'Ptr' is used uninitialized in this function

I do not understand if I declare the pointer as a structure variable, why is become uninitialized ?

program code

Code:

---

#include <stdio.h>
#include <stdlib.h>
struct person
{
  char code;
  int age; 
};
 
int main()
{
    struct person *Ptr;
   
    Ptr->age = 30;
    Ptr->code='X';
   
    printf("Age: %d\n", Ptr->age);
    printf("code: %c",  Ptr->code);
     
    return 0;
}

---

complier give warning

'Ptr' is used uninitialized in this function


if I allocate dynamic memory for pointer variable then code run fine

Code:

---

struct person *Ptr = malloc (sizeof(Ptr));

---

give output

Age: 30
code: X

What is the problem with the previous code line ? why compiler doesn't allocate memory for pointer ? Is it necessary to always allocate dynamic memory if variable is pointer

It's not always necessary to allocate dynamic memory but the pointer has to point to something valid before you use it (called dereferencing). E.g.

Code:

---

#include <stdio.h>

#include <stdlib.h>

struct person

{
  char code;

  int age; 

};
 
int main()
{

    struct person person;

    struct person *Ptr = &person;

     

    Ptr->age = 30;
    Ptr->code='X';
     
    printf("Age: %d\n", Ptr->age);

    printf("code: %c",  Ptr->code);

     

    return 0;

}

---

is perfectly fine because you're initializing Ptr to point to an object of type struct person. Something that exists. In your example you're allocating memory for the structure and Ptr points to that, so that's fine as well. But if you don't point it to something (leave it unitialized) then it points to something random and that's not useful.

Quote:

---

Originally Posted by Hodor

It's not always necessary to allocate dynamic memory but the pointer has to point to something valid before you use it (called dereferencing). E.g..

---

big thanks I have query I think there is only three variables in code Ptr,

Ptr->age and

Ptr->code

What is person ? Is it a variable that occupy memory ?

Code:

---

#include <stdio.h>
 
#include <stdlib.h>
 
struct person
 
{
  char code;
 
  int age; 
 
};
   
int main()
{
 
    struct person person;
 
    struct person *Ptr = &person;
 
     
 
    Ptr->age = 30;
    Ptr->code='X';
 
 printf("address of person : %p \n", &person);
 
    printf("address of Ptr : %p \n", &Ptr);
 
    printf("address of  Ptr->age = %p \n", & Ptr->age);
 
    printf("address of Ptr->code = %p \n", &Ptr->code);
     
    printf("Age: %d\n", Ptr->age);
 
    printf("code: %c",  Ptr->code);
     
    return 0;
 
}

---

address of person : 0061FF28
address of Ptr : 0061FF24
address of Ptr->age = 0061FF2C
address of Ptr->code = 0061FF28
Age: 30
code: X

Yes, it's in memory

Code:

---

printf("address of person : %p \n", &person);

 

 printf("what Ptr points to : %p \n", Ptr);

---

Quote:

---

Originally Posted by Hodor

Yes, it's in memory

Code:

---

printf("address of person : %p \n", &person);

 printf("what Ptr points to : %p \n", Ptr);

---

---

address of person : 0061FF24

what Ptr points to: 0061FF24

address of Ptr->code = 0061FF24

address of Ptr->age = 0061FF28

memory location of all three variables are same why ?

@gajya:

You need to study a good book on the C Programming Language, cover to cover, and do all the exercises at the end of each chapter! Choose one of the three listed below:

C Programming, A Modern Approach
Author: K. N. King

C Primer Plus, 6th Edition
Stephen Prata

C How to Program, 8/e
Deitel & Deitel

These books would explain in detail structs, pointers and all the other features of C and how to use them. Online "tutorials", and YouTube videos are painfully inadequate.