Joining paths in CMake - cmake

Question

I am trying to join two paths together:
SET(CMAKE_INSTALL_RPATH "$ORIGIN/../${CMAKE_INSTALL_LIBDIR}/inkscape")
but a string concatenation does not really do it when CMAKE_INSTALL_LIBDIR contains an absolute path.
Is there a CMake function that takes multiple path arguments and joins relative paths right of the rightmost absolute path to the absolute path, like Python’s os.path.join does?
Examples from Python interpreter showing desired behaviour:
>>> from os.path import join
>>> join("/foo/bar", "/baz/qux")
'/baz/qux'
>>> join("foo/bar", "/baz/qux")
'/baz/qux'
>>> join("/foo/bar", "./baz/qux")
'/foo/bar/./baz/qux'
>>> join("/foo/bar", "../baz/qux")
'/foo/bar/../baz/qux'
>>> join("./foo/bar", "baz/qux")
'./foo/bar/baz/qux'
I need to handle both the cases where the prefix is absolute (e.g. CMAKE_INSTALL_PREFIX), and where it is relative (e.g. $ORIGIN/.. or ${prefix} often needed for pkg-config files). And orthogonally, I need to handle both Linux distributions that use relative CMAKE_INSTALL_LIBDIR, and those that use an absolute one.

Answer 1

EDIT: UPDATED BASED ON OP FEEDBACK
Nothing in CMake supports exactly what you want out of the box. However, you can easily create your own if statements (using IS_ABSOLUTE) for the four scenarios you describe:
if(IS_ABSOLUTE ${PREFIX_DIR})
if(IS_ABSOLUTE ${CMAKE_INSTALL_LIBDIR})
# Both absolute.
set(CMAKE_INSTALL_PKGLIBDIR ....)
else()
# Prefix is absolute, but LIBDIR is relative.
set(CMAKE_INSTALL_PKGLIBDIR ....)
endif()
else()
if(IS_ABSOLUTE ${CMAKE_INSTALL_LIBDIR})
# Prefix is relative, but LIBDIR is absolute.
set(CMAKE_INSTALL_PKGLIBDIR ....)
else()
# Both are relative.
set(CMAKE_INSTALL_PKGLIBDIR ....)
endif()
endif()
This can be generalized and made into a function, called directory_join() or something, and could be used throughout your CMake files wherever you need it.
Assuming I'm interpreting your question correctly, you can accomplish a join mechanism with CMake. If you have some absolute path and some relative path(s) you want to join, the get_filename_component() command can help. Here's a general example:
set(ABS_PATH "C:/the/absolute/path")
set(REL_PATH "../../some/other/relative/path")
# Concatenate your absolute and relative path(s) here.
get_filename_component(COMBINED_PATH ${ABS_PATH}/${REL_PATH} ABSOLUTE)
# Print our merged path to verify.
message(STATUS "COMBINED_PATH: ${COMBINED_PATH}")
We can see by the print-out that the new variable COMBINED_PATH joins the two paths, resolving any relative-ness.
COMBINED_PATH: C:/the/some/other/relative/path

Answer 2

September 2020: cmake_path command has just been merged: https://gitlab.kitware.com/cmake/cmake/-/merge_requests/5158
I have sent an example implementation to the upstream issue. It supports multiple arguments like the Python’s os.path.join and works on Linux at least:
# Modelled after Python’s os.path.join
# https://docs.python.org/3.7/library/os.path.html#os.path.join
# Windows not supported
function(join_paths joined_path first_path_segment)
set(temp_path "${first_path_segment}")
foreach(current_segment IN LISTS ARGN)
if(NOT ("${current_segment}" STREQUAL ""))
if(IS_ABSOLUTE "${current_segment}")
set(temp_path "${current_segment}")
else()
set(temp_path "${temp_path}/${current_segment}")
endif()
endif()
endforeach()
set(${joined_path} "${temp_path}" PARENT_SCOPE)
endfunction()
It would still be nice if CMake supported such essential functionality out of the box.