Today we released a Conan package Boost/1.66.0@conan/stable on conan-center.

This package contains binaries for more than 150 different configurations: Windows (different flavors of Visual Studio), Linux (gcc and clang compilers), OSX. In all systems, it is possible to use different architectures, build types, or choose if we want to link statically, dynamically, or even to use boost header-only libraries.

But this package also includes large improvements for cross-building boost to different platforms, like Raspberry PI, or Android. This amazing work has been done thanks to many contributions from the (conan) community, thank you all!!!

Conan-center only includes the most mainstream binaries, those for Windows, Linux and OSX, but this post explain how you can use conan to easily cross-build Boost to those platforms.

The conan model

Conan packages are defined by recipes, which are python scripts, describing how to build and package the library. With one conan recipe, many different binary packages can be created, i.e: one for Windows Visual Studio 14, another one for Linux GCC 6 and so on. Conan package recipes are responsible for translating the user settings (os, architecture, compiler, etc) and call the underlying library’s build system with the right options/flags, generating a different binary package for different input settings.

Both package recipes and binaries for all platforms and configurations can be uploaded to the same conan server, to share them with the team. From now on, if any developer wants to work with this library and there is already a binary generated for the requested configuration (settings/options) then the library is going to be retrieved directly from the server. It will save a huge amount of time to any developer of CI process, especially when we are talking about big and complex libraries like boost.

When the user runs “conan install” to retrieve the dependencies for his project, Conan will download the recipe, evaluate it with their settings/options, and download the binary matching the user’s configuration.

Cross building Boost with Boost Build (b2)

The Boost Build system (aka b2) accepts some arguments to define the cross-compilation:

architecture, address-model, binary-format, mfloat-abi, abi, target-os

In our case, if we want to cross-build to a Android/ARM system, the arguments to pass to “b2” are:

architecture=arm address-model=32 binary-format=elf abi=appcs target-os=android

Hence the command like command would be:

$ b2 architecture=arm address-model=32 binary-format=elf abi=aapcs target-os=android link=static variant=release --without-python -j8 --abbreviate-paths -d2 --debug-configuration --build-dir=”my_build_folder"

But this is not enough because boost has some third-party dependencies, like zlib or bzip2. The user also needs to set some environment configuration, the build tools and compilers paths, the details of the toolchain and compilations flags has to be specified. To do it the best approach is to define an user-config.jam file with the details for cross-building, which could look like:

using zlib : 1.2.11 : <include>/.conan/data/zlib/1.2.11/conan/stable/package/39a53587004d75943e385925ca011baeab537de0/include <search>/.conan/data/zlib/1.2.11/conan/stable/package/39a53587004d75943e385925ca011baeab537de0/lib ; using clang : 5 : "arm-linux-androideabi-clang++" : <archiver>"/path/to/arm_21_toolchain/bin/arm-linux-androideabi-ar" <ranlib>"/path/to//arm_21_toolchain/bin/arm-linux-androideabi-ranlib" <cxxflags>"-fPIC -I/path/to/arm_21_toolchain/include/c++/4.9.x" <cflags>"-fPIC -I/path/to/arm_21_toolchain/include/c++/4.9.x" <ldflags>"" ;

This is an important detail if we want to cross-build Boost for Android, it should link against cross-built versions of zlib and bzip2 for the same configuration we are cross-building Boost.

It could be a huge challenge for any developer and a real impediment. Fortunately, conan-center repository already contains zlib and bzip2 packages with pre-built binaries for hundreds of configurations, but most importantly: they also know how to cross-build themselves, using their own build systems.

Creating Boost packages with native and cross-build binaries

There are two ways of creating binary packages. The first one is using “conan create”, typically used by package creators to explicitly build and test packages before uploading them. This process usually starts cloning a repository that contains the conan package recipe:

$ git clone -b release/1.66.0 https://github.com/lasote/conan-boost $ conan create . myuser/testing

The second way is to consume existing packages. End users can create a conanfile.txt or conanfile.py , declaring the necessary package dependency, and then use the command, “conan install” (more info getting started).

A conanfile.txt to consume the boost library looks like:

[requires] boost/1.66.0@conan/stable

The “conan install” command will try to download a pre-built binary package for developer’s default configuration (more info default profile), but it can fail if there is no binary for the requested configuration. The “–build missing” argument should be used in this case forcing conan to build the library from sources with the library recipe. The command should be:

$ conan install . --build missing

In both cases, to specify which configuration do you want to build, it is very handy to use profiles:

$ conan install . --build missing --profile my_clang_profile

The profiles are plain text files defining settings, options, environment variables, and build requirements.

