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Re: [i386] Why g++ _always_ link an executable with libm.so?


Denis Zaitsev <zzz@anda.ru> writes:

| On Tue, Jan 04, 2005 at 05:05:23PM -0500, Daniel Jacobowitz wrote:
| > On Wed, Jan 05, 2005 at 03:01:02AM +0500, Denis Zaitsev wrote:
| > > I've found some strange behaviour of g++: if it's used to produce an
| > > executable, i.e.:
| > > 
| > >         g++ xxx.C -o XXX
| > > 
| > > then that XXX is linked with libm.so, regardless of the fact that math
| > > is not used in the program.  Also, libstdc++.so and libgcc_s.so are
| > > linked too, even though they aren't needed as well.
| > > 
| > > But if g++ is used to compilation only, and the link stage is done by
| > > gcc or ld, i.e:
| > > 
| > >         g++ -c xxx.C -o xxx.o
| > >         gcc xxx.o -o XXX
| > > 
| > > then neither of that 3 libs are linked (of course, if they aren't
| > > needed).
| > > 
| > > Why g++ does so?  Is it intentional?  Or how this can be solved?
| > 
| > drow@nevyn:~% readelf -d /usr/lib/libstdc++.so.6 | grep NEEDED
| >  0x00000001 (NEEDED)                     Shared library: [libm.so.6]
| >  0x00000001 (NEEDED)                     Shared library: [libgcc_s.so.1]
| >  0x00000001 (NEEDED)                     Shared library: [libc.so.6]
| 
| Thanks.
| 
| > Libstdc++ needs libm.
| 
| Then the other questions:
| 
|   a) why g++ assumes that libstdc++ is always needed?

Because that is the way it is designed.  If you don't want libstdc++,
say -nostdlib as explained in our documentation.

|   b) why is libm _always_ needed by libstdc++?

Because libstdc++ needs the mathematical functions.

|  It's rather strange.

I guess it is a matter of perspective.  From my part, requiring users
to explicitly supply -lm is a bug.  Simply because the mathematical
functions are part of the standard library; we do not require users to
say -lstr when they use strcpy() and friend; we do not require users
to say -lstdio when they use fprintf() and friends.  YMMV.

-- Gaby


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