Thread: int split...

i have the next code:

Code:

unsigned short int port_n;

port_n = htons(6667);

printf("%x\n%x", (char) port_n,  (char) *(&port_n+1));

what i'm trying to do is to split the 2 bytes of unsigned short int into two single chars. the thing is, *(&port_n+1) does not give me the right value.
int is 2 bytes long, what am i doing wrong?




thank you.

Code:

#include <stdio.h>

int main(void)
{
   unsigned short int port_n = 0x1234;
   printf("%x\n%x\n", *(char*)&port_n,  *((char*)&port_n + 1));
   return 0;
}

/* my output
34
12
*/

can you please explain why: *((char*)&port_n + 1)

Originally Posted by Devil Panther

can you please explain why: *((char*)&port_n + 1)

'&port_n' is the address of port_n. Its type is 'short*'.

'(char*)' is a typecast that changes the type of the address to 'char *'.

'+1' adds one the this address (it's now a char*)

'*(<expression>)' is the dereferencement operator. It allow to reach the pointed value.

Why not ??

union intchar
{
short int i;
char cs[2];
};

So in your case its really (address of port_n + 2)

i'm sorry but it's wrong.


if port_n was a pointer, i would have understood it as:

&port_n - the address of the pointer port_n.

by doing (char *) &port_n i get the addess of the value.

so by increasing the (char *) &port_n by one byte i get the address of next cell from *port_n... and then i get it's value:

* ( (char *)&port_n + 1 ) = 0x12



but since port_n is not a pointer, i don't understand it

Originally Posted by Devil Panther

i'm sorry but it's wrong.

I don't think so.

if port_n was a pointer, i would have understood it as:

&port_n - the address of the pointer port_n.

You think wrong. 'port_n' is an object. Like any object, it has an address &<object>. A pointer is just another object, but it's not necessary to have one there. Actually, according to your original post, this object is a short.

by doing (char *) &port_n i get the addess of the value.

Bad wording. A value is not an object. It has no address at all.
Read my post again, and don't change the words. The semantics is not an option. Every word is important. If there is one you don't understand, please ask for details. Don't assume. Be sure.

Originally Posted by Thantos

but

Code:

&port_n

is a pointer

According to the defintion given by the original poster, port_n is a short, hence &port_n is a value of type short*. Eventually it is a pointer constant of type short*.

A pointer is just another object. It has an address. It happens that the value of a pointer is also an address.

&object has no address (&&object is plain wrong like are &NULL or &0 or &((char*)123)).

Originally Posted by Thantos

I try to keep things simple in my explantions which is why I said what I said

I understand your concern, but the problem is that willing to be too simple leads to misunderstanding. Pointers are a difficult matter where the meaning of the words is essential to be consistent.

There is a big difference between a pointer (object) and a pointer constant (constant). I think important to have it clear in mind

According to the defintion given by the original poster, port_n is a short, hence &port_n is a value of type short*. Eventually it is a pointer constant of type short*.

but &port_n is the address of the value...

There is a big difference between a pointer (object) and a pointer constant (constant). I think important to have it clear in mind

can you please explain me the difference?

but &port_n is the address of the value...

As said &port_n is the address of the object which in turns holds the value.

One difference between &<object> and type * is that in &<object> you can't change the value of the pointer itself (ie: (&<object>)++) while in a pointer you can.

I see your point Emmanuel.

Originally Posted by Devil Panther

but &port_n is the address of the value...

I told you already that this is plain wrong. Don't you read the responses you received? A value has no address.

What is the address of 'a' or 123 ?

Originally Posted by -ed-

difference between a pointer (object) and a pointer constant (constant)

can you please explain me the difference?

A pointer is an object (an other name for variable). It has a memory representation, it occupies a certain number of consecutive bytes. It has a memory location know as 'address'.

The value is the contents of the object (the status of the bits in the bytes of the object). It happens that the kind of value that a pointer holds is an address (the memory location of some other object). Some other objects hold integers, floating points, string of characters... Whatever.

A constant is a literal value, also know as immediate value. It also can be the result of a constant expression.

123 is an integer constant.
'H' is a character constant
(void*)0 is a pointer constant (aka the null pointer)
123.456 is a floating point constant
sizeof (double) is a constant expression
int x; is an object, but
&x is a pointer constant .