dereferencing void pointers by typecasting

[sritejv]

Hello everyone,

I am having a problem with typecasting void pointers.I have read the pointer basics but still cant understand why the following test code doesnt work.

void *xyz;

struct abcd
{
int a;
};

struct abcd a,*ptra;
ptra=&a;
xyz=(struct abcd *)ptra; //doesnt work but no error shown.
.
.
printf("%d \n",xyz->a);

}

warning: dereferencing void * pointer (obviously,type casting didnt work)

error : request for member a in something not a structure or union;

Am i missing some pointer concepts here?


Thanks,
Sritej

[newb16]

Hello everyone,

I am having a problem with typecasting void pointers.I have read the pointer basics but still cant understand why the following test code doesnt work.

void *xyz;

struct abcd
{
int a;
};

struct abcd a,*ptra;
ptra=&a;
xyz=(struct abcd *)ptra; //doesnt work but no error shown.
.
.
printf("%d \n",xyz->a);

}

Information about structure type is not stored in pointer, pointer is just an address.
So when you write xyz->a compiler has no idea on what ->a is and what is its offset within memory block pointed by xyz

ptra=&a;
xyz=ptra; // no need to typecast here
printf("%d \n",((struct abcd *)xyz)->a);

where (struct abcd *)xyz is of type "struct abcd*" and can be used as an argument to ->

[sritejv]

thanks a lot for the correction..
so initial declaration of pointer is taken into account while dereferencing so that offsets can be computed .Thats why typecasting is done while dereferencing.

Thanks,
Sritej

[AmeL]

void *xyz;
struct abcd
{
int a;
};
struct abcd a,*ptra;
ptra=&a;
xyz=(struct abcd *)ptra; //doesnt work but no error shown.
.
.
printf("%d \n",xyz->a);
}

Let review your code a bit more!!!
You declared xyz as a pointer to void.
Then after you do like :
xyz = (struct abcd *)ptra; that ptra's type is a pointer to struct abcd.
This instruction means you're trying to typecast from struct abcd pointer to struct abcd pointer ( again ) and trying to assign that to xyz that was declared as pointer to void.

Thanks
/AmeL