1 Frequently Asked Questions about the GNU C Library
3 This document tries to answer questions a user might have when
4 installing and using glibc. Please make sure you read this before
5 sending questions or bug reports to the maintainers.
7 The GNU C library is very complex. The installation process has not
8 been completely automated; there are too many variables. You can do
9 substantial damage to your system by installing the library
10 incorrectly. Make sure you understand what you are undertaking before
13 If you have any questions you think should be answered in this document,
18 ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
22 1.1. What systems does the GNU C Library run on?
23 1.2. What compiler do I need to build GNU libc?
24 1.3. When I try to compile glibc I get only error messages.
26 1.4. Do I need a special linker or archiver?
27 1.5. What tools do I need for powerpc?
28 1.6. Do I need some more things to compile GNU C Library?
29 1.7. When I run `nm -u libc.so' on the produced library I still
30 find unresolved symbols. Can this be ok?
31 1.8. What are these `add-ons'?
32 1.9. My XXX kernel emulates a floating-point coprocessor for me.
33 Should I enable --with-fp?
34 1.10. When compiling GNU libc I get lots of errors saying functions
35 in glibc are duplicated in libgcc.
36 1.11. Why do I get messages about missing thread functions when I use
37 the librt? I don't even use threads.
38 1.12. What's the problem with configure --enable-omitfp?
40 2. Installation and configuration issues
42 2.1. Can I replace the libc on my Linux system with GNU libc?
43 2.2. How do I configure GNU libc so that the essential libraries
44 like libc.so go into /lib and the other into /usr/lib?
45 2.3. How should I avoid damaging my system when I install GNU libc?
46 2.4. Do I need to use GNU CC to compile programs that will use the
48 2.5. When linking with the new libc I get unresolved symbols
49 `crypt' and `setkey'. Why aren't these functions in the
51 2.6. When I use GNU libc on my Linux system by linking against
52 the libc.so which comes with glibc all I get is a core dump.
53 2.7. Looking through the shared libc file I haven't found the
54 functions `stat', `lstat', `fstat', and `mknod' and while
55 linking on my Linux system I get error messages. How is
56 this supposed to work?
57 2.8. How can I compile gcc 2.7.2.1 from the gcc source code using
59 2.9. The `gencat' utility cannot process the catalog sources which
60 were used on my Linux libc5 based system. Why?
61 2.10. I have set up /etc/nis.conf, and the Linux libc 5 with NYS
62 works great. But the glibc NIS+ doesn't seem to work.
63 2.11. After installing glibc name resolving doesn't work properly.
64 2.12. I have /usr/include/net and /usr/include/scsi as symlinks
65 into my Linux source tree. Is that wrong?
66 2.13. Programs like `logname', `top', `uptime' `users', `w' and
67 `who', show incorrect information about the (number of)
68 users on my system. Why?
69 2.14. After upgrading to glibc 2.1 with symbol versioning I get
70 errors about undefined symbols. What went wrong?
71 2.15. When I start the program XXX after upgrading the library
73 XXX: Symbol `_sys_errlist' has different size in shared
74 object, consider re-linking
75 Why? What should I do?
77 3. Source and binary incompatibilities, and what to do about them
79 3.1. I expect GNU libc to be 100% source code compatible with
80 the old Linux based GNU libc. Why isn't it like this?
81 3.2. Why does getlogin() always return NULL on my Linux box?
82 3.3. Where are the DST_* constants found in <sys/time.h> on many
84 3.4. The prototypes for `connect', `accept', `getsockopt',
85 `setsockopt', `getsockname', `getpeername', `send',
86 `sendto', and `recvfrom' are different in GNU libc from
87 any other system I saw. This is a bug, isn't it?
88 3.5. On Linux I've got problems with the declarations in Linux
90 3.6. I don't include any kernel headers myself but the compiler
91 still complains about redeclarations of types in the kernel
93 3.7. Why don't signals interrupt system calls anymore?
94 3.8. I've got errors compiling code that uses certain string
96 3.9. I get compiler messages "Initializer element not constant" with
97 stdin/stdout/stderr. Why?
98 3.10. I can't compile with gcc -traditional (or
99 -traditional-cpp). Why?
100 3.11. I get some errors with `gcc -ansi'. Isn't glibc ANSI compatible?
104 4.1. After I changed configure.in I get `Autoconf version X.Y.
105 or higher is required for this script'. What can I do?
106 4.2. When I try to compile code which uses IPv6 headers and
107 definitions on my Linux 2.x.y system I am in trouble.
108 Nothing seems to work.
111 ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
115 1.1. What systems does the GNU C Library run on?
