Programming in C

312 Chapter 13 The Preprocessor

The #Operator

If you place a #in front of a parameter in a macro definition, the preprocessor creates a

constant string out of the macro argument when the macro is invoked. For example, the

definition

#define str(x) # x

causes the subsequent invocation

str (testing)

to be expanded into

"testing"

by the preprocessor.The printfcall

printf (str (Programming in C is fun.\n));

is therefore equivalent to

printf ("Programming in C is fun.\n");

The preprocessor literally inserts double quotation marks around the actual macro argu-

ment. Any double quotation marks or backslashes in the argument are preserved by the

preprocessor. So

str ("hello")

produces

"\"hello\""

A more practical example of the use of the #operator might be in the following macro

definition:

#define printint(var) printf (# var " = %i\n", var)

This macro is used to display the value of an integer variable. If countis an integer vari-

able with a value of 100 , the statement

printint (count);

is expanded into

printf ("count" " = %i\n", count);

which, after string concatenation is performed on the two adjacent strings, becomes

printf ("count = %i\n", count);

So the #operator gives you a means to create a character string out of a macro argu-

ment. Incidentally, a space between the #and the parameter name is optional.