What Is the C Preprocessor?

If there is a constant appearing in several places in your program, it's a good idea to associate a symbolic name to the
constant, and then use the symbolic name to replace the constant throughout the program. There are two advantages
to doing so. First, your program will be more readable. Second, it's easier to maintain your program. For instance, if
the value of the constant needs to be changed, find the statement that associates the constant with the symbolic name
and replace the constant with the new one. Without using the symbolic name, you have to look everywhere in your
program to replace the constant. Sounds great, but can we do this in C?
Well, C has a special program called the C preprocessor that allows you to define and associate symbolic names with
constants. In fact, the C preprocessor uses the terminology macro names and macro body to refer to the symbolic
names and the constants. The C preprocessor runs before the compiler. During preprocessing, the operation to replace
a macro name with its associated macro body is called macro substitution or macro expansion.
You can put a macro definition anywhere in your program. However, a macro name has to be defined before it can be
used in your program.
In addition, the C preprocessor gives you the ability to include other source files. For instance, we've been using the
preprocessor directive #include to include C header files, such as stdio.h, stdlib.h, and string.h, in the programs
throughout this book. Also, the C preprocessor enables you to compile different sections of your program under
specified conditions.