[CMake] Generated source files and dependencies(+) (Wojciech Migda)

[Wojciech Migda]

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Hi,

some time ago there was a discussion, which has highlighted problem I
am facing now, but it did not provide final conclusions
(http://www.cmake.org/pipermail/cmake/2008-September/024093.html).

Take this example:

- --------------------------
INCLUDE_DIRECTORIES( ${CMAKE_CURRENT_SOURCE_DIR} )

SET_SOURCE_FILE_PROPERTIES( ${CMAKE_CURRENT_SOURCE_DIR}/a.h PROPERTIES
GENERATED TRUE )

ADD_CUSTOM_COMMAND(
  OUTPUT "${CMAKE_CURRENT_SOURCE_DIR}/a.h"
  COMMAND echo \"\#define STAMP \\\"\" `date` \"\\\"\"  > a.h
  DEPENDS "${CMAKE_CURRENT_SOURCE_DIR}/a.txt"
)

ADD_CUSTOM_TARGET( a_h_gen DEPENDS ${CMAKE_CURRENT_SOURCE_DIR}/a.h )

ADD_LIBRARY( foo STATIC a.c )
- ---------------------------------

Here we have file a.c which includes file a.h.
a.h is generated, with dependency on a.txt.

What I'd expect from the above (and what ZNV hinted in the 2008
thread) building foo library should check dependencies of a.c and if
a.h doesn't exist generate it or it a.txt has changed regenerate a.h

However, files generated with cmake 2.6.4 show that:

1. in Makefile2 a.c has no dependencies on a.h or a_h_gen
2. depend.make for foo library has dependency on a.h only if a.h
existed at the time of cmake execution or during dependency scanning
for a.c

The result is that building foo doesn't generate a.h nor a.h is
refreshed if a.txt changed even though depend.make for foo lists a.h
as its dependency. If I add the following line at the end:

ADD_DEPENDENCIES( foo a_h_gen )

a dependency for foo on a_h_gen will appear in Makefile2 and
everything will work as expected.

This however, in a real life example I am working on there are
hundreds of source files and tens of libraries which indirectly may
depend on a generated header and thus manual specification of
dependency between built library and generated file is simply impractical.

Thus, my question is whether I am missing something in my example
CMakeLists.txt or cmake at this stage is simply unable to handle such
situation.

In the 2008 discussion ZNV illustrated exactly the same situation by
the single vs. two-level dependency illustration, and Esben's response
was that properly constructed CMakeLists.txt file shall enforce single
level of dependencies. Nonetheless, the example above shows that it is
not the case.

Any hints allowing me to reach the desired cmake behaviour will be
appreciated. THANKS.

- -Wojciech
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