Variables

external and static variables are guaranteed to be initialized to zero if not explicitly declared.

• The value of these variables is determined conceptually before the program begins execution

Automatic and register variables have undefined (i.e., garbage) initial values, if not explicitly declared.

• The value of these variables is determined when the function or block is entered.

Static

• Decalaring a variable or fn static does 2 things:

1. it becomes scoped to wherever it was defined (scoped to source file, fn etc)
2. When static is declared inside a function, then that variable will live on between calls of the function, giving us permanent stoage.

• Static variables are allocated memory in data segment, not stack segment
• Unless initialized with a value, static variables default at ‘0’

Register

• Declaring a variable with register advises the compiler that it will be heavily used, placing it in a machine register for quicker access
• Only a few variables in each function may be kept in registers, and only certain types are allowed.
  • Excess register declarations are harmless, however, since the word register is ignored for excess or disallowed declarations
    • The actual limit varies from machine to machine.
  • It is impossible to get the memory address of a register variable.