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
38 mips*-*-linux-gnu Linux-2.x on MIPS
39 ia64-*-linux-gnu Linux-2.x on ia64
40 s390-*-linux-gnu Linux-2.x on IBM S/390
42 Ports to other Linux platforms are in development, and may in fact work
43 already, but no one has sent us success reports for them. Currently no
44 ports to other operating systems are underway, although a few people have
47 If you have a system not listed above (or in the `README' file) and you are
48 really interested in porting it, contact
52 ??binsize What compiler do I need to build GNU libc?
54 {UD} You must use GNU CC to compile GNU libc. A lot of extensions of GNU CC
55 are used to increase portability and speed.
57 GNU CC is found, like all other GNU packages, on
59 ftp://ftp.gnu.org/pub/gnu
61 and the many mirror sites. ftp.gnu.org is always overloaded, so try to find
64 You should always try to use the latest official release. Older versions
65 may not have all the features GNU libc requires. The current releases of
66 gcc (2.95 or newer) should work with the GNU C library (for powerpc see
67 ?powerpc; for ARM see ?arm; for MIPS see ?mips).
69 Please note that gcc 2.95 and 2.95.x cannot compile glibc on Alpha due to
70 problems in the complex float support.
72 ?? When I try to compile glibc I get only error messages.
75 {UD} You definitely need GNU make to build GNU libc. No other make
76 program has the needed functionality.
78 We recommend version GNU make version 3.79 or newer. Older versions have
79 bugs and/or are missing features.
81 ?? Do I need a special linker or assembler?
83 {ZW} If you want a shared library, you need a linker and assembler that
84 understand all the features of ELF, including weak and versioned symbols.
85 The static library can be compiled with less featureful tools, but lacks key
88 For Linux or Hurd, you want binutils 2.8.1.0.23, 2.9.1, or 2.9.1.0.15 or
89 higher. These are the only versions we've tested and found reliable. Other
90 versions after 2.8.1.0.23 may work but we don't recommend them, especially
91 not when C++ is involved. Earlier versions do not work at all.
93 Other operating systems may come with system tools that have all the
94 necessary features, but this is moot because glibc hasn't been ported to
97 ??powerpc Which compiler should I use for powerpc?
99 {GK} You want to use at least gcc 2.95 (together with the right versions
100 of all the other tools, of course). See also question ?excpt.
102 ??arm Which tools should I use for ARM?
104 {PB} You should use egcs 1.1 or a later version. For ELF systems some
105 changes are needed to the compiler; a patch against egcs-1.1.x can be found
108 <ftp://ftp.netwinder.org/users/p/philb/egcs-1.1.1pre2-diff-981126>
110 Binutils 2.9.1.0.16 or later is also required.
112 ?? Do I need some more things to compile the GNU C Library?
114 {UD} Yes, there are some more :-).
116 * GNU gettext. This package contains the tools needed to construct
117 `message catalog' files containing translated versions of system
118 messages. See ftp://ftp.gnu.org/pub/gnu or better any mirror
119 site. (We distribute compiled message catalogs, but they may not be
120 updated in patches.) Please note that the required minimal version
121 (0.10.35) of gettext is alpha software and available from
122 ftp://alpha.gnu.org/gnu .
124 * Some files are built with special tools. E.g., files ending in .gperf
125 need a `gperf' program. The GNU version (now available in a separate
126 package, formerly only as part of libg++) is known to work while some
127 vendor versions do not.
129 You should not need these tools unless you change the source files.
131 * Perl 5 is needed if you wish to test an installation of GNU libc
132 as the primary C library.
134 * When compiling for Linux, the header files of the Linux kernel must
135 be available to the compiler as <linux/*.h> and <asm/*.h>.
137 * lots of disk space (~400MB for i?86-linux; more for RISC platforms).
139 * plenty of time. Compiling just the shared and static libraries for
140 35mins on a 2xPIII@550Mhz w/ 512MB RAM. On a 2xUltraSPARC-II@360Mhz
141 w/ 1GB RAM it takes about 14 minutes. Multiply this by 1.5 or 2.0
142 if you build profiling and/or the highly optimized version as well.
143 For Hurd systems times are much higher.
145 You should avoid compiling in a NFS mounted filesystem. This is
148 James Troup <J.J.Troup@comp.brad.ac.uk> reports a compile time for
149 an earlier (and smaller!) version of glibc of 45h34m for a full build
150 (shared, static, and profiled) on Atari Falcon (Motorola 68030 @ 16 Mhz,
151 14 Mb memory) and Jan Barte <yann@plato.uni-paderborn.de> reports
152 22h48m on Atari TT030 (Motorola 68030 @ 32 Mhz, 34 Mb memory)
154 A full build of the PowerPC library took 1h on a PowerPC 750@400Mhz w/
155 64MB of RAM, and about 9h on a 601@60Mhz w/ 72Mb.
157 If you have some more measurements let me know.
159 ?? What version of the Linux kernel headers should be used?
161 {AJ,UD} The headers from the most recent Linux kernel should be used. The
162 headers used while compiling the GNU C library and the kernel binary used
163 when using the library do not need to match. The GNU C library runs without
164 problems on kernels that are older than the kernel headers used. The other
165 way round (compiling the GNU C library with old kernel headers and running
166 on a recent kernel) does not necessarily work. For example you can't use
167 new kernel features if you used old kernel headers to compile the GNU C
170 {ZW} Even if you are using a 2.0 kernel on your machine, we recommend you
171 compile GNU libc with 2.2 kernel headers. That way you won't have to
172 recompile libc if you ever upgrade to kernel 2.2. To tell libc which
173 headers to use, give configure the --with-headers switch
174 (e.g. --with-headers=/usr/src/linux-2.2.0/include).
176 Note that you must configure the 2.2 kernel if you do this, otherwise libc
177 will be unable to find <linux/version.h>. Just change the current directory
178 to the root of the 2.2 tree and do `make include/linux/version.h'.
180 ?? The compiler hangs while building iconvdata modules. What's
183 {} Removed. Does not apply anymore.
185 ?? When I run `nm -u libc.so' on the produced library I still
186 find unresolved symbols. Can this be ok?
188 {UD} Yes, this is ok. There can be several kinds of unresolved symbols:
190 * magic symbols automatically generated by the linker. These have names
191 like __start_* and __stop_*
193 * symbols starting with _dl_* come from the dynamic linker
195 * weak symbols, which need not be resolved at all (fabs for example)
197 Generally, you should make sure you find a real program which produces
198 errors while linking before deciding there is a problem.
200 ??addon What are these `add-ons'?
202 {UD} To avoid complications with export rules or external source code some
203 optional parts of the libc are distributed as separate packages, e.g., the
204 linuxthreads package.
206 To use these packages as part of GNU libc, just unpack the tarfiles in the
207 libc source directory and tell the configuration script about them using the
208 --enable-add-ons option. If you give just --enable-add-ons configure tries
209 to find all the add-on packages in your source tree. This may not work. If
210 it doesn't, or if you want to select only a subset of the add-ons, give a
211 comma-separated list of the add-ons to enable:
213 configure --enable-add-ons=linuxthreads
217 Add-ons can add features (including entirely new shared libraries), override
218 files, provide support for additional architectures, and just about anything
219 else. The existing makefiles do most of the work; only some few stub rules
220 must be written to get everything running.
