i) Header file:

• A file with extension .h
• It contains C function declarations and macro definitions to be shared between several source files.
• There are two types of header files: files that programmer writes and files that come with compiler.
• It is a simple practice in C or C++ programs to keep all constants, macros, system wide global variables and functions prototype in header file and include that header file whenever it is required.
• Example:

#include <stdio.h>

Here
#include…. Preprocessor directive to call header file

<stdio.h>…. Header file

ii) Macros:

• It is a collection of codes that is defined in C program by a name.
• It differs from function in sense that once a macro is defined by a name, the compiler will put the corresponding codes for it at every place where that macro name appears.
• Advantages of using macro are that the speed of execution of program increases. There is a decrease in the length of the program. Also there is a decrease in errors caused by repetitive coding.