions from the other library. Sorry,
774 but this is what a major release is for. It's better to have a cut now than
775 having no means to support the new techniques later.
777 {MK} There is however a (partial) solution for this problem. Please take a
778 look at the file `login/README.utmpd'.
780 ?? Where are the DST_* constants found in <sys/time.h> on many
783 {UD} These constants come from the old BSD days and are not used anymore
784 (libc5 does not actually implement the handling although the constants are
787 Instead GNU libc contains zone database support and compatibility code for
788 POSIX TZ environment variable handling.
790 ?? The prototypes for `connect', `accept', `getsockopt',
791 `setsockopt', `getsockname', `getpeername', `send',
792 `sendto', and `recvfrom' are different in GNU libc from
793 any other system I saw. This is a bug, isn't it?
795 {UD} No, this is no bug. This version of GNU libc already follows the new
796 Single Unix specifications (and I think the POSIX.1g draft which adopted the
797 solution). The type for a parameter describing a size is now `socklen_t', a
800 ??kerhdr On Linux I've got problems with the declarations in Linux
803 {UD,AJ} On Linux, the use of kernel headers is reduced to the minimum. This
804 gives Linus the ability to change the headers more freely. Also, user
805 programs are not insulated from changes in the size of kernel data
808 For example, the sigset_t type is 32 or 64 bits wide in the kernel. In
809 glibc it is 1024 bits wide. This guarantees that when the kernel gets a
810 bigger sigset_t (for POSIX.1e realtime support, say) user programs will not
811 have to be recompiled. Consult the header files for more information about
814 Therefore you shouldn't include Linux kernel header files directly if glibc
815 has defined a replacement. Otherwise you might get undefined results because
818 ?? I don't include any kernel headers myself but the compiler
819 still complains about redeclarations of types in the kernel
822 {UD} The kernel headers before Linux 2.1.61 and 2.0.32 don't work correctly
823 with glibc. Compiling C programs is possible in most cases but C++ programs
824 have (due to the change of the name lookups for `struct's) problems. One
825 prominent example is `struct fd_set'.
827 There might be some problems left but 2.1.61/2.0.32 fix most of the known
828 ones. See the BUGS file for other known problems.
830 ??signal Why don't signals interrupt system calls anymore?
832 {ZW} By default GNU libc uses the BSD semantics for signal(), unlike Linux
833 libc 5 which used System V semantics. This is partially for compatibility
834 with other systems and partially because the BSD semantics tend to make
835 programming with signals easier.
837 There are three differences:
839 * BSD-style signals that occur in the middle of a system call do not
840 affect the system call; System V signals cause the system call to
841 fail and set errno to EINTR.
843 * BSD signal handlers remain installed once triggered. System V signal
844 handlers work only once, so one must reinstall them each time.
846 * A BSD signal is blocked during the execution of its handler. In other
847 words, a handler for SIGCHLD (for example) does not need to worry about
848 being interrupted by another SIGCHLD. It may, however, be interrupted
851 There is general consensus that for `casual' programming with signals, the
852 BSD semantics are preferable. You don't need to worry about system calls
853 returning EINTR, and you don't need to worry about the race conditions
854 associated with one-shot signal handlers.
856 If you are porting an old program that relies on the old semantics, you can
857 quickly fix the problem by changing signal() to sysv_signal() throughout.
858 Alternatively, define _XOPEN_SOURCE before including <signal.h>.
860 For new programs, the sigaction() function allows you to specify precisely
861 how you want your signals to behave. All three differences listed above are
862 individually switchable on a per-signal basis with this function.
864 If all you want is for one specific signal to cause system calls to fail and
865 return EINTR (for example, to implement a timeout) you can do this with
869 ??string I've got errors compiling code that uses certain string
872 {AJ} glibc 2.1 has special string functions that are faster than the normal
873 library functions. Some of the functions are additionally implemented as
874 inline functions and others as macros.
876 The optimized string functions are only used when compiling with
877 optimizations (-O1 or higher). The behavior can be changed with two feature
880 * __NO_STRING_INLINES: Don't do any string optimizations.
881 * __USE_STRING_INLINES: Use assembly language inline functions (might
882 increase code size dramatically).
884 Since some of these string functions are now additionally defined as macros,
885 code like "char *strncpy();" doesn't work anymore (and is unnecessary, since
886 <string.h> has the necessary declarations). Either change your code or
887 define __NO_STRING_INLINES.
889 {UD} Another problem in this area is that gcc still has problems on machines
890 with very few registers (e.g., ix86). The inline assembler code can require
891 almost all the registers and the register allocator cannot always handle
894 One can disable the string optimizations selectively. Instead of writing
896 cp = strcpy (foo, "lkj");
900 cp = (strcpy) (foo, "lkj");
902 This disables the optimization for that specific call.
904 ?? I get compiler messages "Initializer element not constant" with
905 stdin/stdout/stderr. Why?
907 {RM,AJ} Constructs like:
908 static FILE *InPtr = stdin;
910 lead to this message. This is correct behaviour with glibc since stdin is
911 not a constant expression. Please note that a strict reading of ISO C does
912 not allow above constructs.
914 One of the advantages of this is that you can assign to stdin, stdout, and
915 stderr just like any other global variable (e.g. `stdout = my_stream;'),
916 which can be very useful with custom streams that you can write with libio
917 (but beware this is not necessarily portable). The reason to implement it
918 this way were versioning problems with the size of the FILE structure.
