more explanation of pthread_once

Author: Vladimir Kotal

threads.tex

@@ -422,10 +422,14 @@
 \end{slide}
 
 \begin{itemize}
-\item In the program itself you probably will not need this function.  You can
+\item In program itself you will probably not need this function.  You can
 just call the initialization function before you create the first thread.
 \item It is undefined if \emph{once\_control} is a local variable or does not
 have an expected value.
+\item This is handy for lazy initialization, i.e. only after one of the threads
+call into APIs of the library (as opposed to when the library is being loaded),
+the library becomes initialized and the semantics of \texttt{pthread\_once}
+will make sure this will happen only once.
 \end{itemize}
 
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