FAQ   [plain text]

                        GCC Frequently Asked Questions

   The   latest   version   of  this  document  is  always  available  at

   This  FAQ  tries  to  answer  specific  questions  concerning GCC. For
   general  information  regarding C, C++, resp. Fortran please check the
   [2]comp.lang.c   FAQ,   [3]comp.std.c++   FAQ,   and   the  [4]Fortran
   Information page.

   Other GCC-related FAQs: [5]libstdc++-v3, and [6]GCJ.


    1. [7]General information
         1. [8]What is the relationship between GCC and EGCS?
         2. [9]What is an open development model?
         3. [10]How do I get a bug fixed or a feature added?
         4. [11]Does GCC work on my platform?
    2. [12]Installation
         1. [13]How to install multiple versions of GCC
         2. [14]Dynamic linker is unable to find GCC libraries
         3. [15]libstdc++/libio tests fail badly with --enable-shared
         4. [16]GCC can not find GNU as/GNU ld
         5. [17]cpp: Usage:... Error
         6. [18]Optimizing the compiler itself
         7. [19]Why does libiconv get linked into jc1 on Solaris?
    3. [20]Testsuite problems
         1. [21]How do I pass flags like -fnew-abi to the testsuite?
         2. [22]How can I run the test suite with multiple options?
    4. [23]Older versions of GCC
         1. [24]Is there a stringstream / sstream for GCC 2.95.2?
    5. [25]Miscellaneous
         1. [26]Friend Templates
         2. [27]dynamic_cast,   throw,  typeid  don't  work  with  shared
         3. [28]Why do I need autoconf, bison, xgettext, automake, etc?
         4. [29]Why can't I build a shared library?
         5. [30]When  building  C++,  the  linker  says  my constructors,
            destructors  or  virtual  tables are undefined, but I defined
         6. [31]Will GCC someday include an incremental linker?

                              General information

What is the relationship between GCC and EGCS?

   In  1990/1991  gcc version 1 had reached a point of stability. For the
   targets  it could support, it worked well. It had limitations inherent
   in  its  design  that would be difficult to resolve, so a major effort
   was  made  to  resolve  those  limitations  and  gcc version 2 was the

   When  we  had  gcc2  in  a  useful  state, development efforts on gcc1
   stopped  and we all concentrated on making gcc2 better than gcc1 could
   ever  be.  This is the kind of step forward we wanted to make with the
   EGCS project when it was formed in 1997.

   In   April   1999  the  Free  Software  Foundation  officially  halted
   development on the gcc2 compiler and appointed the EGCS project as the
   official  GCC  maintainers.  The net result was a single project which
   carries  forward  GCC  development  under  the ultimate control of the
   [32]GCC Steering Committee.

What is an open development model?

   We  are  using  a bazaar style [33][1] approach to GCC development: we
   make  snapshots publicly available to anyone who wants to try them; we
   welcome  anyone  to  join  the  development  mailing  list. All of the
   discussions on the development mailing list are available via the web.
   We're  going  to  be making releases with a much higher frequency than
   they have been made in the past.

   In  addition  to  weekly  snapshots of the GCC development sources, we
   have  the sources readable from a CVS server by anyone. Furthermore we
   are  using  remote CVS to allow remote maintainers write access to the

   There  have  been  many  potential GCC developers who were not able to
   participate  in  GCC  development in the past. We want these people to
   help  in  any  way  they  can;  we  ultimately want GCC to be the best
   compiler in the world.

   A  compiler  is  a  complicated piece of software, there will still be
   strong  central  maintainers  who will reject patches, who will demand
   documentation  of  implementations,  and  who  will  keep the level of
   quality  as high as it is today. Code that could use wider testing may
   be integrated--code that is simply ill-conceived won't be.

   GCC  is  not  the first piece of software to use this open development
   process;  FreeBSD, the Emacs lisp repository, and the Linux kernel are
   a few examples of the bazaar style of development.

