#include<stdio.h>
struct s {
char *a1;
int a;
};
int main(){
struct s p={"asdv",11};
struct s p1=p;
p1.a1="vshaj";
printf("%d %s",p.a,p.a1);
}
In above program Does p1.a1 and p.a1 point to same memory address?
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2 Answers
1) Struct p1 is a copy of p
2) HOWEVER - since a1 is a pointer, the copied pointers both point to the same memory. Until you reassign p1.a1 to the address of "vshaj".
3) Don't ever, ever do anything like this in real code ;)
4 Comments
Carl Norum
1.
p1.a1 and p.a1 are the same after the initialization of p1, but not after p1.a1 is reassigned. 2) What's wrong with this code?
greatwolf
he's assigning string literals to non-const
char *.Carl Norum
That's perfectly legitimate, even in "real code". What if you have to deal with one of the (many many many) APIs that don't have
const correctness in their interfaces? Better to have a simple char * and avoid casting in a thousand places in your code.
paulsm4
@Asap - 1)
p1=p;: These are two different objects, p1 is a copy of p; they occupy different memory. 2) Both p.a1 and p1.a1 initially point to the same memory object. So yes, they both point to the same memory object. Right up until p1.a1="vshaj";. 3) Bad things can happen if you try to modify the contents of either p.a1 or p1.a1, because the string you're pointing to is a read-only string literal. 'Hope that helps :)Yes, they do, until you reassign p1.a1, then of course they don't. You could just print them out to prove it.
Example code:
#include <stdio.h>
struct s
{
char *a1;
int a;
};
int main(void)
{
struct s p = { "asdv", 11 };
struct s p1 = p;
printf("They're the same: %p %p\n", p.a1, p1.a1);
p1.a1 = "vshaj";
printf("%d %s\n",p.a,p.a1);
printf("They're different: %p %p\n", p.a1, p1.a1);
return 0;
}
Example run:
$ make example
cc example.c -o example
$ ./example
They're the same: 0x10e258f2a 0x10e258f2a
11 asdv
They're different: 0x10e258f2a 0x10e258f48
4 Comments
Asap
is it possible if i make changes to p1.a1 and that changes are reflected in p.a1?
Carl Norum
No, it's not.
p1 and p are different objects. You can point both at the same string and modify that string, if that's what you want.Asap
If i use this function strupr(p1.a1) it gives runtime error.But if i make char array in struct it does not why?
Carl Norum
That's because you're trying to modify a string literal, which causes undefined behaviour. Don't do that. There are hundreds (seriously, go look) of questions about just that on this site.