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OS/2 Thread cookbook. Great tutorial on how to use threads.
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OS/2 Thread cookbook. Great tutorial on how to use threads.
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Contents of the COOKBOOK.TXT file





















OS/2 THREADS COOKBOOK
_____________________





Version 1.1















Stephen Best
P.O. Box 3097
Manuka A.C.T. 2603
Australia

Phone: 61-6-281-2147
FidoNet: 3:620/243.4
CompuServe: 100033,340











Copyright (c) 1991 Stephen Best
















This document is an attempt to collect together and share a
number of my observations and ideas about programming for OS/2
Presentation Manager using multiple threads that have evolved
over time and been gleaned (gratefully) from other explorers
in this area. A thorough understanding of the use of threads
is essential for construction of all but the most trivial
Presentation Manager programs and it is hoped that the ideas
contained herein with be of aid to programmers beginning to
tap into the exciting possibilities that the use of multiple
threads introduce.


If you would like a copy of the latest revision of this
document (nicely printed on my laser) together with a floppy
disk (3.5 inch only) containing the full 32 bit source of the
examples given, please send $35 ($150 for corporations) in
either Australian or US currency to me at the address above.
This amount also entitles the licensee to use the source from
the examples given in any programs of their own.

Also, if you have any comments at all regarding the material
contained herein, including errors and omissions, I would be
more that happy to hear of them.



Stephen Best

24 July, 1991








Contents
________



Introduction 3

What is a thread? 4

Message queues 5

Performance and restrictions 7

Managing threads 9

Window data 10

Example 1 10

Example 2 12

Other Possibilities 14

Conclusion 14

References 16





OS/2 Threads Cookbook page 3



Introduction
____________

OS/2 as a single user system has the potential to
substantially change the user's perspective as to how a
personal computer should work. Programs using multiple threads
can not only increase execution performance (both perceived
and actual) but also change the emphasis in user-application
interaction to one where the user has more control and
flexibility and where the application itself takes on a
passive role. The program should always be receptive to
interaction with the user even if this is just the capability
for that user to change his/her mind after initiating a
lengthy activity.

Users that repeatedly tell you that "they don't need to
multitask" will have great difficulty in reverting to single
threaded software after having had the luxury of using a well
designed and responsive multi-threaded application. Thus
anyone wishing to compete in the market may have a hard time
selling their product in an increasingly aware public arena.
It is also hoped that all programmers will want to wring the
maximum result from an environment for their efforts, and I
think multiple threads have the potential for good returns in
this area.

It is also true, though not universally appreciated, that
programming to the multi-threaded model has significant impact
on the overall program design, and it is important to have
this in mind up front to avoid major restructuring of the
program at a later stage.


This document is aimed at the OS/2 programmer wishing to tap
into the power that programming with multiple threads
provides. As such, I will attempt to cover all the essential
issues related to threads, a guide to where and when I think
threads are applicable and some substantial coding examples
that I think demonstrate this. These examples are in C and are
for OS/2 2.x, though conversion to other languages and/or OS/2
1.x should not be too difficult once the concepts are
understood.

Note that my coding style dictates that maximum use be made of
the native OS/2 API and as such no Microsoft specific
extensions (such as _beginthread) are used. My hope is that
the life of the document (and the coding examples) will carry
into OS/2's future on RISC platforms.

All code has been developed and tested under the MS OS/2 2.0
SDK Pre-release 3 running on an IBM PS/2 Model 80. The
Microsoft C 386 compiler, linker etc included with this SDK





OS/2 Threads Cookbook page 4


have been used (with the only modification being replacement
of the C run-time prefix and malloc functions with self
developed routines available elsewhere).


It is my belief that practically ALL programs for OS/2
Presentation Manager will benefit from using multiple threads
in their design, and indeed have a responsibility to do so
given the message switching architecture of PM. Comments
(especially those from sources with a vested interest in
promoting second rate software products) that there is only a
minimal requirement for multi-threaded program design should
be considered in the light of the immediate and obvious
benefits that their proper use can achieve.

