🚀 go-pugleaf

This is a list of frequently asked questions for g++ users; thanks to
all those who sent suggestions for improvements.

I'm looking for new questions, better answers, or both.  One thing that's
missing is a section on templates and template problems with g++; I may
just hold off on that until 2.3 is released, since apparently 2.3 will
be much better on that score.

This FAQ is intended to supplement, not replace, Marshall Cline's
excellent FAQ for comp.lang.c++.  Especially if g++ is the first C++
compiler you've ever used, the question "How do I do <X> with g++?"
is probably really "How do I do <X> in C++?".  You can obtain the
C++ FAQ by anonymous FTP from sun.soe.clarkson.edu [128.153.12.3],
in the file ~ftp/pub/C++/FAQ.

First, the questions:
---------------------------------------------------------------------
OBTAINING SOURCE CODE
1.  How do I get a copy of g++ for Unix?
2.  How do I get g++ for other platforms?
3.  I keep hearing people talking about g++ 2.2.2 (or some other number
    starting with 2), but the latest version I can find is g++ 1.42.
4.  What is the latest version of gcc, g++, and libg++?

INSTALLATION ISSUES AND PROBLEMS
5.  I obtained gcc-2.x.y and g++ 1.x.y and I'm trying to build it, but
    I'm having major problems.
6.  OK, I've obtained gcc; what else do I need?
7.  Should I use the Gnu linker, or should I use "collect"?
8.  Should I use the Gnu assembler, or my native assembler?
9.  Should I use the Gnu C library?
10. I am having trouble building libg++ on Ultrix [and possibly other
    systems].  I get errors referring to "dummy.o".  Help!

USER PROBLEMS
11. g++ reports undefined symbols for all my static data members when I
    link.
12. I have a program that uses the "placement syntax" of operator new...
    and g++ won't accept it.
13. I think I have found a bug in g++, but I'm not sure.  How do I know,
    and who should I tell?
14. I have a program that runs on <some other C++ compiler>, and I want
    to get it running under g++.  Is there anything I should watch out
    for?
15. Why does g++ mangle names differently from other C++ compilers?
16. Why can't I link g++-compiled programs against libraries compiled by
    some other C++ compiler?
17. What documentation exists for g++?
18. What are the differences between g++ and the ARM specification of C++?
19. Will g++ compile InterViews?  The NIH class library? (etc)
20. When I use the -g flag on C++ code on a System V Release 4 system,
    I get lots of undefined symbols at link time.  Why?  Help!

Now, the questions and answers:
---------------------------------------------------------------------
1. How do I get a copy of g++ for Unix?

First, you may already have it if you have gcc for your platform; see
the answer to question 3.

If you're trying to find g++, you can get it by anonymous FTP, by
anonymous UUCP, by buying a tape from the Free Software Foundation,
or by getting it from a friend that has it.

Here is a list of anonymous FTP archive sites for Gnu software.

        Japan: ftp.cs.titech.ac.jp, utsun.s.u-tokyo.ac.jp:ftpsync/prep
        Australia: archie.au:gnu
        Europe: src.doc.ic.ac.uk:gnu, ftp.informatik.tu-muenchen.de,
          ftp.informatik.rwth-aachen.de:pub/gnu,
          nic.funet.fi:pub/gnu, ugle.unit.no, isy.liu.se,
          ftp.stacken.kth.se, sunic.sunet.se, ftp.win.tue.nl,
          ftp.diku.dk, ftp.eunet.ch, archive.eu.net
        United States: wuarchive.wustl.edu ftp.cs.widener.edu,
          uxc.cso.uiuc.edu, col.hp.com, gatekeeper.dec.com:pub/GNU,
          ftp.uu.net:packages/gnu

The "official site" is prep.ai.mit.edu, but your transfer will probably
go faster if you use one of the above machines.

UUNET customers can get Gnu sources from UUNET via UUCP.  For information
on how to order tapes from FSF, write to gnu@prep.ai.mit.edu.

UUCP-only sites can get Gnu sources by "anonymous UUCP" from site
"osu-cis" at Ohio State University.  You pay for the long-distance call
to OSU; the price isn't too bad on weekends at 9600 bps.  Send mail to 
uucp@cis.ohio-state.edu or osu-cis!uucp for more information.

Don't forget to retrieve libg++ as well!

