Basically, a pointer holds address; address of some variable, pointer variable, function, structure, array etc. When pointer holds address of some other pointer variable it’s called pointer-to-pointer or double pointer. For example:
/* 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; }
Notice the output below:
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
So, what you observed while accessing integer variable ‘a’ using double pointer ‘**c’ and further modified ‘a’ using **c? Let’s understand this. ‘*b’ is a pointer to an integer, here to integer ‘a’.
int *b = &a; /* *b points to int a */
And, ‘**c’ is a pointer-to-pointer-to-integer i.e.
int **c = &b; /* **c is assigned address of pointer *b */
O key! We further unravel ‘**c’. What is given below segment of code, for example,
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");
Let’s understand the output,
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
Notice that c is a pointer-to-pointer-to-integer, so c holds address of another pointer, here we say ‘b’. What happens when we perform indirection ‘*’ on c. This causes go to the location of pointer ‘*b’ and access the value in that location i.e. address of integer ‘a’ which was assigned to pointer ‘*b’. And when we further perform indirection on ‘*c’ i.e. ‘*(*c)’, we accessed integer ‘a’.
We make an inference from above that as many levels of pointer to pointer we add, an equal no. of ‘*’ operators precede the pointer.
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