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 egcs (1.0.3 and 1.1.1) should work with the GNU C library (for powerpc see
67 ?powerpc; for ARM see ?arm; for MIPS see ?mips).
69 While the GNU CC should be able to compile glibc it is nevertheless adviced
70 to use EGCS. Comparing the sizes of glibc on Intel compiled with a recent
71 EGCS and gcc 2.8.1 shows this:
73 text data bss dec hex filename
74 egcs-2.93.10 862897 15944 12824 891665 d9b11 libc.so
75 gcc-2.8.1 959965 16468 12152 988585 f15a9 libc.so
77 Make up your own decision.
79 GNU CC versions 2.95 and above are derived from egcs, and they may do even
82 Please note that gcc 2.95 and 2.95.x cannot compile glibc on Alpha due to
83 problems in the complex float support.
85 ?? When I try to compile glibc I get only error messages.
88 {UD} You definitely need GNU make to build GNU libc. No other make
89 program has the needed functionality.
91 We recommend version GNU make version 3.79 or newer. Older versions have
92 bugs and/or are missing features.
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 linuxthreads package.
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=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 linuxthreads
241 add-on has the same numbering scheme as the libc and will in general only
242 work with the corresponding libc.
244 {AJ} With glibc 2.2 the crypt add-on and with glibc 2.1 the localedata
245 add-on have been integrated into the normal glibc distribution, crypt and
246 localedata are therefore not anymore add-ons.
248 ?? My XXX kernel emulates a floating-point coprocessor for me.
249 Should I enable --with-fp?
251 {ZW} An emulated FPU is just as good as a real one, as far as the C library
252 is concerned. You only need to say --without-fp if your machine has no way
253 to execute floating-point instructions.
255 People who are interested in squeezing the last drop of performance
256 out of their machine may wish to avoid the trap overhead, but this is
257 far more trouble than it's worth: you then have to compile
258 *everything* this way, including the compiler's internal libraries
259 (libgcc.a for GNU C), because the calling conventions change.
261 ?? When compiling GNU libc I get lots of errors saying functions
262 in glibc are duplicated in libgcc.
264 {EY} This is *exactly* the same problem that I was having. The problem was
265 due to the fact that configure didn't correctly detect that the linker flag
266 --no-whole-archive was supported in my linker. In my case it was because I
267 had run ./configure with bogus CFLAGS, and the test failed.
269 One thing that is particularly annoying about this problem is that once this
270 is misdetected, running configure again won't fix it unless you first delete
273 {UD} Starting with glibc-2.0.3 there should be a better test to avoid some
274 problems of this kind. The setting of CFLAGS is checked at the very
275 beginning and if it is not usable `configure' will bark.
277 ?? Why do I get messages about missing thread functions when I use
278 librt? I don't even use threads.
280 {UD} In this case you probably mixed up your installation. librt uses
281 threads internally and has implicit references to the thread library.
282 Normally these references are satisfied automatically but if the thread
283 library is not in the expected place you must tell the linker where it is.
284 When using GNU ld it works like this:
286 gcc -o foo foo.c -Wl,-rpath-link=/some/other/dir -lrt
288 The `/some/other/dir' should contain the thread library. `ld' will use the
289 given path to find the implicitly referenced library while not disturbing
292 ?? What's the problem with configure --enable-omitfp?
294 {AJ} When --enable-omitfp is set the libraries are built without frame
295 pointers. Some compilers produce buggy code for this model and therefore we
296 don't advise using it at the moment.
298 If you use --enable-omitfp, you're on your own. If you encounter problems
299 with a library that was build this way, we advise you to rebuild the library
300 without --enable-omitfp. If the problem vanishes consider tracking the
301 problem down and report it as compiler failure.
303 Since a library built with --enable-omitfp is undebuggable on most systems,
304 debuggable libraries are also built - you can use them by appending "_g" to
307 The compilation of these extra libraries and the compiler optimizations slow
308 down the build process and need more disk space.
310 ?? I get failures during `make check'. What should I do?
312 {AJ} The testsuite should compile and run cleanly on your system; every
313 failure should be looked into. Depending on the failures, you probably
314 should not install the library at all.
316 You should consider using the `glibcbug' script to report the failure,
317 providing as much detail as possible. If you run a test directly, please
318 remember to set up the environment correctly. You want to test the compiled
319 library - and not your installed one. The best way is to copy the exact
320 command line which failed and run the test from the subdirectory for this
323 There are some failures which are not directly related to the GNU libc:
324 - Some compilers produce buggy code. No compiler gets single precision
325 complex numbers correct on Alpha. Otherwise, the egcs 1.1 release should be
326 ok; gcc 2.8.1 might cause some failures; gcc 2.7.2.x is so buggy that
327 explicit checks have been used so that you can't build with it.
328 - The kernel might have bugs. For example on Linux/Alpha 2.0.34 the
329 floating point handling has quite a number of bugs and therefore most of
330 the test cases in the math subdirectory will fail. Linux 2.2 has
331 fixes for the floating point support on Alpha. The Linux/SPARC kernel has
332 also some bugs in the FPU emulation code (as of Linux 2.2.0).
333 - Other tools might have problems. For example bash 2.03 gives a
334 segmentation fault running the tst-rpmatch.sh test script.
336 ?? What is symbol versioning good for? Do I need it?
338 {AJ} Symbol versioning solves problems that are related to interface
339 changes. One version of an interface might have been introduced in a
340 previous version of the GNU C library but the interface or the semantics of
341 the function has been changed in the meantime. For binary compatibility
342 with the old library, a newer library needs to still have the old interface
343 for old programs. On the other hand, new programs should use the new
344 interface. Symbol versioning is the solution for this problem. The GNU
345 libc version 2.1 uses symbol versioning by default if the installed binutils
348 We don't advise building without symbol versioning, since you lose binary
349 compatibility - forever! The binary compatibility you lose is not only
350 against the previous version of the GNU libc (version 2.0) but also against
353 ?? How can I compile on my fast ix86 machine a working libc for my slow
354 i386? After installing libc, programs abort with "Illegal
357 {AJ} glibc and gcc might generate some instructions on your machine that
358 aren't available on i386. You've got to tell glibc that you're configuring
359 for i386 with adding i386 as your machine, for example:
361 ../configure --prefix=/usr i386-pc-linux-gnu
363 And you need to tell gcc to only generate i386 code, just add `-mcpu=i386'
364 (just -m386 doesn't work) to your CFLAGS.
366 {UD} This applies not only to the i386. Compiling on a i686 for any older
367 model will also fail if the above methods are not used.
369 ?? `make' complains about a missing dlfcn/libdl.so when building
370 malloc/libmemprof.so. How can I fix this?
372 {AJ} Older make version (<= 3.78.90) have a bug which was hidden by a bug in
373 glibc (<= 2.1.2). You need to upgrade make to a newer or fixed version.
375 After upgrading make, you should remove the file sysd-sorted in your build
376 directory. The problem is that the broken make creates a wrong order for
377 one list in that file. The list has to be recreated with the new make -
378 which happens if you remove the file.
