The following two code snippets compiles without any errors/warnings but while running it crashes. Kindly enlighten me.
Program 1
int main( )
{
char *p= "Hello" ;
*p = 'B' ;
printf("\n%s",p);
return 0;
}
Program 2
int main( )
{
char *p= "Hello" ;
Char *q="mug"
*q = *p ;
printf("\n%s",q);
return 0;
}
For program 2 i expected output to be 'Hug'.
When you do:
char *p= "Hello";
You are defining a string literal. String literals are constant data and as you've found out, modifying them results in undefined behavior (often a crash). It should be declared as:
const char *p = "Hello";
So the compiler will throw an error if you try to modify it.
Now if you define it instead as:
char p[] = "Hello";
The memory is then allocated on the stack and you can modify it.
int main(int argc, char *argv[])
{
char p[] = "Hello" ;
*p = 'B' ;
printf("\n%s",p);
return 0;
}
Outputs Bello
For program 2, note only q needs to be on the stack. p can remain a const pointer to a string literal, since you're only reading from it.
int main( )
{
const char *p = "Hello" ;
char q[] = "mug";
*q = *p ;
printf("\n%s",q);
return 0;
}
Outputs Hug
In both samples you are modifying string literals, which yields undefined behavior.
When you create a static string in the form of char *p = "test" the contents of the pointer cannot be changed. In your case trying to modify the contents of the pointer yields to the error that you are observing.
I changed program 2 to not use string literals. It shows "Hug" as you expected.
#include <string.h>
#include <stdio.h>
int main( )
{
char p[10];
char q[10];
strcpy(p,"Hello");
strcpy(q,"mug");
*q = *p ;
printf("\n%s",q);
return 0;
}
What you should write is:
char p[] = "Hello";
The form above (char p [] = "Hello") tells the compiler, "I've got an array of values coming up, please allocate as much space as is needed for them." It also works with ints, for example:
int i [] = { 1, 2, 5, 100, 50000 };
You'll end up with i being a pointer to an array of 5 values.
The strings "Hello" and "mug" are stored in read-only memory and you are trying to write there.
$ gcc -S a.c
$ cat a.s
.file "a.c"
.section .rodata
.LC0:
.string "Hello"
.LC1:
Note that the section is "rodata" (read-only data).
