So, *never* have any #defines before system header files, unless you know exactl   y what
   you are doing.  It could have horrible consequences in many ways:

   #define pid_t void
   #include <sys/types.h>

This is a bogus example, though.  POSIX specifically says that any name
ending in `_t' is reserved for the implementation; thus, and program
overriding such a definition is in error.

However, a program is free to do whatever it pleases with a name such as
`nbytes'.