[CMake] Embedding up-to-date version info in built library

[Kolja Waschk]

Hi,

I know that similar questions have been posted before, but as far as I
have been able to follow, no real solution was ever given.

The goal is to embed revision information in a C++ library "mylib".

I already managed to use "create_custom_target" so that an external
program is executed to create a "version.cc" file each time that "make
mylib" (using GNU make) is called. The corresponding object is
re-compiled and the library is put together from this plus 100 other
object files.

However, collecting all these object files takes some time, especially
on a Windows host. Within the same source tree, we build applications
using the library. Due to the forced version.cc update, this time is
always taken, regardless whether only application code or library code
changed. Now I tried hard to make my custom target (or custom command)
dependend only on changes to the library code, but failed so far.

Assume I have a list of all *.cc source files in variable SOURCES:

Try #1: add_custom_command(OUTPUT version.cc DEPENDS ${SOURCES} ...)

This isn't sufficient because it would ignore changes to the header
files that ${SOURCES} depend on.

Try #2: as above plus IMPLICIT_DEPENDS CXX ${SOURCES} ...

As someone else here noted just a few days ago, the IMPLICIT_DEPENDS
seems to work only if the ${SOURCES} come with full path info. Actually,
it started to work after I added the path info in a foreach() loop to
the list of sources. But another problem came up: Only the first single
list item seems to be scanned for implicit dependencies. And I failed to
come up with a list or command syntax that would allow me to have the
other 99+ sources scanned as well.

Try #3: gather the list of dependencies with output_required_files().

It failed because output_required_files() seems to output empty files in
any case, regardless of how input files are specified.

Try #4: Build another library without the version information, make
a custom_command to update version.cc with that library as a target and
for POST_BUILD, and then create the actual library with version
information dependent on the other library. That works, but now
of course all the object files are compiled twice. But no library is
recompiled if just app code changed. The app developer is happy while
the lib developer gets his sleep ;)

Now if you have any idea to workaround any of the problems described
above, I't really appreciate your help! The solution should not be
limited to Unix systems. I'm currently using CMake 2.6.4.

Kolja