what is the difference between “unsigned int *ptr” and signed int *ptr"?

Question

I know that integer pointer will dereference 4 bytes, then what signed and unsigned do with pointers ? what is the meaning of "unsigned int *ptr" and "signed int *ptr"

Answer 1

Type of pointer suggests what kind of variables address it can keep. So, unsigned int *ptr should keep address of unsigned int and signed int *ptr should keep address of signed int

Please look at following piece of code,

  int main()
  {
    unsigned int * ptr1;
    signed int * ptr2;

    unsigned int i;
    signed int s;

    ptr1 = &s;
    ptr2 = &i;

  }

It gives me following errors in Visual Studio,

Error 1 error C2440: '=' : cannot convert from 'int *' to 'unsigned int *' [...]

Error 2 error C2440: '=' : cannot convert from 'unsigned int *' to 'int *' [...]

Answer 2

When you have

unsigned int* ptr;

the value of ptr is assumed to hold an unsigned int . Similarly, when you have

signed int* ptr;

the value of ptr is assumed to hold an signed int .

Here's a simple experiment:

#include <stdio.h>

int main()
{
   signed int i = -10;
   signed int* p1 = &i;
   unsigned int* p2 = (unsigned int*)p1;

   printf("Value of p1: 0x%p\n", p1);
   printf("Value of p2: 0x%p\n", p2);

   printf("Value of *p1: %d\n", *p1);
   printf("Value of *p2: %u\n", *p2);
}

A sample output of that program:

Value of p1: 0x0x7fff1b52bd6c
Value of p2: 0x0x7fff1b52bd6c
Value of *p1: -10
Value of *p2: 4294967286

Even though the numeric value of the address held by p1 and p2 are same, when dereferenced, they evaluate to vastly different values.

Answer 3

A pointer, in C/C++, is nothing more than a memory location; their sizes are the same for a given architecture. The type of pointer such as void *, int *, unsigned int *, etc... tells the compiler how to handle pointer math.

Visualizing it this way always helps me...

char *ptrMyString = "test";
int *ptrMyINTS[] = {1,2,3,4}

The memory for each of the arrays above looks like this:

Offsets        0   1   2   3   4   5   6   7   8   9  10  11  12  13  14  15 
ptrMyString = [t] [e] [s] [t] [0]
ptrMyINTS   = [0] [0] [0] [1] [0] [0] [0] [2] [0] [0] [0] [3] [0] [0] [0] [4]

sizeof(ptrMyString) should be equal to sizeof(ptrMyINTS) since they are just memory pointers. However, ptrMyString++ will increment by 1 where ptrMYINTS will increment by 4.

Now I'm over simplifying a lot of things and ignoring stuff like virtual memory but the basic idea is, for example, if both pointers start at 0x00000000 (32bit address) then the pointer's value for each offset would be:

ptrMyString+0 = 0x00000000 = t
ptrMyString+1 = 0x00000001 = e
ptrMyString+2 = 0x00000002 = s
ptrMyString+3 = 0x00000003 = t

ptrMyINTS+0 = 0x00000000 = 1
ptrMyINTS+1 = 0x00000004 = 2
ptrMyINTS+2 = 0x00000008 = 3
ptrMyINTS+3 = 0x0000000C = 4

Notice the size is the same for all locations but the amount we increment by, 1 and 4, comes from the type of pointers, char * and int * respectively. I made the assumption that an int is 4 bytes here because it is on 32 bit X86 and ARM architectures.