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Hey there, and welcome!
Today, we’re going to walk through exactly how a C program gets executed — from writing the code to getting it loaded into memory and running on your machine.
Now, C might seem like a straightforward language when you write it, but there’s actually a multi-step process happening behind the scenes. And each step plays a critical role in transforming your code into an actual program your computer can understand.
Let’s walk through it together — step by step.
Step 1 – Writing the Code
It all begins with your C source code. This is the code you write in a text editor — using the C programming language. It’s the human-readable part, where you define things like variables, functions, and logic.
Once you’ve written the code, the file is saved with a “dot C” extension — usually something like program.c. But at this point, the code can’t run yet. It needs to go through a series of transformations to become something the computer can execute.
And the first step in that process is preprocessing.
Step 2 – Preprocessing
The preprocessor is like a filter that prepares your code before it gets compiled.
It handles all the lines that start with a hash symbol — like include statements and macros. These tell the preprocessor to bring in external code from header files, or to replace certain expressions with reusable text or values.
After this step, your code is still written in C, but now it’s expanded and ready for the compiler. So the output of the preprocessor is a modified version of your C code — but with everything fully included and cleaned up for the next step.
Step 3 – Compilation
Next up is the compiler.
The compiler takes the preprocessed C code and translates it into assembly code — which is much closer to what the computer can understand.
At this point, your human-readable logic has been transformed into a low-level set of instructions. These instructions tell the computer what to do — step by step — using the basic operations the hardware understands.
But it still isn’t ready to run just yet.
Step 4 – Assembling
Now it’s the assembler’s turn.
The assembler takes that assembly code and converts it into object code — which is essentially raw machine code, but still not the final product.
Object code is made up of binary instructions — the ones and zeros — but it might still be missing pieces. For example, maybe your code depends on functions from another library.
And that’s where the linker comes in.
Step 5 – Linking
The linker is responsible for combining all the object code together and making sure every function, every variable, and every dependency is properly connected.
It links your object code with any required libraries — whether they’re built-in standard libraries or third-party ones — and creates a single executable file.
Now the program is finally complete. All the pieces have been brought together, and the result is a fully compiled machine code file that your operating system can run.
But it still hasn’t actually been executed.
Step 6 – Loading and Execution
To run the program, we need to load it into memory.
That’s the job of the loader — a part of the operating system that takes your final executable file and loads it into primary memory, or RAM.
Once it’s in memory, the CPU can read the instructions and begin executing your program.
And that’s when your C code actually comes to life — printing output, calculating results, or interacting with users.
From the editor to the processor, your code has gone through an entire journey.
Wrap-Up
So to sum it all up — when you write a C program, it doesn’t just run instantly.
First, the source code goes through preprocessing, where macros and include files are handled. Then it’s compiled into assembly code. That gets passed to the assembler, which produces object code. The linker takes care of connecting all the parts, including external libraries, and generates the final executable.
Finally, the loader places the program into memory, and the processor begins to run it.
This full process — from source code to running application — is what makes C both powerful and efficient. You get full control over what’s happening, and a deeper understanding of how computers really work.
Thanks for reading— and I’ll see you in the next lesson!
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