[CMake] How to append arbitrary linker options?

[Bartlett, Roscoe A]

Hello Brad,

The hack:

  set(CMAKE_CXX_LINK_EXECUTABLE
   "${CMAKE_CXX_LINK_EXECUTABLE} ${NASTY_FLAGS}")

does not work because my libraries still come after these flags.

However, the second suggestion of creaking the 'last' library seems to be working.  I am a little concerned about using:

    target_link_libraries(last ${NASTY_FLAGS})

because the documentation for target_link_libraries(...) does not seem to suggest that you can do this.  I just says that you can list other libraries as:

    target_link_libraries(<target> [lib1 [lib2 [...]]]
                          [[debug|optimized|general] <lib>] ...)

it does not suggest that you can pass in any old arbitrary link options.

Will this behavior that seems to be working be preserved and can it be officially documented?  Are there unit tests in the CMake development test suite that protect this type of behavior?  Will this work on MS Windows Visual Studio and with other generators?

This is somewhat of a hack but until CMake can address every issue automatically (like Fortran libraries in mixed-language programs and appending -lm for C programs), we need to be able to append arbitrary libraries at the end of a link line.

Thanks,

- Ross





> -----Original Message-----
> From: Brad King [mailto:brad.king at kitware.com] 
> Sent: Friday, January 30, 2009 12:43 PM
> To: Bartlett, Roscoe A
> Cc: cmake at cmake.org; Perschbacher, Brent M
> Subject: Re: [CMake] How to append arbitrary linker options?
> 
> Bartlett, Roscoe A wrote:
> > Hello,
> >  
> > I would like to be able to append arbitrary linker options 
> to the end 
> > of my link lines on Unix/Linux systems.  However, the 
> options set in 
> > the CMAKE_EXE_LINKER_FLAGS variable are listed *before* all of the 
> > libraries that CMake knows about.  I need to be able to 
> append a bunch 
> > of nasty options like Fortran libraries, MPI libraries (in 
> some nasty 
> > cases) and other libraries that must come after all other libraries.
> >  
> > The problem is that while I could carefully list the libraries that 
> > need to be appended and I could use find_library(...) to get them 
> > correctly I may just have a glob of libraries and other 
> linker options 
> > that someone gives me and I just want to apply them without 
> having to 
> > parse everything out.
> >  
> > Is there some way to force CMake on Unix/Linux systems to append 
> > arbitrary linker options?
> 
> There is currently no explicit feature for this.  You can 
> hack it for Makefile generators by writing
> 
>   set(CMAKE_CXX_LINK_EXECUTABLE
>     "${CMAKE_CXX_LINK_EXECUTABLE} ${NASTY_FLAGS}")
> 
> and similarly for the other languages.  There is no 
> equivalent for Visual Studio or Xcode though.
> 
> Another (hack) approach is to convince CMake's link 
> dependency analysis to put your flags at the end using a 
> helper target:
> 
>   add_library(last STATIC dummy.c)
>   target_link_libraries(last ${NASTY_FLAGS})
>   # ... link every target to 'last'
>   add_library(mylib ...)
>   target_link_libraries(mylib last)
> 
> This will guarantee that 'last' comes after all other targets 
> on link lines and that ${NASTY_FLAGS} comes after that.
> 
> I suggest doing the work to find the libraries and specify 
> them properly.  Blindly passing flags from foo-config scripts 
> could lead to trouble, especially when using multiple such 
> flag sets.  For example, consider when foo-config returns
> 
>   -L/path/to/foo/lib -lfoo
> 
> and bar-config returns
> 
>   -L/path/to/bar/lib -lbar
> 
> but /path/to/bar/lib contains a (wrong) version of libfoo.  
> If these two sets of flags get appended in the wrong order 
> the wrong foo will be found.  On the other hand if the 
> outputs of foo-config and bar-config are parsed and used to 
> find the corresponding libraries with full paths, CMake will 
> compute a safe link line for you.
> 
> -Brad
> 
> 
>