The Factor garbage collector can move byte arrays around, and it is only safe to pass byte arrays to C functions if the garbage collector will not run while C code still has a reference to the data.
In particular, a byte array can only be passed as a parameter if the the C function does not use the parameter after one of the following occurs:
the C function returns
the C function calls Factor code via a callback
Returning from C to Factor, as well as invoking Factor code via a callback, may trigger garbage collection, and if the function had stored a pointer to the byte array somewhere, this pointer may cease to be valid.