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This sequence tests the name SYSTEM to decide which version of a header to include:

#define HDR "sysv.h"

#elif SYSTEM == BSD

#define HDR "bsd.h"

#elif SYSTEM == MSDOS

#define HDR "msdos.h"

#else

#define HDR "default.h"

#endif

#include HDR

The

and

lines are specialized forms that test

whether a name is defined. The first example of

above could have

been written

#define HDR

/* contents of hdr.h go here */

#endif

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