Question

I’ve always wondered. I know that compilers convert the code you write into binaries but what do linkers do? They’ve always been a mystery to me.

I roughly understand what ‘linking’ is. It is when references to libraries and frameworks are added to the binary. I don’t understand anything beyond that. For me it “just works”. I also understand the basics of dynamic linking but nothing too deep.

Could someone explain the terms?

Answer 1

To understand linkers, it helps to first understand what happens "under the hood" when you convert a source file (such as a C or C++ file) into an executable file (an executable file is a file that can be executed on your machine or someone else’s machine running the same machine architecture).

Under the hood, when a program is compiled, the compiler converts the source file into object byte code. This byte code (sometimes called object code) is mnemonic instructions that only your computer architecture understands. Traditionally, these files have an .OBJ extension.

After the object file is created, the linker comes into play. More often than not, a real program that does anything useful will need to reference other files. In C, for example, a simple program to print your name to the screen would consist of:

printf("Hello Kristina!\n");

When the compiler compiled your program into an obj file, it simply puts a reference to the printf function. The linker resolves this reference. Most programming languages have a standard library of routines to cover the basic stuff expected from that language. The linker links your OBJ file with this standard library. The linker can also link your OBJ file with other OBJ files. You can create other OBJ files that have functions that can be called by another OBJ file. The linker works almost like a word processor’s copy and paste. It "copies" out all the necessary functions that your program references and creates a single executable. Sometimes other libraries that are copied out are dependent on yet other OBJ or library files. Sometimes a linker has to get pretty recursive to do its job.

Note that not all operating systems create a single executable. Windows, for example, uses DLLs that keep all these functions together in a single file. This reduces the size of your executable, but makes your executable dependent on these specific DLLs. DOS used to use things called Overlays (.OVL files). This had many purposes, but one was to keep commonly used functions together in 1 file (another purpose it served, in case you’re wondering, was to be able to fit large programs into memory. DOS has a limitation in memory and overlays could be "unloaded" from memory and other overlays could be "loaded" on top of that memory, hence the name, "overlays"). Linux has shared libraries, which is basically the same idea as DLLs (hard core Linux guys I know would tell me there are MANY BIG differences).