Assembler

As we explained before, a platform has to have an assembler in order to produce object files that contain correct machine-level instructions. In a Unix-like operating system, the assembler can be invoked by using the as utility program. In the rest of this section, we are going to discuss what can be put in an object file by the assembler.

If you install two different Unix-like operating systems on the same architecture, the installed assemblers might not be the same, which is very important. What this means is that, despite the fact that the machine-level instructions are the same, because of being on the same hardware, the produced object files can be different!

If you compile a program and produce the corresponding object file on Linux for an AMD64 architecture, it could be different from if you had tried to compile the same program in a different operating system such as FreeBSD or macOS, and on the same hardware. This implies that while the object files cannot be the same, they do contain the same machine-level instructions. This proves that object files can have different formats in various operating systems.

In other words, each operating system defines its own specific binary format or object file format when it comes to storing machine-level instructions within object files. Therefore, there are two factors that specify the contents of an object file: the architecture (or hardware) and the operating system. Typically, we will use the term platform for such a combination.

To round off this section, we usually say that object files, hence the assembler generating them, are platform-specific. In Linux, we use the Executable and Linking Format (ELF). As the name implies, all executable files, object files, and shared libraries should use this format. In other words, in Linux, the assembler produces ELF object files. In the upcoming chapter, Object Files, we will discuss object files and their formats in greater detail.

In the following section, we will take a deeper look at the linker component. We will demonstrate and explain how the component actually produces the final products in a C project.