[SOLVED] How does CMake decide to make a lib or lib64 directory for installations?

Issue

If I install the same CMake-based package (cxxopts) with the same compiler (gcc12) on two different Unix machines (CentOS & MacOS), one time I get a prefix/lib and the other a prefix/lib64 directory.

Is there a way to predict what the lib directory is going to be?

Solution

How does CMake decide to make a lib or lib64 directory for installations?

It's not so much up to CMake as it is up to the maintainers of a project where a target gets installed to (see the DESTINATION parameter of the install(...) command).

cxxopts uses GNUInstallDirs, which is a helper module provided by CMake defining conventional installation locations for different types of things. See its CMakeLists.txt (include(GNUInstallDirs)) and its cxxopts.cmake file (install(TARGETS cxxopts EXPORT ${targets_export_name} DESTINATION ${CMAKE_INSTALL_LIBDIR})).

From the GNUInstallDirs docs:

LIBDIR
object code libraries (lib or lib64)

---

Is there a way to predict what the lib directory is going to be?

If the project uses GNUInstallDirs, yes. From the CMake source code:

Override this default 'lib' with 'lib64' iff:

• we are on Linux system but NOT cross-compiling
• we are NOT on debian
• we are NOT building for conda
• we are on a 64 bits system

reason is: amd64 ABI: https://github.com/hjl-tools/x86-psABI/wiki/X86-psABI For Debian with multiarch, use lib/${CMAKE_LIBRARY_ARCHITECTURE} if CMAKE_LIBRARY_ARCHITECTURE is set (which contains e.g. "i386-linux-gnu" and CMAKE_INSTALL_PREFIX is "/usr". See http://wiki.debian.org/Multiarch

If you want to do a check at configure time of what this is, then check the CMAKE_INSTALL_LIBDIR variable. Ex. if("${CMAKE_INSTALL_LIBDIR}" STREQUAL "lib64") ....

Answered By - starball

Answer Checked By - Dawn Plyler (WPSolving Volunteer)