typecasting void *pointers

Code:

	int a[2] = {0,1};
	void *b;

	b = (int *)&a;
	b++;

	printf("a's address: %p\n", b);

Hi all, i typecasted the void pointer and try to increment (b++) it to point to the next address but there is this error: "error C2036: 'void *' : unknown size".

Can someone give me some advise on this problem.

You typecasted a not b. This is correct.

Code:

(int *) b = &a;

No actually neither one of you are right. You never have to typecast assignments to void pointers. What it is complaining about is you incrementing b, not assigning it a value.

Code:

((int*)b)++;

gcc gives a warning about lvalue casts being depreciated, but in this case, it's wrong.


Quzah.

I'm sure someone will be along with the right answer eventually.

Hint: consider what sizeof(void) means.

Code:

int a[2] = {0,1};
void *b;

b = &a;
((int*)b)++;
printf("a's address: %p\n", b);

Thanks guys, i know that the problem is about the incrementing b , and i tried

Code:

((int*)b)++;

before but it still couldn't work: "error C2105: '++' needs l-value".

Originally Posted by Salem

I'm sure someone will be along with the right answer eventually.

Hint: consider what sizeof(void) means.

I'll assume you're not talking about my post. If so, what's the correct way? This works just fine, according to a pedantic-ansi-GCC.

Code:

#include<stdio.h>
int main( void )
{
	int a[] = { 1, 2 };
	void* b;

	b = a;
	((int*)b)++;
	printf( "%d\n", *(int*)b );
	return 0;
}

gcc -o inc inc.c -Wall -pedantic -ansi
inc.c: In function `main':
inc.c:8: warning: use of cast expressions as lvalues is deprecated
./inc
2

Quzah.

> "error C2036: 'void *' : unknown size".
char *p ; p++;
This advances p through memory by sizeof(char) bytes

int *p; p++;
This advances p through memory by sizeof(int) bytes

Now, given that void has no size, what do you expect
void *p ; p++
to do?
void has no size (that's what the error message says), so the compiler doesn't know what to add.

> b = &a;
Do you know the difference between this, and say
b = a;
b = &a[0];

I bet you tried this somewhere along the line, and got yet another error message.
int *b;
b = &a; // wrong
But rather than fixing it, you reached for void* as some magic cure-all.

How about

Code:

int a[2] = {0,1};
int *b;
b = a;
b++;
printf("a's address: %p\n", b);

Hi Salem, appreciated the help. I was actually trying out the use of void pointers. You see, i actually need to process a array in terms of int then char:

Code:

int a[2] = {0,1};
void *b;

b = a;
// do some processing
((int*)b)++;


((char*)b)++;
// do some processing

((char*)b)++;
// do some processing

((char*)b)++;
// do some processing

((char*)b)++;
// do some processing

As for
int *b;
int a[2] = {1,0};
>b = a;
>b = &a[0];

I believe b is pointing to the 1st int member in a's array.

In that case, Eavan, I suggest it is unnecessary to mess around with a void pointer;

Code:

int a[2] = {0,1};
char *b;

b = a;
// do some processing

*((int *)b) = 42;    //    Example;  sets first int to 42


b += sizeof(int);

++b;
// do some processing

++b;
// do some processing

++b;
// do some processing

++b;
// do some processing

Makes it a bit more obvious what you're doing, without having to jump through as many hoops with conversions.

How about
b = (&a) [+1]

Originally Posted by puppy

How about
b = (&a) [+1]

I don't get it.

Sorry ...

Your original program compiles fine (gcc version 4.1.0)
I have taken the liberty to slightly modify your prog to

Code:

#include <stdio.h>

int main()
{

int a[2] = {0,1};
void *b;

        b = (int *)&a;
        b++;

        printf("a's address: %p\n", &a);
        printf("b's address: %p\n", b);
}

the o/p is
a's address: 0xbfdf66a8
b's address: 0xbfdf66a9
so the address has been incremented by 1 byte ...(in this case from 8 to 9)

> Your original program compiles fine (gcc version 4.1.0)
Apparently, you need to add more compiler options to prevent you relying on all sorts of compiler extensions

Code:

$ gcc bar.c
$ gcc -W -Wall -ansi -pedantic -O2 bar.c
bar.c: In function ‘main’:
bar.c:10: warning: wrong type argument to increment
bar.c:12: warning: format ‘%p’ expects type ‘void *’, but argument 2 has type ‘int (*)[1u]’
bar.c:14: warning: control reaches end of non-void function

Yes, it does "compile fine" if you have a lazy approach to diagnosing potential problems,

sorry ..
I used make bar
therefor no errors ?

Originally Posted by puppy

sorry ..
I used make bar
therefor no errors ?

Possibly. But makefiles are just plain text and can be adjusted.
Kurt