2. How do I get a copy of g++ for (some other platform)?

The standard gcc/g++ distribution includes VMS support.  Since the
FSF people don't use VMS, it's likely to be somewhat less solid than
the Unix version.  Precompiled copies of G++ and libg++ in VMS-installable
form are available by FTP from mango.rsmas.miami.edu.

DJ Delorie has ported gcc/g++ to DOS; this port is popularly known as
"DJGPP" (the P's stand for "plus").  It can be found on many FTP archive
sites; its "home" is on grape.ecs.clarkson.edu, directory
~ftp/pub/msdos/djgpp.  Make sure you're retrieving the current version,
which should indicate that it is a port of gcc-2.2.2.  It is also
available on site wuarchive.wustl.edu, in directory mirrors/msdos/djgpp,
and many other places as well.

For information on Amiga ports of gcc/g++, retrieve the file
/pub/gnu/MicrosPorts/Amiga from prep.ai.mit.edu, or write
to Leonard Norrgard <vinsci@nic.funet.fi>, who I hope won't be too upset
that I mentioned his name here. [ The file mentions gcc but says
nothing about g++ specifically.  Does it work? ]

There are two different ports of gcc-2.2.2 (and g++) to OS/2, the
so-called EMX port, which requires a particular Unix emulator, and
a port called "gcc/2", which runs native.  The latter port uses
a rather buggy port of the BSD libc.  For more information ask
around on comp.os.os2.programmer.  gcc/2 can be obtained by FTP
from
    ftp-os2.nmsu.edu (128.123.35.151) in /pub/os2/2.0/programming/gcc2-222
    luga.latrobe.edu.au (131.172.2.2) in /pub/os2/2.0/programming/gcc2-222

Eberhard Mattes did the EMX port (mattes@azu.informatik.uni-stuttgart.de).
Trevor Lampre did the gcc/2 port (mmtl@cc.flinders.edu.au).

Because the legal policies of Apple threaten the long-term goals of FSF,
as well as the concept of free software, no support will be lent to
efforts to port Gnu software to Macintosh or other Apple hardware.

3. I keep hearing people talking about g++ 2.2.2 (or some other number
   starting with 2), but the latest version I can find is g++ 1.42.
   Where is it?

As of gcc 2.0, C, C++, and Objective-C as well are all combined into a
single distribution called gcc.  If you get gcc you already have g++.  The
standard installation procedure for any gcc version 2 compiler will
install the C++ compiler as well.

One could argue that we shouldn't even refer to "g++-2.x.y" but it's a
convention.  It means "the C++ compiler included with gcc-2.x.y".

4. What is the latest version of gcc, g++, and libg++?

The latest "2.x" version of gcc/g++ is 2.2.2.  The latest version of
libg++ is 2.2.  2.3 is expected "real soon now", however FSF almost
never announces release dates in advance.

The latest "1.x" version of gcc is 1.42, and the latest "1.x" version of
g++ is 1.42.0.

5. I obtained gcc-2.x.y and g++ 1.x.y and I'm trying to build it, but
   I'm having major problems.  What's going on?

If you wish to build g++-1.42, you must obtain gcc-1.42 first.  The
installation instructions for g++ version 1 leave a lot to be desired,
unfortunately, and I would recommend that, unless you have a special
reason for needing the 1.x compiler, that C++ users use g++-2.2.2, as it
is the version that is being actively maintained.

6. OK, I've obtained gcc; what else do I need?

First off, you'll want libg++ as you can do almost nothing without it
(unless you replace it).

Second, depending on your platform, you may need "gas", the Gnu assembler,
or the Gnu linker (see next question).

7. Should I use the Gnu linker, or should I use "collect"?

First off, for novices: special measures must be taken with C++ to arrange
for the calling of constructors for global or static objects before the
execution of your program, and for the calling of destructors at the end.
(Exception: System VR3 and System VR4 linkers support user-defined
segments; g++ on these systems requires neither the Gnu linker nor
collect.  So if you have such a system, the answer is that you don't
need either one).

If you have experience with AT&T's "cfront", this function is performed
there by programs named "patch" or "munch".  With Gnu C++, it is performed
either by the Gnu linker or by a program known as "collect".  The collect
program is part of the gcc-2.x distribution; you can obtain the Gnu linker
separately as part of the "binutils" package.

(To be technical, it's "collect2"; there were originally several
alternative versions of collect, and this is the one that survived).

There are advantages and disadvantages to either choice.

