cmake: cannot link against static ilmbase libraries

[chadrik]

I was trying to build openexr and link it against static ilmbase libraries, but it does not work. I dug through the CMakeLists.txt files a bit, and the basic problem is that the full paths to libraries are not used when calling TARGET_LINK_LIBRARIES().

Cmake encourages authors to be very explicit about exactly what libraries are used. Calling TARGET_LINK_LIBRARIES() without full paths defers to the native linking tool's own search system, which makes building unpredictable across platforms and gives the user less control for specifying what libraries to use (see below for more on this). This is what is being done in openexr:

TARGET_LINK_LIBRARIES ( dwaLookups
  Half
  Iex${ILMBASE_LIBSUFFIX}
  IlmThread${ILMBASE_LIBSUFFIX}
  ${PTHREAD_LIB}
)

The typical way to use cmake is to write a FindFoo.cmake which fills out a set of variables. Unfortunately, the exact naming convention varies, but it is commonly Foo_FOUND, Foo_LIBRARIES, and Foo_INCLUDE_DIRS (there is no solid convention for whether Foo should be all caps or not. see the native FindBoost vs FindOpenSSL). If a package provides many libraries, there will often be variables for each individual library: Foo_Bar_LIBRARY for libBar, etc (again, no solid convention on whether to make Bar all caps).

If you don't make a FindFoo.cmake file, at the very, very least, you should use FIND_LIBRARY() to get the path to the library, so that users can affect this search using environment variables like CMAKE_PREFIX_PATH and CMAKE_LIBRARY_PATH. More importantly, using the cmake find functions, like FIND_LIBRARY(), ensures that all of your variables contain the full path to the file or directory, not just a name.

Then, when you link, you provide the variable holding list of full paths to the libraries, like so:

TARGET_LINK_LIBRARIES ( dwaLookups
  ${ILMBase_LIBRARIES}
  ${PTHREAD_LIB}
)

Or specify individual libraries, if needed:

TARGET_LINK_LIBRARIES ( dwaLookups
  ${ILMBase_Half_LIBRARY}  ${ILMBase_Iex_LIBRARY}   ${ILMBase_IlmThread_LIBRARY}
  ${PTHREAD_LIB}
)

Here is an example FindILMBase.cmake: https://github.com/nerdvegas/vfxcmake/blob/master/cmake/FindILMBase.cmake

One nice feature of this module is that it gives the user control over whether to search for static libraries, using ILMBase_USE_STATIC_LIBS (a convention borrowed from FindBoost.cmake).

I co-maintain this project, so let me talk to the other author and see if we can get the license changed to BSD so you can just use it.

chad.

[kdt3rd]

A complete refactor of the cmake has taken place, modernizing cmake to match the "Modern CMake" guidelines and practices, and no longer uses the Find* mechanisms, but rather should use the suggested cmake config mechanisms that go into /usr/lib/cmake// and find_package knows how to search for and generate import libraries. We also have switched to standard BUILD_SHARED_LIBS, but also have an option to generate both static and shared, and when you use the imported libraries, you can target the default or the static (both are there as explicit library names if you do both).

Thanks for the report!