From the D Blog -- Interfacing D with C: Strings Part One

[Ali Çehreli]

On 5/25/21 9:00 PM, Виталий Фадеев wrote:

 >      immutable(char)* toStringz( ref string s )
 >      {
 >          if ( s.capacity <= s.length )
 >              s.reserve( s.length + 1 );
 >
 >          char* cptr = cast( char* ) s.ptr; // C ptr
 >          char* zptr = cptr + s.length;     // zero ptr
 >          *zptr = '\0';

That's undefined behavior because that location does not belong to the 
string. Here is an example that defeats the proposed toStringz:

void main()
{
     string s;
     s = "D string";

     auto c_string = toStringz( s );

     auto other = s;
     other ~= 'X';    // <-- Seemingly unrelated operation

// ...
}

puts accesses that unrelated 'X' and more bytes after that:

   C string: D stringX1^


Ali