   With  GCC, we are adding new features and optimizations at a rate that
   has  not  been  done  since  the  creation  of  gcc2;  these additions
   inevitably  have  a temporarily destabilizing effect. With the help of
   developers  working  together  with this bazaar style development, the
   resulting  stability  and quality levels will be better than we've had

     [1]  We've  been discussing different development models a lot over
     the past few months. The paper which started all of this introduced
     two   terms:   A   cathedral  development  model  versus  a  bazaar
     development  model.  The paper is written by Eric S. Raymond, it is
     called  ``The  Cathedral  and  the  Bazaar''. The paper is a useful
     starting point for discussions.

How do I get a bug fixed or a feature added?

   There  are  lots of ways to get something fixed. The list below may be
   incomplete,  but  it covers many of the common cases. These are listed
   roughly  in  order  of decreasing difficulty for the average GCC user,
   meaning  someone who is not skilled in the internals of GCC, and where
   difficulty  is  measured in terms of the time required to fix the bug.
   No  alternative  is  better  than any other; each has its benefits and
     * Fix  it yourself. This alternative will probably bring results, if
       you  work  hard enough, but will probably take a lot of time, and,
       depending  on  the quality of your work and the perceived benefits
       of  your  changes,  your  code may or may not ever make it into an
       official release of GCC.
     * [34]Report  the  problem  to  the GCC bug tracking system and hope
       that  someone will be kind enough to fix it for you. While this is
       certainly  possible, and often happens, there is no guarantee that
       it  will. You should not expect the same response from this method
       that  you  would  see from a commercial support organization since
       the  people  who read GCC bug reports, if they choose to help you,
       will be volunteering their time.
     * Hire  someone  to  fix it for you. There are various companies and
       individuals  providing  support  for  GCC.  This alternative costs
       money, but is relatively likely to get results.

Does GCC work on my platform?

   The   host/target   specific   installation   notes  for  GCC  include
   information  about  known  problems  with  installing  or using GCC on
   particular  platforms. These are included in the sources for a release
   in   INSTALL/specific.html,  and  the  [35]latest  version  is  always
   available  at  the  GCC web site. Reports of [36]successful builds for
   several versions of GCC are also available at the web site.


How to install multiple versions of GCC

   It  may  be  desirable to install multiple versions of the compiler on
   the  same  system. This can be done by using different prefix paths at
   configure time and a few symlinks.

   Basically,   configure  the  two  compilers  with  different  --prefix
   options,  then  build and install each compiler. Assume you want "gcc"
   to be the latest compiler and available in /usr/local/bin; also assume
   that  you want "gcc2" to be the older gcc2 compiler and also available
   in /usr/local/bin.

   The  easiest  way  to  do  this  is  to  configure  the  new  GCC with
   --prefix=/usr/local/gcc      and      the      older     gcc2     with
   --prefix=/usr/local/gcc2.  Build and install both compilers. Then make
   a  symlink  from /usr/local/bin/gcc to /usr/local/gcc/bin/gcc and from
   /usr/local/bin/gcc2  to  /usr/local/gcc2/bin/gcc. Create similar links
   for the "g++", "c++" and "g77" compiler drivers.

   An   alternative   to   using   symlinks   is   to  configure  with  a
   --program-transform-name  option.  This option specifies a sed command
   to  process  installed  program  names  with.  Using  it  you can, for
   instance, have all the new GCC programs installed as "new-gcc" and the
   like.  You  will  still have to specify different --prefix options for
   new  GCC  and old GCC, because it is only the executable program names
   that are transformed. The difference is that you (as administrator) do
   not  have  to set up symlinks, but must specify additional directories
   in your (as a user) PATH. A complication with --program-transform-name
   is  that the sed command invariably contains characters significant to
   the  shell,  and  these  have  to be escaped correctly, also it is not
   possible  to  use  "^"  or  "$"  in the command. Here is the option to
   prefix "new-" to the new GCC installed programs:


   With the above --prefix option, that will install the new GCC programs
   into  /usr/local/gcc/bin  with  names  prefixed by "new-". You can use
   --program-transform-name  if  you  have  multiple versions of GCC, and
   wish to be sure about which version you are invoking.

   If  you use --prefix, GCC may have difficulty locating a GNU assembler
   or  linker on your system, [37]GCC can not find GNU as/GNU ld explains
   how to deal with this.

