A cross-compiler is in fact a collection of different tools set up to tightly work together. The tools are arranged in a way that they are chained, in a kind of cascade, where the output from one becomes the input to another one, to ultimately produce the actual binary code that runs on a machine. So, we call this arrangement a “toolchain”. When a toolchain is meant to generate code for a machine different from the machine it runs on, this is called a cross-toolchain.

The essence of a cross-compiler lies in its ability to generate code for a machine architecture distinct from the host system on which the compiler itself operates. This divergence between the development environment and the target environment necessitates a cross-toolchain, enabling developers to write and compile code on one platform while producing binaries optimized for execution on another.

The cross-toolchain typically consists of several components, including compilers, linkers, assemblers, and libraries, meticulously configured and interconnected to facilitate the compilation process. Each tool in the toolchain contributes to the transformation of source code into executable code, with intermediate outputs seamlessly flowing from one stage to the next until the final binary is produced.

1️⃣ What is Cross Compiler?

A cross compiler is a specialized toolchain that generates executable code for a platform different from the one on which it runs. Unlike native compilers, which produces the code for the same architecture as the host system, cross compilers enable developers to compile code for target environments that may have different instruction sets, operating systems, or hardware configurations.

2️⃣ What are the components in a toolchain?

A toolchain comprises several essential components, each playing a specific role in the compilation process. These components work together seamlessly to transform source code into executable binaries. Here are the key elements typically found in a toolchain:

Ⅰ Compiler

The components that play a role in the toolchain are first and foremost the compiler itself. The compiler turns source code (in C, C++, whatever) into assembly code.

The compiler is the heart of the toolchain. It translates high-level source code into machine code or intermediate representations. Different compilers cater to different programming languages, such as C, C++, Rust, or Go. In a cross-compiler toolchain, the compiler is configured to generate code for the target architecture.

Ⅱ Assembler

 The assembler converts assembly language code into machine code or object code. It translates mnemonic instructions into binary instructions that the target architecture's processor can execute. The output of the assembler is typically in the form of object files.

The assembly code is interpreted by the assembler to generate object code. This is done by the binary utilities, such as the GNU binutils.

Ⅲ Linker

Once the different object code files have been generated, they got to get aggregated together to form the final executable binary. This is called linking, and is achieved with the use of a linker. The GNU binutils also come with a linker.

The linker takes one or more object files, along with any required libraries, and combines them into a single executable file or shared library. It resolves references between different parts of the program, such as function calls and variable accesses, ensuring that all dependencies are satisfied.

Ⅳ Libraries

 Libraries contain precompiled code that provides common functionality or interfaces to system resources. They include standard libraries like libc for C programs, as well as platform-specific libraries for accessing operating system functions, file I/O, networking, and other system services.

Ⅴ Header Files

Header files contain declarations and definitions of functions, data types, constants, and macros used in the source code. They provide interfaces to libraries and system functions, allowing the compiler to check the correctness of function calls and data types during compilation.

3️⃣ Build vs Host vs Target Machines

In the context of cross-compilation, the three terms have specific meanings:

• Build: The machine on which the compilation process runs.
• Host: The machine on which the compiled toolchain runs.
• Target: The machine or architecture for which the compiler will generate code.

When you configure GCC, you specify these with --build=, --host=, and --target=

Lets say You have a PowerPC machine making a compiler that you will use (run) on an x86 machine that will make binaries that run on an ARM.

That makes the PPC the build, the x86 the host, and the target is the ARM.

It's less common to have a build and host that are different, but it certainly does happen. Sometimes the build and host are even the same architecture, but there's something different about the environments that cause this. Making a custom toolchain on my x86 will mean that build and host are x86, but the host may have different libraries, or versions of dependencies, than the build.

⊳ Build Machine:

The build machine is the system where the compilation process takes place. It hosts the development environment, including the necessary tools, libraries, and resources for building software. Developers write, compile, and test their code on the build machine before deploying it to other platforms.

Characteristics:

• Architecture: The build machine may have its own architecture, which may or may not match the target architecture for which the software is being developed.
• Development Tools: It is equipped with compilers, linkers, assemblers, debuggers, and other tools necessary for software development.
• Libraries and Headers: The build machine contains libraries and header files required for compiling and linking software.

⊳ Host Machine:

The host machine is the system where the compiled software runs and executes. It is the environment in which the compiled binaries are deployed and tested. The host machine may or may not be the same as the build machine.

Characteristics:

• Architecture: The host machine's architecture determines the type of binaries that can be executed on it. The compiled software must be compatible with the host machine's architecture.
• Operating System: The host machine may run a different operating system than the build machine. Compatibility between the compiled binaries and the host operating system is essential.
• Execution Environment: The host machine provides the runtime environment for executing the compiled software, including system libraries, kernel, and hardware resources.

⊳ Target Machine:

The target machine is the system for which the software is being developed or compiled. It is the platform where the compiled binaries will ultimately run and execute. In cross-compilation scenarios, the target machine's architecture and operating system may differ from those of the build and host machines.

Characteristics:

• Architecture: The target machine's architecture determines the type of binaries that need to be generated by the cross-compiler. The compiled software must be compatible with the target machine's architecture.
• Operating System: The target machine may run a different operating system than the build and host machines. Cross-compilation involves generating binaries for the target operating system.
• Hardware Configuration: The target machine's hardware configuration, including processor architecture, memory layout, and peripheral devices, influences software development and compilation decisions.

Relationship:

• Build vs. Host: The build machine is where the compilation process occurs, while the host machine is where the compiled software is executed. In some cases, the build and host machines may be the same, but they can also be different, especially in cross-compilation scenarios.
• Build vs. Target: The build machine generates binaries for the target machine. In cross-compilation, the build machine's architecture may differ from that of the target machine, requiring a cross-compiler to generate compatible binaries.

Extras:

• If build, host, and target are all the same, this is called a native.
• If build and host are the same but target is different, this is called a cross.
• If build, host, and target are all different this is called a canadian (for obscure reasons dealing with Canada’s political party and the background of the person working on the build at that time).
• If host and target are the same, but build is different, you are using a cross-compiler to build a native for a different system.
  • Some people call this a host-x-host, crossed native, or cross-built native.
• If build and target are the same, but host is different, you are using a cross compiler to build a cross compiler that produces code for the machine you’re building on.
  • This is rare, so there is no common way of describing it. There is a proposal to call this a crossback.

Common Scenarios

Key Takeaways

• If build == host == target, it is a native build.
• If build == host ≠ target, it is a standard cross-compilation (compiler runs on the same system but targets another).
• If build ≠ host ≠ target, it is a Canadian cross (building a cross-compiler for a different machine).