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-*-linux Linux-2.x on ARM
37 arm-*-linuxaout Linux-2.x on ARM using a.out binaries
39 Ports to other Linux platforms are in development, and may in fact work
40 already, but no one has sent us success reports for them. Currently no
41 ports to other operating systems are underway, although a few people have
44 If you have a system not listed above (or in the `README' file) and you are
45 really interested in porting it, contact
49 ??binsize What compiler do I need to build GNU libc?
51 {UD} You must use GNU CC to compile GNU libc. A lot of extensions of GNU CC
52 are used to increase portability and speed.
54 GNU CC is found, like all other GNU packages, on
56 ftp://ftp.gnu.org/pub/gnu
58 and the many mirror sites. ftp.gnu.org is always overloaded, so try to find
61 You should always try to use the latest official release. Older versions
62 may not have all the features GNU libc requires. The current releases of
63 egcs (1.0.3 and 1.1.1) should work with the GNU C library (for powerpc see
64 ?powerpc; for ARM see ?arm).
66 While the GNU CC should be able to compile glibc it is nevertheless adviced
67 to use EGCS. Comparing the sizes of glibc on Intel compiled with a recent
68 EGCS and gcc 2.8.1 shows this:
70 text data bss dec hex filename
71 egcs-2.93.10 862897 15944 12824 891665 d9b11 libc.so
72 gcc-2.8.1 959965 16468 12152 988585 f15a9 libc.so
74 Make up your own decision.
76 GNU CC versions 2.95 and above are derived from egcs, and they may do even
79 Please note that gcc 2.95 and 2.95.x cannot compile glibc on Alpha due to
80 problems in the complex float support.
82 ?? When I try to compile glibc I get only error messages.
85 {UD} You definitely need GNU make to build GNU libc. No other make
86 program has the needed functionality.
88 We recommend version GNU make version 3.75 or 3.77. Versions before 3.75
89 have bugs and/or are missing features. Version 3.76 has bugs which
90 appear when building big projects like GNU libc. 3.76.1 appears to work but
91 some people have reported problems. If you build GNU make 3.77 from source,
92 please read ?make first.
94 ?? Do I need a special linker or assembler?
96 {ZW} If you want a shared library, you need a linker and assembler that
97 understand all the features of ELF, including weak and versioned symbols.
98 The static library can be compiled with less featureful tools, but lacks key
101 For Linux or Hurd, you want binutils 2.8.1.0.23, 2.9.1, or 2.9.1.0.15 or
102 higher. These are the only versions we've tested and found reliable. Other
103 versions after 2.8.1.0.23 may work but we don't recommend them, especially
104 not when C++ is involved. Earlier versions do not work at all.
106 Other operating systems may come with system tools that have all the
107 necessary features, but this is moot because glibc hasn't been ported to
110 ??powerpc Which compiler should I use for powerpc?
112 {GK} You want to use at least gcc 2.95 (together with the right versions
113 of all the other tools, of course). See also question ?excpt.
115 ??arm Which tools should I use for ARM?
117 {PB} You should use egcs 1.1 or a later version. For ELF systems some
118 changes are needed to the compiler; a patch against egcs-1.1.x can be found
121 <ftp://ftp.netwinder.org/users/p/philb/egcs-1.1.1pre2-diff-981126>
123 Binutils 2.9.1.0.16 or later is also required.
125 ?? Do I need some more things to compile the GNU C Library?
127 {UD} Yes, there are some more :-).
129 * GNU gettext. This package contains the tools needed to construct
130 `message catalog' files containing translated versions of system
131 messages. See ftp://ftp.gnu.org/pub/gnu or better any mirror
132 site. (We distribute compiled message catalogs, but they may not be
133 updated in patches.) Please note that the required minimal version
134 (0.10.35) of gettext is alpha software and available from
135 ftp://alpha.gnu.org/gnu .
137 * Some files are built with special tools. E.g., files ending in .gperf
138 need a `gperf' program. The GNU version (now available in a separate
139 package, formerly only as part of libg++) is known to work while some
140 vendor versions do not.
142 You should not need these tools unless you change the source files.
144 * Perl 5 is needed if you wish to test an installation of GNU libc
145 as the primary C library.
147 * When compiling for Linux, the header files of the Linux kernel must
148 be available to the compiler as <linux/*.h> and <asm/*.h>.
150 * lots of disk space (~400MB for i?86-linux; more for RISC platforms).
152 * plenty of time. Compiling just the shared and static libraries for
153 i?86-linux takes approximately 1h on an AMD-K6@225MHz w/ 96MB of RAM,
154 45mins on a Celeron@400MHz w/ 128MB, and 55mins on a Alpha@533MHz w/ 256MB.
155 Multiply this by 1.5 or 2.0 if you build profiling and/or the highly
156 optimized version as well. For Hurd systems times are much higher.
158 You should avoid compiling in a NFS mounted filesystem. This is
161 James Troup <J.J.Troup@comp.brad.ac.uk> reports a compile time of
162 45h34m for a full build (shared, static, and profiled) on Atari
163 Falcon (Motorola 68030 @ 16 Mhz, 14 Mb memory) and Jan Barte
164 <yann@plato.uni-paderborn.de> reports 22h48m on Atari TT030
165 (Motorola 68030 @ 32 Mhz, 34 Mb memory)
167 A full build of the PowerPC library took 1h on a PowerPC 750@400Mhz w/
168 64MB of RAM, and about 9h on a 601@60Mhz w/ 72Mb.
170 If you have some more measurements let me know.
172 ?? What version of the Linux kernel headers should be used?
174 {AJ,UD} The headers from the most recent Linux kernel should be used. The
175 headers used while compiling the GNU C library and the kernel binary used
176 when using the library do not need to match. The GNU C library runs without
177 problems on kernels that are older than the kernel headers used. The other
178 way round (compiling the GNU C library with old kernel headers and running
179 on a recent kernel) does not necessarily work. For example you can't use
180 new kernel features if you used old kernel headers to compile the GNU C
183 {ZW} Even if you are using a 2.0 kernel on your machine, we recommend you
184 compile GNU libc with 2.2 kernel headers. That way you won't have to
185 recompile libc if you ever upgrade to kernel 2.2. To tell libc which
186 headers to use, give configure the --with-headers switch
187 (e.g. --with-headers=/usr/src/linux-2.2.0/include).
189 Note that you must configure the 2.2 kernel if you do this, otherwise libc
190 will be unable to find <linux/version.h>. Just change the current directory
191 to the root of the 2.2 tree and do `make include/linux/version.h'.
193 ?? The compiler hangs while building iconvdata modules. What's
196 {ZW} This is a problem with old versions of GCC. Initialization of large
197 static arrays is very slow. The compiler will eventually finish; give it
200 The problem is fixed in egcs 1.1.
202 ?? When I run `nm -u libc.so' on the produced library I still
203 find unresolved symbols. Can this be ok?
205 {UD} Yes, this is ok. There can be several kinds of unresolved symbols:
207 * magic symbols automatically generated by the linker. These have names
208 like __start_* and __stop_*
210 * symbols starting with _dl_* come from the dynamic linker
212 * weak symbols, which need not be resolved at all (fabs for example)
214 Generally, you should make sure you find a real program which produces
215 errors while linking before deciding there is a problem.
217 ??addon What are these `add-ons'?
219 {UD} To avoid complications with export rules or external source code some
220 optional parts of the libc are distributed as separate packages (e.g., the
221 crypt package, see ?crypt).