920 To fix those programs you've got to initialize the variable at run time.
921 This can be done, e.g. in main, like:
929 or by constructors (beware this is gcc specific):
932 static void inPtr_construct (void) __attribute__((constructor));
933 static void inPtr_construct (void) { InPtr = stdin; }
936 ?? I can't compile with gcc -traditional (or
937 -traditional-cpp). Why?
939 {AJ} glibc2 does break -traditional and -traditonal-cpp - and will continue
940 to do so. For example constructs of the form:
946 are useful for debugging purposes (you can use foo with your debugger that's
947 why we need the enum) and for compatibility (other systems use defines and
950 ?? I get some errors with `gcc -ansi'. Isn't glibc ANSI compatible?
952 {AJ} The GNU C library is compatible with the ANSI/ISO C standard. If
953 you're using `gcc -ansi', the glibc includes which are specified in the
954 standard follow the standard. The ANSI/ISO C standard defines what has to be
955 in the include files - and also states that nothing else should be in the
956 include files (btw. you can still enable additional standards with feature
959 The GNU C library is conforming to ANSI/ISO C - if and only if you're only
960 using the headers and library functions defined in the standard.
962 ?? I can't access some functions anymore. nm shows that they do
963 exist but linking fails nevertheless.
965 {AJ} With the introduction of versioning in glibc 2.1 it is possible to
966 export only those identifiers (functions, variables) that are really needed
967 by application programs and by other parts of glibc. This way a lot of
968 internal interfaces are now hidden. nm will still show those identifiers
969 but marking them as internal. ISO C states that identifiers beginning with
970 an underscore are internal to the libc. An application program normally
971 shouldn't use those internal interfaces (there are exceptions,
972 e.g. __ivaliduser). If a program uses these interfaces, it's broken. These
973 internal interfaces might change between glibc releases or dropped
979 ?? After I changed configure.in I get `Autoconf version X.Y.
980 or higher is required for this script'. What can I do?
982 {UD} You have to get the specified autoconf version (or a later one)
983 from your favorite mirror of ftp.gnu.org.
985 ?? When I try to compile code which uses IPv6 headers and
986 definitions on my Linux 2.x.y system I am in trouble.
987 Nothing seems to work.
989 {UD} The problem is that IPv6 development still has not reached a point
990 where the headers are stable. There are still lots of incompatible changes
991 made and the libc headers have to follow.
993 Also, make sure you have a suitably recent kernel. As of the 970401
994 snapshot, according to Philip Blundell <Philip.Blundell@pobox.com>, the
995 required kernel version is at least 2.1.30.
997 ?? When I set the timezone by setting the TZ environment variable
998 to EST5EDT things go wrong since glibc computes the wrong time
999 from this information.
1001 {UD} The problem is that people still use the braindamaged POSIX method to
1002 select the timezone using the TZ environment variable with a format EST5EDT
1003 or whatever. People, read the POSIX standard, the implemented behaviour is
1004 correct! What you see is in fact the result of the decisions made while
1005 POSIX.1 was created. We've only implemented the handling of TZ this way to
1006 be POSIX compliant. It is not really meant to be used.
1008 The alternative approach to handle timezones which is implemented is the
1009 correct one to use: use the timezone database. This avoids all the problems
1010 the POSIX method has plus it is much easier to use. Simply run the tzselect
1011 shell script, answer the question and use the name printed in the end by
1012 making a symlink to /usr/share/zoneinfo/NAME (NAME is the returned value
1013 from tzselect) from the file /etc/localtime. That's all. You never again
1016 So, please avoid sending bug reports about time related problems if you use
1017 the POSIX method and you have not verified something is really broken by
1018 reading the POSIX standards.
1020 ?? What other sources of documentation about glibc are available?
1022 {AJ} The FSF has a page about the GNU C library at
1023 <http://www.gnu.org/software/libc/>. The problem data base of open and
1024 solved bugs in GNU libc is available at
1025 <http://www-gnats.gnu.org:8080/cgi-bin/wwwgnats.pl>. Eric Green has written
1026 a HowTo for converting from Linux libc5 to glibc2. The HowTo is accessable
1027 via the FSF page and at <http://www.imaxx.net/~thrytis/glibc>. Frodo
1028 Looijaard describes a different way installing glibc2 as secondary libc at
1029 <http://huizen.dds.nl/~frodol/glibc>.
1031 Please note that this is not a complete list.
1034 Answers were given by:
1035 {UD} Ulrich Drepper, <drepper@cygnus.com>
1036 {DMT} David Mosberger-Tang, <davidm@AZStarNet.com>
1037 {RM} Roland McGrath, <roland@gnu.org>
1038 {AJ} Andreas Jaeger, <aj@arthur.rhein-neckar.de>
1039 {EY} Eric Youngdale, <eric@andante.jic.com>
1040 {PB} Phil Blundell, <Philip.Blundell@pobox.com>
1041 {MK} Mark Kettenis, <kettenis@phys.uva.nl>
1042 {ZW} Zack Weinberg, <zack@rabi.phys.columbia.edu>
1043 {TK} Thorsten Kukuk, <kukuk@vt.uni-paderborn.de>
1044 {GK} Geoffrey Keating, <geoffk@ozemail.com.au>
1045 {HJ} H.J. Lu, <hjl@gnu.org>
1049 outline-regexp:"\\?"