如何使用cmake链接库

Question

I need to use external library in my program. ( https://github.com/davidmoreno/onion ) I am new to CMake (and to this way of compiling programs as well).

The library contains CMakeLists.txt files, so I guess I should be able to use it. But when I try to compile, eg the hello world program example (/onion/examples/hello/). I get an error that CMake is not able to link (or even find) used libraries like onion.h.

To compile I am creating a subdirectory "build" from which I execute commands cmake .. and then cmake --build .

Here are the source files of hello world program. ( https://github.com/davidmoreno/onion/tree/master/examples/hello )

CMakeLists.txt:

include_directories (${PROJECT_SOURCE_DIR}/src/) 

add_executable(hello hello.c)
target_link_libraries(hello onion)

hello.c (only header):

#include <onion/onion.h>
#include <onion/log.h>
#include <onion/version.h>
#include <signal.h>
#include <netdb.h>
#include <stdlib.h>
#include <unistd.h>

Here is the build output:

open# cd build/

open# cmake ..
CMake Warning (dev) in CMakeLists.txt:
  No cmake_minimum_required command is present.  A line of code such as

    cmake_minimum_required(VERSION 3.10)

  should be added at the top of the file.  The version specified may be lower
  if you wish to support older CMake versions for this project.  For more
  information run "cmake --help-policy CMP0000".
This warning is for project developers.  Use -Wno-dev to suppress it.

-- Configuring done
-- Generating done
-- Build files have been written to: /home/user1/onion/examples/hello/build

open# cmake --build .
Scanning dependencies of target hello
[ 50%] Building C object CMakeFiles/hello.dir/hello.o
/home/user1/onion/examples/hello/hello.c:18:10: fatal error: 'onion/onion.h'
      file not found
#include <onion/onion.h>
         ^~~~~~~~~~~~~~~
1 error generated.
*** Error 1 in . (CMakeFiles/hello.dir/build.make:63 'CMakeFiles/hello.dir/hello.o': /usr/bin/cc  -I/home/user1/onion/examples/hello/src   -...)
*** Error 1 in . (CMakeFiles/Makefile2:68 'CMakeFiles/hello.dir/all')
*** Error 1 in /home/user1/onion/examples/hello/build (Makefile:84 'all')

I have tried many solutions but none worked to me.

Thanks for your help.

Answer 1

Based on this repository setup, you should be compiling this example by using the top-level CMakeLists.txt file in the repository, not by running CMake on the file in the /onion/examples/hello/ directory. By running from the top-level, you have the option to include all examples (they are enabled by default).

By running CMake from the /onion/examples/hello/ directory, the include_directories() command resolves relative to the current directory. This is evidenced by the "include" line in your compilation stream here:

-I/home/user1/onion/examples/hello/src

The hello example points to <onion/onion.h> , but header files do not exist here in the directory you included. Rather they are here:

/home/user1/onion/src/onion/onion.h

So, the easiest way to fix the issue would be to compile your example by running your cmake command from the root of the repository.