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      Declaring a auto variable as a register variable,suggesting compiler that a variable be stored in processor register rather than in regular memory.

    register int x;

Limitations on register variable

• Register keyword can be applied on local auto variables
• Register variables works with number types only, not arrays or structures
• It can't work with static or external Storage Class
• declaring a pointer to register variable is an error

       extern keyword is used when global variables to be accessed in functions of same file(optional) or in another file

A variable declared outside of the function is called as Global variables. In order to access these variables in functions, programmer must specify extern keyword. extern keyword dictates/instructs the compiler to use external Linkage for these variables.

extern variables cannot have initializer, if it has it is an error