Cannot find API header file for shared library with cmake - c

Question

I am building a shared library in one project and using it in another. They share a prefix, but I'm not building them together (e.g., <prefix>/mylib and <prefix>/myproject). Both mylib and myproject have src and include directories.
The CMakeList.txt for the shared library:
cmake_minimum_required(VERSION 3.5)
project(mylib)
add_library(mylib SHARED
src/mylib.c
)
target_include_directories(mylib PRIVATE include)
set_target_properties(mylib PROPERTIES PUBLIC_HEADER include/mylib.h)
install(TARGETS
mylib
LIBRARY DESTINATION lib
PUBLIC_HEADER DESTINATION include
RUNTIME DESTINATION bin)
This results in mylib.so being installed in install/mylib/lib/mylib.so and mylib.h being installed in install/mylib/include/mylib.h, which is what I intended.
I then want to build a project that uses mylib:
#include "mylib.h"
int main(void)
{
// use some functions in mylib
}
The associated CMakeList.txt file for main.c:
cmake_minimum_required(VERSION 3.5)
project(myproject)
find_package(mylib REQUIRED)
add_executable(myproject src/main.c)
target_link_libraries(myproject mylib)
install(TARGETS
myproject
DESTINATION lib/${PROJECT_NAME})
This produces:
main.c: fatal error: mylib.h: No such file or directory
#include "mylib.h"
^~~~~~~~~
If I change CMakeList.txt to include the following:
find_path(MYLIB_INCLUDE_DIR mylib.h)
...
target_include_directories(myproject PUBLIC ${MYLIB_INCLUDE_DIR})
Then it finds the header, but not the library. I get a linker error:
/usr/bin/ld: cannot find -lmylib
If I change CMakeList.txt to include the following:
find_library(MYLIB_LIB mylib)
...
target_link_libraries(myproject ${MYLIB_LIB})
Then it builds.
I (think I) understand why finding the library and include files manually works, but that seems to be the wrong way to go about things...
find_package(mylib) does seem to find the mylib package (I can print cmake cache variables and mylib_FOUND=1), but doesn't find the library and header in such a way that they are built with myproject.

Answer 1

You need to specify include directories for both "build" and "install" variants in target_include_directories in your library project:
target_include_directories(mylib PUBLIC
$<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include> # build variant
$<INSTALL_INTERFACE:include> # install variant
)
BTW, such example is provided in documentation for target_include_directories command.
Additionally, you need to EXPORT the library during installation:
install(
TARGETS mylib
EXPORT mylib
LIBRARY DESTINATION lib
PUBLIC_HEADER DESTINATION include
RUNTIME DESTINATION bin)
See this answer for a tutorial on exporting libraries with cmake.