On 9/9/05, _z33 <[EMAIL PROTECTED]> wrote:
> Steve Graegert wrote:
> > On 9/9/05, _z33 <[EMAIL PROTECTED]> wrote:
> >
> >>Steve Graegert wrote:
> >>
> >>> char *s;
> >>> int *i;
> >>>
> >>> s = (int *)malloc(128 * sizeof(char));
> >>> i = (char *)malloc(128 * sizeof(int));
> >>>
> >>> s = "malloc is cool!";
> >>> printf("s: %s\n", s);
> >>> printf("s: %d\n", s);
> >>>
> >>> i = "malloc is cool!";
> >>> printf("i: %s\n", i);
> >>> printf("i: %d\n", i);
> >>>
> >>>This piece of code prints:
> >>>
> >>> s: malloc is cool!
> >>> s: 4350328
> >>> i: malloc is cool!
> >>> i: 4350328
> >>>
> >>
> >> The illustration was too simple and good, for me to understand.
> >> Thanks :)
> >>
> >>
> >>>You see, casting changes the interpretation of bits when they are
> >>>read. The assigment of a particular type to a variable of another
> >>>type destroys the original type information (i.e. loss of precision).
> >>>
> >>
> >> destroys the original type information? Couldn't understand this part.
> >> You are still able to recover the string, regardless of what kind of
> >>datatype you store in. The only disadvantage I that, this obscures the
> >>logic of the program.
> >
> >
> > Didn't mean this specific example, but when casting a float or double
> > to int then you loose some information which can not be restored
> > afterwards.
> >
> >
> >>>Yes function pointers are legal. ANSI C99 says:
> >>>
> >>>"J.5.7 Function pointer casts
> >>>1 A pointer to an object or to void may be cast to a pointer to a
> >>>function, allowing data to be invoked as a function (6.5.4).
> >>>2 A pointer to a function may be cast to a pointer to an object or to
> >>>void, allowing a
> >>>function to be inspected or modified (for example, by a debugger) (6.5.4)."
> >>>
> >>>A function name is just a pointer to the memory location where the
> >>>function is found at runtime. It can be queried, modified and cast to
> >>>other types. It behaves like a variable. Take a look at the
> >>>sigaction structure:
> >>>
> >>> struct sigaction {
> >>> /* SIG_DFL, SIG_IGN or pointer to function */
> >>> void (*sa_handler)(int);
> >>> ... /* some other fields */
> >>> };
> >>>
> >>>You can define sigaction as follows:
> >>>
> >>> void handler(int signo) {
> >>> doneflag = 1;
> >>> }
> >>>
> >>> struct sigaction sa;
> >>> sa.sa_handler = handler;
> >>> ...
> >>>
> >>>Here, sa_handler is registered as a function to be called when the
> >>>specified signal occurrs.
> >>>
> >>>Just handle function pointers as simple pointer variables.
> >>
> >> Have still one silly question ---
> >> since you say function pointers are similar to simple pointer
> >>variables, and that's why the typecast works, what would the following
> >>code mean?
> >>
> >> void handler1 (int a) {
> >>
> >> /* body of handler - 1 */
> >> printf ("HANDLER - 1 \n");
> >>
> >> }
> >>
> >> void handler2 (int a) {
> >>
> >> /* body of handler - 2 */
> >> printf ("HANDLER - 2 \n");
> >>
> >> }
> >>
> >> struct sigaction sa;
> >> sa.sa_handler = (handler2 *) handler1;
> >>
> >> Is this possible? if so, what does it mean?
> >
> >
> > In this case you are trying to cast to a __name__ rather than a type.
>
>
>
> > If you write
> >
> > sa.sa_handler = (void *)handler1;
> >
> > it would be OK, but nothing would happen. Why? Because no
> > typecasting takes place (handler1 is of type void already) and
> > therefore nothing changes. handler1 would be associated to
> > sa_handler.
> >
>
> hmm.. fine
>
> > A final example:
>
> .."final"..
> Got scared seeing this :-O
> my silly questions are annoying you???
No, of course not. Just hoped that I now can make myself as clear as
prossible to answer all your questions with a "final" example. No
reason to be scared. No stupid questions to be asked.
> >
> > #include <stdio.h>
> >
> > void func1(int);
> > void func2(int);
> >
> > void func1(int i) {
> > printf("func1: %d\n", i);
> > }
> >
> > void func2(int i) {
> > printf("func2: %d\n", i);
> > }
> >
> > int main (void) {
> > void (*f1)(int) = (void *)func1;
> > /* the same as above */
> > void (*f1)(int) = func1;
> > void (*f2)(int) = func2;
> > f1(1);
> > f2(2);
> >
> > getc(stdin);
> >
> > return (0);
> > }
> >
> > Everything fine now?
>
> yes :)
>
Cool then. Don't forget: never stop asking!
Regards
\Steve
--
Steve Graegert <[EMAIL PROTECTED]>
Software Consultancy {C/C++ && Java && .NET}
Mobile: +49 (176) 21248869
Office: +49 (9131) 7126409
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