Custom FindXXX module not working in CMake? - c

Question

I have written a basic FindUsb CMake module to find libusb, however it does not seem to be linked correctly when used in a library.
My project structure is as follows:
MyProject
├── MyLibrary
│   ├── CMakeLists.txt
│   ├── cmake
│   │   └── Modules
│   │   └── FindLibUsb.cmake
│   ├── include
│   │   └── MyLibrary
│   │   └── library.h
│   └── src
│   └── library.c
│
└── MyProject
├── CMakeLists.txt
└── src
└── project.c
Where MyProject depends on MyLibrary and MyLibrary depends on libusb.
Here is the contents of MyLibrary/CMakeLists.txt:
cmake_minimum_required(VERSION 2.8.11 FATAL_ERROR)
set(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} "${CMAKE_CURRENT_LIST_DIR}/cmake/Modules")
set(MyLibrary_SOURCES
src/library.c
)
set(MyLibrary_INCLUDES
include
)
# Add source to library
add_library(MyLibrary STATIC ${MyLibrary_SOURCES} ${MyLibrary_INCLUDES})
# Find libusb
find_package(LibUsb REQUIRED)
# Add dependencies
target_link_libraries(MyLibrary ${LIBUSB_LIBRARY})
# Add include directories to library
target_include_directories(MyLibrary PUBLIC ${MyLibrary_INCLUDES} ${LIBUSB_INCLUDE_DIR})
And MyLibrary/cmake/Modules/FindLibUsb.cmake:
# FindLibUsb
# ----------
# Tries to find libusb
#
# Variables:
# LIBUSB_ROOT_DIR - Set this variable to the root installation of CMocka
#
# Read-Only variables:
# LIBUSB_FOUND - system has libusb
# LIBUSB_INCLUDE_DIRS - the libusb include directories
# LIBUSB_INCLUDE_DIR - for backwards compatiblity, the same as LIBUSB_INCLUDE_DIRS
# LIBUSB_LIBRARY - libusb library location
# LIBUSB_DEFINITIONS - compiler switches required for using libusb
#
include(FindPackageHandleStandardArgs)
set(LIBUSB_ROOT_DIR ${LIBUSB_ROOT_DIR} CACHE PATH "Root installation directory of libusb")
find_path(LIBUSB_INCLUDE_DIRS
NAMES libusb.h
PATHS ${LIBUSB_ROOT_DIR}/include
PATH_SUFFIXES libusb-1.0
)
find_library(LIBUSB_LIBRARY
NAMES usb
PATHS ${LIBUSB_ROOT_DIR}/lib
)
set(LIBUSB_INCLUDE_DIR ${LIBUSB_INCLUDE_DIRS})
find_package_handle_standard_args(LibUsb DEFAULT_MSG LIBUSB_LIBRARY LIBUSB_INCLUDE_DIR)
mark_as_advanced(LIBUSB_INCLUDE_DIR LIBUSB_LIBRARY)
I include libusb.h inside MyLibrary/include/MyLibrary/library.h:
#include <libusb.h>
void do_something(libusb_context **context);
And use it inside MyLibrary/src/library.c:
#include "MyLibrary/library.h"
void do_something(libusb_context **context) {
libusb_init(context);
// Do something
libusb_exit(*context);
}
I then include MyLibrary in MyProject from MyProject/CMakeLists.txt like so:
cmake_minimum_required(VERSION 2.8.11 FATAL_ERROR)
set(CMAKE_RUNTIME_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/bin)
set(CMAKE_LIBRARY_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/lib)
set(CMAKE_ARCHIVE_OUTPUT_DIRECTORY ${CMAKE_LIBRARY_OUTPUT_DIRECTORY})
project(MyProject)
add_subdirectory(../MyLibrary ${CMAKE_BINARY_DIR}/MyLibrary)
set(MyProject_SOURCES
src/project.c
)
set(MyProject_INCLUDES
include
)
# Set include directory
include_directories(${MyProject_INCLUDES})
# Add source to executable
add_executable(MyProject ${MyProject_SOURCES})
# Add dependencies
target_link_libraries(MyProject MyLibrary)
And here is the content of MyProject/src/project.c:
#include "MyLibrary/library.h"
int main(int argc, char *argv[]) {
libusb_context *context;
do_something(&context);
}
When compiling the project from MyLibrary/build using cmake .. and make, the project is compiled successfully, however when compiling from MyProject/build I get the following error:
[ 25%] Building C object MyLibrary/CMakeFiles/MyLibrary.dir/src/library.c.o
[ 50%] Linking C static library ../lib/libMyLibrary.a
[ 50%] Built target MyLibrary
[ 75%] Building C object CMakeFiles/MyProject.dir/src/project.c.o
[100%] Linking C executable bin/MyProject
Undefined symbols for architecture x86_64:
"_libusb_exit", referenced from:
_do_something in libMyLibrary.a(library.c.o)
"_libusb_init", referenced from:
_do_something in libMyLibrary.a(library.c.o)
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make[2]: *** [bin/MyProject] Error 1
make[1]: *** [CMakeFiles/MyProject.dir/all] Error 2
make: *** [all] Error 2
It looks like the actual library binary for libusb isn't being linked, which is confirmed when I run make VERBOSE=1 (after a make clean) inside MyProject/build, which has the following output:
/usr/local/Cellar/cmake/3.3.2/bin/cmake -H/Users/jack/Documents/Development/Languages/C/issue/MyProject -B/Users/jack/Documents/Development/Languages/C/issue/MyProject/build --check-build-system CMakeFiles/Makefile.cmake 0