Polemics over, let's learn about OS/2 threads.


What is a thread?
_________________

The thread is the basic level of execution under OS/2 and is
roughly equivalent to the task of other systems. A program (or
process) has a single thread at the beginning of its execution
and can optionally split the activity of that program over a
number of threads. Each thread of execution will be time-
sliced on the processor (CPU) of the computer together with
other threads of that application, and those of other
applications active concurrently. A priority mechanism exists
to ensure that the thread with the highest priority is always
active, with control passing to other threads of a lower
priority when the higher 'blocks' or is waiting on an event.
On top of this OS/2 has a sophisticated scheduler to
dynamically alter thread priority to achieve responsive
overall performance or multitasking within the system.

Note that splitting a single processor intensive task over a
number of threads does not in itself achieve anything as the
processor itself is a finite resource which cannot be driven
beyond its capacity. Indeed the housekeeping in alternately
dispatching threads may slow down execution in this case. (It
may be worthwhile though to keep in mind that a future version
of OS/2 may well support multiple physical processors, and the
requirement for dividing compute bound tasks will change in
this case).

The criteria for dividing a process into threads as discussed
herein is aimed at isolating the activity of a program by
either priority, functional units or access to a resource.

Consider an application which presents the user with a number
of child windows, of which only one (the active window) can
receive the keyboard focus at a time. An example of this is





OS/2 Threads Cookbook page 5


MDI (Multiple Document Interface) applications. It would in
this case make sense to give the active/focus window a higher
priority than the others, if concurrent activity in other
windows is likely to impede the responsiveness of the one with
which the user is interacting at that time. This can be
achieved quite easily by assigning each window its own
'worker' thread and setting the priority of the active/focus
window thread higher than that of its siblings. In this case
the thread for the active window will receive the processor
resource that it requires without interference from the other
windows, aiding in the perceived responsiveness of the
application.

Actual overall efficiency can be achieved by overlapping
processor intensive tasks with those for input/output eg. disk
I/O. OS/2, as a true pre-emptive multitasking system, can
balance the priorities between processes to maximize
throughput but it is the application's responsibility to
separate within itself lengthy I/O tasks from processor
intensive ones, and especially those likely to interfere with
servicing of the system message queue (more on this important
area later). For example, if a user initiates a lengthy file
open/save operation or printing activity it should be possible
to interrupt this activity if the user changes his/her mind,
or still interact with other facets of the application in
parallel with the I/O activity. Failure to split this activity
off from the primary thread can even inhibit the user's
ability to switch to another unrelated application on the

desktop. In this case, it may be advantageous to spawn a
thread specifically for servicing the disk or printer
asynchronously. The main thread could then off-load such tasks
and 'queue' them to a background thread, and get on business
of interacting with the user. Note that in this case it makes
no sense to have a number of threads for a single resource
(like a printer) as no efficiency is gained.

It may be helpful to think of a one for one correspondence
between threads and 'resources' (be they windows or the disk
or a printer), with a 'master' thread interacting with the
user (and hence the system message queue). It is this concept
of resource based threads that will be expounded upon in the
following.


Message queues
______________

Presentation Manager (among other GUI systems) has a message
switching architecture to facilitate the routing of messages
of different types among the 'windows' that make up the
presentation layer for OS/2. An application can receive
messages from the system eg. when a user attempts to re-size a
window, or can send messages to itself or other windows in the





OS/2 Threads Cookbook page 6


system. Messages can either be SENT (explicitly with
WinSendMsg or implicitly with a large number of other API
calls eg. WinSetWindowText) or POSTed (with WinPostMsg). Sent
messages (and those API calls that result in sent messages)
will be turned into direct calls to the window procedure for
the window specified in the send. Posted messages on the other
hand will be queued in the application's message queue for
deferred execution.

