[Insight-developers] passing include dirs and libs list through

[Brad King]

On 05/14/2011 07:31 AM, Alexandre GOUAILLARD wrote:
> then, in ITK,  In the case of GDCM, this is not happening, and the
> list of librairies are hardcoded in
> Modules/ThirdParty/GDCM/CMakeList.txt
> Can you confirm that we are suppose to copy over the list to
> ITK-GDCM_LIBRARIES and that some cmake magic will take care of using
> those when needed?

It's not a copy.  It is a list of the libraries that ITK-GDCM intends
to provide whether from the system or from its own source tree.  Every
module that provides libraries sets that list explicitly to the libraries
it provides.

> that, and that we should use the "GDCM_LIBRARY_DIRS" for that.
[snip]
> If I look in other third party module ... they do not seem to use it

The ITK-VNL module in The Modules/ThirdParty/VNL/CMakeLists.txt does.
In Modules/ThirdParty/GDCM/CMakeLists.txt add:

   set(ITK-GDCM_LIBRARIES  gdcmDICT gdcmMSFF )
+  set(ITK-GDCM_SYSTEM_LIBRARY_DIRS "${GDCM_LIBRARY_DIRS}")
   set(ITK-GDCM_NO_SRC 1)

to propagate library directories from the system.  Of course since
GDCMConfig does not provide GDCM_LIBRARY_DIRS this won't actually
help.  However, GDCM exports its targets in GDCMTargets.cmake and
loads them from GDCMConfig so you shouldn't ever need link dirs
at all.  See below.

> What am I missing here?

Nothing.  This is due to an oversight in our design for modules that
can switch between building a library or finding it on the system :(
The design does not account for third-party packages on the system
that actually provide a targets file like GDCM does.

What GDCM intends to happen is that the dependent package runs
find_package(GDCM) which loads GDCMConfig.cmake which then loads
GDCMTargets.cmake to import the GDCM library targets as logical
names:

  http://www.cmake.org/Wiki/CMake/Tutorials/Exporting_and_Importing_Targets

Even though GDCM is providing the right stuff ITK is not using it
correctly.  The problem is that imported targets have scope only in
the directory where they are imported, and below.  Therefore in
ITK the GDCM logical target names are meaningful only in the
Modules/ThirdParty/GDCM directory and below.  When the string names
are passed to other directories through ITK-GDCM_LIBRARIES and used
in target_link_libraries calls, CMake has no idea that the names are
supposed to be interpreted as imported targets.  Therefore it just
passes them with -l options.

The patch below works around this problem by creating a normal
library target "ITK-GDCM" that links to the imported libraries in
a scope where they are available.  When other directories reference
${ITK-GDCM_LIBRARIES} they will get just ITK-GDCM which is a real
target.  CMake follows the transitive dependencies of it correctly
in the proper scope and links the real gdcm libraries.

-Brad


diff --git a/Modules/ThirdParty/GDCM/CMakeLists.txt b/Modules/ThirdParty/GDCM/CMakeLists.txt
index 2c68408..004f530 100644
--- a/Modules/ThirdParty/GDCM/CMakeLists.txt
+++ b/Modules/ThirdParty/GDCM/CMakeLists.txt
@@ -9,8 +9,7 @@ if(ITK_USE_SYSTEM_GDCM)
   set(ITK-GDCM_SYSTEM_INCLUDE_DIRS
     ${GDCM_INCLUDE_DIRS}
     )
-  set(ITK-GDCM_LIBRARIES  gdcmDICT gdcmMSFF )
-  set(ITK-GDCM_NO_SRC 1)
+  set(ITK-GDCM_LIBRARIES ITK-GDCM)
 else()
   set(ITK-GDCM_INCLUDE_DIRS
     ${ITK-GDCM_BINARY_DIR}
diff --git a/Modules/ThirdParty/GDCM/src/CMakeLists.txt b/Modules/ThirdParty/GDCM/src/CMakeLists.txt
index 2eec2dc..9170bb0 100644
--- a/Modules/ThirdParty/GDCM/src/CMakeLists.txt
+++ b/Modules/ThirdParty/GDCM/src/CMakeLists.txt
@@ -1,3 +1,10 @@
+if(ITK_USE_SYSTEM_GDCM)
+  add_library(ITK-GDCM ITK-GDCM.cxx)
+  target_link_libraries(ITK-GDCM gdcmDICT gdcmMSFF)
+  itk_module_target(ITK-GDCM)
+  return()
+endif()
+
 set(GDCM_TARGETS_NAME ${ITK-GDCM-targets})
 set(GDCM_INSTALL_BIN_DIR ${ITK-GDCM_INSTALL_RUNTIME_DIR})
 set(GDCM_INSTALL_LIB_DIR ${ITK-GDCM_INSTALL_LIBRARY_DIR})
diff --git a/Modules/ThirdParty/GDCM/src/ITK-GDCM.cxx b/Modules/ThirdParty/GDCM/src/ITK-GDCM.cxx
new file mode 100644
index 0000000..286c861
--- /dev/null
+++ b/Modules/ThirdParty/GDCM/src/ITK-GDCM.cxx
@@ -0,0 +1 @@
+int ITK_GDCM(void) { return 0; }