380 You might encounter this bug also in other situations where make scans
381 directories. I strongly advise to upgrade your make version to 3.79 or
385 ??mips Which tools should I use for MIPS?
387 {AJ} You should use the current development version of gcc 2.97 from CVS.
388 gcc 2.95.x does not work correctly on mips-linux.
390 You need also recent binutils, anything before and including 2.10 will not
391 work correctly. Either try the Linux binutils 2.10.0.33 from HJ Lu or the
392 current development version from CVS of binutils.
394 For details check also my page <http://www.suse.de/~aj/glibc-mips.html>.
396 ? Installation and configuration issues
398 ?? Can I replace the libc on my Linux system with GNU libc?
400 {UD} You cannot replace any existing libc for Linux with GNU libc. It is
401 binary incompatible and therefore has a different major version. You can,
402 however, install it alongside your existing libc.
404 For Linux there are three major libc versions:
406 libc-5 original ELF libc
409 You can have any combination of these three installed. For more information
410 consult documentation for shared library handling. The Makefiles of GNU
411 libc will automatically generate the needed symbolic links which the linker
414 ?? How do I configure GNU libc so that the essential libraries
415 like libc.so go into /lib and the other into /usr/lib?
417 {UD,AJ} Like all other GNU packages GNU libc is designed to use a base
418 directory and install all files relative to this. The default is
419 /usr/local, because this is safe (it will not damage the system if installed
420 there). If you wish to install GNU libc as the primary C library on your
421 system, set the base directory to /usr (i.e. run configure --prefix=/usr
422 <other_options>). Note that this can damage your system; see ?safety for
425 Some systems like Linux have a filesystem standard which makes a difference
426 between essential libraries and others. Essential libraries are placed in
427 /lib because this directory is required to be located on the same disk
428 partition as /. The /usr subtree might be found on another
429 partition/disk. If you configure for Linux with --prefix=/usr, then this
430 will be done automatically.
432 To install the essential libraries which come with GNU libc in /lib on
433 systems other than Linux one must explicitly request it. Autoconf has no
434 option for this so you have to use a `configparms' file (see the `INSTALL'
435 file for details). It should contain:
440 The first line specifies the directory for the essential libraries, the
441 second line the directory for system configuration files.
443 ??safety How should I avoid damaging my system when I install GNU libc?
445 {ZW} If you wish to be cautious, do not configure with --prefix=/usr. If
446 you don't specify a prefix, glibc will be installed in /usr/local, where it
447 will probably not break anything. (If you wish to be certain, set the
448 prefix to something like /usr/local/glibc2 which is not used for anything.)
450 The dangers when installing glibc in /usr are twofold:
452 * glibc will overwrite the headers in /usr/include. Other C libraries
453 install a different but overlapping set of headers there, so the effect
454 will probably be that you can't compile anything. You need to rename
455 /usr/include out of the way before running `make install'. (Do not throw
456 it away; you will then lose the ability to compile programs against your
459 * None of your old libraries, static or shared, can be used with a
460 different C library major version. For shared libraries this is not a
461 problem, because the filenames are different and the dynamic linker
462 will enforce the restriction. But static libraries have no version
463 information. You have to evacuate all the static libraries in
464 /usr/lib to a safe location.
466 The situation is rather similar to the move from a.out to ELF which
467 long-time Linux users will remember.
469 ?? Do I need to use GNU CC to compile programs that will use the
472 {ZW} In theory, no; the linker does not care, and the headers are supposed
473 to check for GNU CC before using its extensions to the C language.
475 However, there are currently no ports of glibc to systems where another
476 compiler is the default, so no one has tested the headers extensively
477 against another compiler. You may therefore encounter difficulties. If you
478 do, please report them as bugs.
480 Also, in several places GNU extensions provide large benefits in code
481 quality. For example, the library has hand-optimized, inline assembly
482 versions of some string functions. These can only be used with GCC. See
485 ??crypt When linking with the new libc I get unresolved symbols
486 `crypt' and `setkey'. Why aren't these functions in the
490 Removed. Does not apply anymore.
492 ?? When I use GNU libc on my Linux system by linking against
493 the libc.so which comes with glibc all I get is a core dump.
495 {UD} On Linux, gcc sets the dynamic linker to /lib/ld-linux.so.1 unless the
496 user specifies a --dynamic-linker argument. This is the name of the libc5
497 dynamic linker, which does not work with glibc.
499 For casual use of GNU libc you can just specify to the linker
500 --dynamic-linker=/lib/ld-linux.so.2
502 which is the glibc dynamic linker, on Linux systems. On other systems the
503 name is /lib/ld.so.1. When linking via gcc, you've got to add
504 -Wl,--dynamic-linker=/lib/ld-linux.so.2
506 to the gcc command line.
508 To change your environment to use GNU libc for compiling you need to change
509 the `specs' file of your gcc. This file is normally found at
511 /usr/lib/gcc-lib/<arch>/<version>/specs
513 In this file you have to change a few things:
515 - change `ld-linux.so.1' to `ld-linux.so.2'
517 - remove all expression `%{...:-lgmon}'; there is no libgmon in glibc
519 - fix a minor bug by changing %{pipe:-} to %|
521 Here is what the gcc-2.7.2 specs file should look like when GNU libc is
524 -----------------------------------------------------------------------
526 %{V} %{v:%{!V:-V}} %{Qy:} %{!Qn:-Qy} %{n} %{T} %{Ym,*} %{Yd,*} %{Wa,*:%*}
532 %{fPIC:-D__PIC__ -D__pic__} %{fpic:-D__PIC__ -D__pic__} %{!m386:-D__i486__} %{posix:-D_POSIX_SOURCE} %{pthread:-D_REENTRANT}
541 %{!shared:crtend.o%s} %{shared:crtendS.o%s} crtn.o%s
544 -m elf_i386 %{shared:-shared} %{!shared: %{!ibcs: %{!static: %{rdynamic:-export-dynamic} %{!dynamic-linker:-dynamic-linker /lib/ld-linux.so.2}} %{static:-static}}}
547 %{!shared: %{pthread:-lpthread} %{profile:-lc_p} %{!profile: -lc}}
553 %{!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}
555 *switches_need_spaces:
559 %{funsigned-char:-D__CHAR_UNSIGNED__}
562 -D__ELF__ -Dunix -Di386 -Dlinux -Asystem(unix) -Asystem(posix) -Acpu(i386) -Amachine(i386)
570 -----------------------------------------------------------------------
572 Things get a bit more complicated if you have GNU libc installed in some
573 other place than /usr, i.e., if you do not want to use it instead of the old
574 libc. In this case the needed startup files and libraries are not found in
575 the regular places. So the specs file must tell the compiler and linker
578 Version 2.7.2.3 does and future versions of GCC will automatically
579 provide the correct specs.
581 ?? Looking through the shared libc file I haven't found the
582 functions `stat', `lstat', `fstat', and `mknod' and while
583 linking on my Linux system I get error messages. How is
584 this supposed to work?
