size of array of pointers

Question

i have a doubt regarding sizeof operator

Code 1:

int main()
{
    int p[10];
    printf("%d",sizeof(p));   //output -- 40
    return 0;
}

Code 2:

int main()
{
    int *p[10];
    printf("%d",sizeof(*p));   //output -- 4
    return 0;
}

in the first code p points to an array of ints. in the second code p points to an array of pointers. i am not able to understand why the first code o/p is 40 but 2nd code o/p is 4 thought both points to an array of the same size ?

Answer 1

The output of the following program will give you some hints and understanding about the size of a type and a pointer to a type.

#include <stdio.h>

int main(void)
{
    int p1[10];
    int *p2[10];
    int (*p3)[10];

    printf("sizeof(int)   = %d\n", (int)sizeof(int));
    printf("sizeof(int *) = %d\n", (int)sizeof(int *));
    printf("sizeof(p1)    = %d\n", (int)sizeof(p1));
    printf("sizeof(p2)    = %d\n", (int)sizeof(p2));
    printf("sizeof(p3)    = %d\n", (int)sizeof(p3));

    return 0;
}

int p[10];      => 10 consecutive memory blocks (each can store data of type int) are allocated and named as p

int *p[10];     => 10 consecutive memory blocks (each can store data of type int *) are allocated and named as p

int (*p)[10];   => p is a pointer to an array of 10 consecutive memory blocks (each can store data of type int) 

Now coming to your question:

>> in the first code p points to an array of ints.
>> in the second code p points to an array of pointers.

You are correct. In the code:2, to get the size of the array where p points to, you need to pass the base address

printf("%d", (int)sizeof(p));

and not the following

printf("%d", (int)sizeof(*p));   //output -- 4

The following are equivalent:

*p, *(p+0), *(0+p), p[0]

---

>> what's the difference between p[10] and 
>> (*p)[10]...they appear same to me...plz explain

The following is the answer to your other question:

int p[10];
 _________________________________________
|   0   |   1   |   2   |         |   9   |
| (int) | (int) | (int) |  ...    | (int) |
|_______|_______|_______|_________|_______|


int (*p)[10]
 _____________________
|                     |
| pointer to array of |
|     10 integers     |
|_____________________|