predict output of C program

Question

main ()
{
    struct xx {
        int x = 3;
        char name[] = "hello";
    };
    struct xx *s;
    printf("%d", s->x);
    printf("%s", s->name);
}

The output is :

• A. 3hello
• B. 104ello
• C. 104hello
• D. Compiler error

Comments

So, both matching? :O

---

As you told *s points to unknown location and we are trying to print an int and string from there. This will lead to compliation error.

 

Am I right?

---

yes, but referencing unknown location is not compile error. It is runtime error in C.

Answer 1

C is a very simple language and it is easy to understand everything about it - at least the most common part. Unfortunately this does not happen in Engineering colleges in India and I also have experienced it. In IISc., making a C compiler is just a subject assignment - because it is only that much. 

To answer the above question only 2 things are needed and both everyone knows. 

What does 

struct xx *s;

This creates a pointer s, which can point to an object of type struct s.

printf("%d", s->x);

What does "->" operator does? s->x is nothing but (*s).x.
*s goes to the memory address contained in s (not the address of s), and takes the value from there (amount of data taken is based on the type of s).

So, now I guess answer should be clear here. Yes, the given code is doing invalid memory access and hence should give runtime error. 

But, the given code has another major error, which is that it is initializing structure member variables inside structure definition. This is not allowed in C and will cause compile error. 

Comments

@arjun: sir what if stuct node *s={3,"hello"}

Now it will print the values?

---

No, that is not valid. {3, "hello"} is of type struct s, but here you are assigning it to a pointer. This kind of assignment works with pointers only in the case of strings.

---

Sir what you are saying is that ..we can't assign a value to a pointer Ex : int x = 3 but we can't assign int * x=3 Sir please correct me if am wrong

---

yes. Though it is still not a compile error- compiler will just warn. 

 stuct node *s={3,"hello"}

is actually same as 

 stuct node *s=3; 

The other parts are simply ignored. 

When we do int * p = 3;

We are using 3 as an address which probably is not valid. We won't get an error here- but if we try to access the content using * operator, we would be getting run time error. That is why compiler warns. 

---

All my doubts have been cleared now:)

Thanku  very much sir for such a nice explanation and giving us your valuable time.

---

Sir why we don,t initialize member inside structure ?

Bcz structure does not declare any variable  it defines a type rt ?

---

@arjun sir :  what does this line of urs  means  " *s goes to the memory address contained in s (not the address of s)    "...??

plzzz explain .. 

---

2000

 S 1000

It means s goes to the memory address 2000 (contained in s)Not the address of s ie 1000

Suppose s is pointing to a variable having address 2000 

S means value contained in s and *s means value at 2000 ie 2

2

 

2000

---

@anjali in ur last line  for :

2

2000

s is a pointer pointing to variable having address 2000

then here 

S and  S* both are 2....??

 

---

Let's take an example

Int x =3     => printf("%d", x)  here printing x means value contained by x is printed ie 3 means referencing to a variable means value inside the variable

Int *y= &x here y is a pointer having address of x

If we want to print y then it will print the address of x

And if we want to print *y then it will print the value at x

---

2

2000

s is a pointer pointing to variable having address 2000

then s=2000

and *s =2

ryt..?

---

I ran this program i got an error "

'struct xx' has no member named 'name'

What does it mean ?

---

Yes it is correct , to the best of my knowledge

---

ok..thanks anjali..

---

@shekhar. Yes, that's because the struct declaration failed. 

int x = 3;

is not allowed, so do any other initialization inside struct. During declaration we are not having a memory location and hence there is no where 3 could be stored here.  i.e., C grammar does not allow "=" after a variable declaration inside a struct declaration. So, the code should go to parser error handler and we can simply assume it as syntax error. 

---

value should be assigned in outside of declaration , rt?

---

value can be assigned when objects are made - because then there is a memory location for it. 

Only when we have built a house we can put a bed inside it. Declaration is just like a plan of the house.