Double Pointer (Pointer to Pointer) in C with Example

int **ptr; //This declares a double pointer to an integer

/* ptr2ptr1.c -- Program shows double pointer, it's declaration and uses */
#include <stdio.h>
int main(void)
{
    int a = 50;   /* 'a' an integer initialized value 50 */
    int *b = &a;  /* b is pointer-to-integer */
    int **c = &b; /* c is pointer-to-pointer-to-int or double pointer */
 
    printf("\nValue of integer a in exp. \"a = 50\" is %d\n", a);
    printf("O key! After exp. \"int *b = &a\", value of a using pointer"
           " *b is %d\n", *b);
 
    printf("\nNow, we use double pointer **c in exp. \"int **c =  &b\" "
           "to find\nvalue of a as %d\n", **c);
 
    printf("\nO key! We now try to modify integer a using double pointer"
           " **c.\n");
    **c = 2 * a;
 
    printf("After \"**c = 2 * a\", value of a %d\n", **c);
    printf("\n");
 
    return 0;
}

 
Value of integer a in exp. "a = 50" is 50
O key! After exp. "int *b = &a", value of a using pointer *b is 50
 
Now, we use double pointer **c in exp. "int **c = &b" to find
value of a as 50
 
O key! We now try to modify integer a using double pointer **c.
After "**c = 2 * a", value of a 100

    int *b = &a;    /* *b points to int a */

    int **c = &b;    /* **c is assigned address of pointer *b */

    c;               /* ? */
    *c;              /* ? */
    **c;             /* this is value of integer a */
 
    printf("c is a double pointer and have address as %p.\n", c);
    printf("*c is value at address (%p) is %p\n", c, *c);
    printf("**c is a value of type int, which is %d\n", **c);
    printf("\n");

    c is a double pointer and have address as 0x7fffbc1f73a8.
    *c is value at the address (0x7fffbc1f73a8) is 0x7fffbc1f73b4
    **c is a value of type int, which is 100

#include <stdio.h>
void main()
{
    int k = 5;
    int *p = &k;
    int **m  = &p;
    printf("%d%d%d\n", k, *p, **m);
}