How to set LD_LIBRARY_PATH from a CMake toolchain file? - linux

Question

On Linux, I want to create a CMake toolchain file for cross-compilation.
The compiler needs some shared libraries that are located in non-standard directories, so I have to set LD_LIBRARY_PATH before invoking it. That worked when calling the compiler from the command line, but not when calling it from CMake.
I tried to set LD_LIBRARY_PATH via set(ENV{LD_LIBRARY_PATH} "${CMAKE_CURRENT_LIST_DIR}/<shared library directory>") from the toolchain file. However the compiler complained that it couldn't find the shared libraries.

Answer 1

Table of contents:
Setting it in a toolchain file isn't going to do what you want.
I'm not sure why setting LD_LIBRARY_PATH before invoking the buildsystem isn't working for you.
There's a more idiomatic way in CMake to do what you want.
Setting it in a toolchain file isn't going to do what you want
I'm pretty sure the approach you are asking how to take won't work because set(ENV) just sets an environment variable that will only be known to CMake at the configure stage (not the build stage). Here's a minimal reproducible example of that:
CMakeLists.txt:
cmake_minimum_required(VERSION 3.23)
project(Foo)
set(ENV{FOO} "hello world!")
message("\$ENV{FOO} at configure time: $ENV{FOO}")
add_custom_target(echo
"${CMAKE_COMMAND}" -P "${CMAKE_CURRENT_SOURCE_DIR}/echo.cmake"
VERBATIM
)
echo.cmake:
cmake_minimum_required(VERSION 3.23)
message("\$ENV{FOO} at build time: $ENV{FOO}")
run the following:
$ cmake -S . -B build --fresh
<...>
$ENV{FOO} at configure time: hello world!
<...>
$ cmake --build build/ --target echo
$ENV{FOO} at build time:
Since environment variables are "passed downward" to child processes and not upward, when you do set(ENV), that's just setting it in the CMake process that performs the configure step (the one invoked by the cmake -S ... command in the example above). As the example shows, CMake doesn't do anything fancy to make those environment variables known at its configure time to the generated buildsystem at build time.
I'm not sure why setting LD_LIBRARY_PATH before invoking the buildsystem isn't working for you
So I have to set LD_LIBRARY_PATH before invoking it. It works when calling the compiler from the command line, but not from CMake.
As long as you're exporting the variable for it to be made available to the shell's child processes (using the correct mechanism for whichever shell you're using), that should work. A minimal reproducible example would help here.
There's a more idiomatic way in CMake to do what you want
Try using the find_library() command. It does what its name says it does at configuration time (instead of build time). If you use it, you'll also get the benefit of your buildsystems being more cross-platform.
It has several ways of tweaking how it searches for libraries. You can read about exactly how it works in the official docs.
For your case here, one of the suitable configuration points to guide the library search would be the CMAKE_LIBRARY_PATH variable, although as you'll read about, there are other configuration points you could use as well.