Question

Possible Duplicate:
Difference between char *str=“STRING” and char str[] = “STRING”?

I wrote the following code:

int main()
{
    char *str = "hello";
    str[0] = 'H';
    printf("%s\n", str);
}

This gives me a segmentation fault, I cant understand why.

str is pointer to char not const char. Even if that’s the case shouldn’t it give a compile error like the following program:

int main()
{
    const char *str = "hello";
    str[0] = 'H';
    printf("%s\n", str);    
}

It gives an error: assignment of read-only location *str.

EDIT

If my code places the pointer to a read only location, shouldn’t I get a compilation error?

Answer 1

You assign a pointer to a constant string (which comes as a part of your text and is thus not writable memory).

Fix with char str[] = "hello"; this will create a r/w copy of the constant string on your stack.

What you do is a perfectly valid pointer assignment. What the compiler does not know is that in a standard system constant strings are placed in read-only memory. On embedded (or other weird) systems this may be different.

Depending on your system you could come with an mprotect and change the VM flags on your pointer destination to writable. So the compiler allows for this code, your OS does not though.