02 The Basic Unit of Compilation & Preprocessing Logic

1. The Compilation Unit (Translation Unit)

The Compilation Unit (or Translation Unit) is the fundamental atomic unit that the compiler processes and optimizes. It is crucial to distinguish between the physical source file and the logical compilation unit.

Definition

A Compilation Unit is not just the .c file you write. It is the result of the preprocessing stage. Formula: $$ \text{Compilation Unit} = \text{Source File (.c)} + \text{Recursively Expanded Headers} - \text{Comments} - \text{Excluded Code blocks} $$

Core Characteristic: Independence

The compiler processes one compilation unit at a time and maintains zero knowledge of other units during this stage. * Isolation: When compiling A.c, the compiler does not know B.c exists. * Consequence: This is why Function Declarations (Prototypes) are mandatory. You must tell the compiler: "I promise this function exists somewhere else, let me pass for now, and the Linker will find it later."

Scope and Linkage

[[static | More about static]] The boundaries of a compilation unit define the behavior of the static keyword. * Internal Linkage (static): Variables/functions marked static are private to the current compilation unit. They are invisible to the linker and other files. * External Linkage (extern): By default, functions and global variables are visible to the entire program.