Programming in C

The #defineStatement 311

to be written.You can even use this macro in a subsequent macro definition to convert

an ASCII character from lowercase to uppercase, leaving any nonlowercase character

unchanged:

#define TO_UPPER(x) ( IS_LOWER_CASE (x)? (x) - 'a' + 'A' : (x) )

The program loop

while ( *string != '\0' )

{

*string = TO_UPPER (*string);

++string;

}

would sequence through the characters pointed to by string, converting any lowercase

characters in the string to uppercase.^5

Va riable Number of Arguments to Macros

A macro can be defined to take an indeterminate or variable number of arguments.This

is specified to the preprocessor by putting three dots at the end of the argument list.The

remaining arguments in the list are collectively referenced in the macro definition by the

special identifier __VA_ARGS_ _. As an example, the following defines a macro called

debugPrintfto take a variable number of arguments:

#define debugPrintf(...) printf ("DEBUG: " _ _VA_ARGS_ _);

Legitimate macro uses would include

debugPrintf ("Hello world!\n");

as well as

debugPrintf ("i = %i, j = %i\n", i, j);

In the first case, the output would be

DEBUG: Hello world!

And in the second case, if ihad the value 100 and jthe value 200 , the output would be

DEBUG: i = 100, j = 200

The printfcall in the first case gets expanded into

printf ("DEBUG: " "Hello world\n");

by the preprocessor, which also concatenates the adjacent character string constants

together. So the final printfcall looks like this:

printf ("DEBUG: Hello world\n");

5.There is a host of functions in the library for doing character tests and conversions. For example,

islowerand toupperserve the same purpose as the macros IS_LOWER_CASEand TO_UPPER.

For more details, consult Appendix B, “The Standard C Library.”

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