# CMP0018

Ignore `CMAKE_SHARED_LIBRARY_<Lang>_FLAGS` variable.

CMake 2.8.8 and lower compiled sources in `SHARED` and `MODULE` libraries using the value of the undocumented `CMAKE_SHARED_LIBRARY_<Lang>_FLAGS` platform variable. The variable contained platform-specific flags needed to compile objects for shared libraries. Typically it included a flag such as `-fPIC` for position independent code but also included other flags needed on certain platforms. CMake 2.8.9 and higher prefer instead to use the `POSITION_INDEPENDENT_CODE` target property to determine what targets should be position independent, and new undocumented platform variables to select flags while ignoring `CMAKE_SHARED_LIBRARY_<Lang>_FLAGS` completely.

The default for either approach produces identical compilation flags, but if a project modifies `CMAKE_SHARED_LIBRARY_<Lang>_FLAGS` from its original value this policy determines which approach to use.

The `OLD` behavior for this policy is to ignore the `POSITION_INDEPENDENT_CODE` property for all targets and use the modified value of `CMAKE_SHARED_LIBRARY_<Lang>_FLAGS` for `SHARED` and `MODULE` libraries.

The `NEW` behavior for this policy is to ignore `CMAKE_SHARED_LIBRARY_<Lang>_FLAGS` whether it is modified or not and honor the `POSITION_INDEPENDENT_CODE` target property.

This policy was introduced in CMake version 2.8.9. CMake version 3.20.3 warns when the policy is not set and uses `OLD` behavior. Use the `cmake_policy()` command to set it to `OLD` or `NEW` explicitly.

Note

The `OLD` behavior of a policy is `deprecated by definition` and may be removed in a future version of CMake.