Karnivaurus Karnivaurus - 20 days ago 10
C++ Question

Cmake cannot find library using "link_directories"

I Ubuntu, I am learning about cmake and make, and just trying a simple example. I have two directories:

src
and
build
. In
src
, I have two files:
main.cpp
, and
CMakeLists.txt
, which has (only) the following text:

add_executable(test main.cpp)
link_directories(/usr/lib/x86_64-linux-gnu)
target_link_libraries(test protobuf)


In
/usr/lib/x86_64-linux-gnu
, there is a shared library called
libprotobuf.so
, which I want to link against. My
main.cpp
uses functions in this library, by including the releveant header file,
#include <google/protobuf/message.h>
.

Now, in my
build
directory, I run
cmake ../src
, and then
make
. However, I then get linker errors telling me that there are undefined references to some of the functions in the protobuf library. If I do a search through all the files and subdirectories in
build
, there is not mention of anything related to protobuf.

However, if I remove the
link_directories
line in my
CMakeLists.txt
file, and instead write the full path to the library when specifying the executable, i.e.
target_link_libraries(test /usr/lib/x86_64-linux-gnu/libprotobuf.so)
, it compiles and links fine.

Why is
link_directories
not allowing cmake to find this library?

Answer

Do not use link_directories like this in CMake.

This is a common beginner's mistake, as many other build environments work like this, but in CMake it's just asking for trouble. Even the manpage specifically advises against it:

Note that this command [link_directories] is rarely necessary. Library locations returned by find_package() and find_library() are absolute paths. Pass these absolute library file paths directly to the target_link_libraries() command. CMake will ensure the linker finds them.

So instead, always pass absolute paths to target_link_libraries and use find_library to resolve the link directory:

find_library(PROTOBUF_LIBRARY protobuf HINTS /usr/lib/x86_64-linux-gnu)
target_link_libraries(test PUBLIC ${PROTOBUF_LIBRARY})

This has the huge benefit that you will probably get a diagnostic at CMake configure time if the expected library cannot be found, instead of a random linker error at compile time. Also, this allows the user to specify a library location via the GUI if the target machine has a non-standard directory layout.

So if it doesn't work right away, be sure to check the result of the find_library call and consult the manpage to track down why it doesn't find your library as intended.

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