Question

I’m thrilled. Why does this works fine:

char ptr[] = "hello world!";
ptr[0] = 'H';
printf("%s\n", ptr); // prints "Hello world!"

and this:

char * ptr = "hello world!";
ptr[0] = 'H';
printf("%s\n", ptr);

raises a Segmentation Fault?

Answer 1

Because the ptr[] is modifiable by the standard but the char * is not. The char * uses a const string which can be used many times over in the program the array actually creates a new array and copies your string to it.

By the way this should give a compile error – you must use

const char *ptr="Hello";

Adding some more – basically the compiler is allowed to look for every string in quotes and place it in a read only string table. Because a 1000 place in your program could use and define the string “this”. The compiler can get smart and convert those 1000 “this” to just 1 because they are all the same – because of this it becomes read only – So now one location cannot modify the fixed string after compile time – because it will break you expected output from your program.