C provides a very limited macro facility. You can say
#define name something
and thereafter anywhere ``name’’ appears as a token, ``something’’ will be substituted. This is particularly useful in parametering the sizes of arrays:
#define ARRAYSIZE 100
int arr[ARRAYSIZE];
...
while( i++ < ARRAYSIZE )...
(now we can alter the entire program by changing only the define) or in setting up mysterious constants:
#define SET 01
#define INTERRUPT 02 /* interrupt bit */
#define ENABLED 04
...
if( x & (SET | INTERRUPT | ENABLED) ) ...
Now we have meaningful words instead of mysterious constants. (The mysterious operators `&’ (AND) and `|’ (OR) will be covered in the next section.) It’s an excellent practice to write programs without any literal constants except in #define statements.
There are several warnings about #define. First, there’s no semicolon at the end of a #define; all the text from the name to the end of the line (except for comments) is taken to be the ``something’’. When it’s put into the text, blanks are placed around it. Good style typically makes the name in the #define upper case—this makes parameters more visible. Definitions affect things only after they occur, and only within the file in which they occur. Defines can’t be nested. Last, if there is a #define in a file, then the first character of the file MUST be a `#’, to signal the preprocessor that definitions exist.
The other control word known to C is #include. To include one file in your source at compilation time, say
#include “filename”
This is useful for putting a lot of heavily used data definitions and #define statements at the beginning of a file to be compiled. As with #define, the first line of a file containing a #include has to begin with a `#’. And #include can’t be nested— an included file can’t contain another #include.
