OK, I've had more time to play with CMake and subdirectory CMakeList.txt files, and I'm getting close to the behavior I want. I can conditionally override key variables to use either local or system versions of a library, and I can trigger the builds conditionally as well. Using the strings defined by subdir cmake builds in place of the relevant variable defined by FIND*.cmake seems to allow other libraries to pull in the local copy of the library.
Where I'm having trouble (or at least don't have a clean solution) is the case where the names used to identify the locally built library to CMake's add_library and target_* functions are internally generated as a result of variable substitution within the local CMake file. libpng is the current example - if I'm using the local build I need to pass "png14" to a target_link_libraries command in another subdirectory. I can do this by defining a variable to be either this value or the results of FindPNG.cmake (in the case where I'm using a system lib) but the "png14" string is generated by libpng's CMake logic internally, and no method I've tried yet has allowed the parent or other subdirectories below the parent to observe the final string results from libpng, let alone update the parent cache with the new value. This is primarily a maintenance headache, since I CAN hard code this value (and for some libs, like zlib, the target name is a fixed string and hence this solution is actually adequate.) But it seems to me the CORRECT way to do this would be to do something like defining a "LISTENING_VARIABLE" in the parent that directly updates in the cache file any change to the set value, by any toplevel or subdirectory logic. This means the child subdirectory wouldn't have to know the variable is "special" in any way (and hence force scope breaking logic in the child) - the parent would be explicitly saying "I want to capture value changes to this variable in the CACHE, always, from everywhere." Maybe provide an option to warn when that happens, e.g. LISTENING_VARIABLE(VARNAME WARN) or LISTENING_VARIABLE(VARNAME QUIET) depending on desired behavior - does anything like this exist in CMake? If it does not, would it be an acceptable feature to add? This seems to me like the least intrusive way to allow a parent to capture information from a subdirectory build to be used for OTHER subdirectory builds, without requiring altering of the child logic (which probably is assuming stand-alone building, not working as part of a local system.) I've seen some discussion of the environmental variable approach, but even assuming I did something wrong when I tried to get that to work polluting the environment variable namespace seems to me like an invitation to odd side effects if you accidentally overwrite a variable something else is using or some other process alters a variable used by CMake mid-configure - there really should be a clean way to do this within CMake itself. Anybody have any thoughts? Would the CMake devs consider this or some other mechanism that allows this behavior? Cheers, CY _______________________________________________ Powered by www.kitware.com Visit other Kitware open-source projects at http://www.kitware.com/opensource/opensource.html Please keep messages on-topic and check the CMake FAQ at: http://www.cmake.org/Wiki/CMake_FAQ Follow this link to subscribe/unsubscribe: http://www.cmake.org/mailman/listinfo/cmake
