Preprocessor Commands
Posted by Xingjuan
Last Updated: July 02, 2012

C is unusual in that it has a pre-processor. This comes from its Unix origins. As its name might suggest, the preprocessor is a phase which occurs prior to compilation of a program. The preprocessor has two main uses: it allows external files, such as header files, to be included and it allows macros to be defined. This useful feature traditionally allowed constant values to be defined in Kernighan and Ritchie C, which had no constants in the language.

Pre-processor commands are distinguished by the hash (number) symbol #. One example of this has already been encountered for the standard header file stdio.h.

#include <stdio.h>

is a command which tells the preprocessor to treat the file stdio.h as if it were the actually part of the program text, in other words to include it as part of the program to be compiled.

Macros are words which can be defined to stand in place of something complicated: they are a way of reducing the amount of typing in a program and a way of making long ungainly pieces of code into short words. For example, the simplest use of macros is to give constant values meaningful names: e.g.

#define TELEPHNUM 720663

This allows us to use the word TELEPHNUM in the program to mean the number 720663. In this particular case, the word is clearly not any shorter than the number it will replace, but it is more meaningful and would make a program read more naturally than if the raw number were used. For instance, a program which deals with several different fixed numbers like a telephone number, a postcode and a street number could write:

printf("%d %d %d",TELEPHNUM,postcode,streetnum);

instead of

printf("%d %d %d",720663,345,14);

Using the macros instead makes the actions much clearer and allows the programmer to forget about what the numbers actually are. It also means that a program is easy to alter because to change a telephone number, or whatever, it is only necessary to change the definition, not to retype the number in every single instance.

The important feature of macros is that they are not merely numerical constants which are referenced at compile time, but are strings which are physically replaced before compilation by the preprocessor! This means that almost anything can be defined:

#define SUM  1 + 2 + 3 + 4

would allow SUM to be used instead of 1+2+3+4. Or

#define STRING "Mary had a little lamb..."

would allow a commonly used string to be called by the identifier "string" instead of typing it out afresh each time. The idea of a define statement then is:

#define macroname  definition on rest of line

Macros cannot define more than a single line to be substituted into a program but they can be used anywhere, except inside strings. (Anything enclosed in string quotes is assumed to be complete and untouchable by the compiler.) Some macros are defined already in the file stdio.h such as:
The end of file character (= -1 for instance)
The null character (zero) = 0

pre-processor is very useful!