In the Solaris environment, function calls inside libraries are either mt-safe or not mt-safe; mt-safe code is also called "re-entrant" code. That is, several threads can simultaneously call a given function in a module and it is up to the function code to handle this. The assumption is that data shared between threads is only accessed by module functions. If mutable global data is available to clients of a module, appropriate locks must also be made visible in the interface. Furthermore, the module function cannot be made re-entrant unless the clients are assumed to use the locks consistently and at appropriate times. See also single-lock strategy.