[Freeciv-Dev] Re: Failing to link with a QPE client -- any hints?

[Marko Lindqvist <marko.lindqvist@xxxxxxxxxxx>]

Jason Dorje Short wrote:

> It's a problem in the linking stage.
>
> I've run into this before, and I don't know why it happens.  The order 
> of the list of .a files seems to matter, so one .a file must come before 
> another so that its functions may be used by that object file (or 
> perhaps vice versa).  You can see I don't really understand the problem 
> :-).
> The workaround is generally to list these .a files twice.  This is 
> already done for some .a files, but not all (this is done for civserver 
> as well as civclient).
 ...
> Perhaps someone can find some documentation on this somewhere?  How is 
> it supposed to be done?
 I don't have any documentation to the matter handy, but this is gcc 
feature (not bug, I remember reading this some years back from some 
mailing list where gcc maintainers claimed it to be intentional, or at 
very least accepted side effect of great concept). While linking static 
libraries, it does not collect all symbols, but only those it already 
knows it needs. So, if library A is linked before B, and all calls to 
certain function in A is from B, it is not collected. When we give A 
second time after B, symbol is collected - and it may depend on some 
not-yet-collected symbol in B...
 I have worked with project where all libraries had to be given four! 
times. Well, probably it could have been avoided by carefully 
reorganizing their order, but as there were lots of small libraries it 
was easier to just make it "$(PROG_LIBS) $(PROG_LIBS) $(PROG_LIBS) 
$(PROG_LIBS)"...
 While I'm at it, I'll mention another related problem I had hard time 
tracking time a while back. For certain project above mentioned 'list 
libraries twice' trick did not work in new environment. I found out that 
new versions of libtool require option '--preserve-dup-deps' or linking 
is performaed as if libraries were give only once. To make it worse, 
older versions of libtool, where this work by default, have no such
option at all, so one can't just blindly use it. Actually I found that 
this behaviour differs between libtool 'versions' with exactly same 
version number. So it's time to implement some configure checks.
  But hey, this is not actually freeciv related as libtool is not used 
here at all. I forgot.
 - Caz