   Another  option  that may be easier is to use the --program-prefix= or
   --program-suffix=  options  to  configure. So if you're installing GCC
   2.95.2  and  don't  want  to  disturb  the  current  version of GCC in
   /usr/local/bin/, you could do

     configure --program-suffix=-2.95.2 <other configure options>

   This should result in GCC being installed as /usr/local/bin/gcc-2.95.2
   instead of /usr/local/bin/gcc.

Dynamic linker is unable to find GCC libraries

   This problem manifests itself by programs not finding shared libraries
   they  depend on when the programs are started. Note this problem often
   manifests  itself  with  failures  in  the libio/libstdc++ tests after
   configuring with --enable-shared and building GCC.

   GCC  does  not  specify  a runpath so that the dynamic linker can find
   dynamic libraries at runtime.

   The  short  explanation  is that if you always pass a -R option to the
   linker,  then  your programs become dependent on directories which may
   be NFS mounted, and programs may hang unnecessarily when an NFS server
   goes down.

   The  problem  is  not  programs that do require the directories; those
   programs  are  going  to  hang  no  matter what you do. The problem is
   programs that do not require the directories.

   SunOS  effectively always passed a -R option for every -L option; this
   was  a  bad  idea,  and  so  it was removed for Solaris. We should not
   recreate it.

   However,  if  you  feel  you  really  need such an option to be passed
   automatically  to  the  linker,  you may add it to the GCC specs file.
   This  file  can  be found in the same directory that contains cc1 (run
   gcc -print-prog-name=cc1 to find it). You may add linker flags such as
   -R  or  -rpath, depending on platform and linker, to the *link or *lib

   Another  alternative is to install a wrapper script around gcc, g++ or
   ld  that  adds  the  appropriate directory to the environment variable
   LD_RUN_PATH or equivalent (again, it's platform-dependent).

   Yet another option, that works on a few platforms, is to hard-code the
   full  pathname  of  the  library  into  its  soname.  This can only be
   accomplished   by   modifying   the   appropriate   .ml   file  within
   libstdc++/config (and also libg++/config, if you are building libg++),
   so  that $(libdir)/ appears just before the library name in -soname or
   -h options.

GCC can not find GNU as/GNU ld

   GCC  searches the PATH for an assembler and a loader, but it only does
   so after searching a directory list hard-coded in the GCC executables.
   Since,  on most platforms, the hard-coded list includes directories in
   which  the  system  assembler and loader can be found, you may have to
   take  one  of  the  following actions to arrange that GCC uses the GNU
   versions of those programs.

   To ensure that GCC finds the GNU assembler (the GNU loader), which are
   required  by  [38]some configurations, you should configure these with
   the same --prefix option as you used for GCC. Then build & install GNU
   as (GNU ld) and proceed with building GCC.

   Another  alternative is to create links to GNU as and ld in any of the
   directories  printed  by  the  command  `gcc -print-search-dirs | grep
   '^programs:''.  The  link  to  `ld'  should be named `real-ld' if `ld'
   already exists. If such links do not exist while you're compiling GCC,
   you  may  have to create them in the build directories too, within the
   gcc directory and in all the gcc/stage* subdirectories.

   GCC  2.95 allows you to specify the full pathname of the assembler and
   the linker to use. The configure flags are `--with-as=/path/to/as' and
   `--with-ld=/path/to/ld'.  GCC  will  try to use these pathnames before
   looking  for  `as'  or `(real-)ld' in the standard search dirs. If, at
   configure-time,  the specified programs are found to be GNU utilities,
   `--with-gnu-as' and `--with-gnu-ld' need not be used; these flags will
   be  auto-detected.  One drawback of this option is that it won't allow
   you  to  override  the  search  path  for  assembler  and  linker with
   command-line options -B/path/ if the specified filenames exist.

cpp: Usage:... Error

   If  you  get  an  error like this when building GCC (particularly when
   building   __mulsi3),  then  you  likely  have  a  problem  with  your
   environment variables.
  cpp: Usage: /usr/lib/gcc-lib/i586-unknown-linux-gnulibc1/
  [switches] input output

   First   look   for   an   explicit   '.'  in  either  LIBRARY_PATH  or
   GCC_EXEC_PREFIX  from your environment. If you do not find an explicit
   '.',  look  for an empty pathname in those variables. Note that ':' at
   either the start or end of these variables is an implicit '.' and will
   cause problems.

