void- and Function-Pointers In GNU C, addition and subtraction operations are supported on pointers to void and on pointers to functions. This is done by treating the size of a void or of a function as 1.
A consequence of this is that sizeof is also allowed on void and on function types, and returns 1.
The option -Wpointer-arith requests a warning if these extensions are used.
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https://gcc.gnu.org/onlinedocs/gcc-5.4.0/gcc/Pointer-Arith.html