FAQ

   1                 Frequently Asked Question on GNU C Library
   2
   3 As every FAQ this one also tries to answer questions the user might have
   4 when using the pacakge.  Please make sure you read this before sending
   5 questions or bug reports to the maintainers.
   6
   7 The GNU C Library is very complex.  The building process exploits the
   8 features available in tools generally available.  But many things can
   9 only be done using GNU tools.  Also the code is sometimes hard to
  10 understand because it has to be portable but on the other hand must be
  11 fast.  But you need not understand the details to use GNU C Library.
  12 This will only be necessary if you intend to contribute or change it.
  13
  14 If you have any questions you think should be answered in this document,
  15 please let me know.
  16
  17                                                   --drepper@cygnus.com
  18 \f
  19 ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
  20 [Q1]    ``What systems does the GNU C Library run on?''
  21
  22 [Q2]    ``What compiler do I need to build GNU libc?''
  23
  24 [Q3]    ``When starting make I get only error messages.
  25           What's wrong?''
  26
  27 [Q4]    ``After I changed configure.in I get `Autoconf version X.Y.
  28           or higher is required for this script'.  What can I do?''
  29
  30 [Q5]    ``Do I need a special linker or archiver?''
  31
  32 [Q6]    ``Do I need some more things to compile GNU C Library?''
  33
  34 [Q7]    ``When I run `nm libc.so|grep " U "' on the produced library
  35           I still find unresolved symbols?  Can this be ok?''
  36
  37 [Q8]    ``I expect GNU libc to be 100% source code compatible with
  38           the old Linux based GNU libc.  Why isn't it like this?''
  39
  40 \f
  41 ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
  42 [Q1]    ``What systems does the GNU C Library run on?''
  43
  44 [A1] {UD} This is difficult to answer.  The file `README' lists the
  45 architectures GNU libc is known to run *at some time*.  This does not
  46 mean that it still can be compiled and run on them in the moment.
  47
  48 The systems glibc is known to work on in the moment and most probably
  49 in the future are:
  50
  51         *-*-gnu                 GNU Hurd
  52         i[3456]86-*-linux       Linux-2.0 on Intel
  53
  54 Other Linux platforms are also on the way to be supported but I need
  55 some success reports first.
  56
  57 If you have a system not listed above (or in the `README' file) and
  58 you are really interested in porting it, contact
  59
  60         <bug-glibc@prep.ai.mit.edu>
  61
  62
  63 ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
  64 [Q2]    ``What compiler do I need to build GNU libc?''
  65
  66 [A2] {UD} It is (almost) impossible to compile GNU C Library using a
  67 different compiler than GNU CC.  A lot of extensions of GNU CC are
  68 used to increase the portability and speed.
  69
  70 But this does not mean you have to use GNU CC for using the GNU C
  71 Library.  In fact you should be able to use the native C compiler
  72 because the success only depends on the binutils: the linker and
  73 archiver.
  74
  75 The GNU CC is found like all other GNU packages on
  76         ftp://prep.ai.mit.edu/pub/gnu
  77 or better one of the many mirrors.
  78
  79 You always should try to use the latest official release.  Older
  80 versions might not have all the features GNU libc could use.
  81
  82
  83 ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
  84 [Q3]    ``When starting make I get only errors messages.
  85           What's wrong?''
  86
  87 [A3] {UD} You definitely need GNU make to translate GNU libc.  No
  88 other make program has the needed functionality.
  89
  90 Versions before 3.74 have bugs which prevent correct execution so you
  91 should upgrade to the latest version before starting the compilation.
  92
  93
  94 ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
  95 [Q4]    ``After I changed configure.in I get `Autoconf version X.Y.
  96           or higher is required for this script'.  What can I do?''
  97
  98 [A4] {UD} You have to get the specified autoconf version (or a later)
  99 from your favourite mirror of prep.ai.mit.edu.
 100
 101
 102 ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
 103 [Q5]    ``Do I need a special linker or archiver?''
 104
 105 [A5] {UD} If your native versions are not too buggy you can probably
 106 work with them.  But GNU libc works best with GNU binutils.
 107
 108 On systems where the native linker does not support weak symbols you
 109 will not get a really ISO C compliant C library.  Generally speaking
 110 you should use the GNU binutils if they provide at least the same
 111 functionality as your system's tools.
 112
 113 Always get the newest release of GNU binutils available.
 114 Older releases are known to have bugs that affect building the GNU C library.
 115
 116
 117 ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
 118 [Q6]    ``Do I need some more things to compile GNU C Library?''
