Blocking calls and threads

[Lynn Winebarger <owinebar@gmail.com>]

The thread on how to make asynchronous behavior explicit made me
curious whether making a blocking system call would cause the lisp
thread to yield to other lisp threads (release the global lock).  Do
blocking system calls yield the lisp thread, or is there any way in
lisp code to call blocking functions so that the lisp thread will
yield while the system thread blocks?

Thanks,
Lynn