CSS Computer Science (Optional) — Paper-I, Section-A, Topic II subtopic. Appeared as a past paper question (2016). Complements Program Development and Execution page.

The Full Pipeline: Source Code → Executable

1. Source code (.c / .cpp) — human-written code
2. Preprocessing — preprocessor directives resolved (#include, #define, macro expansion) → produces expanded source
3. Compilation — compiler translates preprocessed source into assembly code
4. Assembly — assembler converts assembly code into object code (machine code, .obj/.o), but not yet linked
5. Linking — linker combines object code with required libraries (standard library, other object files) and resolves external references → produces the final executable (.exe)
6. Loading — at run time, the OS loader loads the executable into memory and transfers control to start execution

Diagram (textual)

Source (.c) → Preprocessor → Expanded Source → Compiler → Assembly (.s) → Assembler → Object Code (.o) → Linker (+ libraries) → Executable (.exe) → Loader → Running Program

Key Distinctions

• Object code is machine code for just one file — may reference functions from other files/libraries it doesn't yet have addresses for
• Linker resolves those references, producing one complete, runnable executable
• Static linking embeds library code directly into the executable; dynamic linking links to shared libraries (.dll/.so) at load/run time

Worked Example

Given a tiny C program:

#include <stdio.h>
#define PI 3.14
int main() {
    printf("%f", PI);
    return 0;
}

1. Preprocessing: #include <stdio.h> is replaced with the contents of the stdio header; PI is textually replaced with 3.14 everywhere it appears
2. Compilation: the expanded source is translated into assembly instructions for printf("%f", 3.14); return 0;
3. Assembly: the assembler turns those assembly instructions into object code (hello.o) — machine code, but printf is only referenced, not yet resolved