scsh-0.6/doc/scsh-manual/threads.tex

%latex -*- latex -*-

\chapter{Concurrent system programming}

The Scheme Shell provides you with support for concurrent programming.
Its interface for concurrent programming consists of several parts:
\begin{itemize}
\item The thread system 
\item Synchronization vehicles 
\item Process state abstractions
\end{itemize}
Whereas the user deals with threads and synchronization explicitly, the
process state abstractions are built into the rest of the system
transparent for the user. Section \ref{sec:ps_interac} describes the
interaction between process state and threads.

\section{Threads}

A thread can be thought of as a procedure that can run independently of
and concurrent to the rest of the system. The calling procedure fires
the thread up and forgets about it.

The current thread interface is completely taken from Scheme\ 48. This
documentation is an extension of the file \texttt{doc/threads.txt}.

The thread structure is named \texttt{threads}, it has to be opened explicitly.

\defun {spawn} {thunk [name]} \undefined 

Create and schedule a new thread that will execute \var{thunk}, a
procedure with no arguments. Note that Scsh's \ex{spawn} does
\textbf{not} return a reference to a thread object. The optional
argument \var{name} is used when printing the thread.

\defun {relinquish-timeslice} {} \undefined

Let other threads run for a while. 

\defun {sleep} {time} \undefined

Puts the current thread into sleep for \var{time} milliseconds. The
time at which the thread is run again may be longer of course.

\defun {terminate-current-thread} {} {does-not-return}

Kill the current thread. 

Mainly for debugging purposes, there is also an interface to the
internal representation of thread objects:

\defun {current-thread} {} {thread-object}

Return the object to which the current thread internally corresponds.
Note that this procedure is exported by the package
\texttt{threads-internal} only.

\defun {thread?} {thing} {\boolean}

Returns true iff \var{thing} is a  thread object. 

\defun {thread-name} {thread} {name}

\var{Name} corresponds to the second parameter that was given to
\ex{spawn} when \var{thread} was created.

\defun{thread-uid} {thread} {\integer}

Returns a unique identifier for the current thread.

\section{Locks}

Locks are a simple mean for mutual exclusion. They implement a concept
commonly known as \textit{semaphores}. Threads can obtain and release
locks. If a thread tries to obtain a lock which is held by another
thread, the first thread is blocked. To access the following
procedures, you must open the structure \texttt{locks}.

\defun{make-lock} {} {lock}

Creates a lock. 

\defun{lock?} {thing} {\boolean}

Returns true iff \var{thing} is a lock.

\defun{obtain-lock} {lock} {\undefined}

Obtain \var{lock}. Causes the thread to block if the lock is held by
a thread.

\defun{maybe-obtain-lock} {lock} {\boolean}

Tries to obtain \var{lock}, but returns false if the lock cannot be
obtained.

\defun{release-lock} {lock} {\boolean}

Releases \var{lock}. Returns true if the lock immediately got a new
owner, false otherwise.

\defun{lock-owner-uid} {lock} {\integer}

Returns the uid of the thread that currently holds \var{lock} or false
if the lock is free.

\section{Placeholders}
Placeholers combine synchronization with value delivery. They can be
thought of as special variables. After creation the value of the
placeholder is undefined. If a thread tries to read the placeholders
value this thread is blocked. Multiple threads are allowed to block on
a single placeholder. They will continue running after another thread
sets the value of the placeholder. Now all reading threads receive the
value and continue executing. Setting a placeholder to two different
values causes an error. The structure \texttt{placeholders} features
the following procedures:

\defun {make-placeholder} {} {placeholder}

Creates a new placeholder.

\defun {placeholder?} {thing} {\boolean}

Returns true iff \var{thing} is a placeholder.

\defun {placeholder-set!} {placeholder value} {\undefined}

Sets the placeholders value to \var{value}. If the placeholder is
already set to a \textit{different} value an exception is risen.

\defun {placeholder-value} {placeholder} {value}

Returns the value of the placeholder. If the placeholder is yet unset,
the current thread is blocked until another thread sets the value by
means of \ex{placeholder-set!}.

\section{The event interface to interrupts}
\label{sec:event-interf-interr}
Scsh provides an synchronous interface to the asynchronous signals
delivered by the operation system\footnote{Olin's paper ``Automatic
  management of operation-system resources'' describes this system in
  detail.}.  The key element in this system is an object called
\textit{sigevent}, which corresponds to a single occurence of a
signal. A sigevent has two fields: the Unix signal that occurred and a
pointer to the next event that occurred. That is, events are kept in a
linked list in increasing-time order. Scsh provides various procedures
to access this list, they are all procided by the structure
\texttt{sigevents}.

\defun {most-recent-sigevent} {} {sigevent}

Returns the most recent sigevent, that is, the head of the sigevent
list.

\defun {sigevent?} {object} {\boolean}

The predicate for sigevents.

\defun {next-sigevent} {pre-event type} {event} 

Returns the next sigevent of type \texttt{type} after sigevent
\texttt{pre-event}. If no such event exists, the procdure blocks.

\defun {next-sigevent-set} {pre-event set} {event}

Returns the next sigevent whose type is in \texttt{set} after
\texttt{pre-event}. If no such event exists, the procdure blocks. 

\defun {next-sigevent/no-wait} {pre-event type} {event or \sharpf}

Same as \texttt{next-sigevent}, but returns \sharpf if no appropriate
event exists.

\defun {next-sigevent-set/no-wait} {set pre-event} {event or \sharpf}

Same as \texttt{next-sigevent-set}, but returns \sharpf if no appropriate
event exists.

As a small example, consider this piece of code that toggles the
variable \texttt{state} by USR1 and USR2:
\begin{code}
(define state #t)

(let lp ((sigevent (most-recent-sigevent)))
  (let ((next (next-sigevent sigevent interrupt/usr1)))
    (set! state #f)
    (let ((next (next-sigevent next interrupt/usr2)))
      (set! state #t)
      (lp next))))
\end{code}
\section{Interaction between threads and process state}
\label{sec:ps_interac}

Global process state is a bad thing: it undermines modularity. In the
case of concurrency however things get even worse.  The simplest
example for this it the current working directory. If this would be
global state, no thread can ever reliably dereference a relative link.

Scsh addresses the problem of process state in a uniform way for
almost all resources. For every global resource there is a
procedure \ex{with-}\textit{resource}\ex{*} \var{thunk} which guarantees that
during the execution of \var{thunk} the resource is 
set to the desired value. There is only one exception: The uid under
which the current process is running. The superuser may change to an
arbitrary user without being prompted for a password, but the way back
is blocked.