   Also note '::' in these paths will also cause similar problems.

Optimizing the compiler itself

   If  you  want to test a particular optimization option, it's useful to
   try  bootstrapping  the  compiler  with  that  option  turned  on. For
   example, to test the -fssa option, you could bootstrap like this:
make BOOT_CFLAGS="-O2 -fssa" bootstrap

Why does libiconv get linked into jc1 on Solaris?

   The  Java  front end requires iconv. If the compiler used to bootstrap
   GCC  finds  libiconv  (because  the  GNU  version of libiconv has been
   installed in the same prefix as the bootstrap compiler), but the newly
   built GCC does not find the library (because it will be installed with
   a  different  prefix), then a link-time error will occur when building
   jc1.  This  problem  does  not show up so often on platforms that have
   libiconv  in  a  default  location  (like  /usr/lib) because then both
   compilers  can  find  a  library  named  libiconv, even though it is a
   different library.

   Using  --disable-nls  at  configure-time does not prevent this problem
   because   jc1   uses  iconv  even  in  that  case.  Solutions  include
   temporarily  removing  the  GNU  libiconv,  copying  it  to  a default
   location   such   as   /usr/lib/,   and  using  --enable-languages  at
   configure-time to disable Java.

                              Testsuite problems

How do I pass flags like -fnew-abi to the testsuite?

   If  you  invoke  runtest directly, you can use the --tool_opts option,
  runtest --tool_opts "-fnew-abi -fno-honor-std" <other options>

   Or,  if you use make check you can use the make variable RUNTESTFLAGS,
  make RUNTESTFLAGS="--tool_opts '-fnew-abi -fno-honor-std'" check-g++

How can I run the test suite with multiple options?

   If you invoke runtest directly, you can use the --target_board option,
  runtest --target_board "unix{-fPIC,-fpic,}" <other options>

   Or,  if you use make check you can use the make variable RUNTESTFLAGS,
  make RUNTESTFLAGS="--target_board 'unix{-fPIC,-fpic,}'" check-gcc

   Either  of  these  examples  will run the tests three times. Once with
   -fPIC, once with -fpic, and once with no additional flags.

   This technique is particularly useful on multilibbed targets.

                        Older versions of GCC and EGCS

Is there a stringstream / sstream for GCC 2.95.2?

   Yes, it's at:


Friend Templates

   In order to make a specialization of a template function a friend of a
   (possibly  template)  class, you must explicitly state that the friend
   function  is  a template, by appending angle brackets to its name, and
   this  template  function  must  have  been declared already. Here's an
template <typename T> class foo {
  friend void bar(foo<T>);

   The  above  declaration declares a non-template function named bar, so
   it  must  be  explicitly  defined  for  each  specialization of foo. A
   template  definition of bar won't do, because it is unrelated with the
   non-template declaration above. So you'd have to end up writing:
void bar(foo<int>) { /* ... */ }
void bar(foo<void>) { /* ... */ }

   If  you  meant  bar  to  be  a  template  function,  you  should  have
   forward-declared it as follows. Note that, since the template function
   declaration  refers  to the template class, the template class must be
   forward-declared too:
template <typename T>
class foo;

template <typename T>
void bar(foo<T>);

template <typename T>
class foo {
  friend void bar<>(foo<T>);

template <typename T>
void bar(foo<T>) { /* ... */ }

   In  this case, the template argument list could be left empty, because
   it  can  be  implicitly  deduced  from the function arguments, but the
   angle  brackets  must  be  present,  otherwise the declaration will be
   taken  as a non-template function. Furthermore, in some cases, you may
   have   to   explicitly  specify  the  template  arguments,  to  remove

   An error in the last public comment draft of the ANSI/ISO C++ Standard
   and  the  fact  that previous releases of GCC would accept such friend
   declarations  as  template declarations has led people to believe that
   the forward declaration was not necessary, but, according to the final
   version of the Standard, it is.

