Storage Classes in C

Storage Classes in C

Storage classes in C define the scope, lifetime, Default Initial Value and visibility of variables and functions. There are four main storage classes in C:

1. auto (Automatic Storage Class)

• Keyword: auto
• Scope: Within block
• Lifetime: Until block exits
• Default Initial Value: Garbage (undefined)
• Use Case: Default for local variables
• Default for local variables inside functions.
• Memory allocated on the stack.
• Scope is limited to the block where it's declared.
• Since auto is the default for local variables, we rarely use it explicitly.

2. register (Register Storage Class)

• Keyword: register
• Scope: Within block
• Lifetime: Until block exits
• Default Initial Value: Garbage (undefined)
  
• Use Case: Faster access (loop counters)
  
• Suggests that the variable be stored in a CPU register (faster access).
• Used for high-speed variables, like loop counters.
• Cannot take the address (&) of a register variable.
• Modern compilers often ignore register, as they optimize automatically.
  

3. static (Static Storage Class)

• Retains its value between function calls.
• Can be used in local or global scope.
• Unlike normal global variables, static global variables cannot be accessed in other files.
  

(a) Static Local Variables

• Maintains value between function calls.

(b) Static Global Variables

• Limited only to the file where they are defined.

4. extern (External Storage Class)

• Used to access global variables from another file.
• Declares a variable without defining it (allocated elsewhere).