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Understanding Macro Functions


A more advanced use of macros is also permitted by the preprocessor. This
involves macros which accept parameters and hand back values. This works
by defining a macro with some dummy parameter, say x. For example: a
macro which is usually defined in one of the standard libraries is abs() which
means the absolute or unsigned value of a number. It is defined below:

 #define ABS(x) ((x) < 0) ? -(x) : (x)

 The result of this is to give the positive (or unsigned) part of any number
or variable. This would be no problem for a function which could accept
parameters, and it is, in fact, no problem for macros. Macros can also be
made to take parameters. Consider the ABS() example. If a programmer
were to write ABS(4) then the preprocessor would substitute 4 for x. If a
program read ABS(i) then the preprocessor would substitute i for x and so
on. (There is no reason why macros can’t take more than one parameter
too. The programmer just includes two dummy parameters with different
names. See the example listing below.) Notice that this definition uses a
curious operator which belongs to C:

 <test> ? <true result> : <false result>




This is like a compact way of writing an ‘if..then..else’ statement, ideal
for macros. But it is also slightly different: it is an expression which returns
a value, where as an ‘if..then..else’ is a statement with no value. Firstly
the test is made. If the test is true then the first statement is carried out,
otherwise the second is carried out. As a memory aid, it could be read as:

if <test> then <true result> else <false result>

(Do not be confused by the above statement which is meant to show what a
programmer might think. It is not a valid C statement.) C can usually pro-
duce much more efficient code for this construction than for a corresponding
if-else statement.
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