223 To use these packages as part of GNU libc, just unpack the tarfiles in the
224 libc source directory and tell the configuration script about them using the
225 --enable-add-ons option. If you give just --enable-add-ons configure tries
226 to find all the add-on packages in your source tree. This may not work. If
227 it doesn't, or if you want to select only a subset of the add-ons, give a
228 comma-separated list of the add-ons to enable:
230 configure --enable-add-ons=crypt,linuxthreads
234 Add-ons can add features (including entirely new shared libraries), override
235 files, provide support for additional architectures, and just about anything
236 else. The existing makefiles do most of the work; only some few stub rules
237 must be written to get everything running.
239 Most add-ons are tightly coupled to a specific GNU libc version. Please
240 check that the add-ons work with the GNU libc. For example the crypt and
241 linuxthreads add-ons have the same numbering scheme as the libc and will in
242 general only work with the corresponding libc.
244 ?? My XXX kernel emulates a floating-point coprocessor for me.
245 Should I enable --with-fp?
247 {ZW} An emulated FPU is just as good as a real one, as far as the C library
248 is concerned. You only need to say --without-fp if your machine has no way
249 to execute floating-point instructions.
251 People who are interested in squeezing the last drop of performance
252 out of their machine may wish to avoid the trap overhead, but this is
253 far more trouble than it's worth: you then have to compile
254 *everything* this way, including the compiler's internal libraries
255 (libgcc.a for GNU C), because the calling conventions change.
257 ?? When compiling GNU libc I get lots of errors saying functions
258 in glibc are duplicated in libgcc.
260 {EY} This is *exactly* the same problem that I was having. The problem was
261 due to the fact that configure didn't correctly detect that the linker flag
262 --no-whole-archive was supported in my linker. In my case it was because I
263 had run ./configure with bogus CFLAGS, and the test failed.
265 One thing that is particularly annoying about this problem is that once this
266 is misdetected, running configure again won't fix it unless you first delete
269 {UD} Starting with glibc-2.0.3 there should be a better test to avoid some
270 problems of this kind. The setting of CFLAGS is checked at the very
271 beginning and if it is not usable `configure' will bark.
273 ?? Why do I get messages about missing thread functions when I use
274 librt? I don't even use threads.
276 {UD} In this case you probably mixed up your installation. librt uses
277 threads internally and has implicit references to the thread library.
278 Normally these references are satisfied automatically but if the thread
279 library is not in the expected place you must tell the linker where it is.
280 When using GNU ld it works like this:
282 gcc -o foo foo.c -Wl,-rpath-link=/some/other/dir -lrt
284 The `/some/other/dir' should contain the thread library. `ld' will use the
285 given path to find the implicitly referenced library while not disturbing
288 ?? What's the problem with configure --enable-omitfp?
290 {AJ} When --enable-omitfp is set the libraries are built without frame
291 pointers. Some compilers produce buggy code for this model and therefore we
292 don't advise using it at the moment.
294 If you use --enable-omitfp, you're on your own. If you encounter problems
295 with a library that was build this way, we advise you to rebuild the library
296 without --enable-omitfp. If the problem vanishes consider tracking the
297 problem down and report it as compiler failure.
299 Since a library built with --enable-omitfp is undebuggable on most systems,
300 debuggable libraries are also built - you can use them by appending "_g" to
303 The compilation of these extra libraries and the compiler optimizations slow
304 down the build process and need more disk space.
306 ?? I get failures during `make check'. What should I do?
308 {AJ} The testsuite should compile and run cleanly on your system; every
309 failure should be looked into. Depending on the failures, you probably
310 should not install the library at all.
312 You should consider using the `glibcbug' script to report the failure,
313 providing as much detail as possible. If you run a test directly, please
314 remember to set up the environment correctly. You want to test the compiled
315 library - and not your installed one. The best way is to copy the exact
316 command line which failed and run the test from the subdirectory for this
319 There are some failures which are not directly related to the GNU libc:
320 - Some compilers produce buggy code. No compiler gets single precision
321 complex numbers correct on Alpha. Otherwise, the egcs 1.1 release should be
322 ok; gcc 2.8.1 might cause some failures; gcc 2.7.2.x is so buggy that
323 explicit checks have been used so that you can't build with it.
324 - The kernel might have bugs. For example on Linux/Alpha 2.0.34 the
325 floating point handling has quite a number of bugs and therefore most of
326 the test cases in the math subdirectory will fail. Linux 2.2 has
327 fixes for the floating point support on Alpha. The Linux/SPARC kernel has
328 also some bugs in the FPU emulation code (as of Linux 2.2.0).
329 - Other tools might have problems. For example bash 2.03 gives a
330 segmentation fault running the tst-rpmatch.sh test script.
332 ?? What is symbol versioning good for? Do I need it?
334 {AJ} Symbol versioning solves problems that are related to interface
335 changes. One version of an interface might have been introduced in a
336 previous version of the GNU C library but the interface or the semantics of
337 the function has been changed in the meantime. For binary compatibility
338 with the old library, a newer library needs to still have the old interface
339 for old programs. On the other hand, new programs should use the new
340 interface. Symbol versioning is the solution for this problem. The GNU
341 libc version 2.1 uses symbol versioning by default if the installed binutils
344 We don't advise building without symbol versioning, since you lose binary
345 compatibility - forever! The binary compatibility you lose is not only
346 against the previous version of the GNU libc (version 2.0) but also against
350 ?? How can I compile on my fast ix86 machine a working libc for my slow
351 i386? After installing libc, programs abort with "Illegal
354 {AJ} glibc and gcc might generate some instructions on your machine that
355 aren't available on i386. You've got to tell glibc that you're configuring
356 for i386 with adding i386 as your machine, for example:
358 ../configure --prefix=/usr i386-pc-linux-gnu
360 And you need to tell gcc to only generate i386 code, just add `-mcpu=i386'
361 (just -m386 doesn't work) to your CFLAGS.
363 {UD} This applies not only to the i386. Compiling on a i686 for any older
364 model will also fail if the above methods are not used.
367 ? Installation and configuration issues
369 ?? Can I replace the libc on my Linux system with GNU libc?
371 {UD} You cannot replace any existing libc for Linux with GNU libc. It is
372 binary incompatible and therefore has a different major version. You can,
373 however, install it alongside your existing libc.
375 For Linux there are three major libc versions:
377 libc-5 original ELF libc
380 You can have any combination of these three installed. For more information
381 consult documentation for shared library handling. The Makefiles of GNU
382 libc will automatically generate the needed symbolic links which the linker
385 ?? How do I configure GNU libc so that the essential libraries
386 like libc.so go into /lib and the other into /usr/lib?
388 {UD,AJ} Like all other GNU packages GNU libc is designed to use a base
389 directory and install all files relative to this. The default is
390 /usr/local, because this is safe (it will not damage the system if installed
391 there). If you wish to install GNU libc as the primary C library on your
392 system, set the base directory to /usr (i.e. run configure --prefix=/usr
393 <other_options>). Note that this can damage your system; see ?safety for
396 Some systems like Linux have a filesystem standard which makes a difference
397 between essential libraries and others. Essential libraries are placed in
398 /lib because this directory is required to be located on the same disk
399 partition as /. The /usr subtree might be found on another
400 partition/disk. If you configure for Linux with --prefix=/usr, then this
401 will be done automatically.
403 To install the essential libraries which come with GNU libc in /lib on
404 systems other than Linux one must explicitly request it. Autoconf has no
405 option for this so you have to use a `configparms' file (see the `INSTALL'
406 file for details). It should contain:
411 The first line specifies the directory for the essential libraries, the
412 second line the directory for system configuration files.
414 ??safety How should I avoid damaging my system when I install GNU libc?
416 {ZW} If you wish to be cautious, do not configure with --prefix=/usr. If
417 you don't specify a prefix, glibc will be installed in /usr/local, where it
418 will probably not break anything. (If you wish to be certain, set the
419 prefix to something like /usr/local/glibc2 which is not used for anything.)
