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\title{The OMNI Thread Abstraction}

\author{Tristan Richardson\\
Olivetti Research Laboratory \\
Cambridge \\
}

\date{{\it Revised} 13 March 1997}

\begin{document}

\maketitle

\section{Introduction}

The OMNI thread abstraction is designed to provide a common set of thread
operations for use in programs written in C++.  Programs written using the
abstraction should be much easier to port between different architectures with
different underlying threads primitives.

The programming interface is designed to be similar to the C language interface
to POSIX threads (IEEE draft standard 1003.1c - previously 1003.4a, often known
as ``pthreads'' \cite{pthreads}).

Much of the abstraction consists of simple C++ object wrappers around pthread
calls.  However for some features such as thread-specific data, a better
interface can be offered because of the use of C++.

Some of the more complex features of pthreads are not supported because of the
difficulty of ensuring the same features can be offered on top of other thread
systems.  Such features include thread cancellation and complex scheduling
control (though simple thread priorities are supported).

The abstraction layer is currently implemented for the following architectures
/ thread systems:

\begin{itemize}

\item Solaris 2.x using pthreads draft 10
\item Solaris 2.x using solaris threads (but pthreads version is now standard)
\item Alpha OSF1 using pthreads draft 4
\item Windows NT using NT threads
\item Linux 2.x using Linuxthread 0.5 (which is based on pthreads draft 10)
\item Linux 2.x using MIT pthreads (which is based on draft 8)
\item ATMos using pthreads draft 6

\end{itemize}

See the {\tt omnithread.h} header file for full details of the API.  The
descriptions below assume you have some previous knowledge of threads, mutexes,
condition variables and semaphores.  Also refer to other documentation
(\cite{birrell}, \cite{pthreads}) for further explanation of these ideas
(particularly condition variables, the use of which may not be particularly
intuitive when first encountered).


\section{Synchronisation objects}

Synchronisation objects are used to synchronise threads within the same
process.  There is no inter-process synchronisation provided.  The
synchronisation objects provided are mutexes, condition variables and counting
semaphores.

\subsection{Mutex}

An object of type {\tt omni\_mutex} is used for mutual exclusion.  It provides
two operations, {\tt lock()} and {\tt unlock()}.  The alternative names {\tt
acquire()} and {\tt release()} can be used if preferred.  Behaviour is
undefined when a thread attempts to lock the same mutex again or when a mutex
is locked by one thread and unlocked by a different thread.


\subsection{Condition Variable}

A condition variable is represented by an {\tt omni\_condition} and is used for
signalling between threads.  A call to {\tt wait()} causes a thread to wait on
the condition variable.  A call to {\tt signal()} wakes up at least one thread
if any are waiting.  A call to {\tt broadcast()} wakes up all threads waiting
on the condition variable.

When constructed, a pointer to an {\tt omni\_mutex} must be given.  A condition
variable {\tt wait()} has an implicit mutex {\tt unlock()} and {\tt lock()}
around it.  The link between condition variable and mutex lasts for the
lifetime of the condition variable (unlike pthreads where the link is only for
the duration of the wait).  The same mutex may be used with several condition
variables.

A wait with a timeout can be achieved by calling {\tt timed\_wait()}.  This is
given an absolute time to wait until.  The routine {\tt
omni\_thread::get\_time()} can be used to turn a relative time into an absolute
time.  {\tt timed\_wait()} returns {\tt ETIMEDOUT} if the time expires before
the condition variable is signalled.


\subsection{Counting semaphores}

An {\tt omni\_semaphore} is a counting semaphore.  When created it is given an
initial unsigned integer value.  When {\tt wait()} is called, the value is
decremented if non-zero.  If the value is zero then the thread blocks instead.
When {\tt post()} is called, if any threads are blocked in {\tt wait()},
exactly one thread is woken. If no threads were blocked then the value of the
semaphore is incremented.

If a thread calls {\tt try\_wait()}, then the thread won't block if the
semaphore's value is 0, returning {\tt EAGAIN} instead.

At present there is no way of querying the value of the semaphore.



\section{Thread object}

A thread is represented by an {\tt omni\_thread} object.  There are broadly two
different ways in which it can be used.

