Standard Library Function Prototypes

Any standard library function used by your program must be prototyped. To
accomplish this, you must include the appropriate header for each library function.
All necessary headers are provided by the C/C++ compiler. In C, all headers are files
that use the .H extension. In C++, headers may be either separate files or built into
the compiler itself. In either case, a header contains two main elements: any definitions
used by the library functions and the prototypes for the library functions. For example,
stdio.h is included in almost all programs in this part of the book because it contains
the prototype for printf() . The headers for the standard library are described in
Part Three.