Storage Classes in C

A storage class defines the scope (visibility), lifetime (duration), and default initial value of a variable.

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The Four Storage Classes

1. auto

• Default storage class for local variables
• Scope: local to the block
• Lifetime: until the block ends
• Default value: garbage

auto int x = 10;   /* same as: int x = 10; */

2. register

• Requests that the variable be stored in a CPU register (faster access)
• Cannot take the address of a register variable
• Scope: local to block
• Lifetime: until block ends

register int counter;
for (register int i = 0; i < 1000000; i++) {
    counter++;
}

Note: The compiler may ignore the register hint.

3. static

• Retains value between function calls
• Global static: scope restricted to file
• Local static: scope local, but lifetime = entire program

void countCalls() {
    static int count = 0;   /* initialised only once */
    count++;
    printf("Called %d times\n", count);
}

int main() {
    countCalls();  /* Called 1 times */
    countCalls();  /* Called 2 times */
    countCalls();  /* Called 3 times */
}

4. extern

• Declares a variable defined in another file or later in the same file
• Used to share variables across multiple files

File 1 (main.c):

#include <stdio.h>
extern int sharedVar;   /* declaration */

int main() {
    printf("%d\n", sharedVar);
    return 0;
}

File 2 (data.c):

int sharedVar = 42;   /* definition */

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Comparison Table