222 Most add-ons are tightly coupled to a specific GNU libc version. Please
223 check that the add-ons work with the GNU libc. For example the linuxthreads
224 add-on has the same numbering scheme as the libc and will in general only
225 work with the corresponding libc.
227 {AJ} With glibc 2.2 the crypt add-on and with glibc 2.1 the localedata
228 add-on have been integrated into the normal glibc distribution, crypt and
229 localedata are therefore not anymore add-ons.
231 ?? My XXX kernel emulates a floating-point coprocessor for me.
232 Should I enable --with-fp?
234 {ZW} An emulated FPU is just as good as a real one, as far as the C library
235 is concerned. You only need to say --without-fp if your machine has no way
236 to execute floating-point instructions.
238 People who are interested in squeezing the last drop of performance
239 out of their machine may wish to avoid the trap overhead, but this is
240 far more trouble than it's worth: you then have to compile
241 *everything* this way, including the compiler's internal libraries
242 (libgcc.a for GNU C), because the calling conventions change.
244 ?? When compiling GNU libc I get lots of errors saying functions
245 in glibc are duplicated in libgcc.
247 {EY} This is *exactly* the same problem that I was having. The problem was
248 due to the fact that configure didn't correctly detect that the linker flag
249 --no-whole-archive was supported in my linker. In my case it was because I
250 had run ./configure with bogus CFLAGS, and the test failed.
252 One thing that is particularly annoying about this problem is that once this
253 is misdetected, running configure again won't fix it unless you first delete
256 {UD} Starting with glibc-2.0.3 there should be a better test to avoid some
257 problems of this kind. The setting of CFLAGS is checked at the very
258 beginning and if it is not usable `configure' will bark.
260 ?? Why do I get messages about missing thread functions when I use
261 librt? I don't even use threads.
263 {UD} In this case you probably mixed up your installation. librt uses
264 threads internally and has implicit references to the thread library.
265 Normally these references are satisfied automatically but if the thread
266 library is not in the expected place you must tell the linker where it is.
267 When using GNU ld it works like this:
269 gcc -o foo foo.c -Wl,-rpath-link=/some/other/dir -lrt
271 The `/some/other/dir' should contain the thread library. `ld' will use the
272 given path to find the implicitly referenced library while not disturbing
275 ?? What's the problem with configure --enable-omitfp?
277 {AJ} When --enable-omitfp is set the libraries are built without frame
278 pointers. Some compilers produce buggy code for this model and therefore we
279 don't advise using it at the moment.
281 If you use --enable-omitfp, you're on your own. If you encounter problems
282 with a library that was build this way, we advise you to rebuild the library
283 without --enable-omitfp. If the problem vanishes consider tracking the
284 problem down and report it as compiler failure.
286 Since a library built with --enable-omitfp is undebuggable on most systems,
287 debuggable libraries are also built - you can use them by appending "_g" to
290 The compilation of these extra libraries and the compiler optimizations slow
291 down the build process and need more disk space.
293 ?? I get failures during `make check'. What should I do?
295 {AJ} The testsuite should compile and run cleanly on your system; every
296 failure should be looked into. Depending on the failures, you probably
297 should not install the library at all.
299 You should consider using the `glibcbug' script to report the failure,
300 providing as much detail as possible. If you run a test directly, please
301 remember to set up the environment correctly. You want to test the compiled
302 library - and not your installed one. The best way is to copy the exact
303 command line which failed and run the test from the subdirectory for this
306 There are some failures which are not directly related to the GNU libc:
307 - Some compilers produce buggy code. No compiler gets single precision
308 complex numbers correct on Alpha. Otherwise, the egcs 1.1 release should be
309 ok; gcc 2.8.1 might cause some failures; gcc 2.7.2.x is so buggy that
310 explicit checks have been used so that you can't build with it.
311 - The kernel might have bugs. For example on Linux/Alpha 2.0.34 the
312 floating point handling has quite a number of bugs and therefore most of
313 the test cases in the math subdirectory will fail. Linux 2.2 has
314 fixes for the floating point support on Alpha. The Linux/SPARC kernel has
315 also some bugs in the FPU emulation code (as of Linux 2.2.0).
316 - Other tools might have problems. For example bash 2.03 gives a
317 segmentation fault running the tst-rpmatch.sh test script.
319 ?? What is symbol versioning good for? Do I need it?
321 {AJ} Symbol versioning solves problems that are related to interface
322 changes. One version of an interface might have been introduced in a
323 previous version of the GNU C library but the interface or the semantics of
324 the function has been changed in the meantime. For binary compatibility
325 with the old library, a newer library needs to still have the old interface
326 for old programs. On the other hand, new programs should use the new
327 interface. Symbol versioning is the solution for this problem. The GNU
328 libc version 2.1 uses symbol versioning by default if the installed binutils
331 We don't advise building without symbol versioning, since you lose binary
332 compatibility - forever! The binary compatibility you lose is not only
333 against the previous version of the GNU libc (version 2.0) but also against
336 ?? How can I compile on my fast ix86 machine a working libc for my slow
337 i386? After installing libc, programs abort with "Illegal
340 {AJ} glibc and gcc might generate some instructions on your machine that
341 aren't available on i386. You've got to tell glibc that you're configuring
342 for i386 with adding i386 as your machine, for example:
344 ../configure --prefix=/usr i386-pc-linux-gnu
346 And you need to tell gcc to only generate i386 code, just add `-mcpu=i386'
347 (just -m386 doesn't work) to your CFLAGS.
349 {UD} This applies not only to the i386. Compiling on a i686 for any older
350 model will also fail if the above methods are not used.
352 ?? `make' complains about a missing dlfcn/libdl.so when building
353 malloc/libmemprof.so. How can I fix this?
355 {AJ} Older make version (<= 3.78.90) have a bug which was hidden by a bug in
356 glibc (<= 2.1.2). You need to upgrade make to a newer or fixed version.
358 After upgrading make, you should remove the file sysd-sorted in your build
359 directory. The problem is that the broken make creates a wrong order for
360 one list in that file. The list has to be recreated with the new make -
361 which happens if you remove the file.
363 You might encounter this bug also in other situations where make scans
364 directories. I strongly advise to upgrade your make version to 3.79 or
368 ??mips Which tools should I use for MIPS?
370 {AJ} You should use the current development version of gcc 2.97 from CVS.
371 gcc 2.95.x does not work correctly on mips-linux.
373 You need also recent binutils, anything before and including 2.10 will not
374 work correctly. Either try the Linux binutils 2.10.0.33 from HJ Lu or the
375 current development version from CVS of binutils.
377 For details check also my page <http://www.suse.de/~aj/glibc-mips.html>.
379 ? Installation and configuration issues
381 ?? Can I replace the libc on my Linux system with GNU libc?
383 {UD} You cannot replace any existing libc for Linux with GNU libc. It is
384 binary incompatible and therefore has a different major version. You can,
385 however, install it alongside your existing libc.
387 For Linux there are three major libc versions:
389 libc-5 original ELF libc
392 You can have any combination of these three installed. For more information
393 consult documentation for shared library handling. The Makefiles of GNU
394 libc will automatically generate the needed symbolic links which the linker
397 ?? How do I configure GNU libc so that the essential libraries
398 like libc.so go into /lib and the other into /usr/lib?
