What is the difference between auto variable and register variable in C?

• Both auto and static variables are local variables.
• Static variables can retain the value of the variable between different function calls.
• But, scope of auto variable is within the function only. It can’t retain the value of the variable between different function calls.

• Type cast function should not be used in const and volatile declaration.
• Because, constant value can’t be modified by the program once they are defined.
• And, volatile variable values might keep on changing without any explicit assignment by the program as operating system will be modifying these values.

• Any number of arguments can be passed to a function. There is no limit on this. Function arguments are stored in stack memory rather than heap memory. Stack memory allocation is depending on the operating system.
• So, any number of arguments can be passed to a function as much as stack has enough memory. Program may crash when stack overflows.

All functions are global by default in a C program/file. But, static keyword makes a function as a local function which can be accessed only by the program/file where it is declared and defined. Other programs/files can’t access these static functions.

• atexit() function can be used if we want to execute any function after program is terminated normally.
• Syntax: int atexit (void (*func)(void));
• The function “func” will be called automatically without any arguments once program is terminated normally.

• exit() function terminates the program/process normally and returns the status code to the parent program/process.
• Syntax: void exit(int status)

• exit() is a system call which terminates current process. exit() is not an instruction of C language.
• Whereas, return() is a C language instruction/statement and it returns from the current function (i.e. provides exit status to calling function and provides control back to the calling function).

#define is a pre-processor directive which is used to define constant value. This constant can be any of the basic data types.

Below is the syntax for comments in C. The characters or words or anything which are given between “/*” and “*/”, won’t be considered by C compiler for compilation process. These will be ignored by C compiler during compilation.

Syntax: /* your comments here */