Quote

thread-safe
This module can safely be used from multiple threads. The effect of calling into a safe module is that the results are valid even when called by multiple threads. However, thread-safe modules can still have global consequences; for example, sending or reading data from a socket affects all threads that are working with that socket. The application has the responsibility to act sane with regards to threads. If one thread creates a file with the name file.tmp then another file which tries to create it will fail; this is not the fault of the module.