The first way is simply to create an {\tt omni\_thread} object, giving a
particular function which the thread should execute.  This is like the POSIX
(or any other) C language interface.

The second method of use is to create a new class which inherits from {\tt
omni\_thread}.  In this case the thread will execute the {\tt run()} member
function of the new class.  One advantage of this scheme is that
thread-specific data can be implemented simply by having data members of the
new class.

When constructed a thread is in the "new" state and has not actually started.
A call to {\tt start()} causes the thread to begin executing.  A static member
function {\tt create()} is provided to construct and start a thread in a single
call.  A thread exits by calling {\tt exit()} or by returning from the thread
function.

Threads can be either detached or undetached.  Detached threads are threads for
which all state will be lost upon exit.  Other threads cannot determine when a
detached thread will disappear, and therefore should not attempt to access the
thread object unless some explicit synchronisation with the detached thread
guarantees that it still exists.

Undetached threads are threads for which storage is not reclaimed until another
thread waits for its termination by calling {\tt join()}.  An exit value can be
passed from an undetached thread to the thread which joins it.

\sloppy{
Detached / undetached threads are distinguished on creation by the type of
function they execute.  Undetached threads execute a function which has a {\tt
void*} return type, whereas detached threads execute a function which has a
{\tt void} return type.  Unfortunately C++ member functions are not allowed to
be distinguished simply by their return type.  Thus in the case of a derived
class of {\tt omni\_thread} which needs an undetached thread, the member
function executed by the thread is called {\tt run\_undetached()} rather than
{\tt run()}, and it is started by calling {\tt start\_undetached()} instead of
{\tt start()}.
}

The abstraction currently supports three priorities of thread, but no guarantee
is made of how this will affect underlying thread scheduling.  The three
priorities are {\tt PRIORITY\_LOW}, {\tt PRIORITY\_NORMAL} and {\tt
PRIORITY\_HIGH}.  By default all threads run at {\tt PRIORITY\_NORMAL}.  A
different priority can be specified on thread creation, or while the thread is
running using {\tt set\_priority().}  A thread's current priority is returned
by {\tt priority()}.

\sloppy{
Other functions provided are {\tt self()} which returns the calling thread's
{\tt omni\_thread} object, {\tt yield()} which requests that other threads be
allowed to run, {\tt id()} which returns an integer id for the thread for use
in debugging, {\tt state()}, {\tt sleep()} and {\tt get\_time()}.
}

\section{Using threads in OMNI programs}

All you need to do is include the {\tt omnithread.h} header file in your source
code, and then link in the omnithread library in to your executable.  The
details of what you need to put in your dir.mk to make the header files and
library available is best explained by example.  You just need something like
this:

\begin{verbatim}

CXXSRCS = wob.cc

CPPFLAGS = $(OMNITHREAD_CPPFLAGS)

wob = $(patsubst %,$(BinPattern),wob)

$(wob): wob.o $(OMNITHREAD_LIB_DEPEND)
        @(libs="$(OMNITHREAD_LIB)"; $(CXXExecutable))

\end{verbatim}

Note that if you use CORBA then it is important that {\tt \$(CORBA\_LIB)}
appears before {\tt \$(OMNITHREAD\_LIB)} in the {\tt libs="..."} assignment.
This is because linking the Solaris thread library before the Orbix library
results in a binary which segmentation faults inside the Orbix library
initialisation.



\section{Threaded I/O shutdown for Unix}

or, how one thread should tell another thread to shut down when it might
be doing a blocking call on a socket.

Unfortunately there doesn't seem to be a standard way of doing this which works
across all Unix systems.  I have investigated the behaviour of our two main
Unix platforms, Solaris 2.5 and Digital Unix 3.2.  On Digital Unix everything
is fine, as the obvious method using shutdown() seems to work OK.
Unfortunately on Solaris shutdown can only be used on a connected socket, so we
need devious means to get around this limitation.  The details are summarised
below:


\subsection{read()}

Thread A is in a loop, doing read(sock), processing the data, then going back
into the read.

Thread B comes along and wants to shut it down - it can't cancel thread A since
(i) working out how to clean up according to where A is in its loop is a
nightmare, and (ii) this isn't available in omnithread anyway.