421 The dangers when installing glibc in /usr are twofold:
423 * glibc will overwrite the headers in /usr/include. Other C libraries
424 install a different but overlapping set of headers there, so the effect
425 will probably be that you can't compile anything. You need to rename
426 /usr/include out of the way before running `make install'. (Do not throw
427 it away; you will then lose the ability to compile programs against your
430 * None of your old libraries, static or shared, can be used with a
431 different C library major version. For shared libraries this is not a
432 problem, because the filenames are different and the dynamic linker
433 will enforce the restriction. But static libraries have no version
434 information. You have to evacuate all the static libraries in
435 /usr/lib to a safe location.
437 The situation is rather similar to the move from a.out to ELF which
438 long-time Linux users will remember.
440 ?? Do I need to use GNU CC to compile programs that will use the
443 {ZW} In theory, no; the linker does not care, and the headers are supposed
444 to check for GNU CC before using its extensions to the C language.
446 However, there are currently no ports of glibc to systems where another
447 compiler is the default, so no one has tested the headers extensively
448 against another compiler. You may therefore encounter difficulties. If you
449 do, please report them as bugs.
451 Also, in several places GNU extensions provide large benefits in code
452 quality. For example, the library has hand-optimized, inline assembly
453 versions of some string functions. These can only be used with GCC. See
456 ??crypt When linking with the new libc I get unresolved symbols
457 `crypt' and `setkey'. Why aren't these functions in the
460 {UD} The US places restrictions on exporting cryptographic programs and
461 source code. Until this law gets abolished we cannot ship the cryptographic
462 functions together with glibc.
464 The functions are available, as an add-on (see ?addon). People in the US
465 may get it from the same place they got GNU libc from. People outside the
466 US should get the code from ftp.gwdg.de [134.76.11.100] in the directory
467 pub/linux/glibc, or another archive site outside the USA. The README explains
468 how to install the sources.
470 If you already have the crypt code on your system the reason for the failure
471 is probably that you did not link with -lcrypt. The crypto functions are in
472 a separate library to make it possible to export GNU libc binaries from the
475 ?? When I use GNU libc on my Linux system by linking against
476 the libc.so which comes with glibc all I get is a core dump.
478 {UD} On Linux, gcc sets the dynamic linker to /lib/ld-linux.so.1 unless the
479 user specifies a --dynamic-linker argument. This is the name of the libc5
480 dynamic linker, which does not work with glibc.
482 For casual use of GNU libc you can just specify to the linker
483 --dynamic-linker=/lib/ld-linux.so.2
485 which is the glibc dynamic linker, on Linux systems. On other systems the
486 name is /lib/ld.so.1. When linking via gcc, you've got to add
487 -Wl,--dynamic-linker=/lib/ld-linux.so.2
489 to the gcc command line.
491 To change your environment to use GNU libc for compiling you need to change
492 the `specs' file of your gcc. This file is normally found at
494 /usr/lib/gcc-lib/<arch>/<version>/specs
496 In this file you have to change a few things:
498 - change `ld-linux.so.1' to `ld-linux.so.2'
500 - remove all expression `%{...:-lgmon}'; there is no libgmon in glibc
502 - fix a minor bug by changing %{pipe:-} to %|
504 Here is what the gcc-2.7.2 specs file should look like when GNU libc is
507 -----------------------------------------------------------------------
509 %{V} %{v:%{!V:-V}} %{Qy:} %{!Qn:-Qy} %{n} %{T} %{Ym,*} %{Yd,*} %{Wa,*:%*}
515 %{fPIC:-D__PIC__ -D__pic__} %{fpic:-D__PIC__ -D__pic__} %{!m386:-D__i486__} %{posix:-D_POSIX_SOURCE} %{pthread:-D_REENTRANT}
524 %{!shared:crtend.o%s} %{shared:crtendS.o%s} crtn.o%s
527 -m elf_i386 %{shared:-shared} %{!shared: %{!ibcs: %{!static: %{rdynamic:-export-dynamic} %{!dynamic-linker:-dynamic-linker /lib/ld-linux.so.2}} %{static:-static}}}
530 %{!shared: %{pthread:-lpthread} %{profile:-lc_p} %{!profile: -lc}}
536 %{!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}
538 *switches_need_spaces:
542 %{funsigned-char:-D__CHAR_UNSIGNED__}
545 -D__ELF__ -Dunix -Di386 -Dlinux -Asystem(unix) -Asystem(posix) -Acpu(i386) -Amachine(i386)
553 -----------------------------------------------------------------------
555 Things get a bit more complicated if you have GNU libc installed in some
556 other place than /usr, i.e., if you do not want to use it instead of the old
557 libc. In this case the needed startup files and libraries are not found in
558 the regular places. So the specs file must tell the compiler and linker
561 Version 2.7.2.3 does and future versions of GCC will automatically
562 provide the correct specs.
564 ?? Looking through the shared libc file I haven't found the
565 functions `stat', `lstat', `fstat', and `mknod' and while
566 linking on my Linux system I get error messages. How is
567 this supposed to work?
569 {RM} Believe it or not, stat and lstat (and fstat, and mknod) are supposed
570 to be undefined references in libc.so.6! Your problem is probably a missing
571 or incorrect /usr/lib/libc.so file; note that this is a small text file now,
572 not a symlink to libc.so.6. It should look something like this:
574 GROUP ( libc.so.6 libc_nonshared.a )
576 ??excpt When I run an executable on one system which I compiled on
577 another, I get dynamic linker errors. Both systems have the same
578 version of glibc installed. What's wrong?
580 {ZW} Glibc on one of these systems was compiled with gcc 2.7 or 2.8, the
581 other with egcs (any version). Egcs has functions in its internal
582 `libgcc.a' to support exception handling with C++. They are linked into
583 any program or dynamic library compiled with egcs, whether it needs them or
584 not. Dynamic libraries then turn around and export those functions again
585 unless special steps are taken to prevent them.
587 When you link your program, it resolves its references to the exception
588 functions to the ones exported accidentally by libc.so. That works fine as
589 long as libc has those functions. On the other system, libc doesn't have
590 those functions because it was compiled by gcc 2.8, and you get undefined
591 symbol errors. The symbols in question are named things like
592 `__register_frame_info'.
594 For glibc 2.0, the workaround is to not compile libc with egcs. We've also
595 incorporated a patch which should prevent the EH functions sneaking into
596 libc. It doesn't matter what compiler you use to compile your program.
598 For glibc 2.1, we've chosen to do it the other way around: libc.so
599 explicitly provides the EH functions. This is to prevent other shared
600 libraries from doing it.
602 {UD} Starting with glibc 2.1.1 you can compile glibc with gcc 2.8.1 or
603 newer since we have explicitly add references to the functions causing the
604 problem. But you nevertheless should use EGCS for other reasons
607 {GK} On some Linux distributions for PowerPC, you can see this when you have
608 built gcc or egcs from the Web sources (gcc versions 2.95 or earlier), then
609 re-built glibc. This happens because in these versions of gcc, exception
610 handling is implemented using an older method; the people making the
611 distributions are a little ahead of their time.
613 A quick solution to this is to find the libgcc.a file that came with the
614 distribution (it would have been installed under /usr/lib/gcc-lib), do
615 `ar x libgcc.a frame.o' to get the frame.o file out, and add a line saying
616 `LDLIBS-c.so += frame.o' to the file `configparms' in the directory you're
617 building in. You can check you've got the right `frame.o' file by running
618 `nm frame.o' and checking that it has the symbols defined that you're
621 This will let you build glibc with the C compiler. The C++ compiler
622 will still be binary incompatible with any C++ shared libraries that
623 you got with your distribution.
625 ?? How can I compile gcc 2.7.2.1 from the gcc source code using
628 {AJ} There's only correct support for glibc 2.0.x in gcc 2.7.2.3 or later.
629 But you should get at least gcc 2.8.1 or egcs 1.1 (or later versions)
632 ?? The `gencat' utility cannot process the catalog sources which
633 were used on my Linux libc5 based system. Why?
635 {UD} The `gencat' utility provided with glibc complies to the XPG standard.
636 The older Linux version did not obey the standard, so they are not
639 To ease the transition from the Linux version some of the non-standard
640 features are also present in the `gencat' program of GNU libc. This mainly
641 includes the use of symbols for the message number and the automatic
642 generation of header files which contain the needed #defines to map the
645 Here is a simple SED script to convert at least some Linux specific catalog
646 files to the XPG4 form:
648 -----------------------------------------------------------------------
649 # Change catalog source in Linux specific format to standard XPG format.
650 # Ulrich Drepper <drepper@cygnus.com>, 1996.
654 s/\$ #\([^ ]*\).*/\1/
656 s/\$ #[^ ]* *\(.*\)/\$ \1/
662 s/\(.*\)\n\(.*\)/\2 \1/
664 -----------------------------------------------------------------------
666 ?? Programs using libc have their messages translated, but other
667 behavior is not localized (e.g. collating order); why?
669 {ZW} Translated messages are automatically installed, but the locale
670 database that controls other behaviors is not. You need to run localedef to
671 install this database, after you have run `make install'. For example, to
672 set up the French Canadian locale, simply issue the command
674 localedef -i fr_CA -f ISO-8859-1 fr_CA
676 Please see localedata/README in the source tree for further details.
678 ?? I have set up /etc/nis.conf, and the Linux libc 5 with NYS
679 works great. But the glibc NIS+ doesn't seem to work.
681 {TK} The glibc NIS+ implementation uses a /var/nis/NIS_COLD_START file for
682 storing information about the NIS+ server and their public keys, because the
683 nis.conf file does not contain all the necessary information. You have to
684 copy a NIS_COLD_START file from a Solaris client (the NIS_COLD_START file is
685 byte order independent) or generate it with nisinit from the nis-tools
686 package; available at
688 http://www.suse.de/~kukuk/linux/nisplus.html
690 ?? I have killed ypbind to stop using NIS, but glibc
693 {TK} For faster NIS lookups, glibc uses the /var/yp/binding/ files from
694 ypbind. ypbind 3.3 and older versions don't always remove these files, so
695 glibc will continue to use them. Other BSD versions seem to work correctly.
696 Until ypbind 3.4 is released, you can find a patch at
698 <ftp://ftp.kernel.org/pub/linux/utils/net/NIS/ypbind-3.3-glibc4.diff.gz>
700 ?? Under Linux/Alpha, I always get "do_ypcall: clnt_call:
701 RPC: Unable to receive; errno = Connection refused" when using NIS.
703 {TK} You need a ypbind version which is 64bit clean. Some versions are not
704 64bit clean. A 64bit clean implementation is ypbind-mt. For ypbind 3.3,
705 you need the patch from ftp.kernel.org (See the previous question). I don't
706 know about other versions.
709 ?? After installing glibc name resolving doesn't work properly.
711 {AJ} You probably should read the manual section describing nsswitch.conf
712 (just type `info libc "NSS Configuration File"'). The NSS configuration
713 file is usually the culprit.
716 ?? How do I create the databases for NSS?
718 {AJ} If you have an entry "db" in /etc/nsswitch.conf you should also create
719 the database files. The glibc sources contain a Makefile which does the
720 necessary conversion and calls to create those files. The file is
721 `db-Makefile' in the subdirectory `nss' and you can call it with `make -f
722 db-Makefile'. Please note that not all services are capable of using a
723 database. Currently passwd, group, ethers, protocol, rpc, services shadow
724 and netgroup are implemented. See also question ?nssdb.
726 ?? I have /usr/include/net and /usr/include/scsi as symlinks
727 into my Linux source tree. Is that wrong?
729 {PB} This was necessary for libc5, but is not correct when using glibc.
730 Including the kernel header files directly in user programs usually does not
731 work (see ?kerhdr). glibc provides its own <net/*> and <scsi/*> header
732 files to replace them, and you may have to remove any symlink that you have
733 in place before you install glibc. However, /usr/include/asm and
734 /usr/include/linux should remain as they were.
736 ?? Programs like `logname', `top', `uptime' `users', `w' and
737 `who', show incorrect information about the (number of)
738 users on my system. Why?
742 ?? After upgrading to glibc 2.1 with symbol versioning I get
743 errors about undefined symbols. What went wrong?
745 {AJ} The problem is caused either by wrong program code or tools. In the
746 versioned libc a lot of symbols are now local that were global symbols in
747 previous versions. It seems that programs linked against older versions
748 often accidentally used libc global variables -- something that should not
751 The only way to fix this is to recompile your program. Sorry, that's the
752 price you might have to pay once for quite a number of advantages with
755 ?? When I start the program XXX after upgrading the library
757 XXX: Symbol `_sys_errlist' has different size in shared
758 object, consider re-linking
759 Why? What should I do?
761 {UD} As the message says, relink the binary. The problem is that a few
762 symbols from the library can change in size and there is no way to avoid
763 this. _sys_errlist is a good example. Occasionally there are new error
764 numbers added to the kernel and this must be reflected at user level,
765 breaking programs that refer to them directly.
767 Such symbols should normally not be used at all. There are mechanisms to
768 avoid using them. In the case of _sys_errlist, there is the strerror()
769 function which should _always_ be used instead. So the correct fix is to
770 rewrite that part of the application.
772 In some situations (especially when testing a new library release) it might
773 be possible that a symbol changed size when that should not have happened.
774 So in case of doubt report such a warning message as a problem.
776 ?? What do I need for C++ development?
778 {HJ,AJ} You need either egcs 1.1 which comes directly with libstdc++ or
779 gcc-2.8.1 together with libstdc++ 2.8.1.1. egcs 1.1 has the better C++
780 support and works directly with glibc 2.1. If you use gcc-2.8.1 with
781 libstdc++ 2.8.1.1, you need to modify libstdc++ a bit. A patch is available
783 <ftp://alpha.gnu.org/gnu/libstdc++-2.8.1.1-glibc2.1-diff.gz>
785 Please note that libg++ 2.7.2 (and the Linux Versions 2.7.2.x) doesn't work
786 very well with the GNU C library due to vtable thunks. If you're upgrading
787 from glibc 2.0.x to 2.1 you have to recompile libstdc++ since the library
788 compiled for 2.0 is not compatible due to the new Large File Support (LFS)
791 {UD} But since in the case of a shared libstdc++ the version numbers should
792 be different existing programs will continue to work.
794 ?? Even statically linked programs need some shared libraries
795 which is not acceptable for me. What can I do?
797 {AJ} NSS (for details just type `info libc "Name Service Switch"') won't
798 work properly without shared libraries. NSS allows using different services
799 (e.g. NIS, files, db, hesiod) by just changing one configuration file
800 (/etc/nsswitch.conf) without relinking any programs. The only disadvantage
801 is that now static libraries need to access shared libraries. This is
802 handled transparently by the GNU C library.
804 A solution is to configure glibc with --enable-static-nss. In this case you
805 can create a static binary that will use only the services dns and files
806 (change /etc/nsswitch.conf for this). You need to link explicitly against
807 all these services. For example:
809 gcc -static test-netdb.c -o test-netdb.c \
810 -lc -lnss_files -lnss_dns -lresolv
812 The problem with this approach is that you've got to link every static
813 program that uses NSS routines with all those libraries.
815 {UD} In fact, one cannot say anymore that a libc compiled with this
816 option is using NSS. There is no switch anymore. Therefore it is
817 *highly* recommended *not* to use --enable-static-nss since this makes
818 the behaviour of the programs on the system inconsistent.
820 ?? I just upgraded my Linux system to glibc and now I get
821 errors whenever I try to link any program.
823 {ZW} This happens when you have installed glibc as the primary C library but
824 have stray symbolic links pointing at your old C library. If the first
825 `libc.so' the linker finds is libc 5, it will use that. Your program
826 expects to be linked with glibc, so the link fails.
828 The most common case is that glibc put its `libc.so' in /usr/lib, but there
829 was a `libc.so' from libc 5 in /lib, which gets searched first. To fix the
830 problem, just delete /lib/libc.so. You may also need to delete other
831 symbolic links in /lib, such as /lib/libm.so if it points to libm.so.5.
833 {AJ} The perl script test-installation.pl which is run as last step during
834 an installation of glibc that is configured with --prefix=/usr should help
835 detect these situations. If the script reports problems, something is
838 ?? When I use nscd the machine freezes.
840 {UD} You cannot use nscd with Linux 2.0.*. There is functionality missing
841 in the kernel and work-arounds are not suitable. Besides, some parts of the
842 kernel are too buggy when it comes to using threads.
844 If you need nscd, you have to use at least a 2.1 kernel.
846 Note that I have at this point no information about any other platform.
848 ?? I need lots of open files. What do I have to do?
850 {AJ} This is at first a kernel issue. The kernel defines limits with
851 OPEN_MAX the number of simultaneous open files and with FD_SETSIZE the
852 number of used file descriptors. You need to change these values in your
853 kernel and recompile the kernel so that the kernel allows to use more open
854 files. You don't necessarily need to recompile the GNU C library since the
855 only place where OPEN_MAX and FD_SETSIZE is really needed in the library
856 itself is the size of fd_set which is used by select.
858 The GNU C library is now select free. This means it internally has no
859 limits imposed by the `fd_set' type. Instead all places where the
860 functionality is needed the `poll' function is used.
862 If you increase the number of file descriptors in the kernel you don't need
863 to recompile the C library.
865 {UD} You can always get the maximum number of file descriptors a process is
866 allowed to have open at any time using
868 number = sysconf (_SC_OPEN_MAX);
870 This will work even if the kernel limits change.
872 ?? How do I get the same behavior on parsing /etc/passwd and
873 /etc/group as I have with libc5 ?
875 {TK} The name switch setup in /etc/nsswitch.conf selected by most Linux
876 distributions does not support +/- and netgroup entries in the files like
877 /etc/passwd. Though this is the preferred setup some people might have
878 setups coming over from the libc5 days where it was the default to recognize
879 lines like this. To get back to the old behaviour one simply has to change
880 the rules for passwd, group, and shadow in the nsswitch.conf file as
891 ??libs What needs to be recompiled when upgrading from glibc 2.0 to glibc
894 {AJ,CG} If you just upgrade the glibc from 2.0.x (x <= 7) to 2.1, binaries
895 that have been linked against glibc 2.0 will continue to work.
897 If you compile your own binaries against glibc 2.1, you also need to
898 recompile some other libraries. The problem is that libio had to be changed
899 and therefore libraries that are based or depend on the libio of glibc,
900 e.g. ncurses, slang and most C++ libraries, need to be recompiled. If you
901 experience strange segmentation faults in your programs linked against glibc
902 2.1, you might need to recompile your libraries.
904 Another problem is that older binaries that were linked statically against
905 glibc 2.0 will reference the older nss modules (libnss_files.so.1 instead of
906 libnss_files.so.2), so don't remove them. Also, the old glibc-2.0 compiled
907 static libraries (libfoo.a) which happen to depend on the older libio
908 behavior will be broken by the glibc 2.1 upgrade. We plan to produce a
909 compatibility library that people will be able to link in if they want
910 to compile a static library generated against glibc 2.0 into a program
911 on a glibc 2.1 system. You just add -lcompat and you should be fine.
913 The glibc-compat add-on will provide the libcompat.a library, the older
914 nss modules, and a few other files. Together, they should make it
915 possible to do development with old static libraries on a glibc 2.1
916 system. This add-on is still in development. You can get it from
917 <ftp://alpha.gnu.org/gnu/glibc/glibc-compat-2.1.tar.gz>
918 but please keep in mind that it is experimental.
920 ?? Why is extracting files via tar so slow?
922 {AJ} Extracting of tar archives might be quite slow since tar has to look up
923 userid and groupids and doesn't cache negative results. If you have nis or
924 nisplus in your /etc/nsswitch.conf for the passwd and/or group database,
925 each file extractions needs a network connection. There are two possible
928 - do you really need NIS/NIS+ (some Linux distributions add by default
929 nis/nisplus even if it's not needed)? If not, just remove the entries.
931 - if you need NIS/NIS+, use the Name Service Cache Daemon nscd that comes
934 ?? Compiling programs I get parse errors in libio.h (e.g. "parse error
935 before `_IO_seekoff'"). How should I fix this?
937 {AJ} You might get the following errors when upgrading to glibc 2.1:
939 In file included from /usr/include/stdio.h:57,
941 /usr/include/libio.h:335: parse error before `_IO_seekoff'
942 /usr/include/libio.h:335: parse error before `_G_off64_t'
943 /usr/include/libio.h:336: parse error before `_IO_seekpos'
944 /usr/include/libio.h:336: parse error before `_G_fpos64_t'
946 The problem is a wrong _G_config.h file in your include path. The
947 _G_config.h file that comes with glibc 2.1 should be used and not one from
948 libc5 or from a compiler directory. To check which _G_config.h file the
949 compiler uses, compile your program with `gcc -E ...|grep G_config.h' and
950 remove that file. Your compiler should pick up the file that has been
951 installed by glibc 2.1 in your include directory.
953 ?? After upgrading to glibc 2.1, libraries that were compiled against
954 glibc 2.0.x don't work anymore.
958 ??nssdb What happened to the Berkeley DB libraries? Can I still use db
959 in /etc/nsswitch.conf?
961 {AJ} Due to too many incompatible changes in disk layout and API of Berkeley
962 DB and a too tight coupling of libc and libdb, the db library has been
963 removed completely from glibc 2.2. The only place that really used the
964 Berkeley DB was the NSS db module.
966 The NSS db module has been rewritten to support a number of different
967 versions of Berkeley DB for the NSS db module. Currently the releases 2.x
968 and 3.x of Berkeley DB are supported. The older db 1.85 library is not
969 supported. You can use the version from glibc 2.1.x or download a version
970 from Sleepycat Software (http://www.sleepycat.com). The library has to be
971 compiled as shared library and installed in the system lib directory
972 (normally /lib). The library needs to have a special soname to be found by
975 If public structures change in a new Berkeley db release, this needs to be
978 Currently the code searches for libraries with a soname of "libdb.so.3"
979 (that's the name from db 2.4.14 which comes with glibc 2.1.x) and
980 "libdb-3.0.so" (the name used by db 3.0.55 as default).
983 ? Source and binary incompatibilities, and what to do about them
985 ?? I expect GNU libc to be 100% source code compatible with
986 the old Linux based GNU libc. Why isn't it like this?
988 {DMT,UD} Not every extension in Linux libc's history was well thought-out.
989 In fact it had a lot of problems with standards compliance and with
990 cleanliness. With the introduction of a new version number these errors can
991 now be corrected. Here is a list of the known source code
994 * _GNU_SOURCE: glibc does not make the GNU extensions available
995 automatically. If a program depends on GNU extensions or some
996 other non-standard functionality, it is necessary to compile it
997 with the C compiler option -D_GNU_SOURCE, or better, to put
998 `#define _GNU_SOURCE' at the beginning of your source files, before
999 any C library header files are included. This difference normally
1000 manifests itself in the form of missing prototypes and/or data type
1001 definitions. Thus, if you get such errors, the first thing you
1002 should do is try defining _GNU_SOURCE and see if that makes the
1005 For more information consult the file `NOTES' in the GNU C library
1008 * reboot(): GNU libc sanitizes the interface of reboot() to be more
1009 compatible with the interface used on other OSes. reboot() as
1010 implemented in glibc takes just one argument. This argument
1011 corresponds to the third argument of the Linux reboot system call.
