How to add compiler arguments using CMake?

Question

I have been using the Clion IDE and am trying to get a simple GTK program to compile using it. I have found that Clion uses CMake, so the issues is here rather than with the IDE itself. I am able to successfully compile and run the program directly from the terminal but have been unsuccessful using CMake.

The problem is simple: when I attempt to compile, the compiler cannot find gtk.h, which is located in /usr/include/gtk-3.0/gtk/gtk.h. I have found that somehow the command compiler argument 'pkg-config --libs --cflags gtk+-3.0' fixes this problem but I have been unable to add this argument using CMake.

I have tried:

set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} `pkg-config --libs --cflags gtk+-3.0`")

But am met with:

Linking CXX executable test
c++: error: `pkg-config: No such file or directory
c++: error: gtk+-3.0`: No such file or directory
c++: error: unrecognized command line option ‘--libs’
c++: error: unrecognized command line option ‘--cflags’
make[3]: *** [test] Error 1
make[2]: *** [CMakeFiles/test.dir/all] Error 2
make[1]: *** [CMakeFiles/test.dir/rule] Error 2
make: *** [test] Error 2

Any suggestions?

---

Further research has revealed this tutorial precisely on the issue I am having. It works like a charm but appears to throw many seemingly undefined variables into the mix. Can anyone explain how and why this works?

# Set the name and the supported language of the project
project(hello-world C)
# Set the minimum version of cmake required to build this project
cmake_minimum_required(VERSION 2.6)
# Use the package PkgConfig to detect GTK+ headers/library files
find_package(PkgConfig REQUIRED)
pkg_check_modules(GTK3 REQUIRED gtk+-3.0)
# Setup CMake to use GTK+, tell the compiler where to look for headers
# and to the linker where to look for libraries
include_directories(${GTK3_INCLUDE_DIRS})
link_directories(${GTK3_LIBRARY_DIRS})
# Add other flags to the compiler
add_definitions(${GTK3_CFLAGS_OTHER})
# Add an executable compiled from hello.c
add_executable(hello main.c)
# Link the target to the GTK+ libraries
target_link_libraries(hello ${GTK3_LIBRARIES})

Answer 1

Use the FindPkgConfig module

cmake_minimum_required(VERSION <your cmake version>)
project(myproject CXX)

# Find the GTK module using pkg-config
include(FindPkgConfig)
pkg_check_modules(GTK REQUIRED "gtk+-3.0")

# Add the path to its header files to the compiler command line
include_directories(${GTK_INCLUDE_DIRS})

# Add any compiler flags it requires
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${GTK_CFLAGS} <other flags>")

# Add the makefile target for your executable and link in the GTK library
add_executable(${CMAKE_PROJECT_NAME} <list of source files>)    
target_link_libraries(${CMAKE_PROJECT_NAME} ${GTK_LDFLAGS} <other libraries>)

the bits in chevrons (<...>) need to be replaced with real values

you can find out more with

cmake --help-module FindPkgConfig

Basically the include(FindPkgConfig) line brings in a few macros - it also ensures that pkg-config is available in the environment. Then a call to pkg_check_modules effectively runs pkg-config, parses the output, and creates a suite of variables using the first argument as a stem.

From the help, this is a basic list (XPREFIX is usually the stem you supply)

      <XPREFIX>_FOUND          ... set to 1 if module(s) exist
      <XPREFIX>_LIBRARIES      ... only the libraries (w/o the '-l')
      <XPREFIX>_LIBRARY_DIRS   ... the paths of the libraries (w/o the '-L')
      <XPREFIX>_LDFLAGS        ... all required linker flags
      <XPREFIX>_LDFLAGS_OTHER  ... all other linker flags
      <XPREFIX>_INCLUDE_DIRS   ... the '-I' preprocessor flags (w/o the '-I')
      <XPREFIX>_CFLAGS         ... all required cflags
      <XPREFIX>_CFLAGS_OTHER   ... the other compiler flags



      <XPREFIX> = <PREFIX>        for common case
      <XPREFIX> = <PREFIX>_STATIC for static linking

Answer 2

You have to understand that when you manually type the compilation command

`pkg-config …`

is not actually an argument to the compiler but makes the shell execute pkg-config with arguments and uses the output of this execution being used as commands to the compiler. I suggest you type just the pkg-config subcommand into your shell to see what it outputs. E.g. on my laptop it is

dw@narfi ~/ % pkg-config --libs --cflags gtk+-3.0

-pthread -I/usr/include/gtk-3.0 -I/usr/include/at-spi2-atk/2.0
-I/usr/include/gtk-3.0 -I/usr/include/gio-unix-2.0/ -I/usr/include/cairo
-I/usr/include/pango-1.0 -I/usr/include/harfbuzz -I/usr/include/pango-1.0
-I/usr/include/atk-1.0 -I/usr/include/cairo -I/usr/include/pixman-1
-I/usr/include/freetype2 -I/usr/include/libdrm -I/usr/include/gdk-pixbuf-2.0
-I/usr/include/libpng16 -I/usr/include/glib-2.0 -I/usr/lib64/glib-2.0/include
-lgtk-3 -lgdk-3 -lpangocairo-1.0 -lpango-1.0 -latk-1.0 -lcairo-gobject -lcairo
-lgdk_pixbuf-2.0 -lgio-2.0 -lgobject-2.0 -lglib-2.0 

and these are the actual arguments passed on to the compiler.

CMake is not a shell through. It has it's own package detection and configuration mechanism. See @kdopen's answer for the details.