The application message queue is created by the application
itself with WinCreateMsgQueue and it is the act of doing so
that distinguishes that application as a Presentation Manager
one (as opposed to a character mode application executing in
its own session). An application may create as many message
queues as desired provided that only one message queue exists
for each thread. Message queues other than the primary one are
optional in multi-threaded applications and the following
examples will attempt to demonstrate where multiple
application message queues might be applicable.

Messages queued on a message queue (by the system or the
application itself) are un-queued with (generally) WinGetMsg
in a message loop and then dispatched to the appropriate
window procedure with WinDispatchMsg. This WinDispatchMsg can
be thought of as turning the POSTed message into a SEND for
immediate execution. In both cases, the window handle given
specifies the appropriate window procedure for that message
... the association between window handle and procedure (for
other than pre-registered classes) is made by the application
with the combination of WinRegisterClass and
WinCreate(Std)Window.

The system message queue (of which there is only one for the
whole Presentation Manager session) is provided to queue those
'messages' that will be subsequently distributed to the
appropriate application message queue(s) at such time that the
context of the message can be determined. The primary
consideration here is user input (both keyboard and mouse
actions) that may occur asynchronously to the application
message flow. The 'problem' for PM programs is that the
application processing of any message can itself change the
destination for keyboard and mouse messages pending in the
system message queue (eg. explicitly with calls WinSetFocus or
WinSetCapture) and thus it is only when PM itself regains
control from prior messages that it is possible to determine
the appropriate application queue in which to place the
keyboard or mouse message. In addition, the program is
responsible for processing messages dealing with loss of focus
and activation before other windows can be activated. The
implication for a PM program is that it should always be
available for processing user input events, and process all
incoming messages quickly.





OS/2 Threads Cookbook page 7



The methodologies discussed in this document are aimed at off-
loading the bulk of the processing requirement for the
application from the 'input' message thread to other 'non-
input' threads, making the application always receptive to
user input, and thus increasing the responsiveness of the
application and the system as a whole. It may be helpful to
consider that serialisation of keyboard and mouse messages in
the system queue is not so much a 'problem' to be overcome
with adding threads, but that the main 'input' thread of the
application is just a vehicle for receiving input from the
system and like all shared resources, to be treated
accordingly.


Performance and restrictions
____________________________

Sent messages can be processed approximately three times
faster than posted messages because they never appear in the
message queue of the application and thus avoid the message
loop altogether. The throughput of inter thread posts has been
measured to be slower by another factor of three. This is not
to say that posts should be avoided, but that it may be
desirable to use sends rather than posts when an clear choice
exists between the two. Sends also have the guarantee that any
dynamic memory area addressed by the message parameter(s) will
remain current for the life of the send, which is a benefit if
more data than the eight bytes permitted with the two 32 bit
message parameters themselves is required. As a bonus, the
return code from the receiving window procedure method is
available upon completion of the send. Sends (because they are
translated into calls to the window procedure) will cause the
window procedure(s) to be called recursively, and thus may
place excessive demands on the program stack with high levels
of recursion.

Posts on the other hand, because of their asynchronous nature
will be serialised in the message queue and processed when the
application itself enters message loop processing. This means
that any dynamic data addressed by message parameters when the
post was issued may no longer be valid. This consideration
requires a number of differing techniques to transfer more
data than the message parameters themselves permit. Another
important point to note about posts is that the message may
not actually be posted should the message queue be full at the
time of the post. The return code from WinPostMsg should thus
be checked to see if the post was in fact accepted and
implementing a delayed retry or some pacing algorithm to
ensure the message is not lost. Despite the above, posts will
play a big part in the interaction of and communication
between threads and thus the techniques for achieving
efficient and reliable use of them is presented herein.





OS/2 Threads Cookbook page 8



Another difference between sends and posts is the context in
which it is valid to issue them. Posts can be issued without
restriction between threads and will appear in the message
queue of the thread with which the window addressed (by the
window handle specified) was created. A variation on
WinPostMsg is WinPostQueueMsg where the handle of the message
queue itself is specified instead of the window handle. This
permits an application to queue messages to another thread
(assuming the receiving thread has created its own message
queue) when no actual window procedure may exist for that
thread. This variation will be explored in one of the
following examples.