400 {UD,AJ} Like all other GNU packages GNU libc is designed to use a base
401 directory and install all files relative to this. The default is
402 /usr/local, because this is safe (it will not damage the system if installed
403 there). If you wish to install GNU libc as the primary C library on your
404 system, set the base directory to /usr (i.e. run configure --prefix=/usr
405 <other_options>). Note that this can damage your system; see ?safety for
408 Some systems like Linux have a filesystem standard which makes a difference
409 between essential libraries and others. Essential libraries are placed in
410 /lib because this directory is required to be located on the same disk
411 partition as /. The /usr subtree might be found on another
412 partition/disk. If you configure for Linux with --prefix=/usr, then this
413 will be done automatically.
415 To install the essential libraries which come with GNU libc in /lib on
416 systems other than Linux one must explicitly request it. Autoconf has no
417 option for this so you have to use a `configparms' file (see the `INSTALL'
418 file for details). It should contain:
423 The first line specifies the directory for the essential libraries, the
424 second line the directory for system configuration files.
426 ??safety How should I avoid damaging my system when I install GNU libc?
428 {ZW} If you wish to be cautious, do not configure with --prefix=/usr. If
429 you don't specify a prefix, glibc will be installed in /usr/local, where it
430 will probably not break anything. (If you wish to be certain, set the
431 prefix to something like /usr/local/glibc2 which is not used for anything.)
433 The dangers when installing glibc in /usr are twofold:
435 * glibc will overwrite the headers in /usr/include. Other C libraries
436 install a different but overlapping set of headers there, so the effect
437 will probably be that you can't compile anything. You need to rename
438 /usr/include out of the way before running `make install'. (Do not throw
439 it away; you will then lose the ability to compile programs against your
442 * None of your old libraries, static or shared, can be used with a
443 different C library major version. For shared libraries this is not a
444 problem, because the filenames are different and the dynamic linker
445 will enforce the restriction. But static libraries have no version
446 information. You have to evacuate all the static libraries in
447 /usr/lib to a safe location.
449 The situation is rather similar to the move from a.out to ELF which
450 long-time Linux users will remember.
452 ?? Do I need to use GNU CC to compile programs that will use the
455 {ZW} In theory, no; the linker does not care, and the headers are supposed
456 to check for GNU CC before using its extensions to the C language.
458 However, there are currently no ports of glibc to systems where another
459 compiler is the default, so no one has tested the headers extensively
460 against another compiler. You may therefore encounter difficulties. If you
461 do, please report them as bugs.
463 Also, in several places GNU extensions provide large benefits in code
464 quality. For example, the library has hand-optimized, inline assembly
465 versions of some string functions. These can only be used with GCC. See
468 ??crypt When linking with the new libc I get unresolved symbols
469 `crypt' and `setkey'. Why aren't these functions in the
473 {} Removed. Does not apply anymore.
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 2.95.2 (or later versions) instead.
631 ?? The `gencat' utility cannot process the catalog sources which
632 were used on my Linux libc5 based system. Why?
634 {UD} The `gencat' utility provided with glibc complies to the XPG standard.
635 The older Linux version did not obey the standard, so they are not
638 To ease the transition from the Linux version some of the non-standard
639 features are also present in the `gencat' program of GNU libc. This mainly
640 includes the use of symbols for the message number and the automatic
641 generation of header files which contain the needed #defines to map the
644 Here is a simple SED script to convert at least some Linux specific catalog
645 files to the XPG4 form:
647 -----------------------------------------------------------------------
648 # Change catalog source in Linux specific format to standard XPG format.
649 # Ulrich Drepper <drepper@cygnus.com>, 1996.
653 s/\$ #\([^ ]*\).*/\1/
655 s/\$ #[^ ]* *\(.*\)/\$ \1/
661 s/\(.*\)\n\(.*\)/\2 \1/
663 -----------------------------------------------------------------------
665 ?? Programs using libc have their messages translated, but other
666 behavior is not localized (e.g. collating order); why?
668 {ZW} Translated messages are automatically installed, but the locale
669 database that controls other behaviors is not. You need to run localedef to
670 install this database, after you have run `make install'. For example, to
671 set up the French Canadian locale, simply issue the command
673 localedef -i fr_CA -f ISO-8859-1 fr_CA
675 Please see localedata/README in the source tree for further details.
677 ?? I have set up /etc/nis.conf, and the Linux libc 5 with NYS
678 works great. But the glibc NIS+ doesn't seem to work.
680 {TK} The glibc NIS+ implementation uses a /var/nis/NIS_COLD_START file for
681 storing information about the NIS+ server and their public keys, because the
682 nis.conf file does not contain all the necessary information. You have to
683 copy a NIS_COLD_START file from a Solaris client (the NIS_COLD_START file is
684 byte order independent) or generate it with nisinit from the nis-tools
685 package; available at
687 http://www.suse.de/~kukuk/linux/nisplus.html
689 ?? I have killed ypbind to stop using NIS, but glibc
692 {TK} For faster NIS lookups, glibc uses the /var/yp/binding/ files from
693 ypbind. ypbind 3.3 and older versions don't always remove these files, so
694 glibc will continue to use them. Other BSD versions seem to work correctly.
695 Until ypbind 3.4 is released, you can find a patch at
697 <ftp://ftp.kernel.org/pub/linux/utils/net/NIS/ypbind-3.3-glibc4.diff.gz>
699 ?? Under Linux/Alpha, I always get "do_ypcall: clnt_call:
700 RPC: Unable to receive; errno = Connection refused" when using NIS.
702 {TK} You need a ypbind version which is 64bit clean. Some versions are not
703 64bit clean. A 64bit clean implementation is ypbind-mt. For ypbind 3.3,
704 you need the patch from ftp.kernel.org (See the previous question). I don't
705 know about other versions.
708 ?? After installing glibc name resolving doesn't work properly.
710 {AJ} You probably should read the manual section describing nsswitch.conf
711 (just type `info libc "NSS Configuration File"'). The NSS configuration
712 file is usually the culprit.
715 ?? How do I create the databases for NSS?
717 {AJ} If you have an entry "db" in /etc/nsswitch.conf you should also create
718 the database files. The glibc sources contain a Makefile which does the
719 necessary conversion and calls to create those files. The file is
720 `db-Makefile' in the subdirectory `nss' and you can call it with `make -f
721 db-Makefile'. Please note that not all services are capable of using a
722 database. Currently passwd, group, ethers, protocol, rpc, services shadow
723 and netgroup are implemented. See also question ?nssdb.
725 ?? I have /usr/include/net and /usr/include/scsi as symlinks
726 into my Linux source tree. Is that wrong?
728 {PB} This was necessary for libc5, but is not correct when using glibc.
729 Including the kernel header files directly in user programs usually does not
730 work (see ?kerhdr). glibc provides its own <net/*> and <scsi/*> header
731 files to replace them, and you may have to remove any symlink that you have
732 in place before you install glibc. However, /usr/include/asm and
733 /usr/include/linux should remain as they were.
735 ?? Programs like `logname', `top', `uptime' `users', `w' and
736 `who', show incorrect information about the (number of)
737 users on my system. Why?
741 ?? After upgrading to glibc 2.1 with symbol versioning I get
742 errors about undefined symbols. What went wrong?