 119
 120 [A6] {UD} Yes, there are some more :-).
 121
 122 * lots of diskspace (for i386-linux this means, e.g., ~70MB).
 123
 124   You should avoid compiling on a NFS mounted device.  This is very
 125   slow.
 126
 127 * plenty of time (approx 1h for i386-linux on i586@133 or 2.5h or
 128   i486@66).
 129
 130   If you are interested in some more measurements let me know.
 131
 132
 133 * When compiling for Linux:
 134
 135   + the header files of the Linux kernel must be available in the
 136     search path of the CPP as <linux/*.h> and <asm/*.h>.
 137
 138
 139 ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
 140 [Q7]    ``When I run `nm libc.so|grep " U "' on the produced library
 141           I still find unresolved symbols?  Can this be ok?''
 142
 143 [A7] {UD} Yes, this is ok.  There can be several kinds of unresolved
 144 symbols:
 145
 146 * magic symbols automatically generated by the linker.  Names are
 147   often like __start_* and __stop_*
 148
 149 * symbols resolved by using libgcc.a
 150   (__udivdi3, __umoddi3, or similar)
 151
 152 * weak symbols, which need not be resolved at all
 153   (currently fabs among others; this gets resolved if the program
 154    is linked against libm, too.)
 155
 156 Generally, you should make sure you find a real program which produces
 157 errors while linking before deciding there is a problem.
 158
 159
 160 ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
 161 [Q8]    ``I expect GNU libc to be 100% source code compatible with
 162           the old Linux based GNU libc.  Why isn't it like this?''
 163
 164 [A8] {DMT} Not every extension in Linux libc's history was well
 165 thought-out.  In fact it had a lot of problems with standards compliance
 166 and with cleanliness.  With the introduction of a new version number these
 167 errors now can be corrected.  Here is a list of the known source code
 168 incompatibilities:
 169
 170 * _GNU_SOURCE: glibc does not automatically define _GNU_SOURCE.  Thus,
 171   if a program depends on GNU extensions or some other non-standard
 172   functionality, it is necessary to compile it with C compiler option
 173   -D_GNU_SOURCE, or better, to put `#define _GNU_SOURCE' at the beginning
 174   of your source files, before any C library header files are included.
 175   This difference normally manifests itself in the form of missing
 176   prototypes and/or data type definitions.  Thus, if you get such errors,
 177   the first thing you should do is try defining _GNU_SOURCE and see if
 178   that makes the problem go away.
 179
 180   For more information consult the file `NOTES' part of the GNU C
 181   library sources.
 182
 183 * reboot(): GNU libc sanitizes the interface of reboot() to be more
 184   compatible with the interface used on other OSes.  In particular,
 185   reboot() as implemented in glibc takes just one argument.  This argument
 186   corresponds to the third argument of the Linux reboot system call.
 187   That is, a call of the form reboot(a, b, c) needs to be changed into
 188   reboot(c).
 189
 190 * errno: If a program uses variable "errno", then it _must_ include header
 191   file <errno.h>.  The old libc often (erroneously) declared this variable
 192   implicitly as a side-effect of including other libc header files.  glibc
 193   is careful to avoid such namespace pollution, which, in turn, means that
 194   you really need to include the header files that you depend on.  This
 195   difference normally manifests itself in the form of the compiler
 196   complaining about the references of the undeclared symbol "errno".
 197
 198 * Linux-specific syscalls: All Linux system calls now have appropriate
 199   library wrappers and corresponding declarations in various header files.
 200   This is because the syscall() macro that was traditionally used to
 201   work around missing syscall wrappers are inherently non-portable and
 202   error-prone.  The following tables lists all the new syscall stubs,
 203   the header-file declaring their interface and the system call name.
 204
 205        syscall name:    wrapper name:   declaring header file:
 206        -------------    -------------   ----------------------
 207        bdflush          bdflush         ???
 208        create_module    create_module   <sys/module.h>
 209        delete_module    delete_module   <sys/module.h>
 210        get_kernel_syms  get_kernel_syms <sys/module.h>
 211        init_module      init_module     <sys/module.h>
 212        syslog           ksyslog_ctl     ???
 213
 214 ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
 215
 216 \f
 217 Answers were given by:
 218 {UD} Ulrich Drepper, <drepper@cygnus.com>
 219 {DMT} David Mosberger-Tang, <davidm@AZStarNet.com>
 220
 221 Amended by:
 222 {RM} Roland McGrath, <roland@gnu.ai.mit.edu>
 223 \f
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