How structure pointer and arrow operator works?

Question

#include <stdio.h> 
#include <stdlib.h> 

str1 
{ 
     int k; 
     struct str1 * ptr1; 
}; 

int main(void) 
{ 
         int * p1, * p2; 
         struct str1 * ptr2; 

         ptr2 = (struct str1 *) malloc(sizeof(struct str1)); 
         ptr2 -> ptr1 = (struct str1 *) malloc(sizeof(struct str1)); 
         ptr2 -> ptr1 -> ptr1 = ptr2; 
         ptr2 -> k = 7; 
         ptr2 -> ptr1 -> k = 25; 
         p1 = (int *) ptr2 -> ptr1; 
         p2 = (int *) ptr2 -> ptr1 -> ptr1;  
         printf("%d  %d\n", * p1, * p2); 

         ptr2 -> ptr1 -> ptr1 -> k ++; 
         printf("%d  %d\n", * p1, * p2); 

         ptr2 -> ptr1 -> ptr1 = ptr2 -> ptr1; 
         ptr2 -> ptr1 -> ptr1 -> k = 44; 
         printf("%d  %d\n", * p1, * p2); 

         * p1 = * p2 = 65; 
         if( ptr2 = = ptr2 -> ptr1 )  printf("Equal\n"); 
         else  printf("Not equal\n"); 
         return 0;  
}

In this code, I did not understand what this ptr2 -> ptr1 -> ptr1 = ptr2; code works. Is it attaching a structure another structure and ptr2 -> ptr1 -> ptr1 = ptr2; What ths code pointing at. Sorry for my bad english. Thank you for your answers.

If you look at the structure definition, and then read each line of code from the top down, you can trace through what it's pointing to pretty easily. — Ken White, Nov 13 '21 at 00:54

score 0 · Answer 1 · answered Nov 13 '21 at 02:40

The str1 struct contains a pointer to another structure of type str1. So it is a recursive function, and you can dereference infinitely (or until you reach a str1 struct that has its pointer equal to NULL.
So ptr2 -> ptr1 -> ptr1 = ptr2; is assigning the current structure of ptr2 to (recursively going up to the last) the last structure. At that point that structure's own pointer is NULL, so you cannot dereference further.

H.S. · Answer 2 · 2021-11-13T05:05:17.643

First of all the the structure str1 declaration is invalid as it misses the keyword struct which introduces a structure declaration. I am assuming it's a typo. The other typo is in equal to operator in this statement

if( ptr2 = = ptr2 -> ptr1 )  printf("Equal\n"); 
         ^^^

it should be ==.

The structure str1 is a self-referential¹⁾ structure as the ptr1 member is pointer to itself i.e. struct str1.

This statement

         ptr2 = (struct str1 *) malloc(sizeof(struct str1));

allocates memory for struct str1 type (assuming malloc is success) and the pointer returned will be assigned to ptr2.

The in-memory view would be something like this:

ptr2---+
       |
       +-------+
       |   |   | 
       +-------+
         k  ptr1

The next statement

         ptr2 -> ptr1 = (struct str1 *) malloc(sizeof(struct str1));

does same as what its previous statement does i.e. allocates memory for struct str1 type. The allocated memory reference will be assigned to pointer ptr2 -> ptr1.

The in-memory view would be something like this:

ptr2---+
       |
       +-------+          +-------+
       |   |   |--------->|   |   |
       +-------+          +-------+
         k  ptr1            k  ptr1

The next statement

ptr2 -> ptr1 -> ptr1 = ptr2;

will assign the pointer ptr2 to ptr2 -> ptr1 -> ptr1. That means, after execution of this statement, both pointers ptr2 and ptr2 -> ptr1 -> ptr1 will point to same memory address and a loop will be created.

The in-memory view would be:

ptr2---+ +-----------------------------+
       | |                             |
       +-------+          +-------+    |
       |   |   |--------->|   |   |----+
       +-------+          +-------+
         k  ptr1            k  ptr1

Note that this statement

         ptr2 -> ptr1 -> ptr1 = ptr2 -> ptr1;

will make pointer ptr2 -> ptr1 -> ptr1 point to structure which it is member of. The in-memory view would be:

ptr2---+                   +-----------+
       |                   |           |
       +-------+          +-------+    |
       |   |   |--------->|   |   |----+
       +-------+          +-------+
         k  ptr1            k  ptr1

Hope this clarifies your doubt. Let me know if you have any further question or confusion.

1). A self-referential structure is a structure in which one of its members is a pointer to the structure itself. It's commonly used in data structures like Linked List, Tree etc. implementation.

How structure pointer and arrow operator works?

2 Answers2