Sends on the other hand can only be issued between threads
each having a message queue, and for reasons following should
be avoided for anything other than intra thread
communications. Firstly, sends to a window created on a
different thread than that from which the send is issued will
still execute in the context of that window's thread and thus
may incur a performance penalty due to the overhead involved
in the required thread switch. In addition, inter thread sends
(and API calls that result in sends to other threads) may
result in a deadlock situation should the receiving thread be
waiting (using say a semaphore) on some event from the calling
thread at the time the send is issued. (Note that
WinMsgMuxSemWait exists specifically to avoid this deadlock
situation.) The temptation may be to think that creating
windows each on separate threads will permit extensive
processing without interference with the overall message flow,
but it must be remembered that all threads that create a (non
object) window are subject to the same input restrictions
discussed above. It is because of these reasons that I propose
creation of all windows on the initial thread and exclusive
use of posts for inter thread communications in this document.

The above brings up the important concept of distribution of
responsibilities within the application. The model I use and
propound herein is that the main (initial) thread be used
almost exclusively for window 'management'. Thus ALL (non
object) windows will be created (or 'owned') by this thread
and any activity likely to involve more than minimal
processing off-loaded to non-window 'service' threads. The
main thread (simply because of the fact that this is where the
windows were created) will be the sole 'input' thread subject
to the keyboard/mouse message restrictions discussed above.
All other threads can thus undertake substantial processing
tasks (or waits) without impacting the application's ability
to appear responsive to user interaction. Using this
demarcation of processing responsibility, it is unlikely that
the problem of using inter thread sends will arise.





OS/2 Threads Cookbook page 9



Managing threads
________________

Threads (over and above the initial one allocated when the
program begins execution) are created explicitly with
DosCreateThread. Each thread will have its own stack
(allocated and committed dynamically with 2.x) but share all
code and data areas of the parent process. Optionally a word
long parameter can be passed to the thread at this time and
this word is normally used to address thread initialization
data (or 'thread parameters'). A thread so created will exist
for the life of the program execution (process) unless it
terminates itself by 'returning' or making a call to DosExit
(with EXIT_THREAD).

Due to any overhead in creating/destroying a thread it is
normal to have the thread life tied to the 'owning' window or
dialog box or failing that, the entire process. There are no
rules as to how many threads should be created in the
'average' program as this will be governed by the activity and
resource requirements of each. One way of deciding the number
(and more importantly, function) of threads to create is to
consider how many of the elements of the program you would
like to run in parallel. Thus a program which creates a number
of windows (all of which require extensive graphics) plus
provides for background printing may create a thread for each
window, with another for servicing the printer queue. Or maybe
all the windows could share a single drawing thread if the
processing requirements are smaller. The final consideration
of thread numbers and function will depend on both the degree
of interactivity and visible feel the programmer wishes to
create with the program and how logically functions are
isolated internally in the program itself. (Realistically, the
same end result may be achieved by creating only a single
'service' thread in addition to the main thread and
alternately allocating time to the respective resources, but
maintaining the desired balance may require duplicating the
function of the OS/2 scheduler itself, and hence be self
defeating.)

A number of other API calls are related to threads.
DosWaitThread (new with 2.x) allows the thread 'owner'
(actually any thread) to wait until the specified thread is
terminated and thus can be used when the owner itself is being
destroyed for clean-up operations. DosSuspendThread and
DosResumeThread allow another thread to temporarily halt
execution of the specified thread, and resume operation at a
later time. Due to the fact that it will probably not be
possible to predict the exact stage of operation of the
specified thread, these calls may not prove to be that useful,
and indeed a similar effect can be achieved by resetting that
thread's priority. DosSetPriority can be used to modify a





OS/2 Threads Cookbook page 10


thread's priority, or to place the thread in a different
dispatching class. DosEnterCritSec and DosExitCritSec can be
used to temporarily disallow execution of all other threads in
the process when serialized access to a resource of some type
must be guaranteed, and using mutex semaphores is not
appropriate. Finally, DosSleep can be used by a thread to
surrender the remainder of its dispatching time slice or to
delay execution for a specified amount of time.

