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?'' ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ [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 i386-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 i386-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 documenation 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. ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ Answers were given by: {UD} Ulrich Drepper, {DMT} David Mosberger-Tang, Amended by: {RM} Roland McGrath, Local Variables: mode:text End: