On Sat, 2008-02-23 at 07:52 +0000, ben lipkowitz wrote:
> On Fri, 22 Feb 2008, Kirk Wallace wrote:
> > Another bit I don't understand:
> >
> > /* STEP 3: allocate shared memory for joystick data */
> > js_data = (hal_js_t *) hal_malloc(sizeof(hal_js_t));
> > if (js_data == 0) {
> > printf( "ERROR: hal_malloc() failed\n");
> > hal_exit(comp_id);
> > return -1;
> > }
> >
> > hal_js_t is a defined variable type.
> > sizeof(hal_js_t) is the memory size of this type.
> > hal_malloc(sizeof(hal_js_t)) returns the memory location of this
> > variable.
> > I don't understand what (hal_js_t *) does. hal_js_t is a type so it is
> > similar to (int *)?
>
> hal_malloc returns a (void *) pointer; this would presumably make
> subsequent code complain about pointer mismatches, and the compiler
> wouldn't know where to look in the data structure for variables like
> &(js_data->axis[n]). the (hal_js_t *) casts the returned pointer to the
> correct pointer type.
Thanks Ben. The cast makes sense. I think of void as being an invention
to allow functions that return nothing, so the function return type
(void *) tells me that a pointer is returned that points to a
nonexistent variable. Is the (void *) just a reminder to always cast the
return pointer to the variable type we happen to need?
--
Kirk Wallace (California, USA
http://www.wallacecompany.com/machine_shop/
Hardinge HNC lathe,
Bridgeport mill conversion, doing XY now,
Zubal lathe conversion pending)
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