1012 That is, a call of the form reboot(a, b, c) needs to be changed into
1013 reboot(c). Beside this the header <sys/reboot.h> defines the needed
1014 constants for the argument. These RB_* constants should be used
1015 instead of the cryptic magic numbers.
1017 * swapon(): the interface of this function didn't change, but the
1018 prototype is in a separate header file <sys/swap.h>. This header
1019 file also provides the SWAP_* constants defined by <linux/swap.h>;
1020 you should use them for the second argument to swapon().
1022 * errno: If a program uses the variable "errno", then it _must_
1023 include <errno.h>. The old libc often (erroneously) declared this
1024 variable implicitly as a side-effect of including other libc header
1025 files. glibc is careful to avoid such namespace pollution, which,
1026 in turn, means that you really need to include the header files that
1027 you depend on. This difference normally manifests itself in the
1028 form of the compiler complaining about references to an undeclared
1031 * Linux-specific syscalls: All Linux system calls now have appropriate
1032 library wrappers and corresponding declarations in various header files.
1033 This is because the syscall() macro that was traditionally used to
1034 work around missing syscall wrappers are inherently non-portable and
1035 error-prone. The following table lists all the new syscall stubs,
1036 the header-file declaring their interface and the system call name.
1038 syscall name: wrapper name: declaring header file:
1039 ------------- ------------- ----------------------
1040 bdflush bdflush <sys/kdaemon.h>
1041 syslog ksyslog_ctl <sys/klog.h>
1043 * lpd: Older versions of lpd depend on a routine called _validuser().
1044 The library does not provide this function, but instead provides
1045 __ivaliduser() which has a slightly different interface. Simply
1046 upgrading to a newer lpd should fix this problem (e.g., the 4.4BSD
1047 lpd is known to be working).
1049 * resolver functions/BIND: like on many other systems the functions of
1050 the resolver library are not included in libc itself. There is a
1051 separate library libresolv. If you get undefined symbol errors for
1052 symbols starting with `res_*' simply add -lresolv to your linker
1055 * the `signal' function's behavior corresponds to the BSD semantic and
1056 not the SysV semantic as it was in libc-5. The interface on all GNU
1057 systems shall be the same and BSD is the semantic of choice. To use
1058 the SysV behavior simply use `sysv_signal', or define _XOPEN_SOURCE.
1059 See ?signal for details.
1061 ??getlog Why does getlogin() always return NULL on my Linux box?
1063 {UD} The GNU C library has a format for the UTMP and WTMP file which differs
1064 from what your system currently has. It was extended to fulfill the needs
1065 of the next years when IPv6 is introduced. The record size is different and
1066 some fields have different positions. The files written by functions from
1067 the one library cannot be read by functions from the other library. Sorry,
1068 but this is what a major release is for. It's better to have a cut now than
1069 having no means to support the new techniques later.
1071 {MK} There is however a (partial) solution for this problem. Please take a
1072 look at the file `login/README.utmpd'.
1074 ?? Where are the DST_* constants found in <sys/time.h> on many
1077 {UD} These constants come from the old BSD days and are not used anymore
1078 (libc5 does not actually implement the handling although the constants are
1081 Instead GNU libc contains zone database support and compatibility code for
1082 POSIX TZ environment variable handling. For former is very much preferred
1085 ?? The prototypes for `connect', `accept', `getsockopt',
1086 `setsockopt', `getsockname', `getpeername', `send',
1087 `sendto', and `recvfrom' are different in GNU libc from
1088 any other system I saw. This is a bug, isn't it?
1090 {UD} No, this is no bug. This version of GNU libc already follows the new
1091 Single Unix specifications (and I think the POSIX.1g draft which adopted the
1092 solution). The type for a parameter describing a size is now `socklen_t', a
1095 ??kerhdr On Linux I've got problems with the declarations in Linux
1098 {UD,AJ} On Linux, the use of kernel headers is reduced to the minimum. This
1099 gives Linus the ability to change the headers more freely. Also, user
1100 programs are now insulated from changes in the size of kernel data
1103 For example, the sigset_t type is 32 or 64 bits wide in the kernel. In
1104 glibc it is 1024 bits wide. This guarantees that when the kernel gets a
1105 bigger sigset_t (for POSIX.1e realtime support, say) user programs will not
1106 have to be recompiled. Consult the header files for more information about
1109 Therefore you shouldn't include Linux kernel header files directly if glibc
1110 has defined a replacement. Otherwise you might get undefined results because
1113 ?? I don't include any kernel headers myself but the compiler
1114 still complains about redeclarations of types in the kernel
1117 {UD} The kernel headers before Linux 2.1.61 and 2.0.32 don't work correctly
1118 with glibc. Compiling C programs is possible in most cases but C++ programs
1119 have (due to the change of the name lookups for `struct's) problems. One
1120 prominent example is `struct fd_set'.
1122 There might be some problems left but 2.1.61/2.0.32 fix most of the known
1123 ones. See the BUGS file for other known problems.
1125 ??signal Why don't signals interrupt system calls anymore?
1127 {ZW} By default GNU libc uses the BSD semantics for signal(), unlike Linux
1128 libc 5 which used System V semantics. This is partially for compatibility
1129 with other systems and partially because the BSD semantics tend to make
1130 programming with signals easier.
1132 There are three differences:
1134 * BSD-style signals that occur in the middle of a system call do not
1135 affect the system call; System V signals cause the system call to
1136 fail and set errno to EINTR.
1138 * BSD signal handlers remain installed once triggered. System V signal
1139 handlers work only once, so one must reinstall them each time.
1141 * A BSD signal is blocked during the execution of its handler. In other
1142 words, a handler for SIGCHLD (for example) does not need to worry about
1143 being interrupted by another SIGCHLD. It may, however, be interrupted
1146 There is general consensus that for `casual' programming with signals, the
1147 BSD semantics are preferable. You don't need to worry about system calls
1148 returning EINTR, and you don't need to worry about the race conditions
1149 associated with one-shot signal handlers.
1151 If you are porting an old program that relies on the old semantics, you can
1152 quickly fix the problem by changing signal() to sysv_signal() throughout.
1153 Alternatively, define _XOPEN_SOURCE before including <signal.h>.
1155 For new programs, the sigaction() function allows you to specify precisely
1156 how you want your signals to behave. All three differences listed above are
1157 individually switchable on a per-signal basis with this function.
1159 If all you want is for one specific signal to cause system calls to fail and
1160 return EINTR (for example, to implement a timeout) you can do this with
1164 ??string I've got errors compiling code that uses certain string
1167 {AJ} glibc 2.1 has special string functions that are faster than the normal
1168 library functions. Some of the functions are additionally implemented as
1169 inline functions and others as macros. This might lead to problems with
1170 existing codes but it is explicitly allowed by ISO C.
1172 The optimized string functions are only used when compiling with
1173 optimizations (-O1 or higher). The behavior can be changed with two feature
1176 * __NO_STRING_INLINES: Don't do any string optimizations.
1177 * __USE_STRING_INLINES: Use assembly language inline functions (might
1178 increase code size dramatically).
1180 Since some of these string functions are now additionally defined as macros,
1181 code like "char *strncpy();" doesn't work anymore (and is unnecessary, since
1182 <string.h> has the necessary declarations). Either change your code or
1183 define __NO_STRING_INLINES.
1185 {UD} Another problem in this area is that gcc still has problems on machines
1186 with very few registers (e.g., ix86). The inline assembler code can require
1187 almost all the registers and the register allocator cannot always handle
1190 One can disable the string optimizations selectively. Instead of writing
1192 cp = strcpy (foo, "lkj");
1196 cp = (strcpy) (foo, "lkj");
1198 This disables the optimization for that specific call.
1200 ?? I get compiler messages "Initializer element not constant" with
1201 stdin/stdout/stderr. Why?
1203 {RM,AJ} Constructs like:
1204 static FILE *InPtr = stdin;
1206 lead to this message. This is correct behaviour with glibc since stdin is
1207 not a constant expression. Please note that a strict reading of ISO C does
1208 not allow above constructs.
1210 One of the advantages of this is that you can assign to stdin, stdout, and
1211 stderr just like any other global variable (e.g. `stdout = my_stream;'),
1212 which can be very useful with custom streams that you can write with libio
1213 (but beware this is not necessarily portable). The reason to implement it
1214 this way were versioning problems with the size of the FILE structure.
1216 To fix those programs you've got to initialize the variable at run time.
1217 This can be done, e.g. in main, like:
1225 or by constructors (beware this is gcc specific):
1228 static void inPtr_construct (void) __attribute__((constructor));
1229 static void inPtr_construct (void) { InPtr = stdin; }
1232 ?? I can't compile with gcc -traditional (or
1233 -traditional-cpp). Why?
1235 {AJ} glibc2 does break -traditional and -traditonal-cpp - and will continue
1236 to do so. For example constructs of the form:
1242 are useful for debugging purposes (you can use foo with your debugger that's
1243 why we need the enum) and for compatibility (other systems use defines and
1246 ?? I get some errors with `gcc -ansi'. Isn't glibc ANSI compatible?
1248 {AJ} The GNU C library is compatible with the ANSI/ISO C standard. If
1249 you're using `gcc -ansi', the glibc includes which are specified in the
1250 standard follow the standard. The ANSI/ISO C standard defines what has to be
1251 in the include files - and also states that nothing else should be in the
1252 include files (btw. you can still enable additional standards with feature
1255 The GNU C library is conforming to ANSI/ISO C - if and only if you're only
1256 using the headers and library functions defined in the standard.
1258 ?? I can't access some functions anymore. nm shows that they do
1259 exist but linking fails nevertheless.
1261 {AJ} With the introduction of versioning in glibc 2.1 it is possible to
1262 export only those identifiers (functions, variables) that are really needed
1263 by application programs and by other parts of glibc. This way a lot of
1264 internal interfaces are now hidden. nm will still show those identifiers
1265 but marking them as internal. ISO C states that identifiers beginning with
1266 an underscore are internal to the libc. An application program normally
1267 shouldn't use those internal interfaces (there are exceptions,
1268 e.g. __ivaliduser). If a program uses these interfaces, it's broken. These
1269 internal interfaces might change between glibc releases or dropped
1272 ?? When using the db-2 library which comes with glibc is used in
1273 the Perl db modules the testsuite is not passed. This did not
1274 happen with db-1, gdbm, or ndbm.
1276 Removed. Does not apply anymore.
1278 ?? The pow() inline function I get when including <math.h> is broken.
1279 I get segmentation faults when I run the program.
1281 {UD} Nope, the implementation is correct. The problem is with egcs version
1282 prior to 1.1. I.e., egcs 1.0 to 1.0.3 are all broken (at least on Intel).
1283 If you have to use this compiler you must define __NO_MATH_INLINES before
1284 including <math.h> to prevent the inline functions from being used. egcs 1.1
1285 fixes the problem. I don't know about gcc 2.8 and 2.8.1.
1287 ?? The sys/sem.h file lacks the definition of `union semun'.
1289 {UD} Nope. This union has to be provided by the user program. Former glibc
1290 versions defined this but it was an error since it does not make much sense
1291 when thinking about it. The standards describing the System V IPC functions
1292 define it this way and therefore programs must be adopted.
1294 ?? Why has <netinet/ip_fw.h> disappeared?
1296 {AJ} The corresponding Linux kernel data structures and constants are
1297 totally different in Linux 2.0 and Linux 2.2. This situation has to be
1298 taken care in user programs using the firewall structures and therefore
1299 those programs (ipfw is AFAIK the only one) should deal with this problem
1302 ?? I get floods of warnings when I use -Wconversion and include
1303 <string.h> or <math.h>.
1305 {ZW} <string.h> and <math.h> intentionally use prototypes to override
1306 argument promotion. -Wconversion warns about all these. You can safely
1307 ignore the warnings.
1309 -Wconversion isn't really intended for production use, only for shakedown
1310 compiles after converting an old program to standard C.
1313 ?? After upgrading to glibc 2.1, I receive errors about
1314 unresolved symbols, like `_dl_initial_searchlist' and can not
1315 execute any binaries. What went wrong?
1317 {AJ} This normally happens if your libc and ld (dynamic linker) are from
1318 different releases of glibc. For example, the dynamic linker
1319 /lib/ld-linux.so.2 comes from glibc 2.0.x, but the version of libc.so.6 is
1322 The path /lib/ld-linux.so.2 is hardcoded in every glibc2 binary but
1323 libc.so.6 is searched via /etc/ld.so.cache and in some special directories
1324 like /lib and /usr/lib. If you run configure with another prefix than /usr
1325 and put this prefix before /lib in /etc/ld.so.conf, your system will break.
1327 So what can you do? Either of the following should work:
1329 * Run `configure' with the same prefix argument you've used for glibc 2.0.x
1330 so that the same paths are used.
1331 * Replace /lib/ld-linux.so.2 with a link to the dynamic linker from glibc
1334 You can even call the dynamic linker by hand if everything fails. You've
1335 got to set LD_LIBRARY_PATH so that the corresponding libc is found and also
1336 need to provide an absolute path to your binary:
1338 LD_LIBRARY_PATH=<path-where-libc.so.6-lives> \
1339 <path-where-corresponding-dynamic-linker-lives>/ld-linux.so.2 \
1340 <path-to-binary>/binary
1342 For example `LD_LIBRARY_PATH=/libold /libold/ld-linux.so.2 /bin/mv ...'
1343 might be useful in fixing a broken system (if /libold contains dynamic
1344 linker and corresponding libc).
1346 With that command line no path is used. To further debug problems with the
1347 dynamic linker, use the LD_DEBUG environment variable, e.g.
1348 `LD_DEBUG=help echo' for the help text.
1350 If you just want to test this release, don't put the lib directory in
1351 /etc/ld.so.conf. You can call programs directly with full paths (as above).
1352 When compiling new programs against glibc 2.1, you've got to specify the
1353 correct paths to the compiler (option -I with gcc) and linker (options
1354 --dynamic-linker, -L and --rpath).
1356 ?? bonnie reports that char i/o with glibc 2 is much slower than with
1357 libc5. What can be done?
1359 {AJ} The GNU C library uses thread safe functions by default and libc5 used
1360 non thread safe versions. The non thread safe functions have in glibc the
1361 suffix `_unlocked', for details check <stdio.h>. Using `putc_unlocked' etc.
1362 instead of `putc' should give nearly the same speed with bonnie (bonnie is a
1363 benchmark program for measuring disk access).
1365 ?? Programs compiled with glibc 2.1 can't read db files made with glibc
1366 2.0. What has changed that programs like rpm break?
1368 Removed. Does not apply anymore.
1370 ?? Autoconf's AC_CHECK_FUNC macro reports that a function exists, but
1371 when I try to use it, it always returns -1 and sets errno to ENOSYS.
1373 {ZW} You are using a 2.0 Linux kernel, and the function you are trying to
1374 use is only implemented in 2.1/2.2. Libc considers this to be a function
1375 which exists, because if you upgrade to a 2.2 kernel, it will work. One
1376 such function is sigaltstack.