Advantages of the Gnu linker:

It's faster than using collect -- collect basically runs the standard Unix
linker on your program twice, inserting some extra code after the first
pass to call the constructors.  This is a sizable time penalty for large
programs.  The Gnu linker does not require this extra pass.

Gnu ld reports undefined symbols using their true names, not the mangled
names.

If there are undefined symbols, Gnu ld reports which object file(s) refer to
the undefined symbol(s).

Advantages of collect:

If your native linker supports shared libraries, you can use shared
libraries with collect.  The Gnu linker does not (yet) support shared
libraries.

The Gnu linker has not been ported to as many platforms as g++ has, so you
may be forced to use collect.

If you use collect, you don't need to get something extra and figure out
how to install it; the standard gcc installation procedure will do it for you.

In conclusion, I don't see a clear win for either alternative at this
point.  Take your pick.

8. Should I use the Gnu assembler, or my native assembler?

This depends on your platform and your decision about the Gnu linker.  For
most platforms, you'll need to use gas if you use the Gnu linker.  For
some platforms, you have no choice; check the gcc installation notes to
see whether you must use gas.  But you can usually use the native
assembler if you don't use the Gnu linker.

The Gnu assembler assembles faster than many native assemblers; however,
on many platforms it cannot support the local debugging format.

9. Should I use the Gnu C library?

At this point in time, no.  The Gnu C library is still very young, and
libg++ still conflicts with it in some places.  Use your native C library
unless you know a lot about the gory details of libg++ and gnu-libc.  This
will probably change in the future.

20. I am having trouble building libg++-2.2 on Ultrix [and possibly other
    systems].  I get errors referring to "dummy.o".  Help!

(The errors on Ultrix end with somehting like

ldopen: cannot open dummy.o
nm: Error: cannot open dummy.o
ldopen: cannot open dummy.o
nm: Error: cannot open dummy.o
nm failed to find FUNC in dummy.o!
sh: -1: bad number
*** Error code 1

and there may be similar problems on other systems).

The fix for this is to make libg++ by saying "make CC=gcc".

11. g++ reports undefined symbols for all my static data members when I link,
    even though the program works correctly for compiler XYZ.  What's going on?

The problem is almost certainly that you don't give definitions for
your static data members.  If you have

class Foo {
	...
	void method();
	static int bar;
};

you have only declared that there is an int named Foo::bar and a member
function named Foo::method that is defined somewhere.  You still need to
defined BOTH method() and bar in some source file.  According to the draft
ANSI standard, you must supply an initializer, such as

int Foo::bar = 0;

in one (and only one) source file.

12. I have a program that uses the "placement syntax" of operator new,
    e.g.

   new (somewhere) T;

   and g++ won't accept it.

g++ accepts an alternate form of the placement syntax, for historical
reasons; use

   new {somewhere} T;

This will be fixed in version 2.3.

13. I think I have found a bug in g++, but I'm not sure.  How do I know,
    and who should I tell?

First, see the excellent section on bugs and bug reports in the gcc manual
(which is included in the gcc distribution).  As a short summary of that
section: if the compiler gets a fatal signal, for any input, it's a bug.
Same thing for producing invalid assembly code.

I will add some extra notes that are C++-specific, since the notes from
gcc are generally C-specific.

First, mail your bug report to "bug-g++@prep.ai.mit.edu".  You may also
post to gnu.bug.g++, but it's better to use mail, particularly if you
any doubt as to whether your news software generates correct reply
addresses.  Don't mail C++ bugs to bug-gcc@prep.ai.mit.edu.

If your bug involves libg++ rather than the compiler, mail to
bug-libg++@prep.ai.mit.edu.  If you're not sure, you could send your bug
to both lists.

Second, if your program does one thing, and you think it should do
something else, it is best to consult a good reference if in doubt.  The
standard reference is "The Annotated C++ Reference Manual", by Ellis and
Stroustrup (copyright 1990, ISBN #0-201-51459-1); the reference manual,
without annotations, also appears in Stroustrup's "The C++ Programming
Language, Second Edition" (copyright 1991, ISBN #0-201-53992-6).  Both are
published by Addison-Wesley.

Note that the behavior of (any version of) AT&T's "cfront" compiler is
NOT the standard for the language.

14. I have a program that runs on <some other C++ compiler>, and I want
    to get it running under g++.  Is there anything I should watch out
    for?

First, see questions 11 and 12.

