describe PTHREAD_BARRIER_SERIAL_THREAD

Author: Vladimir Kotal

threads.tex

@@ -1204,14 +1204,18 @@
 \item The barrier API has been defined since SUSv3, see for example
 \texttt{pthread\_\-barrier\_init}, however they can be created using
 mutexes and condition variables.
-\item The barrier API is non-mandatory part of POSIXu (it belongs to
+\item The barrier API is non-mandatory part of POSIX (it belongs to
 Advanced realtime threads extension) and hence SUS certified system does not
 have to implement it, which is e.g. the case of macOS.
 \item The barrier can be used e.g. in situation when it is necessary to perform
 some initialization between individual phases of processing. The threads
 need to wait for each other because the initialization can only begin
 once the previous phase is over.
 Example \example{pthreads/pthread-barrier.c} shows the use in such case.
+\item \texttt{pthread\_barrier\_wait} returns
+the \texttt{PTHREAD\_BARRIER\_SERIAL\_THREAD} value
+in the last thread that reached the barrier so e.g. a collection of
+results from the last phase of the run can be done.
 \item The barrier condition is for example value of a counter to be 0.
 Each thread that reaches the barrier decrements the counter which is
 initialized to the number of threads in the beginning.