In addition to the above, OS/2 has a rich set of inter-process
communication facilities, such as semaphores and pipes which
may be used for thread control and transferring data between
threads.


Window data
___________

Each window procedure associated with a window class will have
some data to be retained over the life of the window, or
between processing of messages. This 'static' data can be
initialized when the window procedure receives its WM_CREATE
or WM_INITDLG message and updated depending on subsequent
message flow. It is common practise to place such 'static'
data in a dynamically allocated area of memory and have this
addressed by a window 'pointer'. Thus an area of the
appropriate size will be allocated (with malloc) when the
window is created and the address of this area saved in a
window 'word' with WinSetWindowPtr. The address of this area
will be retrieved with WinQueryWindowPtr immediately prior to
processing of all other messages for the window, and the
memory area disposed of (with free) in WM_DESTROY processing.
Thus if multiple 'instances' of the window are created, each

window can be assured of integrity of its own data. This can
have an added benefit in reducing the total EXE file size, and
more importantly promotes what I believe to be a good 'object
oriented' programming style. Though not directly related to
using threads, the concept of data encapsulation will be
rigidly exploited in the coding examples contained herein.


Example 1
_________

The first example below is the complete window procedure for a
file search dialog. This dialog provides the user with a means
to search a number of disks for a specified file, or ones
matching the given 'mask' criteria. The user enters the
desired file name (with or without free characters), selects a
number of disks and presses the 'start' button. Once the
search is initiated, the 'start' button changes its function
to 'stop' to enable the user to interrupt the active search.
As files are found that match the search criteria, they will
be added to a list box which can be scrolled and an entry





OS/2 Threads Cookbook page 11


selected even though the search is still active, enabling the
user to exit with the selected file without waiting for the
search to complete. The 'stop' button reverts to its 'start'
function when the search is complete. Whilst this search is in
progress, the user can move the dialog window or interact with
other applications on the desktop.

The virtue of using a separate thread for this type of dialog
is that the I/O intensive logic for scanning the directory
list(s) for the specified files can be segregated from that of
interacting with the user. The end result is that maximum
flexibility of interaction is achieved without impacting the
speed of the actual search.


This dialog window procedure creates the search thread in the
WM_INITDLG processing and terminates the thread in WM_DESTROY,
thus the thread exists for the life of the dialog session. The
search thread issues a mux wait on two event semaphores: a
'trigger' to initiate a new search and a 'terminate' event to
signal thread termination. Once the search is active, it can
be interrupted by setting the fInterrupt flag TRUE, and this
flag is checked periodically in the search process.

As files are found that match the specified criteria, the
search thread posts a UM_SEARCHUPDATE message to the 'owning'
thread to signal that the found entry should be added to the
list box. In this case, we cannot use the message parameters
on the post to fully contain the data to be transferred as the
file name length clearly exceeds the eight bytes available.
What has been done in this example is to use a simplified form
of circular buffer, with an 'in' and 'out' count. Thus entries
can be added to the buffer when the 'in' count does not exceed
the 'out' count by the total number of entries in the buffer,
otherwise we would overlay data that had not been accepted by
the owning thread. As the buffer and counters are accessible
by both threads, all that is required is to signal the owning
thread that new data has been added to the list and should be
processed. This is done here by equating UM_SEARCHUPDATE to
WM_SEM2 and using message parameter 1 as a progress flag, with
TRUE indicating completion of the search. The WM_SEM1-4
messages are special in that the messages are not stacked in
the message queue, but accumulated into one message with the
message parameter 1 seen by the recipient being the or'ed
result from all the messages parameters posted. WM_SEM2
(rather than WM_SEM1) was selected as the priority of this
message is lower than that of keyboard/mouse messages thus
avoiding any impact on user interaction whilst transferring
data. (If you move the mouse pointer around rapidly you will
notice that the search will slow down.)





