Re: ld: common symbols not allowed with MH_DYLIB outputformat with the -multi_module option

["Sam Lauber" <sam124 at operamail dot com>]

> 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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