Your declaration is completely wrong:
char* s = "teststring";
"teststring" is stored in the code segment, which is read-only, like code. And, s is a pointer to "teststring", at the same time, you're trying to change the value of a read-only memory range. Thus, segmentation fault.
But with:
char s[] = "teststring";
s is initialized with "teststring", which of course is in the code segment, but there is an additional copy operation going on, to the stack in this case.