/usr/local/Cellar/cmake/3.3.2/bin/cmake -E cmake_progress_start /Users/jack/Documents/Development/Languages/C/issue/MyProject/build/CMakeFiles /Users/jack/Documents/Development/Languages/C/issue/MyProject/build/CMakeFiles/progress.marks
/Applications/Xcode.app/Contents/Developer/usr/bin/make -f CMakeFiles/Makefile2 all
/Applications/Xcode.app/Contents/Developer/usr/bin/make -f MyLibrary/CMakeFiles/MyLibrary.dir/build.make MyLibrary/CMakeFiles/MyLibrary.dir/depend
cd /Users/jack/Documents/Development/Languages/C/issue/MyProject/build && /usr/local/Cellar/cmake/3.3.2/bin/cmake -E cmake_depends "Unix Makefiles" /Users/jack/Documents/Development/Languages/C/issue/MyProject /Users/jack/Documents/Development/Languages/C/issue/MyLibrary /Users/jack/Documents/Development/Languages/C/issue/MyProject/build /Users/jack/Documents/Development/Languages/C/issue/MyProject/build/MyLibrary /Users/jack/Documents/Development/Languages/C/issue/MyProject/build/MyLibrary/CMakeFiles/MyLibrary.dir/DependInfo.cmake --color=
/Applications/Xcode.app/Contents/Developer/usr/bin/make -f MyLibrary/CMakeFiles/MyLibrary.dir/build.make MyLibrary/CMakeFiles/MyLibrary.dir/build
[ 25%] Building C object MyLibrary/CMakeFiles/MyLibrary.dir/src/library.c.o
cd /Users/jack/Documents/Development/Languages/C/issue/MyProject/build/MyLibrary && /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/cc -I/Users/jack/Documents/Development/Languages/C/issue/MyLibrary/include -I/usr/local/include/libusb-1.0 -o CMakeFiles/MyLibrary.dir/src/library.c.o -c /Users/jack/Documents/Development/Languages/C/issue/MyLibrary/src/library.c
[ 50%] Linking C static library ../lib/libMyLibrary.a
cd /Users/jack/Documents/Development/Languages/C/issue/MyProject/build/MyLibrary && /usr/local/Cellar/cmake/3.3.2/bin/cmake -P CMakeFiles/MyLibrary.dir/cmake_clean_target.cmake
cd /Users/jack/Documents/Development/Languages/C/issue/MyProject/build/MyLibrary && /usr/local/Cellar/cmake/3.3.2/bin/cmake -E cmake_link_script CMakeFiles/MyLibrary.dir/link.txt --verbose=1
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/ar cq ../lib/libMyLibrary.a CMakeFiles/MyLibrary.dir/src/library.c.o
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/ranlib ../lib/libMyLibrary.a
[ 50%] Built target MyLibrary
/Applications/Xcode.app/Contents/Developer/usr/bin/make -f CMakeFiles/MyProject.dir/build.make CMakeFiles/MyProject.dir/depend
cd /Users/jack/Documents/Development/Languages/C/issue/MyProject/build && /usr/local/Cellar/cmake/3.3.2/bin/cmake -E cmake_depends "Unix Makefiles" /Users/jack/Documents/Development/Languages/C/issue/MyProject /Users/jack/Documents/Development/Languages/C/issue/MyProject /Users/jack/Documents/Development/Languages/C/issue/MyProject/build /Users/jack/Documents/Development/Languages/C/issue/MyProject/build /Users/jack/Documents/Development/Languages/C/issue/MyProject/build/CMakeFiles/MyProject.dir/DependInfo.cmake --color=
/Applications/Xcode.app/Contents/Developer/usr/bin/make -f CMakeFiles/MyProject.dir/build.make CMakeFiles/MyProject.dir/build
[ 75%] Building C object CMakeFiles/MyProject.dir/src/project.c.o
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/cc -I/Users/jack/Documents/Development/Languages/C/issue/MyProject/include -I/Users/jack/Documents/Development/Languages/C/issue/MyLibrary/include -I/usr/local/include/libusb-1.0 -o CMakeFiles/MyProject.dir/src/project.c.o -c /Users/jack/Documents/Development/Languages/C/issue/MyProject/src/project.c
[100%] Linking C executable bin/MyProject
/usr/local/Cellar/cmake/3.3.2/bin/cmake -E cmake_link_script CMakeFiles/MyProject.dir/link.txt --verbose=1
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/cc -Wl,-search_paths_first -Wl,-headerpad_max_install_names CMakeFiles/MyProject.dir/src/project.c.o -o bin/MyProject lib/libMyLibrary.a
Undefined symbols for architecture x86_64:
"_libusb_exit", referenced from:
_do_something in libMyLibrary.a(library.c.o)
"_libusb_init", referenced from:
_do_something in libMyLibrary.a(library.c.o)
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make[2]: *** [bin/MyProject] Error 1
make[1]: *** [CMakeFiles/MyProject.dir/all] Error 2
make: *** [all] Error 2
Which lacks any references to the actual libusb library.
Printing LIBUSB_LIBRARY gives the path /usr/local/lib/libusb.dylib which is valid and exists.
Compiling MyLibrary dynamically does not help either. It looks like MyLibrary just isn't being linked to libusb at all? There's no references to it in the built libMyLibrary.a or libMyLibrary.dylib.

Answer 1

I guess the following line
# Add dependencies
message(MyLibrary ${LIBUSB_LIBRARY})
should be replaced with
# Add dependencies
target_link_libraries(MyLibrary ${LIBUSB_LIBRARY})