Why patching a string using .ptr fails under Linux64 but not under Win32?

Question

Why the small sample below fails under Linux64 but not under Windows32?

module test;

import std.string, std.stdio;

void main(string[] args)
{
    string a = "abcd=1234";
    auto b = &a;
    auto Index = indexOf(*b, '=');

    if (Index != -1)
        *cast (char*) (b.ptr + Index) = '#';

    writeln(*b);
    readln;
}

Answer 1

one thing to remember is that string is an alias to (immutable char)[] which means that trying to write to the elements is undefined behavior

one of the reasons that I can think the behavior differs is that under linux64 the compiler puts the string data in write-protected memory, which means that *cast (char*) (b.ptr + Index) = '#'; fails (either silently or with segfault)