pointer to structure in c

I have written code to understand pointer to structure

Code:

#include<stdio.h> #include<stdlib.h> struct point //define structure { int x; //structure member type integer char y; // structure member type charactr }; void main () { struct point *p = NULL; // p is pointer to structure variable and the value stored in pointer is NULL p = malloc(sizeof(*p)); // Allocate dyanamic memory for pointer to structure variable p p->x = 8; // p is pointer to structure pointing to variable x that is integer type and value store in x is 8 p->y = 'A'; // p is pointer to structure pointing to variable y that is char type and value store in y is 'A' }

I have doubt, do my last two comments match with the code lines ?

if not so what should i write in comments

Code:

p->x = 8; // p is pointer to structure pointing to variable x that is integer type and value stored in x is 8 p->y = 'A'; // p is pointer to structure pointing to variable y that is char type and value stored in y is 'A'

If not what should i write in comments

Why would the type of p change?

Code:

struct point //define structure { int x; //structure member type integer char y; // structure member type charactr }; int main (void) { struct point *p = NULL; // p points to an object of type struct point but currently points to nothing p = malloc(sizeof(*p)); // Allocate dynamic memory for a variable of type struct point. If malloc() succeeds it will point to an object of type struct point // The type of p has not changed (it's still a pointer to an object of struct point; i.e. it's a pointer to a "variable" that has the type struct point... assuming that malloc() didn't fail and p is not NULL) p->x = 8; // p is a pointer to an object of struct point. The type of p->x is int. Store the value 8 in it. p->y = 'A'; // p is a pointer to an object of struct point. The type of p->y is char. Store the value 'A' in it. }

Edit: I'm not really sure why you think that the type of what p points to can change... under normal circumstances (and in this example) it doesn't (you can change it, but this example doesn't and normally you probably wouldn't; you'd need to use an explicit cast if for some reason you needed to)

I've made the type of p purple

Quote:

Originally Posted by Hodor

Why would the type of p change?
Edit: I'm not really sure why you think that the type of what p points to can change... under normal circumstances (and in this example) it doesn't (you can change it, but this example doesn't and normally
I've made the type of p purple

my another doubts

p->x is initialized to 8 or only x is initialized to 8

Code:

p->x ; // p is a pointer to an object of struct point. it point to the object of structure x but p->x is not initialized p->x = 8; // p is a pointer to an object of struct point. it point to the object of structure x and p->x is initialized to 8 or only x is initialized to 8//

Only p is a pointer

p->x is the same as (*p).x

The only pointer is p. Assuming p is not NULL it points to a structure of type struct point.

Code:

struct point //define structure { int x; //structure member type integer char y; // structure member type charactr }; int main(void) { struct point my_struct; my_struct.x = 8; // my_struct.x is not a structure. x is a member of the struct (type int) // Similarly struct point *p = &my_struct; // p is a pointer to an object of type struct point p->x = 8; // p is a pointer to an object of type struct point. x is a member of the object being pointed to by p (x has type int, as above) (*p).x = 8; // this is the same as the previous line return 0; }

I guess I've forgotten how to explain pointers. I don't think about the intermediate steps anymore (maybe never have because I experienced indirection long before I learned C) and without writing a huge wall of text I can't explain it. And I don't want to write a wall of text... maybe someone else can.

But, consider

Code:

struct point //define structure { int x; //structure member type integer char y; // structure member type charactr }; int main(void) { struct point my_struct; // What type is 'point'? struct point *p; // What type is p? my_struct.x; // What type is x? (it's not a pointer) my_struct.y; // What type is y? (it's not a pointer) (*p).x; // What type is x? (it's not a pointer) return 0; }

p->x is just shorthand for (*p).x

Quote:

Originally Posted by Hodor

I guess I've forgotten how to explain pointers. I don't think about the intermediate steps anymore (maybe never have because I experienced indirection long before I learned C) and without writing a huge wall of text I can't explain it. And I don't want to write a wall of text... maybe someone else can.

But, consider

I didn't understand

Code:

p->x = 8;

I understand p is pointer and it pointing to the object x of structure. p hold the memory location of x and value stored in x is 8

Quote:

Originally Posted by skyr6546

I didn't understand

Code:

p->x = 8;

I understand p is pointer and it pointing to the object x of structure. p hold the memory location of x and value stored in x is 8

p holds the "memory location" of the structure, not member x of the structure belonging to the structure being pointed to

(better to think of p as pointing to a structure than being a memory location, but.. yeah)

Quote:

Originally Posted by Hodor

p holds the "memory location" of the structure, not member x of the structure belonging to the structure being pointed to

(better to think of p as pointing to a structure than being a memory location, but.. yeah)

Output of program

address of p : 00000000
address of p : 007113A8
address of p->x : 007113A8
value of p->x : A
value of p->x : B
address of p : 007113A8
address of p->x : 007113A8
address of p : 007113A8

Code:

#include<stdio.h> #include<stdlib.h> struct point //define structure { char x; //structure member type integer }; void main () { struct point *p = NULL; printf("address of p : %p \n", (void*)p); p = malloc(sizeof(*p)); printf("address of p : %p \n", (void*)p); printf("address of p->x : %p \n", (void*)&p->x); p->x = 'A'; printf("value of p->x : %c \n", p->x); p->x = 'B'; printf("value of p->x : %c \n", p->x); printf("address of p : %p \n", (void*)p); printf("address of p->x : %p \n", (void*)&p->x); free(p); printf("address of p : %p \n", (void*)p); }

first time assigning 'A'
Memory location = 007113A8 = A

second time assigning 'B'
Memory location = 007113A8 = B

only value will change but address of pointer will not change