Re: [PATCH] C undefined behavior fix

dewar
2002-01-06T13:06:27Z

<<* Given a pointer, I need a way to determine the address (as an int of
the appropriate size) that the CPU will present to the MMU when I
dereference that pointer.
>>

This is in general ill-defined, a compiler might generate code that sometimes
does byte access to a particular byte, anmd sometimes gets the entire word
in which the byte resides.

This is often a nasty issue, and is one of the few things in this area that
Ada does not properly address.

If you have a memory mapped byte, you really want a way of saying

"when I read this byte, do a byte read, it will not work to do a word read"

pragma Atomic in Ada (volatile gets close in C, but is not close enough) will
ensure a byte store in practice, but may not ensure byte reads.