[CMake] Adding static libraries

[James Bigler]

The mac linker is unique in how it searches for libraries.  The  
linker will *always* pick the shared library over the static one if  
it finds a shared one first or at the same time.  Also the order it  
searches the library path list is different than on other *nix systems.

By default the linker searches the paths in a depth first search.   
Take the following example:

/usr/lib/libmylib.so
/home/user/libmylib.so

-L/home/user -lmylib

This will pick /home/user/libmylib.so

Now if you had this:

/usr/lib/libmylib.so
/home/user/libmylib.a

-L/home/user -lmylib

This will pick up /usr/lib/libmylib.so, not /home/user/libmylib.a.

This is because the linker looks through all the paths for a .so,  
then it looks through all the paths for a .a.

You can fix this by adding a flag: -search_paths_first

Doing a man on ld produces this:

        -search_paths_first
               By  default  when  the  -dynamic  flag is in effect,  
the -lx and
               -weak-lx  options  first  search  for  a  file   of    
the   form
               `libx.dylib'  in each directory in the library search  
path, then
               a file of the form `libx.a'  is  searched  for  in   
the  library
               search  paths.   This  option  changes  it  so that in  
each path
               `libx.dylib' is searched for then `libx.a' before the  
next  path
               in the library search path is searched.

I mentioned this on the mailing list a while back, and I was told  
that this flag was going to be added by default.  I don't know if  
that ever happened or what version it made it in.

Also, note that if you have a .a and .dylib in the same path it will  
always pick the .dylib file.  No magic flags for that, I'm afraid.

James

On Dec 13, 2007, at 1:58 PM, Steven Van Ingelgem wrote:

> Hi,
>
>
> I was talking to Miguel about the recent FindwxWidgets changes because
> they didn't work out on for static libraries on a Mac.
>
> Summarizing the issue is that CMake makes the (wrong) assumption that
> what you want to add are shared libraries (at least on a Mac).
>
> Let me explain in more detail what happens:
>
> The outcome of the Find-file is:
> wxWidgets_LIBRARIES: -arch ppc;-arch i386;;-framework
> QuickTime;-framework IOKit;-framework Carbon;-framework
> Cocoa;-framework
> System;;/usr/lib/libwx_macu_adv-2.9.a;/usr/lib/ 
> libwx_macu_core-2.9.a;/usr/lib/libwx_base_carbonu_net-2.9.a;/usr/ 
> lib/libwx_base_carbonu-2.9.a;/usr/lib/libwx_base_carbonu_xml-2.9.a;- 
> framework
> WebKit;-lwxregexu-2.9;-lwxexpat-2.9;-lwxtiff-2.9;-lwxjpeg-2.9;- 
> lwxpng-2.9;-lwxzlib-2.9;-lpthread;-liconv
>
> As you might notice it's a mix of static and dynamic libraries.
>
> Then I add them with the command
> TARGET_LINK_LIBRARIES(...)
>
>
> Then when I want to link them on a Linux it adds -Wl,-Bstatic -l...
> where ... is the NAME of the library, not the path that was passed in.
> Example: -Wl,-Bstatic -lwx_macu_core-2.9 - 
> lwx_base_carbonu_net-2.9 ... etc ...
>
> Now on a Mac (XCode 2.5, GCC 4.0.1) this -Wl flag doesn't exist, but
> the library list STILL includes "-lwx_macu_core-2.9
> -lwx_base_carbonu_net-2.9", so the directory where it needs to look
> for the static libraries is lost.
>
>
> My proposal is to change the behaviour of CMake to add the static
> libraries as they are given on the command line... The user knows
> best! For example if I would have a static library in
> /home/steven/test/libtest.a, and one in /usr/lib/libtest.a with BOTH
> the same name, and BOTH static, but I add the
> /home/steven/test/libtest.a, then CMake will end up using the
> /usr/lib/libtest.a... Even though I said to use the one in "home".
>
>
> I hope I made myself completely clear here... And I hope I'm not going
> completely nuts here or is this really a bug in CMake?
>
>
> Steven, out.
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