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 -u libc.so' on the produced library I still
  35           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 [Q9]    ``Why does getlogin() always return NULL on my Linux box?''
  41
  42 \f
  43 ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
  44 [Q1]    ``What systems does the GNU C Library run on?''
  45
  46 [A1] {UD} This is difficult to answer.  The file `README' lists the
  47 architectures GNU libc is known to run *at some time*.  This does not
  48 mean that it still can be compiled and run on them in the moment.
  49
  50 The systems glibc is known to work on in the moment and most probably
  51 in the future are:
  52
  53         *-*-gnu                 GNU Hurd
  54         i[3456]86-*-linux       Linux-2.0 on Intel
  55         m68k-*-linux            Linux-2.0 on Motorola 680x0
  56         alpha-*-linux           Linux-2.0 on DEC Alpha
  57
  58 Other Linux platforms are also on the way to be supported but I need
  59 some success reports first.
  60
  61 If you have a system not listed above (or in the `README' file) and
  62 you are really interested in porting it, contact
  63
  64         <bug-glibc@prep.ai.mit.edu>
  65
  66
  67 ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
  68 [Q2]    ``What compiler do I need to build GNU libc?''
  69
  70 [A2] {UD} It is (almost) impossible to compile GNU C Library using a
  71 different compiler than GNU CC.  A lot of extensions of GNU CC are
  72 used to increase the portability and speed.
  73
  74 But this does not mean you have to use GNU CC for using the GNU C
  75 Library.  In fact you should be able to use the native C compiler
  76 because the success only depends on the binutils: the linker and
  77 archiver.
  78
  79 The GNU CC is found like all other GNU packages on
  80         ftp://prep.ai.mit.edu/pub/gnu
  81 or better one of the many mirror sites.
  82
  83 You always should try to use the latest official release.  Older
  84 versions might not have all the features GNU libc could use.
  85
  86
  87 ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
  88 [Q3]    ``When starting `make' I get only errors messages.
  89           What's wrong?''
  90
  91 [A3] {UD} You definitely need GNU make to translate GNU libc.  No
  92 other make program has the needed functionality.
  93
  94 Versions before 3.74 have bugs which prevent correct execution so you
  95 should upgrade to the latest version before starting the compilation.
  96
  97
  98 ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
  99 [Q4]    ``After I changed configure.in I get `Autoconf version X.Y.
 100           or higher is required for this script'.  What can I do?''
 101
 102 [A4] {UD} You have to get the specified autoconf version (or a later)
 103 from your favourite mirror of prep.ai.mit.edu.
 104
 105
 106 ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
 107 [Q5]    ``Do I need a special linker or archiver?''
 108
 109 [A5] {UD} If your native versions are not too buggy you can probably
 110 work with them.  But GNU libc works best with GNU binutils.
 111
 112 On systems where the native linker does not support weak symbols you
 113 will not get a really ISO C compliant C library.  Generally speaking
 114 you should use the GNU binutils if they provide at least the same
 115 functionality as your system's tools.
 116
 117 Always get the newest release of GNU binutils available.
 118 Older releases are known to have bugs that affect building the GNU C
 119 Library.
 120
 121
 122 ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
 123 [Q6]    ``Do I need some more things to compile GNU C Library?''
 124
 125 [A6] {UD} Yes, there are some more :-).
 126
 127 * GNU gettext; the GNU libc is internationalized and partly localized.
 128   For bringing the messages for the different languages in the needed
 129   form the tools from the GNU gettext package are necessary.  See
 130   ftp://prep.ai.mit.edu/pub/gnu or better any mirror site.
 131
 132 * lots of diskspace (for i386-linux this means, e.g., ~70MB).
 133
 134   You should avoid compiling on a NFS mounted device.  This is very
 135   slow.
 136
 137 * plenty of time (approx 1h for i386-linux on i586@133 or 2.5h or
 138   i486@66).
 139
 140   If you have some more measurements let me know.
 141
 142 * Some files depend on special tools.  E.g., files ending in .gperf
 143   need a `gperf' program.  The GNU version (part of libg++) is known
 144   to work while some vendor versions do not.
 145
 146 * When compiling for Linux:
 147
 148   + the header files of the Linux kernel must be available in the
 149     search path of the CPP as <linux/*.h> and <asm/*.h>.
 150
 151 * Some files depend on special tools.  E.g., files ending in .gperf
 152   need a `gperf' program.  The GNU version (part of libg++) is known
 153   to work while some vendor versions do not.
 154
 155 ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
 156 [Q7]    ``When I run `nm -u libc.so' on the produced library I still
 157           find unresolved symbols?  Can this be ok?''
 158
 159 [A7] {UD} Yes, this is ok.  There can be several kinds of unresolved
 160 symbols:
 161
 162 * magic symbols automatically generated by the linker.  Names are
 163   often like __start_* and __stop_*
 164
 165 * symbols starting with _dl_* come from the dynamic linker
 166
 167 * symbols resolved by using libgcc.a
 168   (__udivdi3, __umoddi3, or similar)
 169
 170 * weak symbols, which need not be resolved at all
 171   (currently fabs among others; this gets resolved if the program
 172    is linked against libm, too.)