744 {AJ} The problem is caused either by wrong program code or tools. In the
745 versioned libc a lot of symbols are now local that were global symbols in
746 previous versions. It seems that programs linked against older versions
747 often accidentally used libc global variables -- something that should not
750 The only way to fix this is to recompile your program. Sorry, that's the
751 price you might have to pay once for quite a number of advantages with
754 ?? When I start the program XXX after upgrading the library
756 XXX: Symbol `_sys_errlist' has different size in shared
757 object, consider re-linking
758 Why? What should I do?
760 {UD} As the message says, relink the binary. The problem is that a few
761 symbols from the library can change in size and there is no way to avoid
762 this. _sys_errlist is a good example. Occasionally there are new error
763 numbers added to the kernel and this must be reflected at user level,
764 breaking programs that refer to them directly.
766 Such symbols should normally not be used at all. There are mechanisms to
767 avoid using them. In the case of _sys_errlist, there is the strerror()
768 function which should _always_ be used instead. So the correct fix is to
769 rewrite that part of the application.
771 In some situations (especially when testing a new library release) it might
772 be possible that a symbol changed size when that should not have happened.
773 So in case of doubt report such a warning message as a problem.
775 ?? What do I need for C++ development?
777 {HJ,AJ} You need either egcs 1.1 which comes directly with libstdc++ or
778 gcc-2.8.1 together with libstdc++ 2.8.1.1. egcs 1.1 has the better C++
779 support and works directly with glibc 2.1. If you use gcc-2.8.1 with
780 libstdc++ 2.8.1.1, you need to modify libstdc++ a bit. A patch is available
782 <ftp://alpha.gnu.org/gnu/libstdc++-2.8.1.1-glibc2.1-diff.gz>
784 Please note that libg++ 2.7.2 (and the Linux Versions 2.7.2.x) doesn't work
785 very well with the GNU C library due to vtable thunks. If you're upgrading
786 from glibc 2.0.x to 2.1 you have to recompile libstdc++ since the library
787 compiled for 2.0 is not compatible due to the new Large File Support (LFS)
790 {UD} But since in the case of a shared libstdc++ the version numbers should
791 be different existing programs will continue to work.
793 ?? Even statically linked programs need some shared libraries
794 which is not acceptable for me. What can I do?
796 {AJ} NSS (for details just type `info libc "Name Service Switch"') won't
797 work properly without shared libraries. NSS allows using different services
798 (e.g. NIS, files, db, hesiod) by just changing one configuration file
799 (/etc/nsswitch.conf) without relinking any programs. The only disadvantage
800 is that now static libraries need to access shared libraries. This is
801 handled transparently by the GNU C library.
803 A solution is to configure glibc with --enable-static-nss. In this case you
804 can create a static binary that will use only the services dns and files
805 (change /etc/nsswitch.conf for this). You need to link explicitly against
806 all these services. For example:
808 gcc -static test-netdb.c -o test-netdb \
809 -Wl,--start-group -lc -lnss_files -lnss_dns -lresolv -Wl,--end-group
811 The problem with this approach is that you've got to link every static
812 program that uses NSS routines with all those libraries.
814 {UD} In fact, one cannot say anymore that a libc compiled with this
815 option is using NSS. There is no switch anymore. Therefore it is
816 *highly* recommended *not* to use --enable-static-nss since this makes
817 the behaviour of the programs on the system inconsistent.
819 ?? I just upgraded my Linux system to glibc and now I get
820 errors whenever I try to link any program.
822 {ZW} This happens when you have installed glibc as the primary C library but
823 have stray symbolic links pointing at your old C library. If the first
824 `libc.so' the linker finds is libc 5, it will use that. Your program
825 expects to be linked with glibc, so the link fails.
827 The most common case is that glibc put its `libc.so' in /usr/lib, but there
828 was a `libc.so' from libc 5 in /lib, which gets searched first. To fix the
829 problem, just delete /lib/libc.so. You may also need to delete other
830 symbolic links in /lib, such as /lib/libm.so if it points to libm.so.5.
832 {AJ} The perl script test-installation.pl which is run as last step during
833 an installation of glibc that is configured with --prefix=/usr should help
834 detect these situations. If the script reports problems, something is
837 ?? When I use nscd the machine freezes.
839 {UD} You cannot use nscd with Linux 2.0.*. There is functionality missing
840 in the kernel and work-arounds are not suitable. Besides, some parts of the
841 kernel are too buggy when it comes to using threads.
843 If you need nscd, you have to use at least a 2.1 kernel.
845 Note that I have at this point no information about any other platform.
847 ?? I need lots of open files. What do I have to do?
849 {AJ} This is at first a kernel issue. The kernel defines limits with
850 OPEN_MAX the number of simultaneous open files and with FD_SETSIZE the
851 number of used file descriptors. You need to change these values in your
852 kernel and recompile the kernel so that the kernel allows more open
853 files. You don't necessarily need to recompile the GNU C library since the
854 only place where OPEN_MAX and FD_SETSIZE is really needed in the library
855 itself is the size of fd_set which is used by select.
857 The GNU C library is now select free. This means it internally has no
858 limits imposed by the `fd_set' type. Instead all places where the
859 functionality is needed the `poll' function is used.
861 If you increase the number of file descriptors in the kernel you don't need
862 to recompile the C library.
864 {UD} You can always get the maximum number of file descriptors a process is
865 allowed to have open at any time using
867 number = sysconf (_SC_OPEN_MAX);
869 This will work even if the kernel limits change.
871 ?? How do I get the same behavior on parsing /etc/passwd and
872 /etc/group as I have with libc5 ?
874 {TK} The name switch setup in /etc/nsswitch.conf selected by most Linux
875 distributions does not support +/- and netgroup entries in the files like
876 /etc/passwd. Though this is the preferred setup some people might have
877 setups coming over from the libc5 days where it was the default to recognize
878 lines like this. To get back to the old behaviour one simply has to change
879 the rules for passwd, group, and shadow in the nsswitch.conf file as
890 ??libs What needs to be recompiled when upgrading from glibc 2.0 to glibc
893 {AJ,CG} If you just upgrade the glibc from 2.0.x (x <= 7) to 2.1, binaries
894 that have been linked against glibc 2.0 will continue to work.
896 If you compile your own binaries against glibc 2.1, you also need to
897 recompile some other libraries. The problem is that libio had to be changed
898 and therefore libraries that are based or depend on the libio of glibc,
899 e.g. ncurses, slang and most C++ libraries, need to be recompiled. If you
900 experience strange segmentation faults in your programs linked against glibc
901 2.1, you might need to recompile your libraries.
903 Another problem is that older binaries that were linked statically against
904 glibc 2.0 will reference the older nss modules (libnss_files.so.1 instead of
905 libnss_files.so.2), so don't remove them. Also, the old glibc-2.0 compiled
906 static libraries (libfoo.a) which happen to depend on the older libio
907 behavior will be broken by the glibc 2.1 upgrade. We plan to produce a
908 compatibility library that people will be able to link in if they want
909 to compile a static library generated against glibc 2.0 into a program
910 on a glibc 2.1 system. You just add -lcompat and you should be fine.
912 The glibc-compat add-on will provide the libcompat.a library, the older
913 nss modules, and a few other files. Together, they should make it
914 possible to do development with old static libraries on a glibc 2.1
915 system. This add-on is still in development. You can get it from
916 <ftp://alpha.gnu.org/gnu/glibc/glibc-compat-2.1.tar.gz>
917 but please keep in mind that it is experimental.
