Question: Explain various storage classes in 'C'.
Answer:
The storage class of a variable determines which parts of a program can access it and how long it stays in existence. The storage class can be classified as automatic, register, static and external.
Automatic variable
All variables by default are auto i.e. the declarations int a and auto int a are equivalent. Auto variables retain their scope till the end of the function in which they are defined. An automatic variable is not created until the function in which it defined is called. When the function exits and control is returned to the calling program, the variables are destroyed and their values are lost. The name automatic is used because the variables are automatically created when a function is called and automatically destroyed when it returns.
Register variable
A register declaration is an auto declaration. A register variable has all the characteristics of an auto variable. The difference is that register variable provides fast access as they are stored inside CPU registers rather than in memory.
Static variable
A static variable has the visibility of a local variable but the lifetime of an external variable. Thus it is visible only inside the function in which it is defined, but it remains in existence for the life of the program.
External variable
A large program may be written by a number of persons in different files. A variable declared global in one file will not be available to a function in another file. Such a variable, if required by functions in both the files, should be declared global in one file and at the same time declared external in the second file.