Link module with an external library

[ugo94490]

Hello,

I'm trying to link a module with an external library libfoo.a but I get the error when i run fprime-util build :

/usr/bin/ld : XComponentImpl.cpp:(.text+0x209) : undefined reference to " functionX(x, y) "

In order to make my module link properly I tried several methods.

First I added include_directories(./libs) so that the module knows where to find my header, then I added the option link_directories(./libs/) and link_libraries(foo) in CMake but I still get the error.

I searched the fprime directory and found the options below but I still get the error.

set(INPUT_DEPS
./libs/libfoo.a
)

set(LINK_DEPS
-lfoo
)

I tried these different techniques in the CMakeList of my module and in the CMakeList at the root of the project.

I already tried what @LeStarch says in this thread but it doesn't work for me.

Has anyone ever succeeded in linking a module depending on an external library?

-Ugo

[ThibFrgsGmz]

I found a project using F' that integrates a static library for one of its components : FS @lewisjaggi.

The component that uses libpng (here) is the PiCamera and here is the cmakefile of its component where it specifies the dependency on libpng:

include_directories(../lib/include)
include_directories(../lib/include/libpng16)

find_library(RASPICAM raspicam /usr/local/lib)
find_library(PNG png ../lib/lib)

target_link_libraries(App_PiCamera ${RASPICAM} ${PNG})

I don't know if it worked in his case, I guess it did, but you can see if it helps?

Maybe you can follow up if you try to implement it.

[ThibFrgsGmz]

I will tackle your problem and gather the above information. Depending on my availability and if I can, I will put the full procedure here so that we are in sync on the full process to follow.

[ThibFrgsGmz]

OK I may have found the source of your problem after reproducing it: you may be using a C library that doesn't have the famous external "c" tag in its header file.

If so, I solved it with @lewisjaggi and @LeStarch's method, but I still have the same problem if I use @astroesteban's method.

And here are the explanations that I could gather:

C and C++ languages use different techniques to handle function names.
Unlike C, C++ has built-in function overloading, which means that the linker must modify the function name to indicate the specific prototype it should call.

On the other hand, declaring a function with external "c" changes the linking requirements so that the C++ compiler does not add the extra processing information to the symbol.
ref

---

I will put in another post a short summary of the procedure to gather the information, even if it doesn't solve your problem, it might help others if it is easily visible.

[astroesteban]

With my approach what happens if you change the target_link_libraries(FOLDER_xxxx PRIVATE foo) part to target_link_libraries(FOLDER_xxxx PUBLIC foo)?

[ThibFrgsGmz]

@astroesteban same result, in my case both versions break the generate chain of the command fprime-util generate.

According to the CMakeError.log the linker has lost the references of the pthread library:

Run Build Command(s):/usr/bin/make cmTC_8ec74/fast && /usr/bin/make -f CMakeFiles/cmTC_8ec74.dir/build.make CMakeFiles/cmTC_8ec74.dir/build
make[1]: Entering directory '/home/ThibFrgsGmz/Documents/CODES/fprime-poc/build-fprime-automatic-native/CMakeFiles/CMakeTmp'
Building C object CMakeFiles/cmTC_8ec74.dir/src.c.o
/usr/bin/cc   -DCMAKE_HAVE_LIBC_PTHREAD   -o CMakeFiles/cmTC_8ec74.dir/src.c.o   -c /home/ThibFrgsGmz/Documents/CODES/fprime-poc/build-fprime-automatic-native/CMakeFiles/CMakeTmp/src.c
Linking C executable cmTC_8ec74
/usr/bin/cmake -E cmake_link_script CMakeFiles/cmTC_8ec74.dir/link.txt --verbose=1
/usr/bin/cc  -DCMAKE_HAVE_LIBC_PTHREAD    CMakeFiles/cmTC_8ec74.dir/src.c.o  -o cmTC_8ec74 
/usr/bin/ld: CMakeFiles/cmTC_8ec74.dir/src.c.o: in function `main':
src.c:(.text+0x46): undefined reference to `pthread_create'
/usr/bin/ld: src.c:(.text+0x52): undefined reference to `pthread_detach'
/usr/bin/ld: src.c:(.text+0x63): undefined reference to `pthread_join'
collect2: error: ld returned 1 exit status
make[1]: *** [CMakeFiles/cmTC_8ec74.dir/build.make:87: cmTC_8ec74] Error 1
make[1]: Leaving directory '/home/ThibFrgsGmz/Documents/CODES/fprime-poc/build-fprime-automatic-native/CMakeFiles/CMakeTmp'
make: *** [Makefile:121: cmTC_8ec74/fast] Error 2

Here is what I had put in my cmake file to link my libQuaternion.a library located in fprime-poc/libs (with its header file):

add_library(Quaternion STATIC IMPORTED GLOBAL)
set_target_properties(Quaternion PROPERTIES IMPORTED_LOCATION "${PROJECT_SOURCE_DIR}/libs")
target_include_directories(Quaternion INTERFACE "${PROJECT_SOURCE_DIR}/libs")
target_link_libraries("${MODULE_NAME}" PUBLIC Quaternion)
# Or 
# target_link_libraries("${MODULE_NAME}" PRIVATE Quaternion)

If I comment out these lines before I run the fprime-util generate command, the generation works. And then if I put them back just before running fprime-util build, I get the undefined reference error.

Maybe I made a mistake somewhere?

[astroesteban]

Interesting. I don't know why it would lose pthreads. Maybe pthread needs to get linked against the Quaternion target but your fix worked so I'd go with your approach.

[ThibFrgsGmz]

If you tried to link an external C library, make sure you have the external "c" tag so that the C++ compiler does not add extra manipulation information to the function name symbols.

Then in the CMakeLists.txt of the component in which you want to use the external library you must add the following lines. In my case I try to link a quaternion calculation library (named libQuaternion.a).

# Retrieve the name of the module that converts the root path of your project to a underscore, i.e. Fw/Types becomes Fw_Types.
get_module_name("${CMAKE_CURRENT_LIST_DIR}")

# Tell the compiler the path to the library header file.
include_directories("${PROJECT_SOURCE_DIR}/libs")

# Find all the libraries contained in the directory you specified.
find_library(LIBQUATERNION Quaternion "${PROJECT_SOURCE_DIR}/libs")
# Note: in my case the LIBQUATERNION equals /home/ThibFrgsGmz/Documents/CODES/fprime-poc/libs/libQuaternion.a

# Specify to the linker the libraries to use when linking the given component and its dependents. 
target_link_libraries("${MODULE_NAME}" ${LIBQUATERNION})

If I remove the external "c" tag from the library header file, I get the same error as you, if I put it back, the error disappears.

Hopefully this solves your problem.

[ugo94490]

Hello,

Thank you very much for the help, my problem was due to the external "C" that I had forgotten to add to my lib.

Thank you all for taking the time to help me.