919 ?? Why is extracting files via tar so slow?
921 {AJ} Extracting of tar archives might be quite slow since tar has to look up
922 userid and groupids and doesn't cache negative results. If you have nis or
923 nisplus in your /etc/nsswitch.conf for the passwd and/or group database,
924 each file extractions needs a network connection. There are two possible
927 - do you really need NIS/NIS+ (some Linux distributions add by default
928 nis/nisplus even if it's not needed)? If not, just remove the entries.
930 - if you need NIS/NIS+, use the Name Service Cache Daemon nscd that comes
933 ?? Compiling programs I get parse errors in libio.h (e.g. "parse error
934 before `_IO_seekoff'"). How should I fix this?
936 {AJ} You might get the following errors when upgrading to glibc 2.1:
938 In file included from /usr/include/stdio.h:57,
940 /usr/include/libio.h:335: parse error before `_IO_seekoff'
941 /usr/include/libio.h:335: parse error before `_G_off64_t'
942 /usr/include/libio.h:336: parse error before `_IO_seekpos'
943 /usr/include/libio.h:336: parse error before `_G_fpos64_t'
945 The problem is a wrong _G_config.h file in your include path. The
946 _G_config.h file that comes with glibc 2.1 should be used and not one from
947 libc5 or from a compiler directory. To check which _G_config.h file the
948 compiler uses, compile your program with `gcc -E ...|grep G_config.h' and
949 remove that file. Your compiler should pick up the file that has been
950 installed by glibc 2.1 in your include directory.
952 ?? After upgrading to glibc 2.1, libraries that were compiled against
953 glibc 2.0.x don't work anymore.
957 ??nssdb What happened to the Berkeley DB libraries? Can I still use db
958 in /etc/nsswitch.conf?
960 {AJ} Due to too many incompatible changes in disk layout and API of Berkeley
961 DB and a too tight coupling of libc and libdb, the db library has been
962 removed completely from glibc 2.2. The only place that really used the
963 Berkeley DB was the NSS db module.
965 The NSS db module has been rewritten to support a number of different
966 versions of Berkeley DB for the NSS db module. Currently the releases 2.x
967 and 3.x of Berkeley DB are supported. The older db 1.85 library is not
968 supported. You can use the version from glibc 2.1.x or download a version
969 from Sleepycat Software (http://www.sleepycat.com). The library has to be
970 compiled as shared library and installed in the system lib directory
971 (normally /lib). The library needs to have a special soname to be found by
974 If public structures change in a new Berkeley db release, this needs to be
977 Currently the code searches for libraries with a soname of "libdb.so.3"
978 (that's the name from db 2.4.14 which comes with glibc 2.1.x) and
979 "libdb-3.0.so" (the name used by db 3.0.55 as default).
981 The nss_db module is now in a separate package since it requires a database
982 library being available.
984 ?? What has do be done when upgrading to glibc 2.2?
986 {AJ} The upgrade to glibc 2.2 should run smoothly, there's in general no
987 need to recompile programs or libraries. Nevertheless, some changes might
988 be needed after upgrading:
989 - The utmp daemon has been removed and is not supported by glibc anymore.
990 If it has been in use, it should be switched off.
991 - Programs using IPv6 have to be recompiled due to incompatible changes in
992 sockaddr_in6 by the IPv6 working group.
993 - The Berkeley db libraries have been removed (for details see ??nssdb).
994 - The format of the locale files has changed, all locales should be
995 regenerated with localedef. All statically linked applications which use
996 i18n should be recompiled, otherwise they'll not be localized.
997 - glibc comes with a number of new applications. For example ldconfig has
998 been implemented for glibc, the libc5 version of ldconfig is not needed
1000 - There's no more K&R compatibility in the glibc headers. The GNU C library
1001 requires a C compiler that handles especially prototypes correctly.
1002 Especially gcc -traditional will not work with glibc headers.
1004 Please read also the NEWS file which is the authoritative source for this
1005 and gives more details for some topics.
1007 ?? The makefiles want to do a CVS commit.
1009 {UD} Only if you are not specifying the --without-cvs flag at configure
1010 time. This is what you always have to use if you are checking sources
1011 directly out of the public CVS repository or you have your own private
1014 ?? When compiling C++ programs, I get a compilation error in streambuf.h.
1016 {BH} You are using g++ 2.95.2? After upgrading to glibc 2.2, you need to
1017 apply a patch to the include files in /usr/include/g++, because the fpos_t
1018 type has changed in glibc 2.2. The patch is at
1019 http://clisp.cons.org/~haible/gccinclude-glibc-2.2-compat.diff
1021 ?? When recompiling GCC, I get compilation errors in libio.
1023 {BH} You are trying to recompile gcc 2.95.2? After upgrading to glibc 2.2,
1024 you need to apply a patch to the gcc sources, because the fpos_t type and
1025 a few libio internals have changed in glibc 2.2. T he patch is at
1026 http://clisp.cons.org/~haible/gcc-glibc-2.2-compat.diff
1029 ? Source and binary incompatibilities, and what to do about them
1031 ?? I expect GNU libc to be 100% source code compatible with
1032 the old Linux based GNU libc. Why isn't it like this?
1034 {DMT,UD} Not every extension in Linux libc's history was well thought-out.
1035 In fact it had a lot of problems with standards compliance and with
1036 cleanliness. With the introduction of a new version number these errors can
1037 now be corrected. Here is a list of the known source code
1040 * _GNU_SOURCE: glibc does not make the GNU extensions available
1041 automatically. If a program depends on GNU extensions or some
1042 other non-standard functionality, it is necessary to compile it
1043 with the C compiler option -D_GNU_SOURCE, or better, to put
1044 `#define _GNU_SOURCE' at the beginning of your source files, before
1045 any C library header files are included. This difference normally
1046 manifests itself in the form of missing prototypes and/or data type
1047 definitions. Thus, if you get such errors, the first thing you
1048 should do is try defining _GNU_SOURCE and see if that makes the
1051 For more information consult the file `NOTES' in the GNU C library
1054 * reboot(): GNU libc sanitizes the interface of reboot() to be more
1055 compatible with the interface used on other OSes. reboot() as
1056 implemented in glibc takes just one argument. This argument
1057 corresponds to the third argument of the Linux reboot system call.
1058 That is, a call of the form reboot(a, b, c) needs to be changed into
1059 reboot(c). Beside this the header <sys/reboot.h> defines the needed
1060 constants for the argument. These RB_* constants should be used
1061 instead of the cryptic magic numbers.
1063 * swapon(): the interface of this function didn't change, but the
1064 prototype is in a separate header file <sys/swap.h>. This header
1065 file also provides the SWAP_* constants defined by <linux/swap.h>;
1066 you should use them for the second argument to swapon().
1068 * errno: If a program uses the variable "errno", then it _must_
1069 include <errno.h>. The old libc often (erroneously) declared this
1070 variable implicitly as a side-effect of including other libc header
1071 files. glibc is careful to avoid such namespace pollution, which,
1072 in turn, means that you really need to include the header files that
1073 you depend on. This difference normally manifests itself in the
1074 form of the compiler complaining about references to an undeclared
1077 * Linux-specific syscalls: All Linux system calls now have appropriate
1078 library wrappers and corresponding declarations in various header files.
