Getting started

To start learning about creating packages, we will create a package from an existing source code repository: https://github.com/memsharded/hello. You can check that project, it is a very simple “hello world” C++ library, using CMake as build system to build a library and an executable. It has nothing related to conan in it.

We are using such github repository as an example, but the same process would apply to other source code origins, like downloading a zip or tarball from the internet.

Note

For this concrete example you will need, besides a C++ compiler, both CMake and git installed and in your path. They are not required by conan, you could use your own build system and version control instead.

Creating the package recipe

First, let’s create a folder for our package recipe, and use the conan new helper command that will create a working package recipe for us:

$ mkdir mypkg && cd mypkg
$ conan new Hello/0.1 -t

This will generate the following files:

conanfile.py
test_package
  conanfile.py
  CMakeLists.txt
  example.cpp

At the root level, there is a conanfile.py which is the main recipe file, the one actually defining our package. Also, there is a test_package folder, which contains a simple example consuming project that will require and link with the created package. It is useful to make sure that our package is correctly created.

Let’s have a look to the root package recipe conanfile.py:

from conans import ConanFile, CMake, tools

class HelloConan(ConanFile):
    name = "Hello"
    version = "0.1"
    settings = "os", "compiler", "build_type", "arch"
    options = {"shared": [True, False]}
    default_options = "shared=False"
    generators = "cmake"

    def source(self):
        self.run("git clone https://github.com/memsharded/hello.git")
        self.run("cd hello && git checkout static_shared")
        # This small hack might be useful to guarantee proper /MT /MD linkage in MSVC
        # if the packaged project doesn't have variables to set it properly
        tools.replace_in_file("hello/CMakeLists.txt", "PROJECT(MyHello)", '''PROJECT(MyHello)
include(${CMAKE_BINARY_DIR}/conanbuildinfo.cmake)
conan_basic_setup()''')

    def build(self):
        cmake = CMake(self)
        cmake.configure(source_folder="hello")
        cmake.build()

        # Explicit way:
        # self.run('cmake "%s/hello" %s' % (self.source_folder, cmake.command_line))
        # self.run("cmake --build . %s" % cmake.build_config)

    def package(self):
        self.copy("*.h", dst="include", src="hello")
        self.copy("*hello.lib", dst="lib", keep_path=False)
        self.copy("*.dll", dst="bin", keep_path=False)
        self.copy("*.so", dst="lib", keep_path=False)
        self.copy("*.dylib", dst="lib", keep_path=False)
        self.copy("*.a", dst="lib", keep_path=False)

    def package_info(self):
        self.cpp_info.libs = ["hello"]

This is a complete package recipe. Without worrying too much about every detail, these are the basics:

  • The settings field defines the configuration that defines the different binary packages. In this example we are defining that any change to the OS, compiler, architecture or build type will generate a different binary package. Remember, Conan generates different binary packages for different introduced configuration (in this case settings) for the same recipe.

    Note that the platform where the recipe is running and the package is being built can be different from the final platform where the code will be running (self.settings.os and self.settings.arch) if the package is being cross-built. So if you want to apply a different build depending on the current build machine, you need to check it:

    def build(self):
    if platform.system() == "Windows":
        cmake = CMake(self)
        cmake.configure(source_folder="hello")
        cmake.build()
    else:
        env_build = AutoToolsBuildEnvironment(self)
        env_build.configure()
        env_build.make()

    Learn more in the Cross building section.

  • This package recipe is also able to create different binary packages for static and shared libraries with the shared option, which is defaulted to False (i.e. by default it will use static linkage).

  • The source() method executes a git clone to retrieve the sources from github. Other origins, as downloading a zip file are also available. As you can see, any manipulation of the code can be done, such as checking out any branch or tag, or patching the source code. In this example, we are adding two lines to the existing CMake code, to ensure binary compatibility. Don’t worry too much about it now, we’ll visit it later.

  • The build() first configures the project, then builds it, with standard CMake commands. The CMake object is just a helper to ease the translation of conan settings to CMake command line arguments. Remember that CMake is not strictly required. You can build packages directly invoking make, MSBuild, SCons or any other build system.

    See also

    Check the existing build helpers.

  • The package() method copies artifacts (headers, libs) from the build folder to the final package folder.

  • Finally, the package_info() method defines that consumer must link with the “hello” library when using this package. Other information as include or lib paths can be defined as well. This information is used for files created by generators to be used by consumers, as conanbuildinfo.cmake.

