<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def decode_str(s, free=False):
 """Decodes a SymbolicStr"""

    try:
        if s.len == 0:
            return u""
        return ffi.unpack(s.data, s.len).decode("utf-8", "replace")
    finally:
        if free:
            lib.semaphore_str_free(ffi.addressof(s))