pkgconfig.md
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pkgconfig.md
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	## Using GoogleTest from various build systems

	GoogleTest comes with pkg-config files that can be used to determine all
	necessary flags for compiling and linking to GoogleTest (and GoogleMock).
	Pkg-config is a standardised plain-text format containing

	* the includedir (-I) path
	* necessary macro (-D) definitions
	* further required flags (-pthread)
	* the library (-L) path
	* the library (-l) to link to

	All current build systems support pkg-config in one way or another. For all
	examples here we assume you want to compile the sample
	`samples/sample3_unittest.cc`.

	### CMake

	Using `pkg-config` in CMake is fairly easy:

	```cmake
	cmake_minimum_required(VERSION 3.0)

	cmake_policy(SET CMP0048 NEW)
	project(my_gtest_pkgconfig VERSION 0.0.1 LANGUAGES CXX)

	find_package(PkgConfig)
	pkg_search_module(GTEST REQUIRED gtest_main)

	add_executable(testapp samples/sample3_unittest.cc)
	target_link_libraries(testapp ${GTEST_LDFLAGS})
	target_compile_options(testapp PUBLIC ${GTEST_CFLAGS})

	include(CTest)
	add_test(first_and_only_test testapp)
	```

	It is generally recommended that you use `target_compile_options` + `_CFLAGS`
	over `target_include_directories` + `_INCLUDE_DIRS` as the former includes not
	just -I flags (GoogleTest might require a macro indicating to internal headers
	that all libraries have been compiled with threading enabled. In addition,
	GoogleTest might also require `-pthread` in the compiling step, and as such
	splitting the pkg-config `Cflags` variable into include dirs and macros for
	`target_compile_definitions()` might still miss this). The same recommendation
	goes for using `_LDFLAGS` over the more commonplace `_LIBRARIES`, which happens
	to discard `-L` flags and `-pthread`.

	### Autotools

	Finding GoogleTest in Autoconf and using it from Automake is also fairly easy:

	In your `configure.ac`:

	```
	AC_PREREQ([2.69])
	AC_INIT([my_gtest_pkgconfig], [0.0.1])
	AC_CONFIG_SRCDIR([samples/sample3_unittest.cc])
	AC_PROG_CXX

	PKG_CHECK_MODULES([GTEST], [gtest_main])

	AM_INIT_AUTOMAKE([foreign subdir-objects])
	AC_CONFIG_FILES([Makefile])
	AC_OUTPUT
	```

	and in your `Makefile.am`:

	```
	check_PROGRAMS = testapp
	TESTS = $(check_PROGRAMS)

	testapp_SOURCES = samples/sample3_unittest.cc
	testapp_CXXFLAGS = $(GTEST_CFLAGS)
	testapp_LDADD = $(GTEST_LIBS)
	```

	### Meson

	Meson natively uses pkgconfig to query dependencies:

	```
	project('my_gtest_pkgconfig', 'cpp', version : '0.0.1')

	gtest_dep = dependency('gtest_main')

	testapp = executable(
	'testapp',
	files(['samples/sample3_unittest.cc']),
	dependencies : gtest_dep,
	install : false)

	test('first_and_only_test', testapp)
	```

	### Plain Makefiles

	Since `pkg-config` is a small Unix command-line utility, it can be used in
	handwritten `Makefile`s too:

	```makefile
	GTEST_CFLAGS = `pkg-config --cflags gtest_main`
	GTEST_LIBS = `pkg-config --libs gtest_main`

	.PHONY: tests all

	tests: all
	./testapp

	all: testapp

	testapp: testapp.o
	$(CXX) $(CXXFLAGS) $(LDFLAGS) $< -o $@ $(GTEST_LIBS)

	testapp.o: samples/sample3_unittest.cc
	$(CXX) $(CPPFLAGS) $(CXXFLAGS) $< -c -o $@ $(GTEST_CFLAGS)
	```

