pointer casting question

[sinbad]

is there anything wrong with the following program. specifically
i am asking about casting pointer to an integer pointer (double
pointer)
to a void pointer and casting it back to double pointer.

1 void func(void *x)
2 {
3 int *y;
4 y = (*(int **)x);
5 y++;
6 *((int**)x) = y;
7
8 return;
9 }
10
11 int main(void)
12 {
13 int z = 10;
14 int *y = &z;
15
16 func(&y);
17
18 return 0;
19 }
~

 

[viza]

Hi

is there anything wrong with the following program. specifically i am
asking about casting pointer to an integer pointer (double pointer)
to a void pointer and casting it back to double pointer.

Please just say pointer pointer. Calling it a double pointer will make
people think you mean a pointer to a double precision floating point
number, (double*).

1 void func(void *x)
2 {
3 int *y;
4 y = (*(int **)x);
5 y++;
6 *((int**)x) = y;
7
8 return;
9 }

That is fine. void* can hold an int**.

The line numbers are a nuisance though, imho.

viza

 

[James Kuyper]

is there anything wrong with the following program. specifically
i am asking about casting pointer to an integer pointer (double
pointer)
to a void pointer and casting it back to double pointer.

1 void func(void *x)
2 {
3 int *y;
4 y = (*(int **)x);
5 y++;
6 *((int**)x) = y;

Lines 3-6 could be replaced by:

(*(int**)x)++;

I presume that this is a simplified version of something that will be
much more complicated, in which line 5 would be replaced by something a
lot more complicated. In that case lines 3, 4, and 6 are all quite
reasonable.

7
8 return;
9 }
10
11 int main(void)
12 {
13 int z = 10;
14 int *y = &z;
15
16 func(&y);
17
18 return 0;
19 }
~

This looks good to me; but I often miss minor details that could make a
big difference.

 

[Andrey Tarasevich]

is there anything wrong with the following program. specifically
i am asking about casting pointer to an integer pointer (double
pointer)
to a void pointer and casting it back to double pointer.

1 void func(void *x)
2 {
3 int *y;
4 y = (*(int **)x);
5 y++;
6 *((int**)x) = y;
7
8 return;
9 }
10
11 int main(void)
12 {
13 int z = 10;
14 int *y = &z;
15
16 func(&y);
17
18 return 0;
19 }

There's nothing formally wrong with this code.

The way you do a simple increment inside 'func' looks a bit weird to my
taste though. You could just do

++*(int **) x;

or, for better readability,

int **pp = x;
++*pp;

The second approach requires no explicit cast, although some compilers
might issue a warning, which can usually be suppressed with an explicit cast

int **pp = (int **) x;
++*pp;

 

[CBFalconer]

is there anything wrong with the following program. specifically
i am asking about casting pointer to an integer pointer (double
pointer) to a void pointer and casting it back to double pointer.

1 void func(void *x)
2 {
3 int *y;
4 y = (*(int **)x);
5 y++;
6 *((int**)x) = y;
7
8 return;
9 }
10
11 int main(void)
12 {
13 int z = 10;
14 int *y = &z;
15
16 func(&y);
17
18 return 0;
19 }

I see no doubles. However, a working rule for casts is "don't". So:

void func(void *x) {
int *y;

y = x;
y++;
x = y;
} /* can eliminate return for a void function */

because the compiler knows how to convert a void* to a something*,
and a something* to a void*.

 

[Andrey Tarasevich]

I see no doubles. However, a working rule for casts is "don't". So:

void func(void *x) {
int *y;

y = x;
y++;
x = y;
} /* can eliminate return for a void function */

because the compiler knows how to convert a void* to a something*,
and a something* to a void*.

Good advice, but bad code. Your code only modifies an automatic variable
- the value of the parameter 'x', which, of course, will have absolutely
no effect on the caller. This was not the OP's intent.

 

[CBFalconer]

Good advice, but bad code. Your code only modifies an automatic
variable - the value of the parameter 'x', which, of course, will
have absolutely no effect on the caller. This was not the OP's
intent.

Well, that is one advantage of c.l.c. Bad code tends to get
corrected immediately. Thanks.

The way to avoid such foolish errors is to return the value wanted
with the function.

void *func(void *x) {
int *y = x;

y++;
return y;
}

 

[ozbear]

Good advice, but bad code. Your code only modifies an automatic variable
- the value of the parameter 'x', which, of course, will have absolutely
no effect on the caller. This was not the OP's intent.

I am uncertain about what the OP's intent actually was/is.
If it were to ultimately modify /z/ in the main program from
10 to 11 then the corrected versions don't do that.

Oz