Re: ld: common symbols not allowed with MH_DYLIB output

[Mathieu Malaterre <mmalater at nycap dot rr dot com>]

Sam,

   Since you seems very knowledgable why does the error desepear when I 
initialize the structure ?

int bar [ 4 * 256 ] = { 0,1,2, ... };

   I did not changed nor any compiler option, neither any declaration. 
I still cannot see the difference in between those two, since the 
declaration is exactly the same. The only difference being a default 
initialization.

Thanks again for your time,
Mathieu

On Jun 6, 2005, at 5:57 PM, Sam Lauber wrote:

> > Hello,
> >
> >     I have a question about a valid C code. I am trying to compile
> > the following code in MacOSX (*). I don't understand what the
> > problem is ? Could someone please explain me what is going on ?
> > Since I declare the variable with extern I should not need to pass
> > -fno-common, right ?
> >
> > Thanks for your help
> > Mathieu
> >
> > foo.h:
> > extern int bar[];
> >
> > foo.c:
> > int bar[4 * 256];
> >
> > And compile lines are:
> > $ gcc -o foo.o   -Wall -W -fPIC    -c foo.c
> > $ gcc -dynamiclib   -o libfoo.dylib foo.o
> > ld: common symbols not allowed with MH_DYLIB output format with the
> > -multi_module option
> > foo.o definition of common _bar (size 4096)
> > /usr/bin/libtool: internal link edit command failed
> >
> > using gcc 3.3 20030304 (Apple Computer, Inc. build 1671)
>
> This is not a problem with GCC.  In fact, it is not a problem at all.  
> It is a
> misunderstanding of the linker.  When you run
>
>   gcc -dynamiclib -o libfoo.dylib foo.o
>
> it really does
>
>   libtool -dynamic -o libfoo.dylib foo.o
>
> which becomes
>
>   ld -dylib -o libfoo.dylib foo.o
>
> However, -multi_module is the default, so it really is
>
>   ld -dylib -o libfoo.dylib foo.o -multi_module
>
> -multi_module is enabled by default, so you can get errors from the 
> linker of
> the form `libfoo.dylib(foo.o)' when there's a link problem.
>
> The error happens because
>
>  a) an `extern' variable is called a `common variable'
>  b) in multipule module mode, common variables are not allowed
>  c) if they were allowed, it would defeat the purpose of that option: 
> better
> diagnostics
>
> To fix it, add -Wl,-single_module to the end of the GCC command line.
> However, note that subsequent linker errors will refer to 
> `libfoo.dylib' instead
> of `libfoo.dylib(foo.o)'.
>
> Samuel Lauber
>
> --
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