Question

I’m trying to move my project to CMake, and at the same time have some optimization on the compilation process.

Here’s the deal:

• I have several subdirs that are (have to be) each compiled into a static library (this works).
• I want to gather all the object files from each subdir into another bigger, complete, static library.

It looks like this:

.
libBig.a  # made from object from subdir1 and subdir2
subdir1/
   src/
   libSubdir1.a
subdir2/
   src/
   libSubdir2.a

Today, I managed to use a global variable in which every subdir CMakeLists.txt will append its own source files. I use this variable as a “source” input in my big library:

# the big library depends on all the source files
# ${all_src} is automatically filled with each subdir's cpp file
get_property( BigLib_src GLOBAL PROPERTY all_src)
add_library( Big STATIC ${BigLib_src}) # recompiles all the sources

Now, this works, not too bad, but the thing is, all my source files get compiled twice: once for the subdir library, and once for the big library.

CMake seems to forget that it has already built them.

I have to keep the subdir libraries and ar can’t merge two static libraries.

Do you know how to do that?

Answer 1

You can use the new OBJECT library feature introduced in CMake 2.8.8. The idea is explained here. Basically, the OBJECT library is a similar concept to the convenience library known from Autotools to group object files.

Check the complete CMake OBJECT library tutorial.