[c] Why casting short to void * get a warning, and from int it's okey?

Question

See I'm using glib, and gpointer is a typedef of void *. (glib did this type to make things clear, I guess).

when I make a signal connect, I have to pass the data as void pointer (void *).

so the code is (something like this) :

...
g_signal_connect (object, function, (gpointer) data);
...

If I use short as data type, I get a warning message of gcc like this:

warning: cast to pointer from integer of different size

If I use int as data type, I get no warnings.

But in both cases, everything works well, so, why I get this warning using short?

Answer 1

On a 32 bit architecture, a pointer is 32 bits. An int is usually 32 bits as well - so the conversion is '1 to 1'. A short is usually 2 bytes, so casting a short to a pointer isn't normally a very safe things to do - which is why the compiler warns you.

Answer 2

A short is 2 Bytes a pointer is platform dependant but usually 4 or 8 Bytes. That should be why you get that error. You probably want to pass a the reference of the short in which would be:

(gpointer) &data);

Answer 3

Gcc is telling you that you cast an integer of different size to a pointer, which is dangerous mainly the other way you have it (eg. from a larger datatype to a smaller one). To silence the warning, you can use intptr_t as an intermediate type.

g_signal_connect (object, function, (gpointer)(intptr_t) data);