The test_package folder

Note

The test_package is different from the library unit or integration tests, which should be more comprehensive. These tests are “package” tests, and validate that the package is properly created, and that package consumers will be able to link against it and reuse it.

If you have a look to the test_package folder, you will realize that the example.cpp and the CMakeLists.txt files don’t have anything special. The test_package/conanfile.py file is just another recipe, you can think of it as the consumer conanfile.txt we have already seen in previous sections:

from conans import ConanFile, CMake
import os

class HelloTestConan(ConanFile):
    settings = "os", "compiler", "build_type", "arch"
    generators = "cmake"

    def build(self):
        cmake = CMake(self)
        cmake.configure()
        cmake.build()

    def imports(self):
        self.copy("*.dll", dst="bin", src="bin")
        self.copy("*.dylib*", dst="bin", src="lib")

    def test(self):
        os.chdir("bin")
        self.run(".%sexample" % os.sep)

The main differences with the above conanfile.py are:

  • It doesn’t have a name and version, because we are not creating a package, so they are not necessary.

  • The package() and package_info() methods are not required, since we are not creating a package.

  • The test() method specifies which binaries have to be run.

  • The imports() method is defined to copy shared libraries to the bin folder, so when dynamic linkage is used, and the test() method launches the example executable, they are found and example runs.

Note

An important difference with respect to normal package recipes, is that this one does not need to declare a requires attribute, to depend on the Hello/0.1@demo/testing package we are testing. This requires will be automatically injected by conan while running. You can however declare it explicitly, it will work, but you will have to remember to bump the version, and possibly the user and channel if you change them.

Creating and testing packages

We can create and test the package with our default settings simply by:

$ conan create . demo/testing
...
Hello world!

If you see “Hello world!”, it worked.

This will perform the following steps:

  • Copy (“export” in conan terms) the conanfile.py from the user folder into the local cache.

  • Install the package, forcing building it from sources.

  • Move to the test_package folder, and create a temporary build folder.

  • Execute there a conan install .., so it installs the requirements of the test_package/conanfile.py. Note that it will build “Hello” from sources.

  • Build and launch the example consuming application, calling the test_package/conanfile.py build() and test() methods respectively.

Using conan commands, the conan create command would be equivalent to:

$ conan export . demo/testing
$ conan install Hello/0.1@demo/testing --build=Hello
# package is created now, use test to test it
$ conan test test_package Hello/0.1@demo/testing

The conan create command receives the same command line parameters as conan install so you can pass to it the same settings, options, and command line switches. If you want to create and test packages for different configurations, you could:

$ conan create . demo/testing -s build_type=Debug
$ conan create . demo/testing -o Hello:shared=True -s arch=x86
$ conan create . demo/testing -pr my_gcc49_debug_profile
...
$ conan create ...

Settings vs. options

We have used settings as os, arch and compiler. But the above package recipe also contains a shared option (defined as options = {"shared": [True, False]}). What is the difference between settings and options?

Settings are project-wide configuration, something that typically affects the whole project that is being built. For example, the Operating System or the architecture would be naturally the same for all packages in a dependency graph, linking a Linux library for a Windows app, or mixing architectures is impossible.

Settings cannot be defaulted in a package recipe. A recipe for a given library cannot say that its default os=Windows. The os will be given by the environment in which that recipe is processed. It is a necessary input.

Settings are configurable. You can edit, add, remove settings or subsettings in your settings.yml file. See the settings.yml reference.

On the other hand, options are package-specific configuration. Being a static or shared library is not something that applies to all packages. Some can be header only libraries. Other packages can be just data, or package executables. Or packages can contain a mixture of different artifacts. shared is a common option, but packages can define and use any options they want.

Options are defined in the package recipe, including their allowed values, and it can be defaulted by the package recipe itself. A package for a library can well define that by default it will be a static library (a typical default). If no one else specifies something different, the package will be static.

There are some exceptions to the above, for example, settings can be defined per-package, like in command line:

$ conan install . -s MyPkg:compiler=gcc -s compiler=clang ..

This will use gcc for MyPkg and clang for the rest of the dependencies (extremely unusual case).

You can also have a very widely-used option in many packages and set its value all at once with patterns, like:

$ conan install . -o *:shared=True

Any doubts? Please check out our FAQ section or write us.