On Solaris 2.5 and Digital Unix 3.2 the following strategy works:

Thread B does shutdown(sock,2).

At this point thread A is either blocked inside read(sock), or is elsewhere in
the loop.  If the former then read will return 0, indicating that the socket is
closed.  If the latter then eventually thread A will call read(sock) and then
this will return 0.  Thread A should close(sock), do any other tidying up, and
exit.

If there is another point in the loop that thread A can block then obviously
thread B needs to be aware of this and be able to wake it up in the appropriate
way from that point.



\subsection{accept()}

Again thread A is in a loop, this time doing an accept on listenSock, dealing
with a new connection and going back into accept.  Thread B wants to cancel it.

On Digital Unix 3.2 the strategy is identical to that for read:

Thread B does shutdown(listenSock,2).  Wherever thread A is in the loop,
eventually it will return ECONNABORTED from the accept call.  It should
close(listenSock), tidy up as necessary and exit.

On Solaris 2.5 thread B can't do shutdown(listenSock,2) - this returns
ENOTCONN.  Instead the following strategy can be used:

First thread B sets some sort of "shutdown flag" associated with listenSock.
Then it does getsockaddr(listenSock) to find out which port listenSock is on
(or knows already), sets up a socket dummySock, does connect(dummySock, this
host, port) and finally does close(dummySock).

Now wherever thread A is in the loop, eventually it will call
accept(listenSock).  This will return successfully with a new socket, say
connSock.  Thread A then checks to see if the "shutdown flag" is set.  If not,
then it's a normal connection.  If it is set, then thread A closes listenSock
and connSock, tidies up and exits.


\subsection{write()}

Thread A may be blocked in write, or about to go in to a potentially-blocking
write.  Thread B wants to shut it down.

On Solaris 2.5:

Thread B does shutdown(sock,2).

If thread A is already in write(sock) then it will return with ENXIO.  If
thread A calls write after thread B calls shutdown this will return EIO.


On Digital Unix 3.2:

Thread B does shutdown(sock,2).

If thread A is already in write(sock) then it will return the number of bytes
written before it became blocked.  A subsequent call to write will then
generate SIGPIPE (or EPIPE will be returned if SIGPIPE is ignored by the
thread).


\subsection{connect()}

Thread A may be blocked in connect, or about to go in to a potentially-blocking
connect.  Thread B wants to shut it down.


On Digital Unix 3.2:

Thread B does shutdown(sock,2).

If thread A is already in connect(sock) then it will return a successful
connection.  Subsequent reading or writing will show that the socket has been
shut down (i.e. read returns 0, write generates SIGPIPE or returns EPIPE).  If
thread A calls connect after thread B calls shutdown this will return EINVAL.


On Solaris 2.5:

There is no way to wake up a thread which is blocked in connect.  Instead
Solaris forces us through a ridiculous procedure whichever way we try it.
One way is this:

First thread A creates a pipe in addition to the socket.  Instead of shutting
down the socket, thread B simply writes a byte to the pipe.

\sloppy{
Thread A meanwhile sets the socket non-blocking using fcntl(sock, F\_SETFL,
O\_NONBLOCK).  Then it calls connect on the socket - this will return
EINPROGRESS.  Then it must call select, waiting for either sock to become
writable or for the pipe to become readable.  If select returns that just sock
is writable then the connection has succeeded.  It then needs to set the socket
back to blocking mode using fcntl(sock, F\_SETFL, 0).  If instead select
returns that the pipe is readable, thread A closes the socket, tidies up and
exits.
}

An alternative method is similar but to use polling instead of the pipe.
Thread B justs sets a flag and thread A calls select with a timeout,
periodically waking up to see if the flag has been set.


\begin{thebibliography}{lo}

\bibitem[POSIX94]{pthreads}
{\em Portable Operating System Interface (POSIX) Threads Extension},
P1003.1c Draft 10,
IEEE,
September 1994.

\bibitem[Birrell89]{birrell}
{\em An Introduction to Programming with Threads},
Research Report 35,
DEC Systems Research Center,
Palo Alto, CA,
January 1989.

\end{thebibliography}

\end{document}