117 {UD} This is difficult to answer. The file `README' lists the
118 architectures GNU libc was known to run on *at some time*. This does
119 not mean that it still can be compiled and run on them now.
121 The systems glibc is known to work on as of this release, and most
122 probably in the future, are:
125 i[3456]86-*-linux-gnu Linux-2.x on Intel
126 m68k-*-linux-gnu Linux-2.x on Motorola 680x0
127 alpha-*-linux-gnu Linux-2.x on DEC Alpha
128 powerpc-*-linux-gnu Linux and MkLinux on PowerPC systems
129 sparc-*-linux-gnu Linux-2.x on SPARC
130 sparc64-*-linux-gnu Linux-2.x on UltraSPARC
132 Ports to other Linux platforms are in development, and may in fact
133 work already, but no one has sent us success reports for them.
134 Currently no ports to other operating systems are underway, although a
135 few people have expressed interest.
137 If you have a system not listed above (or in the `README' file) and
138 you are really interested in porting it, contact
143 1.2. What compiler do I need to build GNU libc?
145 {UD} You must use GNU CC to compile GNU libc. A lot of extensions of
146 GNU CC are used to increase portability and speed.
148 GNU CC is found, like all other GNU packages, on
149 ftp://prep.ai.mit.edu/pub/gnu
150 and the many mirror sites. prep is always overloaded, so try to find
151 a local mirror first.
153 You always should try to use the latest official release. Older
154 versions may not have all the features GNU libc requires. On most
155 supported platforms (for powerpc see question question 1.5), 2.7.2.3 is
156 the earliest version that works at all.
159 1.3. When I try to compile glibc I get only error messages.
162 {UD} You definitely need GNU make to translate GNU libc. No
163 other make program has the needed functionality.
165 We recommend version GNU make version 3.75. Versions 3.76 and 3.76.1
166 have bugs which appear when building big projects like GNU libc.
167 Versions before 3.74 have bugs and/or are missing features.
170 1.4. Do I need a special linker or archiver?
172 {UD} You may be able to use your system linker, but GNU libc works
173 best with GNU binutils.
175 On systems where the native linker does not support weak symbols you
176 will not get a fully ISO C compliant C library. Generally speaking
177 you should use the GNU binutils if they provide at least the same
178 functionality as your system's tools.
180 Always get the newest release of GNU binutils available. Older
181 releases are known to have bugs that prevent a successful compilation.
183 {ZW} As of release 2.1 a linker supporting symbol versions is
184 required. For Linux, get binutils-2.8.1.0.17 or later. Other systems
185 may have native linker support, but it's moot right now, because glibc
186 has not been ported to them.
189 1.5. What tools do I need for powerpc?
191 {GK} For a successful installation you definitely need the most recent
192 tools. You can safely assume that anything earlier than binutils
193 2.8.1.0.17 and egcs-1.0 will have problems. We'd advise at the moment
194 binutils 2.8.1.0.18 and egcs-1.0.1.
196 In fact, egcs 1.0.1 currently has two serious bugs that prevent a
197 clean make; one relates to switch statement folding, for which there
198 is a temporary patch at
200 <http://discus.anu.edu.au/~geoffk/egcs-1.0-geoffk.diff.gz>
202 and the other relates to 'forbidden register spilled', for which the
205 CFLAGS-condvar.c += -fno-inline
207 in configparms. Later versions of egcs may fix these problems.
210 1.6. Do I need some more things to compile GNU C Library?
212 {UD} Yes, there are some more :-).
214 * GNU gettext. This package contains the tools needed to construct
215 `message catalog' files containing translated versions of system
216 messages. See ftp://prep.ai.mit.edu/pub/gnu or better any mirror
217 site. (We distribute compiled message catalogs, but they may not be
220 * Some files depend on special tools. E.g., files ending in .gperf
221 need a `gperf' program. The GNU version (part of libg++) is known
222 to work while some vendor versions do not.
224 You should not need these tools unless you change the source files.
226 * Some scripts need perl5 - but at the moment those scripts are not
227 vital for building and installing GNU libc (some data files will not
230 * When compiling for Linux, the header files of the Linux kernel must
231 be available to the compiler as <linux/*.h> and <asm/*.h>.
233 * lots of disk space (~170MB for i?86-linux; more for RISC platforms).
