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.
A final example:
#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?
Regards
\Steve
--
Steve Graegert <[EMAIL PROTECTED]>
Software Consultancy {C/C++ && Java && .NET}
Mobile: +49 (176) 21248869
Office: +49 (9131) 7126409
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