structure

Question

i have a doubt regarding this 
   

#include <stdio.h>
typedef struct {
    char *a;
    char *b;
    } t;
void f1 (t s);
void f2 (t *p);
main()
{
    static t s = {"A", "B"};
    printf ("%s %s\n", s.a, s.b);
    f1(s);
    printf ("%s %s\n", s.a, s.b);
    f2(&s);
}
void f1 (t s)
{
    s.a = "U";
    s.b = "V";
    printf ("%s %s\n", s.a, s.b);
    return;
}
void f2(t *p)
{
    p -> a  = "V";
    p -> b = "W";
    printf("%s %s\n", p -> a, p -> b);
    return;
}
o/p of above code is

A B  U V  A B  V W

for char *s= "string":
          *(s+3)='a'; //this gives an error as i know that we can't change contetn of a string constant.
  but in above code we are replacing "A" to "v" so why there is no any problem loke above ??
code given above is from 

https://gateoverflow.in/3704/gate2004-it_61

 

Answer 1

Yes, in C language we cannot modify a string literal as it is meant to be a constant.

A string literal is something which is not allocated a memory by the programmer so that he can modify it. For example when one uses

printf("a=%d", a);

"a=%d" is a string literal which is stored in the process address space but not in a writable region so that one could modify it. Similar is the case when one do

char *a = "hello";

Now, in the given code "A" and "B" are string literals. So, we cannot modify their contents. i.e., we cannot do

*(s.a) = 'f';

But we can do

s.a = "U";

as here we have another string literal "U" and s.a is made to point there without actually modifying "A".

Comments

@arjun sir thnx for the explanation 
but i have a little doubt as u said " "a=%d" is a string literal which is stored in the process address space but not in a writable region so that one could modify it."
so this process address space is part of RAM or ROM.
 i think it is in ROM plzz explain.....
 

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No, it is RAM only. All our process runs in RAM only. ROM as the name says is Read Only- so ur process cannot write to it even once. We can say that we use a part of RAM virtually as Read Only, once the process starts execution. One way to do this is by segmentation and using a Data Segment as RO.

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ok ..... thnx.....

Answer 2

#include <stdio.h>
int main() {
	char *var = "a_string";
	//*(var+1) = 'c'; // not allowed
	var = "another_string"; // allowed
	printf("%s\n",var );
}

O/P = another_string

• Changing the characters of the string literal is not allowed.
• But we can change the location pointed by the char pointer var
• 
• red_arrow = pointing to old string literal (  "a_string" )
• green_arrow = pointing to new string literal (  "another_string"  ) 

Comments

thnx i m also expecting the same but i needs a confirmation.  In previous gate question that is given in link here it is a string of only one character so why we r not able to differentiate. as i m getting (old->new) it is changing the base address am i right??  .
thnx Bro...

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The base addresses of each string literals are not touched here. If "a_string" has an address 10 and "another_string" has an address 20, then ;

• var points to 10 (first time)
• var points to 20 (later)