Re: ld: once multiple symbol definitions are allowed, both definitions end up in the executable

[Jim Wilson]

On 01/11/2017 01:49 AM, Pavel Shishpor wrote:
> Could please someone advice is it a bug or a feature when we get both
> bodies of the functions with the same name in the executable once
> multiple symbol definitions are allowed? Here is the example showing the
> behavior:

The only thing that the --allow-multiple-definitions option does is 
disable the error message that you would normally get.  It doesn't 
change how the linker output is created.  So yes, you will end up with 
both definitions in the output.

It is best to avoid getting multiple definitions in the first place.

If you want the linker to discard unused functions, then each function 
must be in its own section via gcc -ffunction-sections and then use the 
linker --gc-sections option.  However, this may cause problems of its 
own, e.g. debug functions may disappear because they appear unused, it 
won't work for libraries where most functions are meant to be used by an 
application it is linked with, etc.  Or alternatively, you can try 
making it a COMDAT function, then duplicates get dropped automatically.

For your example, the duplicate f function is in the same text section 
as the main function, which can't be deleted.  The assembler may resolve 
intra-section references itself.  Which means editing the text section 
to remove the f function may break other things in the same section, 
which is unsafe, and hence the linker can't do it.  Thus the requirement 
that we can only delete functions that are in their own section.

Jim