 173
 174 Generally, you should make sure you find a real program which produces
 175 errors while linking before deciding there is a problem.
 176
 177
 178 ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
 179 [Q8]    ``I expect GNU libc to be 100% source code compatible with
 180           the old Linux based GNU libc.  Why isn't it like this?''
 181
 182 [A8] {DMT,UD} Not every extension in Linux libc's history was well
 183 thought-out.  In fact it had a lot of problems with standards compliance
 184 and with cleanliness.  With the introduction of a new version number these
 185 errors now can be corrected.  Here is a list of the known source code
 186 incompatibilities:
 187
 188 * _GNU_SOURCE: glibc does not automatically define _GNU_SOURCE.  Thus,
 189   if a program depends on GNU extensions or some other non-standard
 190   functionality, it is necessary to compile it with C compiler option
 191   -D_GNU_SOURCE, or better, to put `#define _GNU_SOURCE' at the beginning
 192   of your source files, before any C library header files are included.
 193   This difference normally manifests itself in the form of missing
 194   prototypes and/or data type definitions.  Thus, if you get such errors,
 195   the first thing you should do is try defining _GNU_SOURCE and see if
 196   that makes the problem go away.
 197
 198   For more information consult the file `NOTES' part of the GNU C
 199   library sources.
 200
 201 * reboot(): GNU libc sanitizes the interface of reboot() to be more
 202   compatible with the interface used on other OSes.  In particular,
 203   reboot() as implemented in glibc takes just one argument.  This argument
 204   corresponds to the third argument of the Linux reboot system call.
 205   That is, a call of the form reboot(a, b, c) needs to be changed into
 206   reboot(c).
 207      Beside this the header <sys/reboot.h> defines the needed constants
 208   for the argument.  These RB_* constants should be used instead of the
 209   cryptic magic numbers.
 210
 211 * swapon(): the interface of this function didn't changed, but the
 212   prototype is in a separate header file <sys/swap.h>.  For the additional
 213   argument of of swapon() you should use the SWAP_* constants from
 214   <linux/swap.h>, which get defined when <sys/swap.h> is included.
 215
 216 * errno: If a program uses variable "errno", then it _must_ include header
 217   file <errno.h>.  The old libc often (erroneously) declared this variable
 218   implicitly as a side-effect of including other libc header files.  glibc
 219   is careful to avoid such namespace pollution, which, in turn, means that
 220   you really need to include the header files that you depend on.  This
 221   difference normally manifests itself in the form of the compiler
 222   complaining about the references of the undeclared symbol "errno".
 223
 224 * Linux-specific syscalls: All Linux system calls now have appropriate
 225   library wrappers and corresponding declarations in various header files.
 226   This is because the syscall() macro that was traditionally used to
 227   work around missing syscall wrappers are inherently non-portable and
 228   error-prone.  The following tables lists all the new syscall stubs,
 229   the header-file declaring their interface and the system call name.
 230
 231        syscall name:    wrapper name:   declaring header file:
 232        -------------    -------------   ----------------------
 233        bdflush          bdflush         <sys/kdaemon.h>
 234        create_module    create_module   <sys/module.h>
 235        delete_module    delete_module   <sys/module.h>
 236        get_kernel_syms  get_kernel_syms <sys/module.h>
 237        init_module      init_module     <sys/module.h>
 238        syslog           ksyslog_ctl     <sys/klog.h>
 239
 240 * lpd: Older versions of lpd depend on a routine called _validuser().
 241   The library does not provide this function, but instead provides
 242   __ivaliduser() which has a slightly different interfaces.  Simply
 243   upgrading to a newer lpd should fix this problem (e.g., the 4.4BSD
 244   lpd is known to be working).
 245
 246
 247 ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
 248 [Q9]    ``Why does getlogin() always return NULL on my Linux box?''
 249
 250 [A9] {UD} The GNU C library has a format for the UTMP and WTMP file
 251 which differs from what your system currently has.  It was extended to
 252 fulfill the needs of the next years when IPv6 is introduced.  So the
 253 record size is different, fields might have a different position and
 254 so reading the files written by functions from the one library cannot
 255 be read by functions from the other library.  Sorry, but this is what
 256 a major release is for.  It's better to have a cut now than having no
 257 means to support the new techniques later.
 258
 259
 260 ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
 261
 262 \f
 263 Answers were given by:
 264 {UD} Ulrich Drepper, <drepper@cygnus.com>
 265 {DMT} David Mosberger-Tang, <davidm@AZStarNet.com>
 266
 267 Amended by:
 268 {RM} Roland McGrath, <roland@gnu.ai.mit.edu>
 269 \f
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