1079 This is because the syscall() macro that was traditionally used to
1080 work around missing syscall wrappers are inherently non-portable and
1081 error-prone. The following table lists all the new syscall stubs,
1082 the header-file declaring their interface and the system call name.
1084 syscall name: wrapper name: declaring header file:
1085 ------------- ------------- ----------------------
1086 bdflush bdflush <sys/kdaemon.h>
1087 syslog ksyslog_ctl <sys/klog.h>
1089 * lpd: Older versions of lpd depend on a routine called _validuser().
1090 The library does not provide this function, but instead provides
1091 __ivaliduser() which has a slightly different interface. Simply
1092 upgrading to a newer lpd should fix this problem (e.g., the 4.4BSD
1093 lpd is known to be working).
1095 * resolver functions/BIND: like on many other systems the functions of
1096 the resolver library are not included in libc itself. There is a
1097 separate library libresolv. If you get undefined symbol errors for
1098 symbols starting with `res_*' simply add -lresolv to your linker
1101 * the `signal' function's behavior corresponds to the BSD semantic and
1102 not the SysV semantic as it was in libc-5. The interface on all GNU
1103 systems shall be the same and BSD is the semantic of choice. To use
1104 the SysV behavior simply use `sysv_signal', or define _XOPEN_SOURCE.
1105 See ?signal for details.
1107 ??getlog Why does getlogin() always return NULL on my Linux box?
1109 {UD} The GNU C library has a format for the UTMP and WTMP file which differs
1110 from what your system currently has. It was extended to fulfill the needs
1111 of the next years when IPv6 is introduced. The record size is different and
1112 some fields have different positions. The files written by functions from
1113 the one library cannot be read by functions from the other library. Sorry,
1114 but this is what a major release is for. It's better to have a cut now than
1115 having no means to support the new techniques later.
1117 ?? Where are the DST_* constants found in <sys/time.h> on many
1120 {UD} These constants come from the old BSD days and are not used anymore
1121 (libc5 does not actually implement the handling although the constants are
1124 Instead GNU libc contains zone database support and compatibility code for
1125 POSIX TZ environment variable handling. For former is very much preferred
1128 ?? The prototypes for `connect', `accept', `getsockopt',
1129 `setsockopt', `getsockname', `getpeername', `send',
1130 `sendto', and `recvfrom' are different in GNU libc from
1131 any other system I saw. This is a bug, isn't it?
1133 {UD} No, this is no bug. This version of GNU libc already follows the new
1134 Single Unix specifications (and I think the POSIX.1g draft which adopted the
1135 solution). The type for a parameter describing a size is now `socklen_t', a
1138 ??kerhdr On Linux I've got problems with the declarations in Linux
1141 {UD,AJ} On Linux, the use of kernel headers is reduced to the minimum. This
1142 gives Linus the ability to change the headers more freely. Also, user
1143 programs are now insulated from changes in the size of kernel data
1146 For example, the sigset_t type is 32 or 64 bits wide in the kernel. In
1147 glibc it is 1024 bits wide. This guarantees that when the kernel gets a
1148 bigger sigset_t (for POSIX.1e realtime support, say) user programs will not
1149 have to be recompiled. Consult the header files for more information about
1152 Therefore you shouldn't include Linux kernel header files directly if glibc
1153 has defined a replacement. Otherwise you might get undefined results because
1156 ?? I don't include any kernel headers myself but the compiler
1157 still complains about redeclarations of types in the kernel
1160 {UD} The kernel headers before Linux 2.1.61 and 2.0.32 don't work correctly
1161 with glibc. Compiling C programs is possible in most cases but C++ programs
1162 have (due to the change of the name lookups for `struct's) problems. One
1163 prominent example is `struct fd_set'.
1165 There might be some problems left but 2.1.61/2.0.32 fix most of the known
1166 ones. See the BUGS file for other known problems.
1168 ??signal Why don't signals interrupt system calls anymore?
1170 {ZW} By default GNU libc uses the BSD semantics for signal(), unlike Linux
1171 libc 5 which used System V semantics. This is partially for compatibility
1172 with other systems and partially because the BSD semantics tend to make
1173 programming with signals easier.
1175 There are three differences:
1177 * BSD-style signals that occur in the middle of a system call do not
1178 affect the system call; System V signals cause the system call to
1179 fail and set errno to EINTR.
1181 * BSD signal handlers remain installed once triggered. System V signal
1182 handlers work only once, so one must reinstall them each time.
1184 * A BSD signal is blocked during the execution of its handler. In other
1185 words, a handler for SIGCHLD (for example) does not need to worry about
1186 being interrupted by another SIGCHLD. It may, however, be interrupted
1189 There is general consensus that for `casual' programming with signals, the
1190 BSD semantics are preferable. You don't need to worry about system calls
1191 returning EINTR, and you don't need to worry about the race conditions
1192 associated with one-shot signal handlers.
1194 If you are porting an old program that relies on the old semantics, you can
1195 quickly fix the problem by changing signal() to sysv_signal() throughout.
1196 Alternatively, define _XOPEN_SOURCE before including <signal.h>.
1198 For new programs, the sigaction() function allows you to specify precisely
1199 how you want your signals to behave. All three differences listed above are
1200 individually switchable on a per-signal basis with this function.
1202 If all you want is for one specific signal to cause system calls to fail and
1203 return EINTR (for example, to implement a timeout) you can do this with
1207 ??string I've got errors compiling code that uses certain string
1210 {AJ} glibc 2.1 has special string functions that are faster than the normal
1211 library functions. Some of the functions are additionally implemented as
1212 inline functions and others as macros. This might lead to problems with
1213 existing codes but it is explicitly allowed by ISO C.
1215 The optimized string functions are only used when compiling with
1216 optimizations (-O1 or higher). The behavior can be changed with two feature
1219 * __NO_STRING_INLINES: Don't do any string optimizations.
1220 * __USE_STRING_INLINES: Use assembly language inline functions (might
1221 increase code size dramatically).
1223 Since some of these string functions are now additionally defined as macros,
1224 code like "char *strncpy();" doesn't work anymore (and is unnecessary, since
1225 <string.h> has the necessary declarations). Either change your code or
1226 define __NO_STRING_INLINES.
1228 {UD} Another problem in this area is that gcc still has problems on machines
1229 with very few registers (e.g., ix86). The inline assembler code can require
1230 almost all the registers and the register allocator cannot always handle
1233 One can disable the string optimizations selectively. Instead of writing
1235 cp = strcpy (foo, "lkj");
1239 cp = (strcpy) (foo, "lkj");
1241 This disables the optimization for that specific call.
1243 ?? I get compiler messages "Initializer element not constant" with
1244 stdin/stdout/stderr. Why?
1246 {RM,AJ} Constructs like:
1247 static FILE *InPtr = stdin;
1249 lead to this message. This is correct behaviour with glibc since stdin is
1250 not a constant expression. Please note that a strict reading of ISO C does
1251 not allow above constructs.
1253 One of the advantages of this is that you can assign to stdin, stdout, and
1254 stderr just like any other global variable (e.g. `stdout = my_stream;'),
1255 which can be very useful with custom streams that you can write with libio
1256 (but beware this is not necessarily portable). The reason to implement it
1257 this way were versioning problems with the size of the FILE structure.
