Obtaining a plain char* from a string in D?

I'm having an absolute hell of a time trying to figure out how to get a plain, mutable C string (a char*) from a D string (a immutable(char)[]) to that I can pass the character data to legacy C code. toStringz doesn't work, as I get an error saying that I "cannot implicitly convert expression (toStringz(this.fileName())) of type immutable(char)* to char*". Do I need to recreate a new, mutable array of char and copy the characters over?