The volatile keyword means a lot of different things on a lot of different compilers, but one of their more important uses has been to represent special memory-mapped hardware. So declaring it undefined allows compilers to keep using it to represent that hardware. Same reason right-shift, divide and modulus of a negative signed integer is not specified exactly: that lets each CPU’s native instruction set work. If you want exactly-defined behavior, there are atomics and div().

The volatile keyword means a lot of different things on a lot of different compilers, but one of their more important uses has been to represent special memory-mapped hardware. So declaring the behavior in that respect undefined allows compilers to keep using it to represent that hardware. Same reason right-shift, divide and modulus of a negative signed integer is not specified exactly: that lets each CPU’s native instruction set work. If you want exactly-defined behavior, there are atomics and div().