1259 To fix those programs you've got to initialize the variable at run time.
1260 This can be done, e.g. in main, like:
1268 or by constructors (beware this is gcc specific):
1271 static void inPtr_construct (void) __attribute__((constructor));
1272 static void inPtr_construct (void) { InPtr = stdin; }
1275 ?? I can't compile with gcc -traditional (or
1276 -traditional-cpp). Why?
1278 {AJ} glibc2 does break -traditional and -traditonal-cpp - and will continue
1279 to do so. For example constructs of the form:
1285 are useful for debugging purposes (you can use foo with your debugger that's
1286 why we need the enum) and for compatibility (other systems use defines and
1289 ?? I get some errors with `gcc -ansi'. Isn't glibc ANSI compatible?
1291 {AJ} The GNU C library is compatible with the ANSI/ISO C standard. If
1292 you're using `gcc -ansi', the glibc includes which are specified in the
1293 standard follow the standard. The ANSI/ISO C standard defines what has to be
1294 in the include files - and also states that nothing else should be in the
1295 include files (btw. you can still enable additional standards with feature
1298 The GNU C library is conforming to ANSI/ISO C - if and only if you're only
1299 using the headers and library functions defined in the standard.
1301 ?? I can't access some functions anymore. nm shows that they do
1302 exist but linking fails nevertheless.
1304 {AJ} With the introduction of versioning in glibc 2.1 it is possible to
1305 export only those identifiers (functions, variables) that are really needed
1306 by application programs and by other parts of glibc. This way a lot of
1307 internal interfaces are now hidden. nm will still show those identifiers
1308 but marking them as internal. ISO C states that identifiers beginning with
1309 an underscore are internal to the libc. An application program normally
1310 shouldn't use those internal interfaces (there are exceptions,
1311 e.g. __ivaliduser). If a program uses these interfaces, it's broken. These
1312 internal interfaces might change between glibc releases or dropped
1315 ?? When using the db-2 library which comes with glibc is used in
1316 the Perl db modules the testsuite is not passed. This did not
1317 happen with db-1, gdbm, or ndbm.
1319 {} Removed. Does not apply anymore.
1321 ?? The pow() inline function I get when including <math.h> is broken.
1322 I get segmentation faults when I run the program.
1324 {UD} Nope, the implementation is correct. The problem is with egcs version
1325 prior to 1.1. I.e., egcs 1.0 to 1.0.3 are all broken (at least on Intel).
1326 If you have to use this compiler you must define __NO_MATH_INLINES before
1327 including <math.h> to prevent the inline functions from being used. egcs 1.1
1328 fixes the problem. I don't know about gcc 2.8 and 2.8.1.
1330 ?? The sys/sem.h file lacks the definition of `union semun'.
1332 {UD} Nope. This union has to be provided by the user program. Former glibc
1333 versions defined this but it was an error since it does not make much sense
1334 when thinking about it. The standards describing the System V IPC functions
1335 define it this way and therefore programs must be adopted.
1337 ?? Why has <netinet/ip_fw.h> disappeared?
1339 {AJ} The corresponding Linux kernel data structures and constants are
1340 totally different in Linux 2.0 and Linux 2.2. This situation has to be
1341 taken care in user programs using the firewall structures and therefore
1342 those programs (ipfw is AFAIK the only one) should deal with this problem
1345 ?? I get floods of warnings when I use -Wconversion and include
1346 <string.h> or <math.h>.
1348 {ZW} <string.h> and <math.h> intentionally use prototypes to override
1349 argument promotion. -Wconversion warns about all these. You can safely
1350 ignore the warnings.
1352 -Wconversion isn't really intended for production use, only for shakedown
1353 compiles after converting an old program to standard C.
1356 ?? After upgrading to glibc 2.1, I receive errors about
1357 unresolved symbols, like `_dl_initial_searchlist' and can not
1358 execute any binaries. What went wrong?
1360 {AJ} This normally happens if your libc and ld (dynamic linker) are from
1361 different releases of glibc. For example, the dynamic linker
1362 /lib/ld-linux.so.2 comes from glibc 2.0.x, but the version of libc.so.6 is
1365 The path /lib/ld-linux.so.2 is hardcoded in every glibc2 binary but
1366 libc.so.6 is searched via /etc/ld.so.cache and in some special directories
1367 like /lib and /usr/lib. If you run configure with another prefix than /usr
1368 and put this prefix before /lib in /etc/ld.so.conf, your system will break.
1370 So what can you do? Either of the following should work:
1372 * Run `configure' with the same prefix argument you've used for glibc 2.0.x
1373 so that the same paths are used.
1374 * Replace /lib/ld-linux.so.2 with a link to the dynamic linker from glibc
1377 You can even call the dynamic linker by hand if everything fails. You've
1378 got to set LD_LIBRARY_PATH so that the corresponding libc is found and also
1379 need to provide an absolute path to your binary:
1381 LD_LIBRARY_PATH=<path-where-libc.so.6-lives> \
1382 <path-where-corresponding-dynamic-linker-lives>/ld-linux.so.2 \
1383 <path-to-binary>/binary
1385 For example `LD_LIBRARY_PATH=/libold /libold/ld-linux.so.2 /bin/mv ...'
1386 might be useful in fixing a broken system (if /libold contains dynamic
1387 linker and corresponding libc).
1389 With that command line no path is used. To further debug problems with the
1390 dynamic linker, use the LD_DEBUG environment variable, e.g.
1391 `LD_DEBUG=help echo' for the help text.
1393 If you just want to test this release, don't put the lib directory in
1394 /etc/ld.so.conf. You can call programs directly with full paths (as above).
1395 When compiling new programs against glibc 2.1, you've got to specify the
1396 correct paths to the compiler (option -I with gcc) and linker (options
1397 --dynamic-linker, -L and --rpath).
1399 ?? bonnie reports that char i/o with glibc 2 is much slower than with
1400 libc5. What can be done?
1402 {AJ} The GNU C library uses thread safe functions by default and libc5 used
1403 non thread safe versions. The non thread safe functions have in glibc the
1404 suffix `_unlocked', for details check <stdio.h>. Using `putc_unlocked' etc.
1405 instead of `putc' should give nearly the same speed with bonnie (bonnie is a
1406 benchmark program for measuring disk access).
1408 ?? Programs compiled with glibc 2.1 can't read db files made with glibc
1409 2.0. What has changed that programs like rpm break?
1411 {} Removed. Does not apply anymore.
1413 ?? Autoconf's AC_CHECK_FUNC macro reports that a function exists, but
1414 when I try to use it, it always returns -1 and sets errno to ENOSYS.
1416 {ZW} You are using a 2.0 Linux kernel, and the function you are trying to
1417 use is only implemented in 2.1/2.2. Libc considers this to be a function
1418 which exists, because if you upgrade to a 2.2 kernel, it will work. One
1419 such function is sigaltstack.
1421 Your program should check at runtime whether the function works, and
1422 implement a fallback. Note that Autoconf cannot detect unimplemented
1423 functions in other systems' C libraries, so you need to do this anyway.
1425 ?? My program segfaults when I call fclose() on the FILE* returned
1426 from setmntent(). Is this a glibc bug?
1428 {GK} No. Don't do this. Use endmntent(), that's what it's for.
1430 In general, you should use the correct deallocation routine. For instance,
1431 if you open a file using fopen(), you should deallocate the FILE * using
1432 fclose(), not free(), even though the FILE * is also a pointer.
