ld: common symbols not allowed with MH_DYLIB output

[Mathieu Malaterre]

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
>
> -- 
> _______________________________________________
> Surf the Web in a faster, safer and easier way:
> Download Opera 8 at http://www.opera.com
>
> Powered by Outblaze
>