219 lines
8.4 KiB
Plaintext
219 lines
8.4 KiB
Plaintext
Frequently Asked Question on GNU C Library
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As every FAQ this one also tries to answer the questions the user
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might when using the pacakge. Please make sure you read this before
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sending questions/bug reports to the maintainers.
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The GNU C Library is very complex. The building process exploits the
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features available in tools generally available. But many things can
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only be done using GNU tools. Also the code is sometimes hard to
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understand because it has to be portable but on the other hand must be
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fast. But you need not understand the details to use GNU C Library.
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This will only be necessary if you intend to contribute or change it.
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If you have any question which you think might be worth answered in
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this document let me know.
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--drepper@cygnus.com
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~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
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[Q1] ``What systems the GNU C Library runs on?''
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[Q2] ``What compiler do I need to translate GNU libc?''
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[Q3] ``When starting make I get only errors messages.
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What's wrong?''
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[Q4] ``After I changed configure.in I get `Autoconf version X.Y.
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or higher is required for this script'. What can I do?''
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[Q5] ``Do I need a special linker or archiver?''
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[Q6] ``Do I need some more things to compile GNU C Library?''
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[Q7] ``When I run `nm libc.so|grep " U "' on the produced library
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I still find unresolved symbols? Can this be ok?''
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[Q8] ``I expect GNU libc to be 100% source code compatible with
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the old Linux based GNU libc. Why isn't it like this?''
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~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
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[Q1] ``What systems the GNU C Library runs on?''
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[A1] {UD} This is difficult to answer. The file `README' lists the
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architectures GNU libc is known to run *at some time*. This does not
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mean that it still can be compiled and run on them in the moment.
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The systems glibc is known to work on in the moment and most probably
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in the future are:
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*-*-gnu GNU Hurd
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i[3456]86-*-linux Linux-2.0 on Intel
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Other Linux platforms are also on the way to be supported but I need
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some success reports first.
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If you have a system not listed above (or in the `README' file) and
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you are really interested in porting it, contact
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Roland McGrath <roland@gnu.ai.mit.edu>
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or Ulrich Drepper <drepper@cygnus.com>
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~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
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[Q2] ``What compiler do I need to translate GNU libc?''
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[A2] {UD} It is (almost) impossible to compile GNU C Library using a
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different compiler than GNU CC. A lot of extensions of GNU CC are
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used to increase the portability and speed.
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But this does not mean you have to use GNU CC for using the GNU C
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Library. In fact you should be able to use the native C compiler
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because the success only depends on the binutils: the linker and
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archiver.
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The GNU CC is found like all other GNU packages on
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ftp://prep.ai.mit.edu/pub/gnu
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or better one of the many mirrors.
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You always should try to use the latest official release. Older
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versions might not have all the features GNU libc could use.
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~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
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[Q3] ``When starting make I get only errors messages.
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What's wrong?''
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[A3] {UD} You definitely need GNU make to translate GNU libc. No
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other make program has the needed functionality.
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Versions before 3.74 have bugs which prevent correct execution so you
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should upgrade to the latest version before starting the compilation.
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~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
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[Q4] ``After I changed configure.in I get `Autoconf version X.Y.
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or higher is required for this script'. What can I do?''
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[A4] {UD} You have to get the specified autoconf version (or a later)
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from your favourite mirror of prep.ai.mit.edu.
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~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
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[Q5] ``Do I need a special linker or archiver?''
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[A5] {UD} If your native versions are not too buggy you can work with
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them. But GNU libc works best with GNU binutils.
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On systems where the native linker does not support weak symbols you
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will not get a really ISO C compliant C library. Generally speaking
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you should use the GNU binutils if they provide at least the same
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functionality as your system's tools.
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~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
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[Q6] ``Do I need some more things to compile GNU C Library?''
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[A6] {UD} Yes, there are some more :-).
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* lots of diskspace (for i386-linux this means, e.g., ~70MB)
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You should avoid compiling on a NFS mounted device. This is very
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slow.
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* plenty of time (approx 1h for i386-linux on i586@133 or 2.5h or
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i486@66).
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If you have some more interested measurements let me know.
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~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
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[Q7] ``When I run `nm libc.so|grep " U "' on the produced library
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I still find unresolved symbols? Can this be ok?''
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[A7] {UD} Yes, this is ok. There can be several kinds of unresolved
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symbols:
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* magic symbols automatically generated by the linker. Names are
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often like __start_* and __stop_*-
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* symbols resolved by using libgcc.a
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(__udivdi3, __umoddi3, or similar)
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* weak symbols, which need not be resolved at all
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(currently fabs among others; this gets resolved if the program
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is linked against libm, too.)
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Generally, you should make sure you find a real program which produces
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errors while linking.
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~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
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[Q8] ``I expect GNU libc to be 100% source code compatible with
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the old Linux based GNU libc. Why isn't it like this?''
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[A8] {DMT} Not every extension in Linux libc's history was well
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thought. In fact it had a lot of problems with standard compliance
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and cleanliness. With the introduction of a new version number these
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errors now can be corrected. The following list shows a list of the
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know source code incompatibilities.
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* _GNU_SOURCE: glibc does not automatically define _GNU_SOURCE. Thus,
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if a program depends on GNU extensions, it is necessary
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to compile it with C compiler option -D_GNU_SOURCE. This difference
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normally mainfests itself in the form of missing prototypes and/or
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data type definitions. Thus, if you get such errors, the first thing you
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should do is grep the header files in /usr/include and /usr/include/sys
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to check whether the functions are really missing or whether it is
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just necessary to add a define of _GNU_SOURCE. Similar comments apply
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to _BSD_SOURCE, _POSIX_SOURCE, _SVID_SOURCE etc (see
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/usr/include/features.h).
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* reboot(): GNU libc sanitizes the interface of reboot() to be more
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compatible with the interface used on other OSes. In particular,
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reboot() as implemented in glibc takes just one argument. This argument
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corresponds to the third argument of the Linux reboot system call.
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That is, a call of the form reboot(a, b, c) needs to be changed into
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reboot(c).
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* errno: If a program uses variable "errno", then it _must_ include header
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file <errno.h>. The old libc often (erroneously) declared this variable
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implicitly as a side-effect of including other libc header files. glibc
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is careful to avoid such namespace pollution, which, in turn, means that
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you really need to include the header files that you depend on. This
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difference normally manifests itself in the form of the compiler
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complaining about the references of the undeclared symbol "errno".
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* Linux-specific syscalls: All Linux system calls now have appropriate
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library wrappers and corresponding declarations in various header files.
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This is because the syscall() macro that was traditionally used to
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work around missing syscall wrappers are inherently non-portable and
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error-prone. The following tables lists all the new syscall stubs,
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the header-file declaring their interface and the system call name.
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syscall name: wrapper name: declaring header file:
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------------- ------------- ----------------------
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bdflush bdflush <unistd.h>
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create_module create_module <sys/module.h>
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delete_module delete_module <sys/module.h>
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get_kernel_syms get_kernel_syms <sys/module.h>
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init_module init_module <sys/module.h>
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syslog ksyslog_ctl <unistd.h>
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To get the Linux-specific declarations in <unistd.h>, you'll need
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to define C pre-processor macro _LINUX_SOURCE during compilation.
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~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
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Answers were given by:
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{UD} Ulrich Drepper, <drepper@cygnus.com>
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{DMT} David Mosberger-Tang, <davidm@AZStarNet.com>
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Local Variables:
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mode:text
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End:
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