What is the difference between variable declaration and variable definition? ( in a line)

A structure containing the same structure pointer variable as its element is called as self-referential structure.

No, the header file only declares function. The definition is in library which is linked by the linker.

Dividing the program in to sub programs (modules/function) to achieve the given task is modular approach. More generic functions definition gives the ability to re-use the functions, such as built-in library functions.

Preprocessor is a directive to the compiler to perform certain things before the actual compilation process begins.

Can be used to input integer in all the supported format.

Escape it using (backslash).

Yes, if it is not appearing as the last case and if we do not want the control to flow to the following case after default if any.

If the structure/union variable is a pointer variable, to access structure/union elements the arrow operator is used.

We can create integer structure members of differing size apart from non-standard size using bit fields. Such structure size is automatically adjusted with the multiple of integer size of the machine.