Also note that under the current implementation, shared variables
use a little more memory and are a little slower than ordinary variables.
+=head1 Threadsafety of System Libraries
+
+Whether various library calls are threadsafe is outside the control
+of Perl. Calls often suffering from not being threadsafe include
+localtime(), gmtime(), get{gr,host,net,proto,serv,pw}*(), readdir(),
+rand(), srand(). If the system Perl is compiled in has threadsafe
+variants of these calls, they will be used, but besides that, Perl is
+at the mercy of the thread safety or unsafety of the calls. Please
+consult your C library call documentation.
+
=head1 Conclusion
A complete thread tutorial could fill a book (and has, many times),