586 {RM} Believe it or not, stat and lstat (and fstat, and mknod) are supposed
587 to be undefined references in libc.so.6! Your problem is probably a missing
588 or incorrect /usr/lib/libc.so file; note that this is a small text file now,
589 not a symlink to libc.so.6. It should look something like this:
591 GROUP ( libc.so.6 libc_nonshared.a )
593 ??excpt When I run an executable on one system which I compiled on
594 another, I get dynamic linker errors. Both systems have the same
595 version of glibc installed. What's wrong?
597 {ZW} Glibc on one of these systems was compiled with gcc 2.7 or 2.8, the
598 other with egcs (any version). Egcs has functions in its internal
599 `libgcc.a' to support exception handling with C++. They are linked into
600 any program or dynamic library compiled with egcs, whether it needs them or
601 not. Dynamic libraries then turn around and export those functions again
602 unless special steps are taken to prevent them.
604 When you link your program, it resolves its references to the exception
605 functions to the ones exported accidentally by libc.so. That works fine as
606 long as libc has those functions. On the other system, libc doesn't have
607 those functions because it was compiled by gcc 2.8, and you get undefined
608 symbol errors. The symbols in question are named things like
609 `__register_frame_info'.
611 For glibc 2.0, the workaround is to not compile libc with egcs. We've also
612 incorporated a patch which should prevent the EH functions sneaking into
613 libc. It doesn't matter what compiler you use to compile your program.
615 For glibc 2.1, we've chosen to do it the other way around: libc.so
616 explicitly provides the EH functions. This is to prevent other shared
617 libraries from doing it.
619 {UD} Starting with glibc 2.1.1 you can compile glibc with gcc 2.8.1 or
620 newer since we have explicitly add references to the functions causing the
621 problem. But you nevertheless should use EGCS for other reasons
624 {GK} On some Linux distributions for PowerPC, you can see this when you have
625 built gcc or egcs from the Web sources (gcc versions 2.95 or earlier), then
626 re-built glibc. This happens because in these versions of gcc, exception
627 handling is implemented using an older method; the people making the
628 distributions are a little ahead of their time.
630 A quick solution to this is to find the libgcc.a file that came with the
631 distribution (it would have been installed under /usr/lib/gcc-lib), do
632 `ar x libgcc.a frame.o' to get the frame.o file out, and add a line saying
633 `LDLIBS-c.so += frame.o' to the file `configparms' in the directory you're
634 building in. You can check you've got the right `frame.o' file by running
635 `nm frame.o' and checking that it has the symbols defined that you're
638 This will let you build glibc with the C compiler. The C++ compiler
639 will still be binary incompatible with any C++ shared libraries that
640 you got with your distribution.
642 ?? How can I compile gcc 2.7.2.1 from the gcc source code using
645 {AJ} There's only correct support for glibc 2.0.x in gcc 2.7.2.3 or later.
646 But you should get at least gcc 2.8.1 or egcs 1.1 (or later versions)
649 ?? The `gencat' utility cannot process the catalog sources which
650 were used on my Linux libc5 based system. Why?
652 {UD} The `gencat' utility provided with glibc complies to the XPG standard.
653 The older Linux version did not obey the standard, so they are not
656 To ease the transition from the Linux version some of the non-standard
657 features are also present in the `gencat' program of GNU libc. This mainly
658 includes the use of symbols for the message number and the automatic
659 generation of header files which contain the needed #defines to map the
662 Here is a simple SED script to convert at least some Linux specific catalog
663 files to the XPG4 form:
665 -----------------------------------------------------------------------
666 # Change catalog source in Linux specific format to standard XPG format.
667 # Ulrich Drepper <drepper@cygnus.com>, 1996.
671 s/\$ #\([^ ]*\).*/\1/
673 s/\$ #[^ ]* *\(.*\)/\$ \1/
679 s/\(.*\)\n\(.*\)/\2 \1/
681 -----------------------------------------------------------------------
683 ?? Programs using libc have their messages translated, but other
684 behavior is not localized (e.g. collating order); why?
686 {ZW} Translated messages are automatically installed, but the locale
687 database that controls other behaviors is not. You need to run localedef to
688 install this database, after you have run `make install'. For example, to
689 set up the French Canadian locale, simply issue the command
691 localedef -i fr_CA -f ISO-8859-1 fr_CA
693 Please see localedata/README in the source tree for further details.
695 ?? I have set up /etc/nis.conf, and the Linux libc 5 with NYS
696 works great. But the glibc NIS+ doesn't seem to work.
698 {TK} The glibc NIS+ implementation uses a /var/nis/NIS_COLD_START file for
699 storing information about the NIS+ server and their public keys, because the
700 nis.conf file does not contain all the necessary information. You have to
701 copy a NIS_COLD_START file from a Solaris client (the NIS_COLD_START file is
702 byte order independent) or generate it with nisinit from the nis-tools
703 package; available at
705 http://www.suse.de/~kukuk/linux/nisplus.html
707 ?? I have killed ypbind to stop using NIS, but glibc
710 {TK} For faster NIS lookups, glibc uses the /var/yp/binding/ files from
711 ypbind. ypbind 3.3 and older versions don't always remove these files, so
712 glibc will continue to use them. Other BSD versions seem to work correctly.
713 Until ypbind 3.4 is released, you can find a patch at
715 <ftp://ftp.kernel.org/pub/linux/utils/net/NIS/ypbind-3.3-glibc4.diff.gz>
717 ?? Under Linux/Alpha, I always get "do_ypcall: clnt_call:
718 RPC: Unable to receive; errno = Connection refused" when using NIS.
720 {TK} You need a ypbind version which is 64bit clean. Some versions are not
721 64bit clean. A 64bit clean implementation is ypbind-mt. For ypbind 3.3,
722 you need the patch from ftp.kernel.org (See the previous question). I don't
723 know about other versions.
726 ?? After installing glibc name resolving doesn't work properly.
728 {AJ} You probably should read the manual section describing nsswitch.conf
729 (just type `info libc "NSS Configuration File"'). The NSS configuration
730 file is usually the culprit.
733 ?? How do I create the databases for NSS?
735 {AJ} If you have an entry "db" in /etc/nsswitch.conf you should also create
736 the database files. The glibc sources contain a Makefile which does the
737 necessary conversion and calls to create those files. The file is
738 `db-Makefile' in the subdirectory `nss' and you can call it with `make -f
739 db-Makefile'. Please note that not all services are capable of using a
740 database. Currently passwd, group, ethers, protocol, rpc, services shadow
741 and netgroup are implemented. See also question ?nssdb.
743 ?? I have /usr/include/net and /usr/include/scsi as symlinks
744 into my Linux source tree. Is that wrong?
746 {PB} This was necessary for libc5, but is not correct when using glibc.
747 Including the kernel header files directly in user programs usually does not
748 work (see ?kerhdr). glibc provides its own <net/*> and <scsi/*> header
749 files to replace them, and you may have to remove any symlink that you have
750 in place before you install glibc. However, /usr/include/asm and
751 /usr/include/linux should remain as they were.
753 ?? Programs like `logname', `top', `uptime' `users', `w' and
754 `who', show incorrect information about the (number of)
755 users on my system. Why?