235 * plenty of time. Compiling just the shared and static libraries for
236 i?86-linux takes approximately 1h on an i586@133, or 2.5h on
237 i486@66, or 4.5h on i486@33. Multiply this by 1.5 or 2.0 if you
238 build profiling and/or the highly optimized version as well. For
239 Hurd systems times are much higher.
241 You should avoid compiling in a NFS mounted filesystem. This is
244 James Troup <J.J.Troup@comp.brad.ac.uk> reports a compile time of
245 45h34m for a full build (shared, static, and profiled) on Atari
246 Falcon (Motorola 68030 @ 16 Mhz, 14 Mb memory) and Jan Barte
247 <yann@plato.uni-paderborn.de> reports 22h48m on Atari TT030
248 (Motorola 68030 @ 32 Mhz, 34 Mb memory)
250 If you have some more measurements let me know.
253 1.7. When I run `nm -u libc.so' on the produced library I still
254 find unresolved symbols. Can this be ok?
256 {UD} Yes, this is ok. There can be several kinds of unresolved
259 * magic symbols automatically generated by the linker. These have names
260 like __start_* and __stop_*
262 * symbols starting with _dl_* come from the dynamic linker
264 * symbols resolved by using libgcc.a
265 (__udivdi3, __umoddi3, or similar)
267 * weak symbols, which need not be resolved at all (fabs for example)
269 Generally, you should make sure you find a real program which produces
270 errors while linking before deciding there is a problem.
273 1.8. What are these `add-ons'?
275 {UD} To avoid complications with export rules or external source
276 code some optional parts of the libc are distributed as separate
277 packages (e.g., the crypt package, see question 2.5).
279 To use these packages as part of GNU libc, just unpack the tarfiles in
280 the libc source directory and tell the configuration script about them
281 using the --enable-add-ons option. If you give just --enable-add-ons
282 configure tries to find all the add-on packages in your source tree.
283 This may not work. If it doesn't, or if you want to select only a
284 subset of the add-ons, give a comma-separated list of the add-ons to
287 configure --enable-add-ons=crypt,linuxthreads
291 Add-ons can add features (including entirely new shared libraries),
292 override files, provide support for additional architectures, and
293 just about anything else. The existing makefiles do most of the work;
294 only some few stub rules must be written to get everything running.
297 1.9. My XXX kernel emulates a floating-point coprocessor for me.
298 Should I enable --with-fp?
300 {ZW} An emulated FPU is just as good as a real one, as far as the C
301 library is concerned. You only need to say --without-fp if your
302 machine has no way to execute floating-point instructions.
304 People who are interested in squeezing the last drop of performance
305 out of their machine may wish to avoid the trap overhead, but this is
306 far more trouble than it's worth: you then have to compile
307 *everything* this way, including the compiler's internal libraries
308 (libgcc.a for GNU C), because the calling conventions change.
311 1.10. When compiling GNU libc I get lots of errors saying functions
312 in glibc are duplicated in libgcc.
314 {EY} This is *exactly* the same problem that I was having. The
315 problem was due to the fact that configure didn't correctly detect
316 that the linker flag --no-whole-archive was supported in my linker.
317 In my case it was because I had run ./configure with bogus CFLAGS, and
320 One thing that is particularly annoying about this problem is that
321 once this is misdetected, running configure again won't fix it unless
322 you first delete config.cache.
324 {UD} Starting with glibc-2.0.3 there should be a better test to avoid
325 some problems of this kind. The setting of CFLAGS is checked at the
326 very beginning and if it is not usable `configure' will bark.
329 1.11. Why do I get messages about missing thread functions when I use
330 the librt? I don't even use threads.
332 {UD} In this case you probably mixed up your installation of the libc.
333 The librt internally uses threads and it has implicit references to
334 the thread library. Normally these references are satisfied
335 automatically but if the thread library belonging to the librt is not
336 in the expected place one has to specify this place. When using GNU
337 ld it works like this:
339 gcc -o foo foo.c -Wl,-rpath-link=/some/other/dir -lrt
341 The `/some/other/dir' should contain the matching thread library and
342 `ld' will use the given path to find the implicitly referenced library
343 while not disturbing any other link path order.
346 1.12. What's the problem with configure --enable-omitfp?
348 {AJ} When --enable-omitfp is set the libraries are built without frame
349 pointers. Some compilers produce buggy code for this model and
350 therefore we don't advise using it at the moment.
352 If you use --enable-omitfp, you're on your own. If you encounter
353 problems with a library that was build this way, we advise you to
354 rebuild the library without --enable-omitfp. If the problem vanishes
355 consider tracking the problem down and report it as compiler failure.
357 Since a library build with --enable-omitfp is undebuggable on most
358 systems, debuggable libraries are also built - you can use it by
359 appending "_g" to the library names.
361 The compilation of these extra libraries and the compiler optimizations
362 slow down the build process and need more disk space.
365 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
367 2. Installation and configuration issues
369 2.1. Can I replace the libc on my Linux system with GNU libc?
371 {UD} You cannot replace any existing libc for Linux with GNU
372 libc. It is binary incompatible and therefore has a different major
373 version. You can, however, install it alongside your existing libc.
375 For Linux there are three major libc versions:
377 libc-5 original ELF libc
380 You can have any combination of these three installed. For more
381 information consult documentation for shared library handling. The
382 Makefiles of GNU libc will automatically generate the needed symbolic
383 links which the linker will use.
386 2.2. How do I configure GNU libc so that the essential libraries
387 like libc.so go into /lib and the other into /usr/lib?
389 {UD,AJ} Like all other GNU packages GNU libc is designed to use a base
390 directory and install all files relative to this. The default is
391 /usr/local, because this is safe (it will not damage the system if
392 installed there). If you wish to install GNU libc as the primary C
393 library on your system, set the base directory to /usr (i.e. run
394 configure --prefix=/usr <other_options>). Note that this can damage
395 your system; see question 2.3 for details.
397 Some systems like Linux have a filesystem standard which makes a
398 difference between essential libraries and others. Essential
399 libraries are placed in /lib because this directory is required to be
400 located on the same disk partition as /. The /usr subtree might be
401 found on another partition/disk. If you configure for Linux with
402 --prefix=/usr, then this will be done automatically.
404 To install the essential libraries which come with GNU libc in /lib on
405 systems other than Linux one must explicitly request it. Autoconf has
406 no option for this so you have to use a `configparms' file (see the
407 `INSTALL' file for details). It should contain:
412 The first line specifies the directory for the essential libraries,
413 the second line the directory for system configuration files.
416 2.3. How should I avoid damaging my system when I install GNU libc?
418 {ZW} If you wish to be cautious, do not configure with --prefix=/usr.
419 If you don't specify a prefix, glibc will be installed in /usr/local,
420 where it will probably not break anything. (If you wish to be
421 certain, set the prefix to something like /usr/local/glibc2 which is
422 not used for anything.)
424 The dangers when installing glibc in /usr are twofold:
426 * glibc will overwrite the headers in /usr/include. Other C libraries
427 install a different but overlapping set of headers there, so the
428 effect will probably be that you can't compile anything. You need to
429 rename /usr/include out of the way first. (Do not throw it away; you
430 will then lose the ability to compile programs against your old libc.)
432 * None of your old libraries, static or shared, can be used with a
433 different C library major version. For shared libraries this is not a
434 problem, because the filenames are different and the dynamic linker
435 will enforce the restriction. But static libraries have no version
436 information. You have to evacuate all the static libraries in
437 /usr/lib to a safe location.
439 The situation is rather similar to the move from a.out to ELF which
440 long-time Linux users will remember.
443 2.4. Do I need to use GNU CC to compile programs that will use the
446 {ZW} In theory, no; the linker does not care, and the headers are
447 supposed to check for GNU CC before using its extensions to the C
450 However, there are currently no ports of glibc to systems where
451 another compiler is the default, so no one has tested the headers
452 extensively against another compiler. You may therefore encounter
453 difficulties. If you do, please report them as bugs.
455 Also, in several places GNU extensions provide large benefits in code
456 quality. For example, the library has hand-optimized, inline assembly
457 versions of some string functions. These can only be used with GCC.
458 See question 3.8 for details.
461 2.5. When linking with the new libc I get unresolved symbols
462 `crypt' and `setkey'. Why aren't these functions in the
465 {UD} The US places restrictions on exporting cryptographic programs
466 and source code. Until this law gets abolished we cannot ship the
467 cryptographic functions together with glibc.
469 The functions are available, as an add-on (see question 1.8). People in the
470 US may get it from the same place they got GNU libc from. People
471 outside the US should get the code from ftp://ftp.ifi.uio.no/pub/gnu,
472 or another archive site outside the USA. The README explains how to
475 If you already have the crypt code on your system the reason for the
476 failure is probably that you did not link with -lcrypt. The crypto
477 functions are in a separate library to make it possible to export GNU
478 libc binaries from the US.
481 2.6. When I use GNU libc on my Linux system by linking against
482 the libc.so which comes with glibc all I get is a core dump.
484 {UD} On Linux, gcc sets the dynamic linker to /lib/ld-linux.so.1
485 unless the user specifies a -dynamic-linker argument. This is the
486 name of the libc5 dynamic linker, which does not work with glibc.
488 For casual use of GNU libc you can just specify
489 -dynamic-linker=/lib/ld-linux.so.2
491 which is the glibc dynamic linker, on Linux systems. On other systems
492 the name is /lib/ld.so.1.
494 To change your environment to use GNU libc for compiling you need to
495 change the `specs' file of your gcc. This file is normally found at
497 /usr/lib/gcc-lib/<arch>/<version>/specs
499 In this file you have to change a few things:
501 - change `ld-linux.so.1' to `ld-linux.so.2'
503 - remove all expression `%{...:-lgmon}'; there is no libgmon in glibc
505 - fix a minor bug by changing %{pipe:-} to %|
507 Here is what the gcc-2.7.2 specs file should look like when GNU libc
508 is installed at /usr:
510 -----------------------------------------------------------------------
512 %{V} %{v:%{!V:-V}} %{Qy:} %{!Qn:-Qy} %{n} %{T} %{Ym,*} %{Yd,*} %{Wa,*:%*}
518 %{fPIC:-D__PIC__ -D__pic__} %{fpic:-D__PIC__ -D__pic__} %{!m386:-D__i486__} %{posix:-D_POSIX_SOURCE} %{pthread:-D_REENTRANT}
527 %{!shared:crtend.o%s} %{shared:crtendS.o%s} crtn.o%s
530 -m elf_i386 %{shared:-shared} %{!shared: %{!ibcs: %{!static: %{rdynamic:-export-dynamic} %{!dynamic-linker:-dynamic-linker /lib/ld-linux.so.2}} %{static:-static}}}
533 %{!shared: %{pthread:-lpthread} %{profile:-lc_p} %{!profile: -lc}}
539 %{!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}
541 *switches_need_spaces:
545 %{funsigned-char:-D__CHAR_UNSIGNED__}
548 -D__ELF__ -Dunix -Di386 -Dlinux -Asystem(unix) -Asystem(posix) -Acpu(i386) -Amachine(i386)
556 -----------------------------------------------------------------------
558 Things get a bit more complicated if you have GNU libc installed in
559 some other place than /usr, i.e., if you do not want to use it instead
560 of the old libc. In this case the needed startup files and libraries
561 are not found in the regular places. So the specs file must tell the
562 compiler and linker exactly what to use.
564 Version 2.7.2.3 does and future versions of GCC will automatically
565 provide the correct specs.
568 2.7. Looking through the shared libc file I haven't found the
569 functions `stat', `lstat', `fstat', and `mknod' and while
570 linking on my Linux system I get error messages. How is
571 this supposed to work?
573 {RM} Believe it or not, stat and lstat (and fstat, and mknod)
574 are supposed to be undefined references in libc.so.6! Your problem is
575 probably a missing or incorrect /usr/lib/libc.so file; note that this
576 is a small text file now, not a symlink to libc.so.6. It should look
579 GROUP ( libc.so.6 ld.so.1 libc.a )
581 or in ix86/Linux and alpha/Linux:
583 GROUP ( libc.so.6 ld-linux.so.2 libc.a )
586 2.8. How can I compile gcc 2.7.2.1 from the gcc source code using
589 {AJ} There's only correct support for glibc 2.0.x in gcc 2.7.2.3
590 or later. You should get at least gcc 2.7.2.3. All previous versions
591 had problems with glibc support.
594 2.9. The `gencat' utility cannot process the catalog sources which
595 were used on my Linux libc5 based system. Why?
597 {UD} The `gencat' utility provided with glibc complies to the XPG
598 standard. The older Linux version did not obey the standard, so they
601 To ease the transition from the Linux version some of the non-standard
602 features are also present in the `gencat' program of GNU libc. This
603 mainly includes the use of symbols for the message number and the automatic
604 generation of header files which contain the needed #defines to map the
607 Here is a simple SED script to convert at least some Linux specific
608 catalog files to the XPG4 form:
610 -----------------------------------------------------------------------
611 # Change catalog source in Linux specific format to standard XPG format.
612 # Ulrich Drepper <drepper@cygnus.com>, 1996.
616 s/\$ #\([^ ]*\).*/\1/
618 s/\$ #[^ ]* *\(.*\)/\$ \1/
624 s/\(.*\)\n\(.*\)/\2 \1/
626 -----------------------------------------------------------------------
629 2.10. I have set up /etc/nis.conf, and the Linux libc 5 with NYS
630 works great. But the glibc NIS+ doesn't seem to work.
632 {TK} The glibc NIS+ implementation uses a /var/nis/NIS_COLD_START
633 file for storing information about the NIS+ server and their public
634 keys, because the nis.conf file does not contain all the necessary
635 information. You have to copy a NIS_COLD_START file from a Solaris
636 client (the NIS_COLD_START file is byte order independent) or generate
637 it with nisinit from the nis-tools package (available at
638 http://www-vt.uni-paderborn.de/~kukuk/linux/nisplus.html).
641 2.11. After installing glibc name resolving doesn't work properly.
643 {AJ} You probably should read the manual section describing
644 nsswitch.conf (just type `info libc "NSS Configuration File"').
645 The NSS configuration file is usually the culprit.
648 2.12. I have /usr/include/net and /usr/include/scsi as symlinks
649 into my Linux source tree. Is that wrong?
651 {PB} This was necessary for libc5, but is not correct when using
652 glibc. Including the kernel header files directly in user programs
653 usually does not work (see question 3.5). glibc provides its own <net/*>
654 and <scsi/*> header files to replace them, and you may have to remove
655 any symlink that you have in place before you install glibc. However,
656 /usr/include/asm and /usr/include/linux should remain as they were.
659 2.13. Programs like `logname', `top', `uptime' `users', `w' and
660 `who', show incorrect information about the (number of)
661 users on my system. Why?
663 {MK} See question 3.2.
666 2.14. After upgrading to glibc 2.1 with symbol versioning I get
667 errors about undefined symbols. What went wrong?
669 {AJ} The problem is caused either by wrong program code or tools. In
670 the versioned libc a lot of symbols are now local that were global
671 symbols in previous versions. It seems that programs linked against
672 older versions often accidentally used libc global variables --
673 something that should not happen.
675 The only way to fix this is to recompile your program. Sorry, that's
676 the price you might have to pay once for quite a number of advantages
677 with symbol versioning.
680 2.15. When I start the program XXX after upgrading the library
682 XXX: Symbol `_sys_errlist' has different size in shared
683 object, consider re-linking
684 Why? What should I do?
686 {UD} As the message says, relink the binary. The problem is that
687 a few symbols from the library can change in size and there is no way
688 to avoid this. _sys_errlist is a good example. Occasionally there are
689 new error numbers added to the kernel and this must be reflected at user
690 level, breaking programs that refer to them directly.
692 Such symbols should normally not be used at all. There are mechanisms
693 to avoid using them. In the case of _sys_errlist, there is the
694 strerror() function which should _always_ be used instead. So the
695 correct fix is to rewrite that part of the application.
697 In some situations (especially when testing a new library release) it
698 might be possible that a symbol changed size when that should not have
699 happened. So in case of doubt report such a warning message as a
703 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
705 3. Source and binary incompatibilities, and what to do about them
707 3.1. I expect GNU libc to be 100% source code compatible with
708 the old Linux based GNU libc. Why isn't it like this?
710 {DMT,UD} Not every extension in Linux libc's history was well
711 thought-out. In fact it had a lot of problems with standards compliance
712 and with cleanliness. With the introduction of a new version number these
713 errors can now be corrected. Here is a list of the known source code
716 * _GNU_SOURCE: glibc does not make the GNU extensions available
717 automatically. If a program depends on GNU extensions or some
718 other non-standard functionality, it is necessary to compile it
719 with the C compiler option -D_GNU_SOURCE, or better, to put
720 `#define _GNU_SOURCE' at the beginning of your source files, before
721 any C library header files are included. This difference normally
722 manifests itself in the form of missing prototypes and/or data type
723 definitions. Thus, if you get such errors, the first thing you
724 should do is try defining _GNU_SOURCE and see if that makes the
727 For more information consult the file `NOTES' in the GNU C library
730 * reboot(): GNU libc sanitizes the interface of reboot() to be more
731 compatible with the interface used on other OSes. reboot() as
732 implemented in glibc takes just one argument. This argument
733 corresponds to the third argument of the Linux reboot system call.
734 That is, a call of the form reboot(a, b, c) needs to be changed into
735 reboot(c). Beside this the header <sys/reboot.h> defines the needed
736 constants for the argument. These RB_* constants should be used
737 instead of the cryptic magic numbers.
739 * swapon(): the interface of this function didn't change, but the
740 prototype is in a separate header file <sys/swap.h>. This header
741 file also provides the SWAP_* constants defined by <linux/swap.h>;
742 you should use them for the second argument to swapon().
744 * errno: If a program uses the variable "errno", then it _must_
745 include <errno.h>. The old libc often (erroneously) declared this
746 variable implicitly as a side-effect of including other libc header
747 files. glibc is careful to avoid such namespace pollution, which,
748 in turn, means that you really need to include the header files that
749 you depend on. This difference normally manifests itself in the
750 form of the compiler complaining about references to an undeclared
753 * Linux-specific syscalls: All Linux system calls now have appropriate
754 library wrappers and corresponding declarations in various header files.
755 This is because the syscall() macro that was traditionally used to
756 work around missing syscall wrappers are inherently non-portable and
757 error-prone. The following table lists all the new syscall stubs,
758 the header-file declaring their interface and the system call name.
760 syscall name: wrapper name: declaring header file:
761 ------------- ------------- ----------------------
762 bdflush bdflush <sys/kdaemon.h>
763 syslog ksyslog_ctl <sys/klog.h>
765 * lpd: Older versions of lpd depend on a routine called _validuser().
766 The library does not provide this function, but instead provides
767 __ivaliduser() which has a slightly different interface. Simply
768 upgrading to a newer lpd should fix this problem (e.g., the 4.4BSD
769 lpd is known to be working).
771 * resolver functions/BIND: like on many other systems the functions of
772 the resolver library are not included in libc itself. There is a
773 separate library libresolv. If you get undefined symbol errors for
774 symbols starting with `res_*' simply add -lresolv to your linker
777 * the `signal' function's behavior corresponds to the BSD semantic and
778 not the SysV semantic as it was in libc-5. The interface on all GNU
779 systems shall be the same and BSD is the semantic of choice. To use
780 the SysV behavior simply use `sysv_signal', or define _XOPEN_SOURCE.
781 See question 3.7 for details.
784 3.2. Why does getlogin() always return NULL on my Linux box?
786 {UD} The GNU C library has a format for the UTMP and WTMP file which
787 differs from what your system currently has. It was extended to
788 fulfill the needs of the next years when IPv6 is introduced. The
789 record size is different and some fields have different positions.
790 The files written by functions from the one library cannot be read by
791 functions from the other library. Sorry, but this is what a major
792 release is for. It's better to have a cut now than having no means to
793 support the new techniques later.
795 {MK} There is however a (partial) solution for this problem. Please
796 take a look at the file `login/README.utmpd'.
799 3.3. Where are the DST_* constants found in <sys/time.h> on many
802 {UD} These constants come from the old BSD days and are not used
803 anymore (libc5 does not actually implement the handling although the
804 constants are defined).
806 Instead GNU libc contains zone database support and compatibility code
807 for POSIX TZ environment variable handling.
810 3.4. The prototypes for `connect', `accept', `getsockopt',
811 `setsockopt', `getsockname', `getpeername', `send',
812 `sendto', and `recvfrom' are different in GNU libc from
813 any other system I saw. This is a bug, isn't it?
815 {UD} No, this is no bug. This version of GNU libc already follows the
816 new Single Unix specifications (and I think the POSIX.1g draft which
817 adopted the solution). The type for a parameter describing a size is
818 now `socklen_t', a new type.
821 3.5. On Linux I've got problems with the declarations in Linux
824 {UD,AJ} On Linux, the use of kernel headers is reduced to the minimum.
825 This gives Linus the ability to change the headers more freely. Also,
826 user programs are now insulated from changes in the size of kernel
829 For example, the sigset_t type is 32 or 64 bits wide in the kernel.
830 In glibc it is 1024 bits wide. This guarantees that when the kernel
831 gets a bigger sigset_t (for POSIX.1e realtime support, say) user
832 programs will not have to be recompiled. Consult the header files for
833 more information about the changes.
835 Therefore you shouldn't include Linux kernel header files directly if
836 glibc has defined a replacement. Otherwise you might get undefined
837 results because of type conflicts.
840 3.6. I don't include any kernel headers myself but the compiler
841 still complains about redeclarations of types in the kernel
844 {UD} The kernel headers before Linux 2.1.61 and 2.0.32 don't work
845 correctly with glibc. Compiling C programs is possible in most cases
846 but C++ programs have (due to the change of the name lookups for
847 `struct's) problems. One prominent example is `struct fd_set'.
849 There might be some problems left but 2.1.61/2.0.32 fix most of the
850 known ones. See the BUGS file for other known problems.
853 3.7. Why don't signals interrupt system calls anymore?
855 {ZW} By default GNU libc uses the BSD semantics for signal(),
856 unlike Linux libc 5 which used System V semantics. This is partially
857 for compatibility with other systems and partially because the BSD
858 semantics tend to make programming with signals easier.
860 There are three differences:
862 * BSD-style signals that occur in the middle of a system call do not
863 affect the system call; System V signals cause the system call to
864 fail and set errno to EINTR.
866 * BSD signal handlers remain installed once triggered. System V signal
867 handlers work only once, so one must reinstall them each time.
869 * A BSD signal is blocked during the execution of its handler. In other
870 words, a handler for SIGCHLD (for example) does not need to worry about
871 being interrupted by another SIGCHLD. It may, however, be interrupted
874 There is general consensus that for `casual' programming with signals, the
875 BSD semantics are preferable. You don't need to worry about system calls
876 returning EINTR, and you don't need to worry about the race conditions
877 associated with one-shot signal handlers.
879 If you are porting an old program that relies on the old semantics, you can
880 quickly fix the problem by changing signal() to sysv_signal() throughout.
881 Alternatively, define _XOPEN_SOURCE before including <signal.h>.
883 For new programs, the sigaction() function allows you to specify precisely
884 how you want your signals to behave. All three differences listed above are
885 individually switchable on a per-signal basis with this function.
887 If all you want is for one specific signal to cause system calls to fail
888 and return EINTR (for example, to implement a timeout) you can do this with
892 3.8. I've got errors compiling code that uses certain string
895 {AJ} glibc 2.1 has special string functions that are faster
896 than the normal library functions. Some of the functions are
897 implemented as inline functions and others as macros.
899 The optimized string functions are only used when compiling with
900 optimizations (-O1 or higher). The behavior can be changed with two
903 * __NO_STRING_INLINES: Don't do any string optimizations.
904 * __USE_STRING_INLINES: Use assembly language inline functions (might
905 increase code size dramatically).
907 Since some of these string functions are now additionally defined as
908 macros, code like "char *strncpy();" doesn't work anymore (and is
909 unnecessary, since <string.h> has the necessary declarations). Either
910 change your code or define __NO_STRING_INLINES.
912 {UD} Another problem in this area is that gcc still has problems on
913 machines with very few registers (e.g., ix86). The inline assembler
914 code can require almost all the registers and the register allocator
915 cannot always handle this situation.
917 One can disable the string optimizations selectively. Instead of writing
919 cp = strcpy (foo, "lkj");
923 cp = (strcpy) (foo, "lkj");
925 This disables the optimization for that specific call.
928 3.9. I get compiler messages "Initializer element not constant" with
929 stdin/stdout/stderr. Why?
931 {RM,AJ} Constructs like:
932 static FILE *InPtr = stdin;
934 lead to this message. This is correct behaviour with glibc since stdin
935 is not a constant expression. Please note that a strict reading of ISO
936 C does not allow above constructs.
938 One of the advantages of this is that you can assign to stdin, stdout,
939 and stderr just like any other global variable (e.g. `stdout =
940 my_stream;'), which can be very useful with custom streams that you
941 can write with libio (but beware this is not necessarily
942 portable). The reason to implement it this way were versioning
943 problems with the size of the FILE structure.
946 3.10. I can't compile with gcc -traditional (or
947 -traditional-cpp). Why?
949 {AJ} glibc2 does break -traditional and -traditonal-cpp - and will continue
950 to do so. For example constructs of the form:
954 are useful for debugging purpuses (you can use foo with your debugger
955 that's why we need the enum) and for compatibility (other systems use
956 defines and check with #ifdef).
959 3.11. I get some errors with `gcc -ansi'. Isn't glibc ANSI compatible?
961 {AJ} The GNU C library is compatible with the ANSI/ISO C standard. If
962 you're using `gcc -ansi', the glibc includes which are specified in
963 the standard follow the standard. The ANSI/ISO C standard defines what
964 has to be in the include files - and also states that nothing else
965 should be in the include files (btw. you can still enable additional
966 standards with feature flags).
968 The GNU C library is conforming to ANSI/ISO C - if and only if you're
969 only using the headers and library functions defined in the standard.
972 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
976 4.1. After I changed configure.in I get `Autoconf version X.Y.
977 or higher is required for this script'. What can I do?
979 {UD} You have to get the specified autoconf version (or a later one)
980 from your favorite mirror of prep.ai.mit.edu.
983 4.2. When I try to compile code which uses IPv6 headers and
984 definitions on my Linux 2.x.y system I am in trouble.
985 Nothing seems to work.
987 {UD} The problem is that IPv6 development still has not reached a
988 point where the headers are stable. There are still lots of
989 incompatible changes made and the libc headers have to follow.
991 Also, make sure you have a suitably recent kernel. As of the 970401
992 snapshot, according to Philip Blundell <Philip.Blundell@pobox.com>, the
993 required kernel version is at least 2.1.30.
996 ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
998 Answers were given by:
999 {UD} Ulrich Drepper, <drepper@cygnus.com>
1000 {DMT} David Mosberger-Tang, <davidm@AZStarNet.com>
1001 {RM} Roland McGrath, <roland@gnu.org>
1002 {AJ} Andreas Jaeger, <aj@arthur.rhein-neckar.de>
1003 {EY} Eric Youngdale, <eric@andante.jic.com>
1004 {PB} Phil Blundell, <Philip.Blundell@pobox.com>
1005 {MK} Mark Kettenis, <kettenis@phys.uva.nl>
1006 {ZW} Zack Weinberg, <zack@rabi.phys.columbia.edu>
1007 {TK} Thorsten Kukuk, <kukuk@vt.uni-paderborn.de>
1008 {GK} Geoffrey Keating, <Geoff.Keating@anu.edu.au>
1012 outline-regexp:"\\?"