What’s a header? What’s a source-code file? Discuss the purpose of each. Solution The word Source File and Header is just a convention in C language. A header file is a file with extension .h which contains C function declarations and macro definitions. These come either with the compiler or are written by the programmers. The header files are always included into a Source file by using \'#include\'. The preprocessor literally just copy and pastes the contents from the header files into the source file, before the compilation begins. Consider the below example for your reference: #include<stdio.h> int main(void) { puts(\"Hello World!\"); return 0; } Without the header, the compiler doesnot know how to deal with the function \"puts()\". The header lets the compiler know how to manage the arguments and return value. The function puts() is a part of the library and to use its functioning, it has been included in the source code file. .