OS/2 Threads Cookbook page 12


A few other observations on this example. Because of the
nature of the WM_SEMx messages, there is no risk of flooding
the application message queue (and hence losing a post) in
that there can be only one message of this type in the queue
at any time. Also, it is likely that a number of found entries
can be transferred for each post the main thread sees, hence
improving the efficiency of the transfer. If the circular
buffer is full (indicated by the value of the difference in
the counters) the search thread issues a DosSleep (0) to
surrender the remainder of its dispatching time slice and thus
allowing the main thread to process the queued entries and
free up the slots required.

Another important element is that the dialog window procedure
has been structured to not have to depend synchronously on the
action of the search thread, allowing the search to be
interrupted and end without the main thread logic having to
issue a wait. If it is possible to avoid such waits, an extra
level of semaphore handshaking can be omitted.


Example 2
_________

The second example is a window procedure (together with its
'service' thread) for utilizing 'shadow' bitmaps to facilitate
fast paints and to off-load the bulk of the processing
requirement to a 'non input' thread. A shadow bitmap (as used
in this example) is the context for the drawing operations
which can proceed offline from the main window procedure and
be quickly transferred to the window context with GpiBitBlt in
the WM_PAINT method. This implementation is ideal when an
application can present the completed drawing, rather than
show the drawing activity in progress. Also, if the
destination window is to be restored (eg. after being covered
by another) a subsequent call to the processor intensive
graphics functions is avoided.

This example differs from the first in that the service thread
allocates its own message queue, and communications between
threads is achieved with posts (rather than semaphores). Thus,
a request for some activity can be 'queued' to the service
thread (with WinPostQueueMsg) by specifying the handle of the
message queue itself. Note that WinPostMsg could not be used
in this case as the service thread has not actually created
any windows and hence no window handle exists to enable PM to
determine which queue is applicable. The service thread has
its own message loop to un-queue the posted requests and route
to the appropriate logic based on message ID, and in this
sense is no different from a normal window procedure. When the
activity is complete, the service thread posts a completion
message to the 'owning' thread to trigger the appropriate
action (eg. paint). Lastly, the service thread is terminated





OS/2 Threads Cookbook page 13


by posting WM_QUIT to its message queue which causes the loop
to terminate.

The service thread in this example exists for the life of its
'owning' window, created in WM_CREATE and terminated in
WM_DESTROY. As the main procedure must insure that the service
thread's message queue is valid, a semaphore is set by the
service thread when the queue handle is available to its
owner.

If multiple instances of this window are required, each will
have its own service thread and this enables a priority
mechanism to exist to ensure that the active window will be
drawn before other, non-active windows. This is achieved in
this example buy raising or lowering the service thread
priority (in WM_ACTIVATE) so that the active window's priority
is always higher that its siblings. The priority mechanism is
absolute in that the service thread for the active window must
'block' (in WinGetMsg) before the other windows will receive
any processor resource. Note that as implemented in this
example this set priority will still be lower than that of the
main 'input' thread to reduce any interference with desktop
operations.

When using this message queue technique, it is possible to
optionally check for pending messages posted in the queue with
a call to WinQueryQueueStatus. In this example, as all output
posts from the service thread are the same, some processing
may be saved if processing of the current message is aborted
in favour of pending messages of the same type. This should
only be attempted when it can be quaranteed that the sequence
of incoming messages is not disturbed.