759 ?? After upgrading to glibc 2.1 with symbol versioning I get
760 errors about undefined symbols. What went wrong?
762 {AJ} The problem is caused either by wrong program code or tools. In the
763 versioned libc a lot of symbols are now local that were global symbols in
764 previous versions. It seems that programs linked against older versions
765 often accidentally used libc global variables -- something that should not
768 The only way to fix this is to recompile your program. Sorry, that's the
769 price you might have to pay once for quite a number of advantages with
772 ?? When I start the program XXX after upgrading the library
774 XXX: Symbol `_sys_errlist' has different size in shared
775 object, consider re-linking
776 Why? What should I do?
778 {UD} As the message says, relink the binary. The problem is that a few
779 symbols from the library can change in size and there is no way to avoid
780 this. _sys_errlist is a good example. Occasionally there are new error
781 numbers added to the kernel and this must be reflected at user level,
782 breaking programs that refer to them directly.
784 Such symbols should normally not be used at all. There are mechanisms to
785 avoid using them. In the case of _sys_errlist, there is the strerror()
786 function which should _always_ be used instead. So the correct fix is to
787 rewrite that part of the application.
789 In some situations (especially when testing a new library release) it might
790 be possible that a symbol changed size when that should not have happened.
791 So in case of doubt report such a warning message as a problem.
793 ?? What do I need for C++ development?
795 {HJ,AJ} You need either egcs 1.1 which comes directly with libstdc++ or
796 gcc-2.8.1 together with libstdc++ 2.8.1.1. egcs 1.1 has the better C++
797 support and works directly with glibc 2.1. If you use gcc-2.8.1 with
798 libstdc++ 2.8.1.1, you need to modify libstdc++ a bit. A patch is available
800 <ftp://alpha.gnu.org/gnu/libstdc++-2.8.1.1-glibc2.1-diff.gz>
802 Please note that libg++ 2.7.2 (and the Linux Versions 2.7.2.x) doesn't work
803 very well with the GNU C library due to vtable thunks. If you're upgrading
804 from glibc 2.0.x to 2.1 you have to recompile libstdc++ since the library
805 compiled for 2.0 is not compatible due to the new Large File Support (LFS)
808 {UD} But since in the case of a shared libstdc++ the version numbers should
809 be different existing programs will continue to work.
811 ?? Even statically linked programs need some shared libraries
812 which is not acceptable for me. What can I do?
814 {AJ} NSS (for details just type `info libc "Name Service Switch"') won't
815 work properly without shared libraries. NSS allows using different services
816 (e.g. NIS, files, db, hesiod) by just changing one configuration file
817 (/etc/nsswitch.conf) without relinking any programs. The only disadvantage
818 is that now static libraries need to access shared libraries. This is
819 handled transparently by the GNU C library.
821 A solution is to configure glibc with --enable-static-nss. In this case you
822 can create a static binary that will use only the services dns and files
823 (change /etc/nsswitch.conf for this). You need to link explicitly against
824 all these services. For example:
826 gcc -static test-netdb.c -o test-netdb \
827 -Wl,--start-group -lc -lnss_files -lnss_dns -lresolv -Wl,--end-group
829 The problem with this approach is that you've got to link every static
830 program that uses NSS routines with all those libraries.
832 {UD} In fact, one cannot say anymore that a libc compiled with this
833 option is using NSS. There is no switch anymore. Therefore it is
834 *highly* recommended *not* to use --enable-static-nss since this makes
835 the behaviour of the programs on the system inconsistent.
837 ?? I just upgraded my Linux system to glibc and now I get
838 errors whenever I try to link any program.
840 {ZW} This happens when you have installed glibc as the primary C library but
841 have stray symbolic links pointing at your old C library. If the first
842 `libc.so' the linker finds is libc 5, it will use that. Your program
843 expects to be linked with glibc, so the link fails.
845 The most common case is that glibc put its `libc.so' in /usr/lib, but there
846 was a `libc.so' from libc 5 in /lib, which gets searched first. To fix the
847 problem, just delete /lib/libc.so. You may also need to delete other
848 symbolic links in /lib, such as /lib/libm.so if it points to libm.so.5.
850 {AJ} The perl script test-installation.pl which is run as last step during
851 an installation of glibc that is configured with --prefix=/usr should help
852 detect these situations. If the script reports problems, something is
855 ?? When I use nscd the machine freezes.
857 {UD} You cannot use nscd with Linux 2.0.*. There is functionality missing
858 in the kernel and work-arounds are not suitable. Besides, some parts of the
859 kernel are too buggy when it comes to using threads.
861 If you need nscd, you have to use at least a 2.1 kernel.
863 Note that I have at this point no information about any other platform.
865 ?? I need lots of open files. What do I have to do?
867 {AJ} This is at first a kernel issue. The kernel defines limits with
868 OPEN_MAX the number of simultaneous open files and with FD_SETSIZE the
869 number of used file descriptors. You need to change these values in your
870 kernel and recompile the kernel so that the kernel allows more open
871 files. You don't necessarily need to recompile the GNU C library since the
872 only place where OPEN_MAX and FD_SETSIZE is really needed in the library
873 itself is the size of fd_set which is used by select.
875 The GNU C library is now select free. This means it internally has no
876 limits imposed by the `fd_set' type. Instead all places where the
877 functionality is needed the `poll' function is used.
879 If you increase the number of file descriptors in the kernel you don't need
880 to recompile the C library.
882 {UD} You can always get the maximum number of file descriptors a process is
883 allowed to have open at any time using
885 number = sysconf (_SC_OPEN_MAX);
887 This will work even if the kernel limits change.
889 ?? How do I get the same behavior on parsing /etc/passwd and
890 /etc/group as I have with libc5 ?
892 {TK} The name switch setup in /etc/nsswitch.conf selected by most Linux
893 distributions does not support +/- and netgroup entries in the files like
894 /etc/passwd. Though this is the preferred setup some people might have
895 setups coming over from the libc5 days where it was the default to recognize
896 lines like this. To get back to the old behaviour one simply has to change
897 the rules for passwd, group, and shadow in the nsswitch.conf file as
908 ??libs What needs to be recompiled when upgrading from glibc 2.0 to glibc
911 {AJ,CG} If you just upgrade the glibc from 2.0.x (x <= 7) to 2.1, binaries
912 that have been linked against glibc 2.0 will continue to work.
914 If you compile your own binaries against glibc 2.1, you also need to
915 recompile some other libraries. The problem is that libio had to be changed
916 and therefore libraries that are based or depend on the libio of glibc,
917 e.g. ncurses, slang and most C++ libraries, need to be recompiled. If you
918 experience strange segmentation faults in your programs linked against glibc
919 2.1, you might need to recompile your libraries.
921 Another problem is that older binaries that were linked statically against
922 glibc 2.0 will reference the older nss modules (libnss_files.so.1 instead of
923 libnss_files.so.2), so don't remove them. Also, the old glibc-2.0 compiled
924 static libraries (libfoo.a) which happen to depend on the older libio
925 behavior will be broken by the glibc 2.1 upgrade. We plan to produce a
926 compatibility library that people will be able to link in if they want
927 to compile a static library generated against glibc 2.0 into a program
928 on a glibc 2.1 system. You just add -lcompat and you should be fine.
