Re: void pointers & void function pointers



Peter Goddard wrote:
> Hello,
> Is it possible, using casting to promote...
>
> void *Ptr;
>
> To behave like...
>
> void (*FnPtr)(void);
>
> ?
>
> i.e. Can the 'void *Ptr' be cast into calling a function once it points to
> one?

No, or "not portably." Function pointers can only point
to functions; data pointers can only point to data. If you
try to coerce a pointer of one class to a pointer of the other,
all bets are off -- on some implementations it will work as
you perhaps hoped, while on others you will be unpleasantly
surprised.

(Usually, people who want to make this conversion have
just filled an array with instructions and want to get the
program to execute them. Quite aside from the difficulties
of converting the pointer, on many machines you will face
other obstacles. For example, a machine that "knows" the
I- and D-spaces are distinct may well have separate cache
mechanisms, and may not bother to synchronize them: you may
wind up executing the garbage that occupied the array before
you stored the instructions, which are still sitting in a
data cache and have not yet been flushed back to memory.
Machines that maintain "permission bits" on regions of memory
may not set the "executable" attribute on any data memory you
can access; you'll get some kind of trap when the CPU tries to
fetch in instruction from a non-executable memory area. And
so on, and so on, into endless machine-dependent territory --
and, perhaps, an appreciation of why the C Standard chose not
to try to enter this particular bramble bush.)

--
Eric.Sosman@xxxxxxx

.

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