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GCC Frequently Asked Questions
The latest version of this document is always available at
[1]http://gcc.gnu.org/faq.html.
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.
_________________________________________________________________
Questions
1. [7]General information
1. [8]How do I get a bug fixed or a feature added?
2. [9]Does GCC work on my platform?
2. [10]Installation
1. [11]How to install multiple versions of GCC
2. [12]Dynamic linker is unable to find GCC libraries
3. [13]libstdc++/libio tests fail badly with --enable-shared
4. [14]GCC can not find GNU as/GNU ld
5. [15]cpp: Usage:... Error
6. [16]Optimizing the compiler itself
7. [17]Why does libiconv get linked into jc1 on Solaris?
3. [18]Testsuite problems
1. [19]How do I pass flags like -fnew-abi to the testsuite?
2. [20]How can I run the test suite with multiple options?
4. [21]Miscellaneous
1. [22]Friend Templates
2. [23]dynamic_cast, throw, typeid don't work with shared libraries
3. [24]Why do I need autoconf, bison, xgettext, automake, etc?
4. [25]Why can't I build a shared library?
5. [26]When building C++, the linker says my constructors, destructors
or virtual tables are undefined, but I defined them
_________________________________________________________________
General information
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 disadvantages.
* 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.
* [27]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 [28]latest version is always available at the GCC web site. Reports
of [29]successful builds for several versions of GCC are also available at
the web site.
_________________________________________________________________
Installation
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:
--program-transform-name='s,\\\\(.*\\\\),new-\\\\1,'
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, [30]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 specs.
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 [31]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-gnulibc1/2.7.2.3/cpp
[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, e.g:
runtest --tool_opts "-fnew-abi -fno-honor-std" <other options>
Or, if you use make check you can use the make variable RUNTESTFLAGS, e.g:
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, e.g:
runtest --target_board "unix{-fPIC,-fpic,}" <other options>
Or, if you use make check you can use the make variable RUNTESTFLAGS, e.g:
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.
_________________________________________________________________
Miscellaneous
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 example:
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 ambiguity.
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) [basic.def.odr].
For more details about the way that GCC implements these and other C++
features, please read the [32]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 SVN 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.
When building from diffs or SVN 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
[33]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 [34]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 done.
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.
References
1. http://gcc.gnu.org/faq.html
2. http://c-faq.com/
3. http://www.comeaucomputing.com/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#support
9. http://gcc.gnu.org/faq.html#platforms
10. http://gcc.gnu.org/faq.html#installation
11. http://gcc.gnu.org/faq.html#multiple
12. http://gcc.gnu.org/faq.html#rpath
13. http://gcc.gnu.org/faq.html#rpath
14. http://gcc.gnu.org/faq.html#gas
15. http://gcc.gnu.org/faq.html#environ
16. http://gcc.gnu.org/faq.html#optimizing
17. http://gcc.gnu.org/faq.html#iconv
18. http://gcc.gnu.org/faq.html#testsuite
19. http://gcc.gnu.org/faq.html#testoptions
20. http://gcc.gnu.org/faq.html#multipletests
21. http://gcc.gnu.org/faq.html#misc
22. http://gcc.gnu.org/faq.html#friend
23. http://gcc.gnu.org/faq.html#dso
24. http://gcc.gnu.org/faq.html#generated_files
25. http://gcc.gnu.org/faq.html#picflag-needed
26. http://gcc.gnu.org/faq.html#vtables
27. http://gcc.gnu.org/bugs.html
28. http://gcc.gnu.org/install/specific.html
29. http://gcc.gnu.org/buildstat.html
30. http://gcc.gnu.org/faq.html#gas
31. http://gcc.gnu.org/install/specific.html
32. http://www.codesourcery.com/cxx-abi/
33. ftp://ftp.gnu.org/gnu/
34. ftp://gcc.gnu.org/pub/gcc/infrastructure/
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