[9fans] Re: constant string

[Richard Bos <rlb@hoekstra-uitgeverij.nl>]

Prem Mallappa <prem_mallappa@hotpop.com> wrote:

> #include <stdio.h>
> 
> int main(void)
> {
> 	char *s = "Hello";
>
> 	s[2] = 'z';
> 	
> 	printf ("%s\n", s);
> 	
> 	return 0;
> }

> my question is when i compile this in Linux (gcc) i get a segmentation 
> fault at the second statement of main(),
> As far as my knowledge i think this is because in 2nd chapter of K&R book
> it is being mentioned that
> ' any thing enclosed between " and " is a string constant' so here i am
> changing a constant, and i get a segmentation fault ( and i also noticed
> that gcc stores the string Hello in read-only datasegment)

<http://www.eskimo.com/~scs/C-faq/q1.32.html>. Basically, yes, you're
correct.

> but when i compile the same thing in plan9 C compiler ( both native "8c"
> and "pcc" i get a output of "Hezlo")
> why this is happenning..

To begin with, IIRC Plan 9 C isn't exactly ISO C. But in any case,
assigning to a constant (in those cases where you don't get a mandatory
warning, and when you ignore the warning and do the assignment anyway,
as well) invokes undefined behaviour. This means that the implementation
is free to behave as it chooses to; this includes crashing, behaving as
if it were correct code, and anything in between.

Richard