What do you mean? A cast to a char pointer is absolutely valid C. For most intents and purposes, a pointer "type" in C is merely the step size, and only void pointers cannot be decremented.
The line of code is basically two lines:
(char*)hash_ptr.parm.v.p = (char*)hash_ptr.parm.v.p - 1; // move the pointer back by one byte
*((char*)hash_ptr.parm.v.p) = fpop(sst); //At the memory location, write the output of fpop(sst)
The line of code is basically two lines: