Frequently Asked Question on GNU C Library As every FAQ this one also tries to answer questions the user might have when using the package. Please make sure you read this before sending questions or bug reports to the maintainers. The GNU C Library is very complex. The building process exploits the features available in tools generally available. But many things can only be done using GNU tools. Also the code is sometimes hard to understand because it has to be portable but on the other hand must be fast. But you need not understand the details to use GNU C Library. This will only be necessary if you intend to contribute or change it. If you have any questions you think should be answered in this document, please let me know. --drepper@cygnus.com ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ [Q1] ``What systems does the GNU C Library run on?'' [Q2] ``What compiler do I need to build GNU libc?'' [Q3] ``When starting make I get only error messages. What's wrong?'' [Q4] ``After I changed configure.in I get `Autoconf version X.Y. or higher is required for this script'. What can I do?'' [Q5] ``Do I need a special linker or archiver?'' [Q6] ``Do I need some more things to compile GNU C Library?'' [Q7] ``When I run `nm -u libc.so' on the produced library I still find unresolved symbols? Can this be ok?'' [Q8] ``Can I replace the libc on my Linux system with GNU libc?'' [Q9] ``I expect GNU libc to be 100% source code compatible with the old Linux based GNU libc. Why isn't it like this?'' [Q10] ``Why does getlogin() always return NULL on my Linux box?'' [Q11] ``Where are the DST_* constants found in on many systems?'' [Q12] ``The `gencat' utility cannot process the input which are successfully used on my Linux libc based system. Why?'' [Q13] ``How do I configure GNU libc so that the essential libraries like libc.so go into /lib and the other into /usr/lib?'' [Q14] ``When linking with the new libc I get unresolved symbols `crypt' and `setkey'. Why aren't these functions in the libc anymore?'' [Q15] ``What are these `add-ons'?'' [Q16] ``When I use GNU libc on my Linux system by linking against to libc.so which comes with glibc all I get is a core dump.'' [Q17] ``Looking through the shared libc file I haven't found the functions `stat', `lstat', `fstat', and `mknod' and while linking on my Linux system I get error messages. How is this supposed to work?'' ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ [Q1] ``What systems does the GNU C Library run on?'' [A1] {UD} This is difficult to answer. The file `README' lists the architectures GNU libc is known to run *at some time*. This does not mean that it still can be compiled and run on them in the moment. The systems glibc is known to work on in the moment and most probably in the future are: *-*-gnu GNU Hurd i[3456]86-*-linux Linux-2.0 on Intel m68k-*-linux Linux-2.0 on Motorola 680x0 alpha-*-linux Linux-2.0 on DEC Alpha Other Linux platforms are also on the way to be supported but I need some success reports first. If you have a system not listed above (or in the `README' file) and you are really interested in porting it, contact ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ [Q2] ``What compiler do I need to build GNU libc?'' [A2] {UD} It is (almost) impossible to compile GNU C Library using a different compiler than GNU CC. A lot of extensions of GNU CC are used to increase the portability and speed. But this does not mean you have to use GNU CC for using the GNU C Library. In fact you should be able to use the native C compiler because the success only depends on the binutils: the linker and archiver. The GNU CC is found like all other GNU packages on ftp://prep.ai.mit.edu/pub/gnu or better one of the many mirror sites. You always should try to use the latest official release. Older versions might not have all the features GNU libc could use. ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ [Q3] ``When starting `make' I get only errors messages. What's wrong?'' [A3] {UD} You definitely need GNU make to translate GNU libc. No other make program has the needed functionality. Versions before 3.74 have bugs which prevent correct execution so you should upgrade to the latest version before starting the compilation. ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ [Q4] ``After I changed configure.in I get `Autoconf version X.Y. or higher is required for this script'. What can I do?'' [A4] {UD} You have to get the specified autoconf version (or a later) from your favourite mirror of prep.ai.mit.edu. ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ [Q5] ``Do I need a special linker or archiver?'' [A5] {UD} If your native versions are not too buggy you can probably work with them. But GNU libc works best with GNU binutils. On systems where the native linker does not support weak symbols you will not get a really ISO C compliant C library. Generally speaking you should use the GNU binutils if they provide at least the same functionality as your system's tools. Always get the newest release of GNU binutils available. Older releases are known to have bugs that affect building the GNU C Library. ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ [Q6] ``Do I need some more things to compile GNU C Library?'' [A6] {UD} Yes, there are some more :-). * GNU gettext; the GNU libc is internationalized and partly localized. For bringing the messages for the different languages in the needed form the tools from the GNU gettext package are necessary. See ftp://prep.ai.mit.edu/pub/gnu or better any mirror site. * lots of diskspace (for i?86-linux this means, e.g., ~70MB). You should avoid compiling on a NFS mounted device. This is very slow. * plenty of time (approx 1h for i?86-linux on i586@133 or 2.5h on i486@66 or 4.5h on i486@33). If you have some more measurements let me know. * Some files depend on special tools. E.g., files ending in .gperf need a `gperf' program. The GNU version (part of libg++) is known to work while some vendor versions do not. * When compiling for Linux: + the header files of the Linux kernel must be available in the search path of the CPP as and . * Some files depend on special tools. E.g., files ending in .gperf need a `gperf' program. The GNU version (part of libg++) is known to work while some vendor versions do not. ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ [Q7] ``When I run `nm -u libc.so' on the produced library I still find unresolved symbols? Can this be ok?'' [A7] {UD} Yes, this is ok. There can be several kinds of unresolved symbols: * magic symbols automatically generated by the linker. Names are often like __start_* and __stop_* * symbols starting with _dl_* come from the dynamic linker * symbols resolved by using libgcc.a (__udivdi3, __umoddi3, or similar) * weak symbols, which need not be resolved at all (currently fabs among others; this gets resolved if the program is linked against libm, too.) Generally, you should make sure you find a real program which produces errors while linking before deciding there is a problem. ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ [Q8] ``Can I replace the libc on my Linux system with GNU libc?'' [A8] {UD} You cannot replace any existing libc for Linux with GNU libc. There are different versions of C libraries and you can run libcs with different major version independently. For Linux there are today two libc versions: libc-4 old a.out libc libc-5 current ELF libc GNU libc will have the major number 6 and therefore you can have this additionally installed. For more information consult documentation for shared library handling. The Makefiles of GNU libc will automatically generate the needed symbolic links which the linker will use. ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ [Q9] ``I expect GNU libc to be 100% source code compatible with the old Linux based GNU libc. Why isn't it like this?'' [A9] {DMT,UD} Not every extension in Linux libc's history was well thought-out. In fact it had a lot of problems with standards compliance and with cleanliness. With the introduction of a new version number these errors now can be corrected. Here is a list of the known source code incompatibilities: * _GNU_SOURCE: glibc does not automatically define _GNU_SOURCE. Thus, if a program depends on GNU extensions or some other non-standard functionality, it is necessary to compile it with C compiler option -D_GNU_SOURCE, or better, to put `#define _GNU_SOURCE' at the beginning of your source files, before any C library header files are included. This difference normally manifests itself in the form of missing prototypes and/or data type definitions. Thus, if you get such errors, the first thing you should do is try defining _GNU_SOURCE and see if that makes the problem go away. For more information consult the file `NOTES' part of the GNU C library sources. * reboot(): GNU libc sanitizes the interface of reboot() to be more compatible with the interface used on other OSes. In particular, reboot() as implemented in glibc takes just one argument. This argument corresponds to the third argument of the Linux reboot system call. That is, a call of the form reboot(a, b, c) needs to be changed into reboot(c). Beside this the header defines the needed constants for the argument. These RB_* constants should be used instead of the cryptic magic numbers. * swapon(): the interface of this function didn't changed, but the prototype is in a separate header file . For the additional argument of of swapon() you should use the SWAP_* constants from , which get defined when is included. * errno: If a program uses variable "errno", then it _must_ include header file . The old libc often (erroneously) declared this variable implicitly as a side-effect of including other libc header files. glibc is careful to avoid such namespace pollution, which, in turn, means that you really need to include the header files that you depend on. This difference normally manifests itself in the form of the compiler complaining about the references of the undeclared symbol "errno". * Linux-specific syscalls: All Linux system calls now have appropriate library wrappers and corresponding declarations in various header files. This is because the syscall() macro that was traditionally used to work around missing syscall wrappers are inherently non-portable and error-prone. The following tables lists all the new syscall stubs, the header-file declaring their interface and the system call name. syscall name: wrapper name: declaring header file: ------------- ------------- ---------------------- bdflush bdflush create_module create_module delete_module delete_module get_kernel_syms get_kernel_syms init_module init_module syslog ksyslog_ctl * lpd: Older versions of lpd depend on a routine called _validuser(). The library does not provide this function, but instead provides __ivaliduser() which has a slightly different interfaces. Simply upgrading to a newer lpd should fix this problem (e.g., the 4.4BSD lpd is known to be working). * resolver functions/BIND: like on many other systems the functions of the resolver library are not included in the libc itself. There is a separate library libresolv. If you find some symbols starting with `res_*' undefined simply add -lresolv to your call of the linker. ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ [Q10] ``Why does getlogin() always return NULL on my Linux box?'' [A10] {UD} The GNU C library has a format for the UTMP and WTMP file which differs from what your system currently has. It was extended to fulfill the needs of the next years when IPv6 is introduced. So the record size is different, fields might have a different position and so reading the files written by functions from the one library cannot be read by functions from the other library. Sorry, but this is what a major release is for. It's better to have a cut now than having no means to support the new techniques later. ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ [Q11] ``Where are the DST_* constants found in on many systems?'' [A11] {UD} These constants come from the old BSD days and are not used today anymore (even the Linux based glibc does not implement the handling although the constants are defined). Instead GNU libc contains the zone database handling and compatibility code for POSIX TZ environment variable handling. ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ [Q12] ``The `gencat' utility cannot process the input which are successfully used on my Linux libc based system. Why?'' [A12] {UD} Unlike the author of the `gencat' program which is distributed with Linux libc I have read the underlying standards before writing the code. It is completely compatible with the specification given in X/Open Portability Guide. To ease the transition from the Linux version some of the non-standard features are also present in the `gencat' program of GNU libc. This mainly includes the use of symbols for the message number and the automatic generation of header files which contain the needed #defines to map the symbols to integers. Here is a simple SED script to convert at least some Linux specific catalog files to the XPG4 form: ----------------------------------------------------------------------- # Change catalog source in Linux specific format to standard XPG format. # Ulrich Drepper , 1996. # /^\$ #/ { h s/\$ #\([^ ]*\).*/\1/ x s/\$ #[^ ]* *\(.*\)/\$ \1/ } /^# / { s/^# \(.*\)/\1/ G s/\(.*\)\n\(.*\)/\2 \1/ } ----------------------------------------------------------------------- ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ [Q13] ``How do I configure GNU libc so that the essential libraries like libc.so go into /lib and the other into /usr/lib?'' [A13] {UD} Like all other GNU packages GNU libc is configured to use a base directory and install all files relative to this. If you intend to really use GNU libc on your system this base directory is /usr. I.e., you run configure --prefix=/usr Some systems like Linux have a filesystem standard which makes a difference between essential libraries and others. Essential libraries are placed in /lib because this directory is required to be located on the same disk partition as /. The /usr subtree might be found on another partition/disk. To install the essential libraries which come with GNU libc in /lib one must explicitly tell this. Autoconf has no option for this so you have to use the file where all user supplied additional information should go in: `configparms' (see the `INSTALL' file). For Linux the `configparms' file should contain: slibdir=/lib sysconfdir=/etc The first line specifies the directory for the essential libraries, the second line the directory for file which are by tradition placed in a directory named /etc. ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ [Q14] ``When linking with the new libc I get unresolved symbols `crypt' and `setkey'. Why aren't these functions in the libc anymore?'' [A14] {UD} Remember the US restrictions of exporting cryptographic programs and source code. Until this law gets abolished we cannot ship the cryptographic function together with the libc. But of course we provide the code and there is an very easy way to use this code. First get the extra package. People in the US way get it from the same place they got the GNU libc from. People outside the US should get the code from ftp.uni-c.dk [129.142.6.74], or another archive site outside the USA. The README explains how to install the sources. If you already have the crypt code on your system the reason for the failure is probably that you failed to link with -lcrypt. The crypto functions are in a separate library to make it possible to export GNU libc binaries from the US. ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ [Q15] ``What are these `add-ons'?'' [A15] {UD} To avoid complications with external or external source code some optional parts of the libc are distributed as separate packages (e.g., the crypt package, see Q14). To ease the use as part of GNU libc the installer just has to unpack the package and tell the configuration script about these additional subdirectories using the --enable-add-ons option. When you add the crypt add-on you just have to use configure --enable-add-ons=crypt,XXX ... where XXX are possible other add-ons and ... means the rest of the normal option list. You can use add-ons also to overwrite some files in glibc. The add-on system dependent subdirs are search first. It is also possible to add banner files (use a file named `Banner') or create shared libraries. Using add-ons has the big advantage that the makefiles of the GNU libc can be used. Only some few stub rules must be written to get everything running. Even handling of architecture dependent compilation is provided. The GNU libc's sysdeps/ directory shows how to use this feature. ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ [Q16] ``When I use GNU libc on my Linux system by linking against to libc.so which comes with glibc all I get is a core dump.'' [A16] {UD} It is not enough to simply link against the GNU libc library itself. The GNU C library comes with its own dynamic linker which really conforms to the ELF API standard. This dynamic linker must be used. Normally this is done by the compiler. The gcc will use -dynamic-linker /lib/ld-linux.so.1 unless the user specifies her/himself a -dynamic-linker argument. But this is not the correct name for the GNU dynamic linker. The correct name is /lib/ld.so.1 which is the name specified in the SVr4 ABi. To change your environment to use GNU libc for compiling you need to change the `specs' file of your gcc. This file is normally found at /usr/lib/gcc-lib///specs In this file you have to change a few things: - change `ld-linux.so.1' to `ld.so.1' (or to ld-linux.so.2, see below) - remove all expression `%{...:-lgmon}'; there is no libgmon in glibc Things are getting a bit more complicated if you have GNU libc installed in some other place than /usr, i.e., if you do not want to use it instead of the old libc. In this case the needed startup files and libraries are not found in the regular places. So the specs file must tell the compiler and linker exactly what to use. Here is for example the gcc-2.7.2 specs file when GNU libc is installed at /home/gnu: ----------------------------------------------------------------------- *asm: %{V} %{v:%{!V:-V}} %{Qy:} %{!Qn:-Qy} %{n} %{T} %{Ym,*} %{Yd,*} %{Wa,*:%*} *asm_final: %{pipe:-} *cpp: %{fPIC:-D__PIC__ -D__pic__} %{fpic:-D__PIC__ -D__pic__} %{!m386:-D__i486__} %{posix:-D_POSIX_SOURCE} -I/home/gnu/include *cc1: *cc1plus: *endfile: %{!shared:crtend.o%s} %{shared:crtendS.o%s} /home/gnu/lib/crtn.o%s *link: -m elf_i386 -rpath=/home/gnu/lib -L/home/gnu/lib %{shared:-shared} %{!shared: %{!ibcs: %{!static: %{rdynamic:-export-dynamic} %{!dynamic-linker:-dynamic-linker=/home/gnu/lib/ld.so.1}} %{static:-static}}} *lib: %{!shared: %{mieee-fp:-lieee} %{p:-lc_p} %{!p:%{pg:-lc_p} %{!pg:-lc}}} *libgcc: %{!shared:-lgcc} *startfile: %{!shared: %{pg:/home/gnu/lib/gcrt1.o%s} %{!pg:%{p:/home/gnu/lib/gcrt1.o} %{!p:/home/gnu/lib/crt1.o%s}}} /home/gnu/lib/crti.o%s %{!shared:crtbegin.o%s} %{shared:crtbeginS.o%s} *switches_need_spaces: *signed_char: %{funsigned-char:-D__CHAR_UNSIGNED__} *predefines: -D__ELF__ -Dunix -Di386 -Dlinux -Asystem(unix) -Asystem(posix) -Acpu(i386) -Amachine(i386) *cross_compile: 0 *multilib: . ; ----------------------------------------------------------------------- The above is currently correct for all systems but ix86/Linux. Because of compatibility issues on this platform the dynamic linker must have a different name: ld-linux.so.2. So you have to replace %{!dynamic-linker:-dynamic-linker=/home/gnu/lib/ld.so.1} by %{!dynamic-linker:-dynamic-linker=/home/gnu/lib/ld-linux.so.2} in the above example specs file. Future versions of GCC will automatically provide the correct specs. ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ [Q17] ``Looking through the shared libc file I haven't found the functions `stat', `lstat', `fstat', and `mknod' and while linking on my Linux system I get error messages. How is this supposed to work?'' [A17] {RM} Believe it or not, stat and lstat (and fstat, and mknod) are supposed to be undefined references in libc.so.6! Your problem is probably a missing or incorrect /usr/lib/libc.so file; note that this is a small text file now, not a symlink to libc.so.6. It should look something like this: GROUP ( libc.so.6 ld.so.1 libc.a ) {UD} The Linux ldconfig file probably generates a link libc.so -> libc.so.6 in /lib. This is not correct. There must not be such a link. The linker script with the above contents is placed in /usr/lib which is enough for the linker. ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ Answers were given by: {UD} Ulrich Drepper, {DMT} David Mosberger-Tang, {RM} Roland McGrath, Local Variables: mode:text End: