Preprocessor directives

One of C's most useful features is its preprocessor. This is a part of compilation which occurs before the main compilation, and performs simple tasks such as including header files. We will only describe two of its features.

#include

The #include preprocessor command inserts a specified file into the file being compiled. It is often used to include header files containing many function declarations:

#include <catam.h> /* access the CCATSL library */

this in turn contains

#include <stdio.h>

which declares the standard IO functions (such as printf).

#define

The simplest use of the #define pre-processor command looks like:

#define MAX_ITERS 1000 /* .. */ int i; /* .. */ for (i=1; i<=MAX_ITERS; i++) { /* .. */ }

This asks the preprocessor to replace occurrences of the word MAX_ITERS with 1000 (this is a common way to define constants in C). #define can also be used to define macros which behave like functions, but we will not discuss these.