#define in C

Simple Define Statements

One of the uses of the #define statement is to define simple constant format is this:

#define SYMBOL value /* comment */

The SYMBOL is any valid C symbol name (by convention, #define names are all uppercase). The value can be a simple number or an expression.

Like variable declarations, a constant declaration needs a comment explains it. This comment helps create a dictionary of constants.

Some examples:

/* Max number of symbols in a procedure */

#define SYMBOL_MAX 500

/* The longest name handled by this system */

#define NAME_LENGTH 50

#define constants are declared like variables. Always put a comment describes the constant after each declaration.

Constant names are all upper-case.

Constant expressions

If the value of a #define statement is a compound expression, you can run problems. The following code looks correct, but it hides a fatal flaw.

/* Length of the object (inches) (partl=10, part2=20) */

#define LENGTH 10 + 20     /* Bad practice */

#define WIDTH 30           /* Width of table (in inches) */

/*..... */

/* Prints out an incorrect width */

printf( "The area i s %d\n" , LENGTH * WIDTH);

Expanding the printf line, you get:

printf( "The area i s %d\n" , LENGTH * WIDTH);

printf( "The area i s %d\n" , 10 + 20 * WIDTH);

printf( "The area i s %d\n" , 10 + 20 * 30);

This another example of how the C preprocessor can hide problems. Clearly LENGTH is 10 + 20, which is 30. So LENGTH is 30, right? Wrong. LENGTH literally 10 + 20 , and:

10 + 20 * 30

is vastly different from:

30 * 30

To avoid problems like this, always surround all #define expressions with parenthesis (  ()  ). Thus, the statement:

/* Length of the object (inches) (partl=10, part2=20) */

#define LENGTH 10 + 20       /* Bad Practice */

Becomes:

/* Length of the object (inches) (partl=10, part2=20) */

#define LENGTH (10 + 20)       /* Good Practice */

If the value of a constant is anything other than a single number, enclose it in parentheses.

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