dynamic_cast, throw, typeid don't work with shared libraries

   The new C++ ABI in the GCC 3.0 series uses address comparisons, rather
   than string compares, to determine type equality. This leads to better
   performance.  Like  other objects that have to be present in the final
   executable,  these  std::type_info  objects  have what is called vague
   linkage  because  they  are  not  tightly  bound to any one particular
   translation  unit  (object file). The compiler has to emit them in any
   translation  unit  that  requires their presence, and then rely on the
   linking  and  loading  process  to  make sure that only one of them is
   active  in  the  final  executable.  With  static linking all of these
   symbols  are  resolved at link time, but with dynamic linking, further
   resolution occurs at load time. You have to ensure that objects within
   a  shared  library  are resolved against objects in the executable and
   other shared libraries.
     * For  a  program  which  is  linked  against  a  shared library, no
       additional precautions are needed.
     * You  cannot  create a shared library with the "-Bsymbolic" option,
       as that prevents the resolution described above.
     * If  you  use dlopen to explicitly load code from a shared library,
       you  must do several things. First, export global symbols from the
       executable  by  linking  it  with  the "-E" flag (you will have to
       specify  this  as  "-Wl,-E"  if you are invoking the linker in the
       usual  manner  from  the compiler driver, g++). You must also make
       the   external   symbols  in  the  loaded  library  available  for
       subsequent  libraries by providing the RTLD_GLOBAL flag to dlopen.
       The symbol resolution can be immediate or lazy.

   Template  instantiations  are  another,  user visible, case of objects
   with vague linkage, which needs similar resolution. If you do not take
   the  above precautions, you may discover that a template instantiation
   with  the same argument list, but instantiated in multiple translation
   units,  has several addresses, depending in which translation unit the
   address  is  taken.  (This  is  not  an exhaustive list of the kind of
   objects  which  have  vague  linkage  and  are expected to be resolved
   during linking & loading.)

   If  you  are  worried  about  different  objects  with  the  same name
   colliding  during  the linking or loading process, then you should use
   namespaces  to  disambiguate them. Giving distinct objects with global
   linkage  the same name is a violation of the One Definition Rule (ODR)

   For more details about the way that GCC implements these and other C++
   features,   please   read   the   [40]ABI   specification.   Note  the
   std::type_info  objects  which must be resolved all begin with "_ZTS".
   Refer   to  ld's  documentation  for  a  description  of  the  "-E"  &
   "-Bsymbolic" flags.

Why do I need autoconf, bison, xgettext, automake, etc?

   If  you're  using  diffs up dated from one snapshot to the next, or if
   you're  using  the  CVS  repository,  you  may need several additional
   programs to build GCC.

   These  include, but are not necessarily limited to autoconf, automake,
   bison, and xgettext.

   This  is  necessary  because  neither  diff  nor  cvs  keep timestamps
   correct.  This causes problems for generated files as "make" may think
   those generated files are out of date and try to regenerate them.

   An  easy  way  to  work  around  this problem is to use the gcc_update
   script  in  the  contrib  subdirectory  of  GCC,  which  handles  this
   transparently  without requiring installation of any additional tools.
   (Note: Up to and including GCC 2.95 this script was called egcs_update

   When  building  from diffs or CVS or if you modified some sources, you
   may also need to obtain development versions of some GNU tools, as the
   production  versions  do not necessarily handle all features needed to
   rebuild GCC.

   In    general,    the   current   versions   of   these   tools   from
   [41]ftp://ftp.gnu.org/gnu/ will work. At present, Autoconf 2.50 is not
   supported, and you will need to use Autoconf 2.13; work is in progress
   to fix this problem. Also look at
   [42]ftp://gcc.gnu.org/pub/gcc/infrastructure/ for any special versions
   of packages.

Why can't I build a shared library?

   When  building  a shared library you may get an error message from the
   linker like `assert pure-text failed:' or `DP relative code in file'.

   This  kind  of error occurs when you've failed to provide proper flags
   to gcc when linking the shared library.

   You can get this error even if all the .o files for the shared library
   were  compiled  with  the  proper  PIC  option. When building a shared
   library,  gcc  will  compile  additional  code  to  be included in the
   library.  That  additional  code must also be compiled with the proper
   PIC option.

