What the advantages of using Unions?

• C programming language is a structure oriented programming language, was developed at Bell Laboratories in 1972 by Dennis Ritchie
• C language features were derived from earlier language called “B” (Basic Combined Programming Language – BCPL)
• In earlier days, programs were written in assembly level language. So, it had happened to write very big programs to perform specific tasks using assembly code.
• But, ‘B’ language could perform the same task in few lines of program and it was faster than assemble language code.
• But, B language did not support some features like data types and structures etc. So, this was a drawback of B language. So, Dennis Ritchie developed C language by keeping most part of the B language and adding many features that produced powerful and effective outputs.
• So, C language was invented for implementing UNIX operating system. Most of the UNIX components were rewritten in C.
• In 1978, Dennis Ritchie and Brian Kernighan published the first edition  “The C Programming Language” and commonly known as K&R C
• In 1983, the American National Standards Institute (ANSI) established a committee to provide a modern, comprehensive definition of C. The resulting definition, the ANSI standard, or “ANSI C”, was completed late 1988.
• C89/C90 standard – First standardized specification for C language was developed by American National Standards Institute in 1989. C89 and C90 standards refer to the same programming language.
• C99 standard – Next revision was published in 1999 that introduced new futures like advanced data types and other changes.
• C11 standard adds new features to C and library like type generic macros, anonymous structures, improved Unicode support, atomic operations, multi-threading, and bounds-checked functions. It also makes some portions of the existing C99 library optional, and improves compatibility with C++.
• Embedded C includes features not available in normal C like fixed-point arithmetic, named address spaces, and basic I/O hardware addressing.

Declaration of a variable/function simply declares that the variable/function exists somewhere in the program but the memory is not allocated for them. But the declaration of a variable/function serves an important role. And that is the type of the variable/function. Therefore, when a variable is declared, the program knows the data type of that variable. In case of function declaration, the program knows what are the arguments to that functions, their data types, the order of arguments and the return type of the function. So that’s all about declaration. Coming to the definition, when we define a variable/function, apart from the role of declaration, it also allocates memory for that variable/function. Therefore, we can think of definition as a super set of declaration. (or declaration as a subset of definition). From this explanation, it should be obvious that a variable/function can be declared any number of times but it can be defined only once. (Remember the basic principle that you can’t have two locations of the same variable/function).

   // This is only declaration. y is not allocated memory by this statement 
  extern int y; 

  // This is both declaration and definition, memory to x is allocated by this statement.
  int x;

int main(void)
{
 if (printf("Hello World")) ;
}