[CMake] Critical Xcode problem with OBJECT libraries

[Paul Smith]

I've built CMake 3.0.0-rc1 and tested it, and I still see this same bug
with that version of CMake.

Anyone have any ideas?  Not having reliable builds is wreaking total
havoc on the Xcode users on our development team.


On Wed, 2014-02-19 at 15:17 -0500, Paul Smith wrote:
> Hi all.  I'm using CMake 2.8.12.1 on Linux, MacOSX, and Windows.
> 
> On my system I need to build libraries in a large number of
> subdirectories and link them (statically) with various executables
> (different executables use different libraries).  Then I also need to
> take the content of all those libraries and include them into a single
> shared library / DLL.  I understand this is tricky for CMake and so the
> special "OBJECT" library type was introduced to help with that.
> 
> Here's a repro case for the problem I see:
> 
>   cmake_minimum_required(VERSION 2.8.12)
>   project(CMTEST CXX)
> 
>   add_library(foo_OBJECTS OBJECT foo.cpp foo.h)
> 
>   add_library(foo STATIC dummy.cpp $<TARGET_OBJECTS:foo_OBJECTS>)
>   add_dependencies(foo foo_OBJECTS)
> 
>   add_executable(hello hello.cpp)
>   target_link_libraries(hello foo)
> 
> I needed to add the "dummy.cpp" because I previously learned that Xcode
> can't handle creating a library only from OBJECTS like this.  It
> contains nothing but a comment.
> 
>       * I run cmake -G Xcode
>       * I run xcodebuild build -config RelWithDebInfo
>       * Everything is built correctly
>       * Now I modify "foo.cpp"
>       * I run xcodebuild build -config RelWithDebInfo
> 
> For the last step I expect that:
>      1. foo.cpp will be recompiled
>      2. libfoo.a library will be updated
>      3. the "hello" program will be rebuilt.
> 
> As a side-effect I guess a hidden "libfoo_OBJECTS.a" will be created,
> but I don't actually care about that.
> 
> This is how it works with Makefile and MSVC.  With Xcode, though, what
> actually happens is that the "libfoo_OBJECTS.a" is rebuild, BUT it does
> NOT rebuild libfoo.a and it does NOT relink the "hello" executable!!
> 
> I've explicitly stated a dependency between the foo library target and
> the foo_OBJECTS target, and it still does not rebuild on changes!
> 
> As you can imagine this is causing my Xcode-based builds to be
> completely unreliable: they fail with weird link errors, or else at
> runtime with strange vtable issues.
> 
> Anyone else able to reproduce this?  Is it a known limitation of Xcode?
> Any ideas for workarounds?
>