	### Help! pkg-config can't find GoogleTest!

	Let's say you have a `CMakeLists.txt` along the lines of the one in this
	tutorial and you try to run `cmake`. It is very possible that you get a failure
	along the lines of:

	```
	-- Checking for one of the modules 'gtest_main'
	CMake Error at /usr/share/cmake/Modules/FindPkgConfig.cmake:640 (message):
	None of the required 'gtest_main' found
	```

	These failures are common if you installed GoogleTest yourself and have not
	sourced it from a distro or other package manager. If so, you need to tell
	pkg-config where it can find the `.pc` files containing the information. Say you
	installed GoogleTest to `/usr/local`, then it might be that the `.pc` files are
	installed under `/usr/local/lib64/pkgconfig`. If you set

	```
	export PKG_CONFIG_PATH=/usr/local/lib64/pkgconfig
	```

	pkg-config will also try to look in `PKG_CONFIG_PATH` to find `gtest_main.pc`.

	### Using pkg-config in a cross-compilation setting

	Pkg-config can be used in a cross-compilation setting too. To do this, let's
	assume the final prefix of the cross-compiled installation will be `/usr`, and
	your sysroot is `/home/MYUSER/sysroot`. Configure and install GTest using

	```
	mkdir build && cmake -DCMAKE_INSTALL_PREFIX=/usr ..
	```

	Install into the sysroot using `DESTDIR`:

	```
	make -j install DESTDIR=/home/MYUSER/sysroot
	```

	Before we continue, it is recommended to always define the following two
	variables for pkg-config in a cross-compilation setting:

	```
	export PKG_CONFIG_ALLOW_SYSTEM_CFLAGS=yes
	export PKG_CONFIG_ALLOW_SYSTEM_LIBS=yes
	```

	otherwise `pkg-config` will filter `-I` and `-L` flags against standard prefixes
	such as `/usr` (see https://bugs.freedesktop.org/show_bug.cgi?id=28264#c3 for
	reasons why this stripping needs to occur usually).

	If you look at the generated pkg-config file, it will look something like

	```
	libdir=/usr/lib64
	includedir=/usr/include

	Name: gtest
	Description: GoogleTest (without main() function)
	Version: 1.10.0
	URL: https://github.com/google/googletest
	Libs: -L${libdir} -lgtest -lpthread
	Cflags: -I${includedir} -DGTEST_HAS_PTHREAD=1 -lpthread
	```

	Notice that the sysroot is not included in `libdir` and `includedir`! If you try
	to run `pkg-config` with the correct
	`PKG_CONFIG_LIBDIR=/home/MYUSER/sysroot/usr/lib64/pkgconfig` against this `.pc`
	file, you will get

	```
	$ pkg-config --cflags gtest
	-DGTEST_HAS_PTHREAD=1 -lpthread -I/usr/include
	$ pkg-config --libs gtest
	-L/usr/lib64 -lgtest -lpthread
	```

	which is obviously wrong and points to the `CBUILD` and not `CHOST` root. In
	order to use this in a cross-compilation setting, we need to tell pkg-config to
	inject the actual sysroot into `-I` and `-L` variables. Let us now tell
	pkg-config about the actual sysroot

	```
	export PKG_CONFIG_DIR=
	export PKG_CONFIG_SYSROOT_DIR=/home/MYUSER/sysroot
	export PKG_CONFIG_LIBDIR=${PKG_CONFIG_SYSROOT_DIR}/usr/lib64/pkgconfig
	```

	and running `pkg-config` again we get

	```
	$ pkg-config --cflags gtest
	-DGTEST_HAS_PTHREAD=1 -lpthread -I/home/MYUSER/sysroot/usr/include
	$ pkg-config --libs gtest
	-L/home/MYUSER/sysroot/usr/lib64 -lgtest -lpthread
	```

	which contains the correct sysroot now. For a more comprehensive guide to also
	including `${CHOST}` in build system calls, see the excellent tutorial by Diego
	Elio Pettenò: https://autotools.io/pkgconfig/cross-compiling.html

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