There are two other reasons why a program that worked under one compiler
might fail under another: your program may depend on the order of
evaluation of side effects in an expression, or it may depend on the
lifetime of a temporary (you may be assuming that a temporary object
"lives" longer than the standard guarantees).  As an example of the
first:

	void func(int,int);

	int i = 3;
	func(i++,i++);

Novice programmers think that the increments will be evaluated in strict
left-to-right order.  Neither C nor C++ guarantees this; the second
increment might happen first, for example.  func might get 3,4, or it
might get 4,3.

The second problem often happens with classes like the libg++ String
class.  Let's say I have

String func1();
void func2(const char*);

and I say

    func2(func1());

because I know that class String has an "operator const char*".  So what
really happens is

    func2(func1().convert());

where I'm pretending I have a convert() method that is the same as the
cast.  This is unsafe, because the temporary String object may be deleted
after its last use (the call to the conversion function), leaving the
pointer pointing to garbage, so by the time func2 is called, it gets an
invalid argument.

If you think this is ugly, you should know that the ANSI C++ committee is
STILL debating the lifetime-of-temporaries problem.

For now, the safe way to write such code is to give the temporary a name,
which forces it to live until the end of the scope of the name. For
example:

	String& tmp = func1();
	func2(tmp);


15. Why does g++ mangle names differently from other C++ compilers?

See the answer to the next question.

16. Why can't I link g++-compiled programs against libraries compiled by
    some other C++ compiler?

Some people think that, if only the FSF and Cygnus folks would stop being
stubborn and mangle names the same way that, say, cfront does, then any
g++-compiled program would link successfully against any cfront-compiled
library and vice versa.  Name mangling is the least of the problems.
Compilers differ as to how objects are laid out, how multiple inheritance
is implemented, how virtual function calls are handled, and so on, so if
the name mangling were made the same, your programs would link against
libraries provided from other compilers but then crash when run.  For this
reason, the ARM *encourages* compiler writers to make their name mangling
different from that of other compilers for the same platform.
Incompatible libraries are then detected at link time, rather than at run
time.

17. What documentation exists for g++ 2.x?

Almost none.  The gcc manual describes the C front end, and also the back
end, which is shared by the C++ compiler, but there is almost no
documentation for the C++ front end.  There is a Unix-style manual entry,
"g++.1", in the gcc-2.x distribution; this describes the extra
command-line options that g++ supports, and the #pragma interface and
#pragma implementation directives.

I understand that work is being done on a g++ internals document.

18. What are the differences between g++ and the ARM specification of C++?

The chief thing missing from g++ that is in the ARM is exceptions (the
other major compilers do not have exceptions either).  There are bits
and pieces of exception code present, but it is not presently usable.

The "placement new" operator is non-standard, see question 12.

The template implementation is still new and shaky.  [I'm looking for
someone who has used the template facility a lot to write about the
quirks of the g++ implementation; perhaps this should wait for 2.3,
which I am assured will have a vastly improved template implementation].

As with any beta-test compiler, there are bugs.  You can help improve
the compiler by submitting detailed bug reports.

[A full bug list would be very long indeed, so I won't put one here.
I may add a list of frequently-reported bugs and "non-bugs" like the
static class members issue mentioned above].

19. Will g++ compile InterViews?  The NIH class library?

The NIH class library uses a non-portable, compiler-dependent hack
to initialize itself, which makes life difficult for g++ users.
It will not work without modification, and I don't know what modifications
are required or whether anyone has done them successfully.

Brendan Kehoe of Cygnus is working on getting NIHCL to build with g++.
He says, "The NIHCL release will hopefully contain patches to gcc 2.3 to
let it build."

[ From Stienar Bang <steinarb@idt.unit.no> ]

The C++ compiler part of gcc-2.3 (when released) *should* be able to
compile InterViews release 3.1 (when released) out of the box (no
patches required on either side).


20. When I use the -g flag on C++ code on a System V Release 4 system,
    I get lots of undefined symbols at link time.  Why?  Help!

[From Ron Guilmette:]
The changes needed to get the g++ front-end to generate proper DWARF style
debugging information for System V Release 4 are not yet completed, nor
will they be until g++ version 2.4 (at the earliest).

There is nothing that you (as an end-user) can do to correct this problem.
(It is actually *many* problems, and they are all very complex.)  Until
the g++ maintainers have time to fix this, you should simply *avoid* using
the -g option when using g++ on SVR4.

--
Joe Buck	jbuck@ohm.berkeley.edu