3.2. Cross-Development Toolchain Generation

The Yocto Project does most of the work for you when it comes to creating cross-development toolchains. This section provides some technical background on how cross-development toolchains are created and used. For more information on toolchains, you can also see the Yocto Project Application Development and the Extensible Software Development Kit (eSDK) manual.

In the Yocto Project development environment, cross-development toolchains are used to build the image and applications that run on the target hardware. With just a few commands, the OpenEmbedded build system creates these necessary toolchains for you.

The following figure shows a high-level build environment regarding toolchain construction and use.

Most of the work occurs on the Build Host. This is the machine used to build images and generally work within the the Yocto Project environment. When you run BitBake to create an image, the OpenEmbedded build system uses the host gcc compiler to bootstrap a cross-compiler named gcc-cross. The gcc-cross compiler is what BitBake uses to compile source files when creating the target image. You can think of gcc-cross simply as an automatically generated cross-compiler that is used internally within BitBake only.

Note

The extensible SDK does not use gcc-cross-canadian since this SDK ships a copy of the OpenEmbedded build system and the sysroot within it contains gcc-cross.

The chain of events that occurs when gcc-cross is bootstrapped is as follows:

     gcc -> binutils-cross -> gcc-cross-initial -> linux-libc-headers -> glibc-initial -> glibc -> gcc-cross -> gcc-runtime

gcc: The build host's GNU Compiler Collection (GCC).
binutils-cross: The bare minimum binary utilities needed in order to run the gcc-cross-initial phase of the bootstrap operation.
gcc-cross-initial: An early stage of the bootstrap process for creating the cross-compiler. This stage builds enough of the gcc-cross, the C library, and other pieces needed to finish building the final cross-compiler in later stages. This tool is a "native" package (i.e. it is designed to run on the build host).
linux-libc-headers: Headers needed for the cross-compiler.
glibc-initial: An initial version of the Embedded GLIBC needed to bootstrap glibc.
gcc-cross: The final stage of the bootstrap process for the cross-compiler. This stage results in the actual cross-compiler that BitBake uses when it builds an image for a targeted device.

Note
If you are replacing this cross compiler toolchain with a custom version, you must replace gcc-cross.

This tool is also a "native" package (i.e. it is designed to run on the build host).
gcc-runtime: Runtime libraries resulting from the toolchain bootstrapping process. This tool produces a binary that consists of the runtime libraries need for the targeted device.

You can use the OpenEmbedded build system to build an installer for the relocatable SDK used to develop applications. When you run the installer, it installs the toolchain, which contains the development tools (e.g., the gcc-cross-canadian), binutils-cross-canadian, and other nativesdk-* tools, which are tools native to the SDK (i.e. native to SDK_ARCH), you need to cross-compile and test your software. The figure shows the commands you use to easily build out this toolchain. This cross-development toolchain is built to execute on the SDKMACHINE, which might or might not be the same machine as the Build Host.

Note

If your target architecture is supported by the Yocto Project, you can take advantage of pre-built images that ship with the Yocto Project and already contain cross-development toolchain installers.

Here is the bootstrap process for the relocatable toolchain:

     gcc -> binutils-crosssdk -> gcc-crosssdk-initial -> linux-libc-headers ->
        glibc-initial -> nativesdk-glibc -> gcc-crosssdk -> gcc-cross-canadian

gcc: The build host's GNU Compiler Collection (GCC).
binutils-crosssdk: The bare minimum binary utilities needed in order to run the gcc-crosssdk-initial phase of the bootstrap operation.
gcc-crosssdk-initial: An early stage of the bootstrap process for creating the cross-compiler. This stage builds enough of the gcc-crosssdk and supporting pieces so that the final stage of the bootstrap process can produce the finished cross-compiler. This tool is a "native" binary that runs on the build host.
linux-libc-headers: Headers needed for the cross-compiler.
glibc-initial: An initial version of the Embedded GLIBC needed to bootstrap nativesdk-glibc.
nativesdk-glibc: The Embedded GLIBC needed to bootstrap the gcc-crosssdk.
gcc-crosssdk: The final stage of the bootstrap process for the relocatable cross-compiler. The gcc-crosssdk is a transitory compiler and never leaves the build host. Its purpose is to help in the bootstrap process to create the eventual relocatable gcc-cross-canadian compiler, which is relocatable. This tool is also a "native" package (i.e. it is designed to run on the build host).
gcc-cross-canadian: The final relocatable cross-compiler. When run on the SDKMACHINE, this tool produces executable code that runs on the target device. Only one cross-canadian compiler is produced per architecture since they can be targeted at different processor optimizations using configurations passed to the compiler through the compile commands. This circumvents the need for multiple compilers and thus reduces the size of the toolchains.

Note

For information on advantages gained when building a cross-development toolchain installer, see the "Building an SDK Installer" section in the Yocto Project Application Development and the Extensible Software Development Kit (eSDK) manual.