[CMake] cmake 3.7.2: add_custom_command can't DEPEND on OBJECT library?

[Jakob van Bethlehem]

Hej,

CMake experts may correct me if I'm wrong, but this is actually indeed not
supported (yet). Even though an object-library looks like any ordinary
library, in fact it is just a kind of 'shortcut' for a bunch of object
files - it is *not* a proper cmake TARGET. Some additional explanation can
be found here:
https://cmake.org/cmake/help/latest/manual/cmake-buildsystem.7.html#object-libraries

So, then the question obviously becomes how to achieve the desired result.
If the creation of out.map only depends on that one source file, I'd expect
you get the wanted result if instead of writing 'DEPENDS liblface', you
write 'DEPENDS iface.cpp' - I'm not totally sure though.

Sincerely,
Jakob


On Sun, Jan 15, 2017 at 12:35 AM, Paul Smith <paul at mad-scientist.net> wrote:

> If I create an OBJECT library:
>
>   add_library(libIface OBJECT iface.cpp)
>
> Then I create a custom command that depends on this:
>
>   add_custom_command(OUTPUT out.map
>       COMMAND touch out.map
>       DEPENDS libIface
>       VERBATIM)
>
> It doesn't work: the dependency is missing so when I edit the iface.cpp
> file the custom command is not run.  I'm using the Makefile generator,
> on Linux, and if I look at the generated makefile it's obvious that
> there's nothing there representing the OBJECT library:
>
>   out.map:
>         @$(CMAKE_COMMAND) -E ... "Generating out.map"
>         touch out.map
>
> The documentation for add_custom_command() clearly says that DEPENDS can
> specify "any target (created by the ... add_library() command)".
>
> Is there something missing from the docs that should say it doesn't work
> with OBJECT libraries, or is this a bug in cmake?
>
>
> Repro:
>
> --------------------------------------------------------------------
> $ cat CMakeLists.txt
> cmake_minimum_required(VERSION 3.5)
> project(MapTest)
>
> add_library(libIface OBJECT iface.cpp)
>
> add_custom_command(OUTPUT out.map
>     COMMAND touch out.map
>     DEPENDS libIface
>     VERBATIM)
>
> add_custom_target(libIfaceMap DEPENDS out.map)
>
> add_library(iface SHARED $<TARGET_OBJECTS:libIface> out.map)
>
> $ touch iface.cpp
>
> $ cmake .
>
> $ make
> Scanning dependencies of target libIface
> [ 33%] Building CXX object CMakeFiles/libIface.dir/iface.cpp.o
> [ 33%] Built target libIface
> [ 66%] Generating out.map
> Scanning dependencies of target iface
> [100%] Linking CXX shared library libiface.so
> [100%] Built target iface
>
> $ touch iface.cpp
>
> $ make
> Scanning dependencies of target libIface
> [ 33%] Building CXX object CMakeFiles/libIface.dir/iface.cpp.o
> [ 33%] Built target libIface
> [ 66%] Linking CXX shared library libiface.so
> [100%] Built target iface
> --------------------------------------------------------------------
>
> Note that in the last step the "out.map" file was not recreated even
> though iface.cpp was modified.
> --
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