This example has been structured so that the main window
thread never has to explicitly wait for completion of a posted
task (other than for thread termination and recovery from
failed posts). If serialisation is necessary, semaphores
'posted' by the service thread can be used to delay execution
until desired. Alternatively, the main thread can wait for a
posted completion message by using WinGetMsg and specifying
the message identity. In the example given

WinGetMsg (pw->hab, &qmsg, hwnd,
UM_WINDOWUPDATE, UM_WINDOWUPDATE);

would delay the main thread until the requested service was
complete. Note that either of the above (using semaphores or
waiting for completion messages) issued from the main input
thread will have the effect of stopping flow in the main
message queue of the program, and delay incoming keyboard and
mouse messages system-wide (as discussed above). The goal





OS/2 Threads Cookbook page 14


should thus be to structure the program so that such
serialised dependencies are minimized (or ideally avoided).


Other Possibilities
___________________

The above two examples represent a sample of the possibilities
of managing program activity with multiple threads. A number
of other methodologies exist which may prove applicable to
different program requirements.

A variation on the shadow bitmap example above is to give
drawing control of the window presentation space to a service
thread. This has the similar benefit in that processor
intensive graphics functions can be off-line from the main
input thread with the bonus that the application user can see
the drawing in progress, rather than wait for the shadow
bitmap to be completed. To do this the program would (probably
in WM_CREATE) associate a presentation space to the window
context with WinOpenWindowDC and GpiCreatePS and pass this
presentation space handle to the drawing thread. The drawing
thread would thus receive requests from the main thread and
invoke the graphics functions required to draw directly upon
the window presentation space. Some provision may need to be
made for retaining the results of the drawing activity should
a full or partial re-paint be required due to window sizing or
restoral.

An extension of using threads with their own message queue is
to create object windows (windows created with a parent of
HWND_OBJECT). Activity in such 'windows' is initiated with
WinPostMsg as the object window handle is specified to
identify the appropriate message queue and window procedure
for that window. In all other respects, this is identical to
the message queue example above. The use of object windows may
be applicable when a thread exists to support a number of
resources and no overlap in processing is required.


Conclusion
__________

It is hoped that by now the reader has understood the
fundamentals of why multiple threads are applicable to OS/2
Presentation Manager programs for improving the overall
responsiveness of the desktop dictated by the message queue
architecture, and the implications for presentation of a
flexible user-application interface. The existence of threads
in OS/2 provides the application designer with a rich set of
techniques to distribute function within the program itself
and co-ordinate activity. The goal of the application designer
should be to identify opportunities for parallel operation,
and to build the program with the appropriate threads to





OS/2 Threads Cookbook page 15


achieve this, whilst allowing the user to interrupt or abort
any lengthy activity in progress.

Multiple threads, I feel, offer the means to totally transform
a user's expectation of how personal computer software should
work and hopefully this document will help bring about this
new age of more responsive and flexible software.





OS/2 Threads Cookbook page 16



References
__________

The following references may be useful in expanding the
reader's understanding of OS/2 multi-threading techniques and
possibilities as applicable to Presentation Manager
programming:



Utilizing OS/2 Multithread Techniques in Presentation Manager
_____________________________________________________________
Applications, Charles Petzold
____________

Microsoft Systems Journal Vol. 3 No. 2


Planning and Writing a Multithreaded OS/2 Program with
______________________________________________________
Microsoft C, Richard Hale Shaw
___________

Microsoft Systems Journal Vol. 4 No. 2


OS/2 PM Programming: A Performance Guide, P.G. Toghill
________________________________________

IBM Personal Systems Developer, Winter 1991


A Multithread CPU Monitor, Marc Cohen
_________________________

OS/2 Notebook, The Best of the IBM Personal Systems Developer,
Microsoft Press


Programming for Multithreaded Drawing, Charles Petzold
_____________________________________

PC Magazine, Vol. 9 Nos. 10-12


Programming the OS/2 Presentation Manager, Charles Petzold
_________________________________________

Microsoft Press


Inside OS/2, Gordon Letwin
___________

Microsoft Press


Microsoft OS/2 Programmer's Reference Vol. 1
____________________________________________

Microsoft Press





OS/2 Threads Cookbook page 17



Programming Guide
_________________

IBM OS/2 Programming Tools and Information, Version 1.2


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