All routines that can potentially be called by a thread from a multithreaded program should be MT-Safe. Therefore, two or more activations of a routine must be able to correctly execute concurrently. So, every library interface that a multithreaded program uses must be MT-Safe.
Not all libraries are now MT-Safe. The commonly used libraries that are MT-Safe are listed in the following table. The libraries are accessed in the /usr/lib directory.
Table 7–3 Some MT-Safe Libraries
Routines in libraries that are not guaranteed to be MT-Safe can safely be called by multithreaded programs only when such calls are single threaded.