1378 Your program should check at runtime whether the function works, and
1379 implement a fallback. Note that Autoconf cannot detect unimplemented
1380 functions in other systems' C libraries, so you need to do this anyway.
1382 ?? My program segfaults when I call fclose() on the FILE* returned
1383 from setmntent(). Is this a glibc bug?
1385 {GK} No. Don't do this. Use endmntent(), that's what it's for.
1387 In general, you should use the correct deallocation routine. For instance,
1388 if you open a file using fopen(), you should deallocate the FILE * using
1389 fclose(), not free(), even though the FILE * is also a pointer.
1391 In the case of setmntent(), it may appear to work in most cases, but it
1392 won't always work. Unfortunately, for compatibility reasons, we can't
1393 change the return type of setmntent() to something other than FILE *.
1398 ?? After I changed configure.in I get `Autoconf version X.Y.
1399 or higher is required for this script'. What can I do?
1401 {UD} You have to get the specified autoconf version (or a later one)
1402 from your favorite mirror of ftp.gnu.org.
1404 ?? When I try to compile code which uses IPv6 headers and
1405 definitions on my Linux 2.x.y system I am in trouble.
1406 Nothing seems to work.
1408 {UD} The problem is that IPv6 development still has not reached a point
1409 where the headers are stable. There are still lots of incompatible changes
1410 made and the libc headers have to follow.
1412 {PB} The 2.1 release of GNU libc aims to comply with the current versions of
1413 all the relevant standards. The IPv6 support libraries for older Linux
1414 systems used a different naming convention and so code written to work with
1415 them may need to be modified. If the standards make incompatible changes in
1416 the future then the libc may need to change again.
1418 IPv6 will not work with a 2.0.x kernel. When kernel 2.2 is released it
1419 should contain all the necessary support; until then you should use the
1420 latest 2.1.x release you can find. As of 98/11/26 the currently recommended
1421 kernel for IPv6 is 2.1.129.
1423 Also, as of the 2.1 release the IPv6 API provided by GNU libc is not
1424 100% complete. In particular the getipnodebyname and getipnodebyaddr
1425 functions are not implemented.
1427 ??tzdb When I set the timezone by setting the TZ environment variable
1428 to EST5EDT things go wrong since glibc computes the wrong time
1429 from this information.
1431 {UD} The problem is that people still use the braindamaged POSIX method to
1432 select the timezone using the TZ environment variable with a format EST5EDT
1433 or whatever. People, if you insist on using TZ instead of the timezone
1434 database (see below), read the POSIX standard, the implemented behaviour is
1435 correct! What you see is in fact the result of the decisions made while
1436 POSIX.1 was created. We've only implemented the handling of TZ this way to
1437 be POSIX compliant. It is not really meant to be used.
1439 The alternative approach to handle timezones which is implemented is the
1440 correct one to use: use the timezone database. This avoids all the problems
1441 the POSIX method has plus it is much easier to use. Simply run the tzselect
1442 shell script, answer the question and use the name printed in the end by
1443 making a symlink /etc/localtime pointing to /usr/share/zoneinfo/NAME (NAME
1444 is the returned value from tzselect). That's all. You never again have to
1447 So, please avoid sending bug reports about time related problems if you use
1448 the POSIX method and you have not verified something is really broken by
1449 reading the POSIX standards.
1451 ?? What other sources of documentation about glibc are available?
1453 {AJ} The FSF has a page about the GNU C library at
1454 <http://www.gnu.org/software/libc/>. The problem data base of open and
1455 solved bugs in GNU libc is available at
1456 <http://www-gnats.gnu.org:8080/cgi-bin/wwwgnats.pl>. Eric Green has written
1457 a HowTo for converting from Linux libc5 to glibc2. The HowTo is accessible
1458 via the FSF page and at <http://www.imaxx.net/~thrytis/glibc>. Frodo
1459 Looijaard describes a different way installing glibc2 as secondary libc at
1460 <http://huizen.dds.nl/~frodol/glibc>.
1462 Please note that this is not a complete list.
1464 ?? The timezone string for Sydney/Australia is wrong since even when
1465 daylight saving time is in effect the timezone string is EST.
1467 {UD} The problem for some timezones is that the local authorities decided
1468 to use the term "summer time" instead of "daylight saving time". In this
1469 case the abbreviation character `S' is the same as the standard one. So,
1472 Eastern Standard Time = EST
1473 Eastern Summer Time = EST
1475 Great! To get this bug fixed convince the authorities to change the laws
1476 and regulations of the country this effects. glibc behaves correctly.
1478 ??make I've build make 3.77 against glibc 2.1 and now make gets
1479 segmentation faults.
1481 {AJ} GNU make 3.77 has support for 64 bit filesystems which is slightly
1482 broken (and one of the new features in the GNU C library 2.1 is 64 bit
1483 filesystem support :-( ). To get a working make you can use either make
1484 3.75 or patch 3.77. A working patch is available via RedHat's Rawhide server
1485 (ftp://rawhide.redhat.com/SRPMS/SRPMS/make-3.77-*src.rpm).
1487 ?? Why do so many programs using math functions fail on my AlphaStation?
1489 {AO} The functions floor() and floorf() use an instruction that is not
1490 implemented in some old PALcodes of AlphaStations. This may cause
1491 `Illegal Instruction' core dumps or endless loops in programs that
1492 catch these signals. Updating the firmware to a 1999 release has
1493 fixed the problem on an AlphaStation 200 4/166.
1495 ?? The conversion table for character set XX does not match with
1498 {UD} I don't doubt for a minute that some of the conversion tables contain
1499 errors. We tried the best we can and relied on automatic generation of the
1500 data to prevent human-introduced errors but this still is no guarantee. If
1501 you think you found a problem please send a bug report describing it and
1502 give an authoritive reference. The latter is important since otherwise
1503 the current behaviour is as good as the proposed one.
1505 Before doing this look through the list of known problem first:
1507 - the GBK (simplified Chinese) encoding is based on Unicode tables. This
1508 is good. These tables, however, differ slightly from the tables used
1509 by the M$ people. The differences are these [+ Unicode, - M$]:
1516 In addition the Unicode tables contain mappings for the GBK characters
1517 0xA8BC, 0xA8BF, 0xA989 to 0xA995, and 0xFE50 to 0xFEA0.
1519 - when mapping from EUC-CN to GBK and vice versa we ignore the fact that
1520 the coded character at position 0xA1A4 maps to different Unicode
1521 characters. Since the iconv() implementation can do whatever it wants
1522 if it cannot directly map a character this is a perfectly good solution
1523 since the semantics and appearance of the character does not change.
1526 Answers were given by:
1527 {UD} Ulrich Drepper, <drepper@cygnus.com>
1528 {DMT} David Mosberger-Tang, <davidm@AZStarNet.com>
1529 {RM} Roland McGrath, <roland@gnu.org>
1530 {AJ} Andreas Jaeger, <aj@suse.de>
1531 {EY} Eric Youngdale, <eric@andante.jic.com>
1532 {PB} Phil Blundell, <Philip.Blundell@pobox.com>
1533 {MK} Mark Kettenis, <kettenis@phys.uva.nl>
1534 {ZW} Zack Weinberg, <zack@rabi.phys.columbia.edu>
1535 {TK} Thorsten Kukuk, <kukuk@suse.de>
1536 {GK} Geoffrey Keating, <geoffk@ozemail.com.au>
1537 {HJ} H.J. Lu, <hjl@gnu.org>
1538 {CG} Cristian Gafton, <gafton@redhat.com>
1539 {AO} Alexandre Oliva, <oliva@lsd.ic.unicamp.br>
1543 outline-regexp:"\\?"
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