   Adding  the  proper PIC option (-fpic or -fPIC) to the link line which
   creates  the  shared  library  will  fix  this problem on targets that
   support PIC in this manner. For example:
        gcc -c -fPIC myfile.c
        gcc -shared -o libmyfile.so -fPIC myfile.o

When building C++, the linker says my constructors, destructors or virtual
tables are undefined, but I defined them

   The  ISO  C++  Standard  specifies that all virtual methods of a class
   that  are  not  pure-virtual must be defined, but does not require any
   diagnostic  for  violations  of  this rule [class.virtual]/8. Based on
   this   assumption,   GCC   will   only  emit  the  implicitly  defined
   constructors,  the assignment operator, the destructor and the virtual
   table  of  a class in the translation unit that defines its first such
   non-inline method.

   Therefore,  if  you  fail to define this particular method, the linker
   may  complain  about  the lack of definitions for apparently unrelated
   symbols.  Unfortunately,  in  order  to improve this error message, it
   might  be  necessary  to  change  the linker, and this can't always be

   The  solution  is to ensure that all virtual methods that are not pure
   are  defined.  Note  that  a  destructor must be defined even if it is
   declared pure-virtual [class.dtor]/7.

Will GCC someday include an incremental linker?

   Incremental  linking is part of the linker, not the compiler. As such,
   GCC doesn't have anything to do with incremental linking. Depending on
   what  platform  you  use,  it  may  be possible to tell GCC to use the
   platform's native linker (e.g., Solaris' ild(1)).


   1. http://gcc.gnu.org/faq.html
   2. http://www.eskimo.com/~scs/C-faq/top.html
   3. http://www.jamesd.demon.co.uk/csc/faq.html
   4. http://www.fortran.com/fortran/info.html
   5. http://gcc.gnu.org/onlinedocs/libstdc++/faq/index.html
   6. http://gcc.gnu.org/java/faq.html
   7. http://gcc.gnu.org/faq.html#general
   8. http://gcc.gnu.org/faq.html#gcc
   9. http://gcc.gnu.org/faq.html#open-development
  10. http://gcc.gnu.org/faq.html#support
  11. http://gcc.gnu.org/faq.html#platforms
  12. http://gcc.gnu.org/faq.html#installation
  13. http://gcc.gnu.org/faq.html#multiple
  14. http://gcc.gnu.org/faq.html#rpath
  15. http://gcc.gnu.org/faq.html#rpath
  16. http://gcc.gnu.org/faq.html#gas
  17. http://gcc.gnu.org/faq.html#environ
  18. http://gcc.gnu.org/faq.html#optimizing
  19. http://gcc.gnu.org/faq.html#iconv
  20. http://gcc.gnu.org/faq.html#testsuite
  21. http://gcc.gnu.org/faq.html#testoptions
  22. http://gcc.gnu.org/faq.html#multipletests
  23. http://gcc.gnu.org/faq.html#old
  24. http://gcc.gnu.org/faq.html#2.95sstream
  25. http://gcc.gnu.org/faq.html#misc
  26. http://gcc.gnu.org/faq.html#friend
  27. http://gcc.gnu.org/faq.html#dso
  28. http://gcc.gnu.org/faq.html#generated_files
  29. http://gcc.gnu.org/faq.html#picflag-needed
  30. http://gcc.gnu.org/faq.html#vtables
  31. http://gcc.gnu.org/faq.html#incremental
  32. http://gcc.gnu.org/steering.html
  33. http://gcc.gnu.org/faq.html#cathedral-vs-bazaar
  34. http://gcc.gnu.org/bugs.html
  35. http://gcc.gnu.org/install/specific.html
  36. http://gcc.gnu.org/buildstat.html
  37. http://gcc.gnu.org/faq.html#gas
  38. http://gcc.gnu.org/install/specific.html
  39. http://gcc.gnu.org/ml/libstdc++/2000-q2/msg00700/sstream
  40. http://www.codesourcery.com/cxx-abi/
  41. ftp://ftp.gnu.org/gnu/
  42. ftp://gcc.gnu.org/pub/gcc/infrastructure/