A default profile for OSX could be:

[settings] os=Macos os_build=Macos arch=x86_64 arch_build=x86_64 compiler=apple-clang compiler.version=9.0 compiler.libcxx=libc++ build_type=Release [options] [build_requires] [env]

If we want to cross-compile to Raspberry PI from Windows, we could install the correct toolchain and define the following profile:

~/.conan/profiles/rpi target_host=arm-linux-gnueabihf standalone_toolchain=C:/sysgcc/raspberry cc_compiler=gcc cxx_compiler=g++ [settings] os_build=Windows arch_build=x86_64 os=Linux arch=armv7 # Change to armv6 if you are using Raspberry 1 compiler=gcc compiler.version=6 compiler.libcxx=libstdc++11 build_type=Release [env] CONAN_CMAKE_FIND_ROOT_PATH=$standalone_toolchain/$target_host/sysroot PATH=[$standalone_toolchain/bin] CHOST=$target_host AR=$target_host-ar AS=$target_host-as RANLIB=$target_host-ranlib LD=$target_host-ld STRIP=$target_host-strip CC=$target_host-$cc_compiler CXX=$target_host-$cxx_compiler CXXFLAGS=-I"$standalone_toolchain/$target_host/lib/include"

And use this profile with conan create which would build the library, in the local cache, with the settings specified in our profile “rpi”:

$ conan create . conan/stable --profile=rpi

You can follow this guide in the conan docs to know more about compiling for Raspberry PI.

If you want to cross build a Conan package you need:

The correct toolchain (compiler and tools)

A profile that describe the settings and the needed environment variables to locate the toolchain.

Creating Boost packages for Android

Preparing the Android toolchain

The Android toolchain can be generated from the Android NDK.

From the NDK version r16 is only supported “clang” compiler and “libc++” as the standard c++ library.

You can download the NDK and invoke the “make_standalone_toolchain.py” script specifying the api level, architecture and standard library:

$ cd build/tools $ python make_standalone_toolchain.py --arch=arm --api=21 --stl=libc++ --install-dir=/myfolder/arm_21_toolchain

Preparing the Conan profile

Copy and paste this profile in your profiles folder (~/.conan/profiles) adjusting the path to the new standalone toolchain (replace “/myfolder/arm_21_toolchain” with your install path):

.conan/profiles/android_21_armeabi-v7a_clang

standalone_toolchain=/myfolder/arm_21_toolchain target_host=arm-linux-androideabi cc_compiler=clang cxx_compiler=clang++ [settings] compiler=clang compiler.version=5.0 compiler.libcxx=libc++ os=Android os.api_level=21 arch=armv7 build_type=Release [env] CONAN_CMAKE_FIND_ROOT_PATH=$standalone_toolchain/sysroot PATH=[$standalone_toolchain/bin] CHOST=$target_host AR=$target_host-ar AS=$target_host-as RANLIB=$target_host-ranlib CC=$target_host-$cc_compiler CXX=$target_host-$cxx_compiler LD=$target_host-ld STRIP=$target_host-strip CFLAGS= -fPIC -I$standalone_toolchain/include/c++/4.9.x CXXFLAGS= -fPIC -I$standalone_toolchain/include/c++/4.9.x LDFLAGS=

Building Boost package for Android

In conan-center you can find binaries for known platforms like Windows, OSX and Linux but not for Android. In this section we are going to build boost for Android and upload the resulting package to a repository where it can be consumed by other developers, not building the boost library from sources again in the process.

We are going to use the “conan create” command, so we need to clone the recipe repository:

git clone -b release/1.66.0 https://github.com/lasote/conan-boost conan create . conan/stable --build missing --profile=android_21_armeabi-v7_clang

With the “–build missing” parameter we are telling conan to build from sources any transitive dependency for the specified profile, in this case Conan will cross-build also zlib and bzip2.

Now we can upload the generated binary packages to a conan-server, Artifactory or Bintray. Check the Uploading packages section in the docs.

> conan upload boost/1.66.0@conan/stable --all -r=myremote > conan upload zlib* --all -r=myremote > conan upload bzip2* --all -r=myremote # or just > conan upload * --all -r=myremote

We could also share profiles, remotes and settings among the team with the conan config install command.

At this point the binary packages for Android are in my-remote (i.e: conan-center) and developers using the above profile can reuse them without building from sources the whole boost library.

In a following blog post we will create an android app, using the boost library, with an Android Studio project and a simple “conanfile.txt”. We will explain how to build the same app for multiple architectures (arm, x86 …) and package everything in a single apk.