Warning:dereferencing `void *' pointer

[g_p]

Hi again!

In my program i have declared a function with this prototype:

int function(void *var) //The second argument is a void * pointer
Also i have a struct 

typedef struct hello hello;
struct hello
{
        char *a;
        char *b;
        int  l;
        int  t;
        struct hello *next;
};


hello *pointer_hello;

When i do: function(pointer_hello);

I get the warnings:

warning: dereferencing `void *' pointer
test.c:516: error: request for member `a' in something not a structure or union
warning: dereferencing `void *' pointer
test.c:517: error: request for member `b' in something not a structure or union
warning: dereferencing `void *' pointer
test.c:518: error: request for member `l' in something not a structure or union
warning: dereferencing `void *' pointer
test.c:519: error: request for member `t' in something not a structure or union


What shall i change in my program?

Well the problem is that the argument of the function "function" has to be void in order
not only to take a pointer to a hello struct but to any other struct!

Thanks, in advance!

[Salem]

It's impossible to tell, since we need the actual order in which you declared things, and in which file(s) they reside.

[MacGyver]

My guess is that you are passing a struct pointer to a function that is defined to take a void pointer. You must cast the void pointer to be that of a struct pointer before actually dereferencing it. The compiler has no idea what a void pointer is pointing to. With regard to size, it could be pointing to a byte, a word, a double word, etc. etc. There is no known way of it knowing you are passing it a struct pointer.

In this case, you must be explicit when dealing with void *'s.

[g_p]

i do

#define  int function( void * var ) ;
typedef struct hello hello;
struct hello
{
        char *a;
        char *b;
        int  l;
        int  t;
        struct hello *next;
};

Then it starts

int main() {

hello *pointer_hello;
function(poinet_hello);
}

That's all!

[g_p]

function(pointer_hello);

[Salem]

> #define int function( void * var ) ;
What the hell is this supposed to mean?

Make function accept an hello* and be done with it.

[g_p]

My guess is that you are passing a struct pointer to a function that is defined to take a void pointer. You must cast the void pointer to be that of a struct pointer before actually dereferencing it. The compiler has no idea what a void pointer is pointing to. With regard to size, it could be pointing to a byte, a word, a double word, etc. etc. There is no known way of it knowing you are passing it a struct pointer.

In this case, you must be explicit when dealing with void *'s.

How can i do this cast?
can you make an example using the example i have given?

Thanks, in advance!

[MacGyver]

struct x
{
	int a;
};

...

int example(void *p)
{
	struct x *q;

	if(p)
	{
		q = (struct x *)p; /* Explicit cast from a void * to a struct x * */
		q.a = 5;
		return 1;
	}
	return 0;
}

...

int main(void)
{
	struct x v1;
	example(&v1);

	return 0;
}

Note: I did not check this code. I would suggest doing what was already suggested: just make your function accept a *hello.

[ssharish2005]

How can i do this cast?

dont use void * pointer for this purpose. It looks really messy for evert struct that u wanted to refer you will have to cast it.

It better u get the argument as struct hello * rather than void *. Which is quite more easy than void. If there are suitation some thing like where a single function need to do the same job for two different struct with same attributes but different names then may be u can think of void pointer.

ssharish2005