[CMake] Disallowing in-source builds

[aaron.meadows at thomsonreuters.com]

Yeah, I think I'll go with something like what you are saying.  I wonder
if I can remove the CMakeFiles directory and the CMakeCache.txt file
from within the CMakeLists script... 

Aaron C. Meadows 

> Hi all.
> 
> 
> 
> Is there a good way to disallow in-source builds?  Ideally, I'd like
to
> prevent it before any cruft is written into the source tree.  I
> experimented with writing a function into my CMakelists file and
calling
> it.  The function checked if CMAKE_BINARY_DIR was equal to
> CMAKE_SOURCE_DIR and messaged a FATAL_ERROR if that was the case.
This
> works ok, but still generates a CMakeFiles directory and a
> CMakeCache.txt file.

I don't think there's a way to prevent that from happening. The bad
thing about this is that if the user doesn't clean away the in-source
CMakeCache.txt file, subsequent out-of-source builds will fail. Perhaps
you can do something like this:

# check for polluted source tree
if(EXISTS ${CMAKE_SOURCE_DIR}/CMakeCache.txt OR
    EXISTS ${CMAKE_SOURCE_DIR}/CMakeFiles)
  message(FATAL_ERROR
    "CMakeCache.txt or CMakeFiles exists in source directory!")
endif()
# make sure the user doesn't play dirty with symlinks
get_filename_component(srcdir "${CMAKE_SOURCE_DIR}" REALPATH)
get_filename_component(bindir "${CMAKE_BINARY_DIR}" REALPATH)
# disallow in-source builds
if(${srcdir} STREQUAL ${bindir})
  message(FATAL_ERROR "In-source builds are forbidden!")
endif()

> 
> The second half of the question is of course, is there an easy way to
> clean out a source tree if an in-source build was accidentally kicked
> off?  (short of dividing the files by their timestamp and removing the
> newer ones, etc..)

No, simply because CMake cannot. Your build system might have something
like the following:

execute_process(COMMAND echo "BOOM" > ${CMAKE_BINARY_DIR}/boom.txt
VERBATIM)

CMake never knows that the file boom.txt is written, and therefor can't
clean it away. The only reasonable way I know of is using git
(http://git-scm.com):

git clean -df

will remove all the files and directories that are not part of the
repository. With tar-ball builds it's easier. Just wipe the source tree
and unpack again.


Michael

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