C Questions #2

Describe about storage allocation and scope of global, extern, static, local and register variables?
C Mostly Asked Questions

What are register variables? What are the advantage of using register variables?
What is the use of typedef?
Can we specify variable field width in a scanf() format string? If possible how?
Out of fgets() and gets() which function is safe to use and why?
Difference between strdup and strcpy?
What is recursion?
Differentiate between a for loop and a while loop? What are it uses?
What are the different storage classes in C?
Write down the equivalent pointer expression for referring the same element a[j][k]?
What is difference between Structure and Unions?
What the advantages of using Unions?
What are the advantages of using pointers in a program?
What is the difference between Strings and Arrays?