Question

int a = 10;    
int *p = &a; 

*p = 20;  /* Is this a valid statement? */

I understand that if I do int *p; and then if I do *p = 10 , it is invalid because I have not assigned any memory to p. However, I was wondering if initializing a pointer to some address allocates memory to that pointer or not?

Answer 1

int a = 10;    
int *p = &a; 

*p = 20;  /* Is this a valid statement? */

The answer is YES in this particular case.

But when you do like this :

int *ptr;  //declaration of pointer variable
*ptr = 20;  // It means you are assigning value 20 to the variable where `ptr` points to.

But actually ptr is not pointing to anywhere, basically it means it has indeterminate value.

so doing *ptr = 20 will put the value 20 to the memory address pointed to by ptr.
So it’s called Undefined bahavior

In you case it’s valid because &a is valid memory location and p started pointing to that variable when we do p= &a.

so *p = 20 means actually changing or assigning the value of a using the pointer p.