Storage Classes in C

Storage Classes in C

Storage classes define where a variable is stored, how long it lives, and who can access it.

auto

• Storage: Stack
• Scope: Block (inside {})
• Lifetime: Only during the function call
• Note: This is the default for all local variables — you almost never write auto explicitly

register

• Storage: CPU Register (if available, otherwise falls back to stack)
• Scope: Block
• Lifetime: Function call
• Note: A hint to the compiler to optimize for speed. Modern compilers largely ignore this since they optimize register usage themselves

static

• Storage: Static Memory (data segment)
• Scope: Block or File (depending on where it’s declared)
• Lifetime: Entire program duration
• Note: Retains its value between function calls — unlike auto, it doesn’t reset. When declared at file level, it’s invisible to other files (internal linkage)

extern

• Storage: Static Memory
• Scope: Global (across files)
• Lifetime: Entire program
• Note: Declared elsewhere — tells the compiler “this variable exists in another translation unit.” Used to share variables across .c files

Quick Mental Model

auto/register  → born when function starts, die when it ends
static         → born at program start, live forever, remember their value
extern         → live in another file, borrowed here