930 The glibc-compat add-on will provide the libcompat.a library, the older
931 nss modules, and a few other files. Together, they should make it
932 possible to do development with old static libraries on a glibc 2.1
933 system. This add-on is still in development. You can get it from
934 <ftp://alpha.gnu.org/gnu/glibc/glibc-compat-2.1.tar.gz>
935 but please keep in mind that it is experimental.
937 ?? Why is extracting files via tar so slow?
939 {AJ} Extracting of tar archives might be quite slow since tar has to look up
940 userid and groupids and doesn't cache negative results. If you have nis or
941 nisplus in your /etc/nsswitch.conf for the passwd and/or group database,
942 each file extractions needs a network connection. There are two possible
945 - do you really need NIS/NIS+ (some Linux distributions add by default
946 nis/nisplus even if it's not needed)? If not, just remove the entries.
948 - if you need NIS/NIS+, use the Name Service Cache Daemon nscd that comes
951 ?? Compiling programs I get parse errors in libio.h (e.g. "parse error
952 before `_IO_seekoff'"). How should I fix this?
954 {AJ} You might get the following errors when upgrading to glibc 2.1:
956 In file included from /usr/include/stdio.h:57,
958 /usr/include/libio.h:335: parse error before `_IO_seekoff'
959 /usr/include/libio.h:335: parse error before `_G_off64_t'
960 /usr/include/libio.h:336: parse error before `_IO_seekpos'
961 /usr/include/libio.h:336: parse error before `_G_fpos64_t'
963 The problem is a wrong _G_config.h file in your include path. The
964 _G_config.h file that comes with glibc 2.1 should be used and not one from
965 libc5 or from a compiler directory. To check which _G_config.h file the
966 compiler uses, compile your program with `gcc -E ...|grep G_config.h' and
967 remove that file. Your compiler should pick up the file that has been
968 installed by glibc 2.1 in your include directory.
970 ?? After upgrading to glibc 2.1, libraries that were compiled against
971 glibc 2.0.x don't work anymore.
975 ??nssdb What happened to the Berkeley DB libraries? Can I still use db
976 in /etc/nsswitch.conf?
978 {AJ} Due to too many incompatible changes in disk layout and API of Berkeley
979 DB and a too tight coupling of libc and libdb, the db library has been
980 removed completely from glibc 2.2. The only place that really used the
981 Berkeley DB was the NSS db module.
983 The NSS db module has been rewritten to support a number of different
984 versions of Berkeley DB for the NSS db module. Currently the releases 2.x
985 and 3.x of Berkeley DB are supported. The older db 1.85 library is not
986 supported. You can use the version from glibc 2.1.x or download a version
987 from Sleepycat Software (http://www.sleepycat.com). The library has to be
988 compiled as shared library and installed in the system lib directory
989 (normally /lib). The library needs to have a special soname to be found by
992 If public structures change in a new Berkeley db release, this needs to be
995 Currently the code searches for libraries with a soname of "libdb.so.3"
996 (that's the name from db 2.4.14 which comes with glibc 2.1.x) and
997 "libdb-3.0.so" (the name used by db 3.0.55 as default).
999 The nss_db module is now in a separate package since it requires a database
1000 library being available.
1002 ?? What has do be done when upgrading to glibc 2.2?
1004 {AJ} The upgrade to glibc 2.2 should run smoothly, there's in general no
1005 need to recompile programs or libraries. Nevertheless, some changes might
1006 be needed after upgrading:
1007 - The utmp daemon has been removed and is not supported by glibc anymore.
1008 If it has been in use, it should be switched off.
1009 - Programs using IPv6 have to be recompiled due to incompatible changes in
1010 sockaddr_in6 by the IPv6 working group.
1011 - The Berkeley db libraries have been removed (for details see ??nssdb).
1012 - The format of the locale files has changed, all locales should be
1013 regenerated with localedef. All statically linked applications which use
1014 i18n should be recompiled, otherwise they'll not be localized.
1015 - glibc comes with a number of new applications. For example ldconfig has
1016 been implemented for glibc, the libc5 version of ldconfig is not needed
1018 - There's no more K&R compatibility in the glibc headers. The GNU C library
1019 requires a C compiler that handles especially prototypes correctly.
1020 Especially gcc -traditional will not work with glibc headers.
1022 Please read also the NEWS file which is the authoritative source for this
1023 and gives more details for some topics.
1026 ?? The makefiles want to do a CVS commit.
1028 {UD} Only if you are not specifying the --without-cvs flag at configure
1029 time. This is what you always have to use if you are checking sources
1030 directly out of the public CVS repository or you have your own private
1034 ? Source and binary incompatibilities, and what to do about them
1036 ?? I expect GNU libc to be 100% source code compatible with
1037 the old Linux based GNU libc. Why isn't it like this?
1039 {DMT,UD} Not every extension in Linux libc's history was well thought-out.
1040 In fact it had a lot of problems with standards compliance and with
1041 cleanliness. With the introduction of a new version number these errors can
1042 now be corrected. Here is a list of the known source code
1045 * _GNU_SOURCE: glibc does not make the GNU extensions available
1046 automatically. If a program depends on GNU extensions or some
1047 other non-standard functionality, it is necessary to compile it
1048 with the C compiler option -D_GNU_SOURCE, or better, to put
1049 `#define _GNU_SOURCE' at the beginning of your source files, before
1050 any C library header files are included. This difference normally
1051 manifests itself in the form of missing prototypes and/or data type
1052 definitions. Thus, if you get such errors, the first thing you
1053 should do is try defining _GNU_SOURCE and see if that makes the
1056 For more information consult the file `NOTES' in the GNU C library
1059 * reboot(): GNU libc sanitizes the interface of reboot() to be more
1060 compatible with the interface used on other OSes. reboot() as
1061 implemented in glibc takes just one argument. This argument
1062 corresponds to the third argument of the Linux reboot system call.
1063 That is, a call of the form reboot(a, b, c) needs to be changed into
1064 reboot(c). Beside this the header <sys/reboot.h> defines the needed
1065 constants for the argument. These RB_* constants should be used
1066 instead of the cryptic magic numbers.
1068 * swapon(): the interface of this function didn't change, but the
1069 prototype is in a separate header file <sys/swap.h>. This header
1070 file also provides the SWAP_* constants defined by <linux/swap.h>;
1071 you should use them for the second argument to swapon().
1073 * errno: If a program uses the variable "errno", then it _must_
1074 include <errno.h>. The old libc often (erroneously) declared this
1075 variable implicitly as a side-effect of including other libc header
1076 files. glibc is careful to avoid such namespace pollution, which,
1077 in turn, means that you really need to include the header files that
1078 you depend on. This difference normally manifests itself in the
1079 form of the compiler complaining about references to an undeclared
1082 * Linux-specific syscalls: All Linux system calls now have appropriate
1083 library wrappers and corresponding declarations in various header files.
1084 This is because the syscall() macro that was traditionally used to
1085 work around missing syscall wrappers are inherently non-portable and
1086 error-prone. The following table lists all the new syscall stubs,
1087 the header-file declaring their interface and the system call name.
1089 syscall name: wrapper name: declaring header file:
1090 ------------- ------------- ----------------------
1091 bdflush bdflush <sys/kdaemon.h>
1092 syslog ksyslog_ctl <sys/klog.h>
1094 * lpd: Older versions of lpd depend on a routine called _validuser().
1095 The library does not provide this function, but instead provides
1096 __ivaliduser() which has a slightly different interface. Simply
1097 upgrading to a newer lpd should fix this problem (e.g., the 4.4BSD
1098 lpd is known to be working).
1100 * resolver functions/BIND: like on many other systems the functions of
1101 the resolver library are not included in libc itself. There is a
1102 separate library libresolv. If you get undefined symbol errors for
1103 symbols starting with `res_*' simply add -lresolv to your linker
1106 * the `signal' function's behavior corresponds to the BSD semantic and
1107 not the SysV semantic as it was in libc-5. The interface on all GNU
1108 systems shall be the same and BSD is the semantic of choice. To use
1109 the SysV behavior simply use `sysv_signal', or define _XOPEN_SOURCE.
1110 See ?signal for details.
1112 ??getlog Why does getlogin() always return NULL on my Linux box?
1114 {UD} The GNU C library has a format for the UTMP and WTMP file which differs
1115 from what your system currently has. It was extended to fulfill the needs
1116 of the next years when IPv6 is introduced. The record size is different and
1117 some fields have different positions. The files written by functions from
1118 the one library cannot be read by functions from the other library. Sorry,
1119 but this is what a major release is for. It's better to have a cut now than
1120 having no means to support the new techniques later.
1122 ?? Where are the DST_* constants found in <sys/time.h> on many
1125 {UD} These constants come from the old BSD days and are not used anymore
1126 (libc5 does not actually implement the handling although the constants are
1129 Instead GNU libc contains zone database support and compatibility code for
1130 POSIX TZ environment variable handling. For former is very much preferred
1133 ?? The prototypes for `connect', `accept', `getsockopt',
1134 `setsockopt', `getsockname', `getpeername', `send',
1135 `sendto', and `recvfrom' are different in GNU libc from
1136 any other system I saw. This is a bug, isn't it?
1138 {UD} No, this is no bug. This version of GNU libc already follows the new
1139 Single Unix specifications (and I think the POSIX.1g draft which adopted the
1140 solution). The type for a parameter describing a size is now `socklen_t', a
1143 ??kerhdr On Linux I've got problems with the declarations in Linux
1146 {UD,AJ} On Linux, the use of kernel headers is reduced to the minimum. This
1147 gives Linus the ability to change the headers more freely. Also, user
1148 programs are now insulated from changes in the size of kernel data
1151 For example, the sigset_t type is 32 or 64 bits wide in the kernel. In
1152 glibc it is 1024 bits wide. This guarantees that when the kernel gets a
1153 bigger sigset_t (for POSIX.1e realtime support, say) user programs will not
1154 have to be recompiled. Consult the header files for more information about
1157 Therefore you shouldn't include Linux kernel header files directly if glibc
1158 has defined a replacement. Otherwise you might get undefined results because
1161 ?? I don't include any kernel headers myself but the compiler
1162 still complains about redeclarations of types in the kernel
1165 {UD} The kernel headers before Linux 2.1.61 and 2.0.32 don't work correctly
1166 with glibc. Compiling C programs is possible in most cases but C++ programs
1167 have (due to the change of the name lookups for `struct's) problems. One
1168 prominent example is `struct fd_set'.
1170 There might be some problems left but 2.1.61/2.0.32 fix most of the known
1171 ones. See the BUGS file for other known problems.
1173 ??signal Why don't signals interrupt system calls anymore?
1175 {ZW} By default GNU libc uses the BSD semantics for signal(), unlike Linux
1176 libc 5 which used System V semantics. This is partially for compatibility
1177 with other systems and partially because the BSD semantics tend to make
1178 programming with signals easier.
1180 There are three differences:
1182 * BSD-style signals that occur in the middle of a system call do not
1183 affect the system call; System V signals cause the system call to
1184 fail and set errno to EINTR.
1186 * BSD signal handlers remain installed once triggered. System V signal
1187 handlers work only once, so one must reinstall them each time.
1189 * A BSD signal is blocked during the execution of its handler. In other
1190 words, a handler for SIGCHLD (for example) does not need to worry about
1191 being interrupted by another SIGCHLD. It may, however, be interrupted
1194 There is general consensus that for `casual' programming with signals, the
1195 BSD semantics are preferable. You don't need to worry about system calls
1196 returning EINTR, and you don't need to worry about the race conditions
1197 associated with one-shot signal handlers.
1199 If you are porting an old program that relies on the old semantics, you can
1200 quickly fix the problem by changing signal() to sysv_signal() throughout.
1201 Alternatively, define _XOPEN_SOURCE before including <signal.h>.
1203 For new programs, the sigaction() function allows you to specify precisely
1204 how you want your signals to behave. All three differences listed above are
1205 individually switchable on a per-signal basis with this function.
1207 If all you want is for one specific signal to cause system calls to fail and
1208 return EINTR (for example, to implement a timeout) you can do this with
1212 ??string I've got errors compiling code that uses certain string
1215 {AJ} glibc 2.1 has special string functions that are faster than the normal
1216 library functions. Some of the functions are additionally implemented as
1217 inline functions and others as macros. This might lead to problems with
1218 existing codes but it is explicitly allowed by ISO C.
1220 The optimized string functions are only used when compiling with
1221 optimizations (-O1 or higher). The behavior can be changed with two feature
1224 * __NO_STRING_INLINES: Don't do any string optimizations.
1225 * __USE_STRING_INLINES: Use assembly language inline functions (might
1226 increase code size dramatically).
1228 Since some of these string functions are now additionally defined as macros,
1229 code like "char *strncpy();" doesn't work anymore (and is unnecessary, since
1230 <string.h> has the necessary declarations). Either change your code or
1231 define __NO_STRING_INLINES.
1233 {UD} Another problem in this area is that gcc still has problems on machines
1234 with very few registers (e.g., ix86). The inline assembler code can require
1235 almost all the registers and the register allocator cannot always handle
1238 One can disable the string optimizations selectively. Instead of writing
1240 cp = strcpy (foo, "lkj");
1244 cp = (strcpy) (foo, "lkj");
1246 This disables the optimization for that specific call.
1248 ?? I get compiler messages "Initializer element not constant" with
1249 stdin/stdout/stderr. Why?
1251 {RM,AJ} Constructs like:
1252 static FILE *InPtr = stdin;
1254 lead to this message. This is correct behaviour with glibc since stdin is
1255 not a constant expression. Please note that a strict reading of ISO C does
1256 not allow above constructs.
1258 One of the advantages of this is that you can assign to stdin, stdout, and
1259 stderr just like any other global variable (e.g. `stdout = my_stream;'),
1260 which can be very useful with custom streams that you can write with libio
1261 (but beware this is not necessarily portable). The reason to implement it
1262 this way were versioning problems with the size of the FILE structure.
1264 To fix those programs you've got to initialize the variable at run time.
1265 This can be done, e.g. in main, like:
1273 or by constructors (beware this is gcc specific):
1276 static void inPtr_construct (void) __attribute__((constructor));
1277 static void inPtr_construct (void) { InPtr = stdin; }
1280 ?? I can't compile with gcc -traditional (or
1281 -traditional-cpp). Why?
1283 {AJ} glibc2 does break -traditional and -traditonal-cpp - and will continue
1284 to do so. For example constructs of the form:
1290 are useful for debugging purposes (you can use foo with your debugger that's
1291 why we need the enum) and for compatibility (other systems use defines and
1294 ?? I get some errors with `gcc -ansi'. Isn't glibc ANSI compatible?
1296 {AJ} The GNU C library is compatible with the ANSI/ISO C standard. If
1297 you're using `gcc -ansi', the glibc includes which are specified in the
1298 standard follow the standard. The ANSI/ISO C standard defines what has to be
1299 in the include files - and also states that nothing else should be in the
1300 include files (btw. you can still enable additional standards with feature
1303 The GNU C library is conforming to ANSI/ISO C - if and only if you're only
1304 using the headers and library functions defined in the standard.
1306 ?? I can't access some functions anymore. nm shows that they do
1307 exist but linking fails nevertheless.
1309 {AJ} With the introduction of versioning in glibc 2.1 it is possible to
1310 export only those identifiers (functions, variables) that are really needed
1311 by application programs and by other parts of glibc. This way a lot of
1312 internal interfaces are now hidden. nm will still show those identifiers
1313 but marking them as internal. ISO C states that identifiers beginning with
1314 an underscore are internal to the libc. An application program normally
1315 shouldn't use those internal interfaces (there are exceptions,
1316 e.g. __ivaliduser). If a program uses these interfaces, it's broken. These
1317 internal interfaces might change between glibc releases or dropped
1320 ?? When using the db-2 library which comes with glibc is used in
1321 the Perl db modules the testsuite is not passed. This did not
1322 happen with db-1, gdbm, or ndbm.
1324 Removed. Does not apply anymore.
1326 ?? The pow() inline function I get when including <math.h> is broken.
1327 I get segmentation faults when I run the program.
1329 {UD} Nope, the implementation is correct. The problem is with egcs version
1330 prior to 1.1. I.e., egcs 1.0 to 1.0.3 are all broken (at least on Intel).
1331 If you have to use this compiler you must define __NO_MATH_INLINES before
1332 including <math.h> to prevent the inline functions from being used. egcs 1.1
1333 fixes the problem. I don't know about gcc 2.8 and 2.8.1.
1335 ?? The sys/sem.h file lacks the definition of `union semun'.
1337 {UD} Nope. This union has to be provided by the user program. Former glibc
1338 versions defined this but it was an error since it does not make much sense
1339 when thinking about it. The standards describing the System V IPC functions
1340 define it this way and therefore programs must be adopted.
1342 ?? Why has <netinet/ip_fw.h> disappeared?
1344 {AJ} The corresponding Linux kernel data structures and constants are
1345 totally different in Linux 2.0 and Linux 2.2. This situation has to be
1346 taken care in user programs using the firewall structures and therefore
1347 those programs (ipfw is AFAIK the only one) should deal with this problem
1350 ?? I get floods of warnings when I use -Wconversion and include
1351 <string.h> or <math.h>.
1353 {ZW} <string.h> and <math.h> intentionally use prototypes to override
1354 argument promotion. -Wconversion warns about all these. You can safely
1355 ignore the warnings.
1357 -Wconversion isn't really intended for production use, only for shakedown
1358 compiles after converting an old program to standard C.
1361 ?? After upgrading to glibc 2.1, I receive errors about
1362 unresolved symbols, like `_dl_initial_searchlist' and can not
1363 execute any binaries. What went wrong?
1365 {AJ} This normally happens if your libc and ld (dynamic linker) are from
1366 different releases of glibc. For example, the dynamic linker
1367 /lib/ld-linux.so.2 comes from glibc 2.0.x, but the version of libc.so.6 is
1370 The path /lib/ld-linux.so.2 is hardcoded in every glibc2 binary but
1371 libc.so.6 is searched via /etc/ld.so.cache and in some special directories
1372 like /lib and /usr/lib. If you run configure with another prefix than /usr
1373 and put this prefix before /lib in /etc/ld.so.conf, your system will break.
1375 So what can you do? Either of the following should work:
1377 * Run `configure' with the same prefix argument you've used for glibc 2.0.x
1378 so that the same paths are used.
1379 * Replace /lib/ld-linux.so.2 with a link to the dynamic linker from glibc
1382 You can even call the dynamic linker by hand if everything fails. You've
1383 got to set LD_LIBRARY_PATH so that the corresponding libc is found and also
1384 need to provide an absolute path to your binary:
1386 LD_LIBRARY_PATH=<path-where-libc.so.6-lives> \
1387 <path-where-corresponding-dynamic-linker-lives>/ld-linux.so.2 \
1388 <path-to-binary>/binary
1390 For example `LD_LIBRARY_PATH=/libold /libold/ld-linux.so.2 /bin/mv ...'
1391 might be useful in fixing a broken system (if /libold contains dynamic
1392 linker and corresponding libc).
1394 With that command line no path is used. To further debug problems with the
1395 dynamic linker, use the LD_DEBUG environment variable, e.g.
1396 `LD_DEBUG=help echo' for the help text.
1398 If you just want to test this release, don't put the lib directory in
1399 /etc/ld.so.conf. You can call programs directly with full paths (as above).
1400 When compiling new programs against glibc 2.1, you've got to specify the
1401 correct paths to the compiler (option -I with gcc) and linker (options
1402 --dynamic-linker, -L and --rpath).
1404 ?? bonnie reports that char i/o with glibc 2 is much slower than with
1405 libc5. What can be done?
1407 {AJ} The GNU C library uses thread safe functions by default and libc5 used
1408 non thread safe versions. The non thread safe functions have in glibc the
1409 suffix `_unlocked', for details check <stdio.h>. Using `putc_unlocked' etc.
1410 instead of `putc' should give nearly the same speed with bonnie (bonnie is a
1411 benchmark program for measuring disk access).
1413 ?? Programs compiled with glibc 2.1 can't read db files made with glibc
1414 2.0. What has changed that programs like rpm break?
1416 Removed. Does not apply anymore.
1418 ?? Autoconf's AC_CHECK_FUNC macro reports that a function exists, but
1419 when I try to use it, it always returns -1 and sets errno to ENOSYS.
1421 {ZW} You are using a 2.0 Linux kernel, and the function you are trying to
1422 use is only implemented in 2.1/2.2. Libc considers this to be a function
1423 which exists, because if you upgrade to a 2.2 kernel, it will work. One
1424 such function is sigaltstack.
1426 Your program should check at runtime whether the function works, and
1427 implement a fallback. Note that Autoconf cannot detect unimplemented
1428 functions in other systems' C libraries, so you need to do this anyway.
