Complete Journey of a C Program: From Source Code to Execution

 When you write a program, a lot happens behind the scenes before the computer can actually run it. Let’s understand the full journey from writing your code to seeing your program in action.

1. Source Code (.c file)

This is where you start as a programmer.

• Source code is the actual code you write in a programming language like C, C++, Java, etc.
• It’s written in human-readable form (using words and symbols).
• Example:

#include<stdio.h>
int main() {
    printf("Hello, World!");
    return 0;
}

• File saved as: program.c

2. Compiler

• A compiler reads your source code and translates it into object code (machine language).
• It checks for errors in your code before translation.
• It converts the entire program at once.

Why?

Computers cannot understand source code directly. They need 0s and 1s (machine code).

3. Object Code (.o file)

• After compilation, the output is called object code.
• Object code is a half-finished version of your program. It’s not yet ready to run.
• It contains machine code but may still be missing some required pieces (like functions from external libraries).
• Example file: program.o

Think of object code as the "ingredients ready but not yet cooked."

4. Linker (combines with libraries)

• A linker takes your object code and adds any missing pieces needed to make the program work.
• It combines:
• Your object code.
• Library files (common functions like printf()).
• Other object files (if your program has multiple parts).

Result:

A complete executable file is created.

5. Executable File (.exe file)

• Now you have a complete, ready-to-run program!
• This is called the executable file.
• On Windows: files like program.exe
• On Linux/Mac: simply called executables (no special extension).

When you double-click this file or run it via terminal, your program starts.

6. Loader (Often Forgotten Step)

• When you run the executable file, your operating system uses a loader.
• Loader’s job:
• Loads the executable file from storage (like hard disk) into main memory (RAM).
• Prepares the program for the CPU to execute.

Why?

The CPU can only work on programs present in RAM (main memory), not directly from the hard disk.

7. Execution (Program Runs)

• Once the loader places the program in memory, the CPU starts reading and executing instructions from there.
• Your program is now running and producing output.