“Magic numbers” are arbitrary constant values that indicate array bounds, maximal number of open files, the size of a memory page, and similar environment and program-specific units. Instead of using hard-coded values of magic numbers, it’s better to use a named constant. Consider the following example:
ioctl(fd, KIOCSOUND, 1193180/hertz); //make a beep
The magical value 1193180 happens to be the number of clock cycles per second on this particular machine. However, readers of this code have no way of guessing what this magic number represents. Furthermore, the use of a hard-coded literal value also compromises code portability since on another machine, the number of clock cycles is likely to be different.
The preferred method of handling magic numbers is to give them a meaningful name, which is both more readable and easier to maintain:
const int CLOCK _CYCLES = 1193180; ioctl(fd, KIOCSOUND, CLOCK _CYCLES /hertz);
When this code is ported to another machine, only the definition of CLOCK_CYCLES should change.