1430 ?? My program segfaults when I call fclose() on the FILE* returned
1431 from setmntent(). Is this a glibc bug?
1433 {GK} No. Don't do this. Use endmntent(), that's what it's for.
1435 In general, you should use the correct deallocation routine. For instance,
1436 if you open a file using fopen(), you should deallocate the FILE * using
1437 fclose(), not free(), even though the FILE * is also a pointer.
1439 In the case of setmntent(), it may appear to work in most cases, but it
1440 won't always work. Unfortunately, for compatibility reasons, we can't
1441 change the return type of setmntent() to something other than FILE *.
1446 ?? After I changed configure.in I get `Autoconf version X.Y.
1447 or higher is required for this script'. What can I do?
1449 {UD} You have to get the specified autoconf version (or a later one)
1450 from your favorite mirror of ftp.gnu.org.
1452 ?? When I try to compile code which uses IPv6 headers and
1453 definitions on my Linux 2.x.y system I am in trouble.
1454 Nothing seems to work.
1456 {UD} The problem is that IPv6 development still has not reached a point
1457 where the headers are stable. There are still lots of incompatible changes
1458 made and the libc headers have to follow.
1460 {PB} The 2.1 release of GNU libc aims to comply with the current versions of
1461 all the relevant standards. The IPv6 support libraries for older Linux
1462 systems used a different naming convention and so code written to work with
1463 them may need to be modified. If the standards make incompatible changes in
1464 the future then the libc may need to change again.
1466 IPv6 will not work with a 2.0.x kernel. When kernel 2.2 is released it
1467 should contain all the necessary support; until then you should use the
1468 latest 2.1.x release you can find. As of 98/11/26 the currently recommended
1469 kernel for IPv6 is 2.1.129.
1471 Also, as of the 2.1 release the IPv6 API provided by GNU libc is not
1474 ??tzdb When I set the timezone by setting the TZ environment variable
1475 to EST5EDT things go wrong since glibc computes the wrong time
1476 from this information.
1478 {UD} The problem is that people still use the braindamaged POSIX method to
1479 select the timezone using the TZ environment variable with a format EST5EDT
1480 or whatever. People, if you insist on using TZ instead of the timezone
1481 database (see below), read the POSIX standard, the implemented behaviour is
1482 correct! What you see is in fact the result of the decisions made while
1483 POSIX.1 was created. We've only implemented the handling of TZ this way to
1484 be POSIX compliant. It is not really meant to be used.
1486 The alternative approach to handle timezones which is implemented is the
1487 correct one to use: use the timezone database. This avoids all the problems
1488 the POSIX method has plus it is much easier to use. Simply run the tzselect
1489 shell script, answer the question and use the name printed in the end by
1490 making a symlink /etc/localtime pointing to /usr/share/zoneinfo/NAME (NAME
1491 is the returned value from tzselect). That's all. You never again have to
1494 So, please avoid sending bug reports about time related problems if you use
1495 the POSIX method and you have not verified something is really broken by
1496 reading the POSIX standards.
1498 ?? What other sources of documentation about glibc are available?
1500 {AJ} The FSF has a page about the GNU C library at
1501 <http://www.gnu.org/software/libc/>. The problem data base of open and
1502 solved bugs in GNU libc is available at
1503 <http://www-gnats.gnu.org:8080/cgi-bin/wwwgnats.pl>. Eric Green has written
1504 a HowTo for converting from Linux libc5 to glibc2. The HowTo is accessible
1505 via the FSF page and at <http://www.imaxx.net/~thrytis/glibc>. Frodo
1506 Looijaard describes a different way installing glibc2 as secondary libc at
1507 <http://huizen.dds.nl/~frodol/glibc>.
1509 Please note that this is not a complete list.
1511 ?? The timezone string for Sydney/Australia is wrong since even when
1512 daylight saving time is in effect the timezone string is EST.
1514 {UD} The problem for some timezones is that the local authorities decided
1515 to use the term "summer time" instead of "daylight saving time". In this
1516 case the abbreviation character `S' is the same as the standard one. So,
1519 Eastern Standard Time = EST
1520 Eastern Summer Time = EST
1522 Great! To get this bug fixed convince the authorities to change the laws
1523 and regulations of the country this effects. glibc behaves correctly.
1525 ??make I've build make 3.77 against glibc 2.1 and now make gets
1526 segmentation faults.
1528 Removed. Does not apply anymore, use make 3.79 or newer.
1530 ?? Why do so many programs using math functions fail on my AlphaStation?
1532 {AO} The functions floor() and floorf() use an instruction that is not
1533 implemented in some old PALcodes of AlphaStations. This may cause
1534 `Illegal Instruction' core dumps or endless loops in programs that
1535 catch these signals. Updating the firmware to a 1999 release has
1536 fixed the problem on an AlphaStation 200 4/166.
1538 ?? The conversion table for character set XX does not match with
1541 {UD} I don't doubt for a minute that some of the conversion tables contain
1542 errors. We tried the best we can and relied on automatic generation of the
1543 data to prevent human-introduced errors but this still is no guarantee. If
1544 you think you found a problem please send a bug report describing it and
1545 give an authoritive reference. The latter is important since otherwise
1546 the current behaviour is as good as the proposed one.
1548 Before doing this look through the list of known problem first:
1550 - the GBK (simplified Chinese) encoding is based on Unicode tables. This
1551 is good. These tables, however, differ slightly from the tables used
1552 by the M$ people. The differences are these [+ Unicode, - M$]:
1559 In addition the Unicode tables contain mappings for the GBK characters
1560 0xA8BC, 0xA8BF, 0xA989 to 0xA995, and 0xFE50 to 0xFEA0.
1562 - when mapping from EUC-CN to GBK and vice versa we ignore the fact that
1563 the coded character at position 0xA1A4 maps to different Unicode
1564 characters. Since the iconv() implementation can do whatever it wants
1565 if it cannot directly map a character this is a perfectly good solution
1566 since the semantics and appearance of the character does not change.
1569 Answers were given by:
1570 {UD} Ulrich Drepper, <drepper@cygnus.com>
1571 {DMT} David Mosberger-Tang, <davidm@AZStarNet.com>
1572 {RM} Roland McGrath, <roland@gnu.org>
1573 {AJ} Andreas Jaeger, <aj@suse.de>
1574 {EY} Eric Youngdale, <eric@andante.jic.com>
1575 {PB} Phil Blundell, <Philip.Blundell@pobox.com>
1576 {MK} Mark Kettenis, <kettenis@phys.uva.nl>
1577 {ZW} Zack Weinberg, <zack@rabi.phys.columbia.edu>
1578 {TK} Thorsten Kukuk, <kukuk@suse.de>
1579 {GK} Geoffrey Keating, <geoffk@ozemail.com.au>
1580 {HJ} H.J. Lu, <hjl@gnu.org>
1581 {CG} Cristian Gafton, <gafton@redhat.com>
1582 {AO} Alexandre Oliva, <oliva@lsd.ic.unicamp.br>
1586 outline-regexp:"\\?"
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