1434 In the case of setmntent(), it may appear to work in most cases, but it
1435 won't always work. Unfortunately, for compatibility reasons, we can't
1436 change the return type of setmntent() to something other than FILE *.
1441 ?? After I changed configure.in I get `Autoconf version X.Y.
1442 or higher is required for this script'. What can I do?
1444 {UD} You have to get the specified autoconf version (or a later one)
1445 from your favorite mirror of ftp.gnu.org.
1447 ?? When I try to compile code which uses IPv6 headers and
1448 definitions on my Linux 2.x.y system I am in trouble.
1449 Nothing seems to work.
1451 {UD} The problem is that IPv6 development still has not reached a point
1452 where the headers are stable. There are still lots of incompatible changes
1453 made and the libc headers have to follow.
1455 {PB} The 2.1 release of GNU libc aims to comply with the current versions of
1456 all the relevant standards. The IPv6 support libraries for older Linux
1457 systems used a different naming convention and so code written to work with
1458 them may need to be modified. If the standards make incompatible changes in
1459 the future then the libc may need to change again.
1461 IPv6 will not work with a 2.0.x kernel. When kernel 2.2 is released it
1462 should contain all the necessary support; until then you should use the
1463 latest 2.1.x release you can find. As of 98/11/26 the currently recommended
1464 kernel for IPv6 is 2.1.129.
1466 Also, as of the 2.1 release the IPv6 API provided by GNU libc is not
1469 ??tzdb When I set the timezone by setting the TZ environment variable
1470 to EST5EDT things go wrong since glibc computes the wrong time
1471 from this information.
1473 {UD} The problem is that people still use the braindamaged POSIX method to
1474 select the timezone using the TZ environment variable with a format EST5EDT
1475 or whatever. People, if you insist on using TZ instead of the timezone
1476 database (see below), read the POSIX standard, the implemented behaviour is
1477 correct! What you see is in fact the result of the decisions made while
1478 POSIX.1 was created. We've only implemented the handling of TZ this way to
1479 be POSIX compliant. It is not really meant to be used.
1481 The alternative approach to handle timezones which is implemented is the
1482 correct one to use: use the timezone database. This avoids all the problems
1483 the POSIX method has plus it is much easier to use. Simply run the tzselect
1484 shell script, answer the question and use the name printed in the end by
1485 making a symlink /etc/localtime pointing to /usr/share/zoneinfo/NAME (NAME
1486 is the returned value from tzselect). That's all. You never again have to
1489 So, please avoid sending bug reports about time related problems if you use
1490 the POSIX method and you have not verified something is really broken by
1491 reading the POSIX standards.
1493 ?? What other sources of documentation about glibc are available?
1495 {AJ} The FSF has a page about the GNU C library at
1496 <http://www.gnu.org/software/libc/>. The problem data base of open and
1497 solved bugs in GNU libc is available at
1498 <http://www-gnats.gnu.org:8080/cgi-bin/wwwgnats.pl>. Eric Green has written
1499 a HowTo for converting from Linux libc5 to glibc2. The HowTo is accessible
1500 via the FSF page and at <http://www.imaxx.net/~thrytis/glibc>. Frodo
1501 Looijaard describes a different way installing glibc2 as secondary libc at
1502 <http://huizen.dds.nl/~frodol/glibc>.
1504 Please note that this is not a complete list.
1506 ?? The timezone string for Sydney/Australia is wrong since even when
1507 daylight saving time is in effect the timezone string is EST.
1509 {UD} The problem for some timezones is that the local authorities decided
1510 to use the term "summer time" instead of "daylight saving time". In this
1511 case the abbreviation character `S' is the same as the standard one. So,
1514 Eastern Standard Time = EST
1515 Eastern Summer Time = EST
1517 Great! To get this bug fixed convince the authorities to change the laws
1518 and regulations of the country this effects. glibc behaves correctly.
1520 ??make I've build make 3.77 against glibc 2.1 and now make gets
1521 segmentation faults.
1523 {} Removed. Does not apply anymore, use make 3.79 or newer.
1525 ?? Why do so many programs using math functions fail on my AlphaStation?
1527 {AO} The functions floor() and floorf() use an instruction that is not
1528 implemented in some old PALcodes of AlphaStations. This may cause
1529 `Illegal Instruction' core dumps or endless loops in programs that
1530 catch these signals. Updating the firmware to a 1999 release has
1531 fixed the problem on an AlphaStation 200 4/166.
1533 ?? The conversion table for character set XX does not match with
1536 {UD} I don't doubt for a minute that some of the conversion tables contain
1537 errors. We tried the best we can and relied on automatic generation of the
1538 data to prevent human-introduced errors but this still is no guarantee. If
1539 you think you found a problem please send a bug report describing it and
1540 give an authoritive reference. The latter is important since otherwise
1541 the current behaviour is as good as the proposed one.
1543 Before doing this look through the list of known problem first:
1545 - the GBK (simplified Chinese) encoding is based on Unicode tables. This
1546 is good. These tables, however, differ slightly from the tables used
1547 by the M$ people. The differences are these [+ Unicode, - M$]:
1554 In addition the Unicode tables contain mappings for the GBK characters
1555 0xA8BC, 0xA8BF, 0xA989 to 0xA995, and 0xFE50 to 0xFEA0.
1557 - when mapping from EUC-CN to GBK and vice versa we ignore the fact that
1558 the coded character at position 0xA1A4 maps to different Unicode
1559 characters. Since the iconv() implementation can do whatever it wants
1560 if it cannot directly map a character this is a perfectly good solution
1561 since the semantics and appearance of the character does not change.
1563 ?? How can I find out which version of glibc I am using in the moment?
1565 {UD} If you want to find out about the version from the command line simply
1566 run the libc binary. This is probably not possible on all platforms but
1567 where it is simply locate the libc DSO and start it as an application. On
1572 This will produce all the information you need.
1574 What always will work is to use the API glibc provides. Compile and run the
1575 following little program to get the version information:
1577 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1579 #include <gnu/libc-version.h>
1580 int main (void) { puts (gnu_get_libc_version ()); return 0; }
1581 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1583 This interface can also obviously be used to perform tests at runtime if
1584 this should be necessary.
1587 Answers were given by:
1588 {UD} Ulrich Drepper, <drepper@cygnus.com>
1589 {DMT} David Mosberger-Tang, <davidm@AZStarNet.com>
1590 {RM} Roland McGrath, <roland@gnu.org>
1591 {AJ} Andreas Jaeger, <aj@suse.de>
1592 {EY} Eric Youngdale, <eric@andante.jic.com>
1593 {PB} Phil Blundell, <Philip.Blundell@pobox.com>
1594 {MK} Mark Kettenis, <kettenis@phys.uva.nl>
1595 {ZW} Zack Weinberg, <zack@rabi.phys.columbia.edu>
1596 {TK} Thorsten Kukuk, <kukuk@suse.de>
1597 {GK} Geoffrey Keating, <geoffk@ozemail.com.au>
1598 {HJ} H.J. Lu, <hjl@gnu.org>
1599 {CG} Cristian Gafton, <gafton@redhat.com>
1600 {AO} Alexandre Oliva, <oliva@lsd.ic.unicamp.br>
1601 {BH} Bruno Haible, <haible@clisp.cons.org>
1605 outline-regexp:"\\?"
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