Programming: Variable Storage Classes

Automatic: auto 

• storage is automatically allocated on function/block entry and automatically freed when the function/block is exited 
• may not be used with global variables (which have storage space that exists for the life of the program) 
• auto is the default for function/block variables 
• auto int a is the same as int a 
• because it is the default, it is almost never used

Optimization Hint: register

• register provides a hint to the compiler that you think a variable will be frequently used
• compiler is free to ignore register hint
• if ignored, the variable is equivalent to an auto variable with the exception that you may not take the address of a register (since, if put in a register, the variable will not have an address)
• rarely used, since any modern compiler will do a better job of optimization than most programmers

Static Storage: static

• if used inside a block or function, the compiler will create space for the variable which lasts for the life of the program

•   int
    counter(void)
    {
   static int cnt = 0;
  
   return cnt++;
    }
  

  causes the counter() function to return a constantly increasing number

External References: extern

• If a variable is declared (with global scope) in one file but referenced in another, the extern keyword is used to inform the compiler of the variable's existence:
  • In declare.c:

    int farvar;
    

  • In use.c:

    {
     extern int farvar;
     int a;
     a = farvar * 2;
    }
    

• Note that the extern keyword is for declarations, not definitions
• An extern declaration does not create any storage; that must be done with a global definition

Private Variables: static

• another use for the static keyword is to ensure that code outside this file cannot modify variables that are globally declared inside this file
  • If declare.c had declared farvar as:

    static int farvar;
    

    then the extern int farvar statement in use.c would cause an error
  • This use of static is commonly used in situations where a group of functions need to share information but do not want to risk other functions changing their internal variables

    static int do_ping = 1; /* start with `PING' */
    
    void
    ping(void)
    {
     if (do_ping == 1) {
      printf("PING ");
      do_ping = 0;
     }
    }
    
    void
    pong(void)
    {
     if (do_ping == 0) {
      printf("PONG\n");
      do_ping = 1;
     }
    }

Variable Initialization

• auto, register and static variables may be initialized at creation:

    int
    main(void)
    {
   int a = 0;
   register int start = 1234;
   static float pi = 3.141593;
    }
  

• Any global and static variables which have not been explicitly initialized by the programmer are set to zero
• If an auto or register variable has not been explicitly initialized, it contains whatever was previously stored in the space that is allocated to it
  • this means that auto and register variables should always be initialized before being used
  • compiler may provide a switch to warn about uninitialized variables