Question

Possible Duplicate:
Why does this Seg Fault?
What is the difference between char a[] = “string”; and char *p = “string”;

Trying to understand why s[0]=’H’ fails. I’m guessing this has something to do with the data segment in the process memory but maybe someone better explain this?

void str2 (void) 
{ 
    char *s = "hello"; 
    printf("%s\n", s); 
    s[0] = 'H';          //maybe this is a problem because content in s is constant? 
    printf("%s\n", s); 
}

int main()
{
    str2();
    return 0;
}

Answer 1

It’s wrong because the C standard says that attempting to modify a string literal gives undefined behavior.

Exactly what will happen can and will vary. In some cases it’ll “work” — the content of the string literal will change to what you’ve asked (e.g., back in the MS-DOS days, it usually did). In other cases, the compiler will merge identical string literals, so something like:

char *a = "1234";
char *b = "1234";

a[1] = 'a';

printf("%s\n", b);

…would print out 1a34, even though you never explicitly modified b at all.

In still other cases (including most modern systems) you can expect the attempted write to fail completely and some sort of exception/signal to be thrown instead.