Re: Should I use mutex in this context?

"Doug Harrison [MVP]" <>
Thu, 30 Oct 2008 21:57:38 -0500
On Thu, 30 Oct 2008 16:40:09 -0400, Tommy <> wrote:

Doug Harrison [MVP] wrote:

On Thu, 30 Oct 2008 14:03:21 -0400, Tommy <> wrote:

The OP asked very clearly, does he need a traditional synchronization
kernel object (event, he pointed out) for a simple 1 read thread, 1
write thread concept?

And the answer for this SIMPLE CASE, you don't.

And the simple answer is wrong. :)

Well, you will never be able to make this fail, and right or wrong,
you would have to admit it is more common place than not.

No offense intended, but your program contains a number of undesirable and
naive practices and assumptions. I'll point out the major ones and try to
avoid nitpicking.

------ CUT HERE: -------------

#include <stdio.h>
#include <windows.h>
#include <conio.h>
#include <process.h>

bool Abort = false;

void thread(void *p)
    while (!Abort) {

Given that Abort is not declared volatile, this while loop is dangerous
because it depends on the compiler being unable to optimize access to Abort
as I explained in a couple of other messages. If you're going to cheat and
use a bool like this, ALWAYS make it volatile; NEVER rely on getting lucky
with the loop body, which is what's happening here. Note also that in some
MP circles, NOT x86, even using volatile means subjecting the detection of
the value change to indefinite delay.

(If you truly understand why your code works under these special
conditions, which I described in earlier messages, I don't understand why
you felt the need to post an example of it to me.)


There is no reason to call _endthread here. If you call _endthread in a C++
thread, the stack won't be unwound, and local objects won't be destroyed.
For this reason, the "end thread" class of functions is nearly useless in
real programs. You should just fall off the end of the thread controlling
function or otherwise return normally from it. If you need to get out of a
thread in a nested call situation, disciplined used of exceptions can help
in C++, and in C, there's good ol' setjmp/longjmp.


void main(char argc, char *argv[])
    HANDLE h = (HANDLE)_beginthread(thread, 0, NULL );

The function _beginthread should never be used, because the thread handle
is closed automatically when threads started by _beginthread terminate.
This means you can't use the handles in wait functions. You should always
use _beginthreadex, and do read MSDN, because it talks about a lot of this.

    printf("-MAIN: Thread Started h: %d\n",h);

    printf("-MAIN: Press Escape To Exit\n");
    while (1) {
       if (_kbhit() && _getch() == 27) break;

    Abort = true;

    printf("-MAIN: Waiting for Thread Completion\n");
    if (WaitForSingleObject(h,INFINITE) != WAIT_OBJECT_0) {

This WFSO call is undefined due to the auto-deletion of the handle. (The
docs explicitly state that closing a handle that is the object of an active
wait is undefined.)

        printf("! Unexpected error\n");
    printf("-MAIN: Complete\n");
------ CUT HERE: -------------

I gave a comprehensive, concise (as
possible, given the complexity of the issue) reply to the OP
in my first message in this thread.

Which is all good and undisputed. But IMV, goes beyond the overall
scope of the OP question.

Answering the OP's question does not go beyond the scope of it. :) As I
said, it's complex, and it depends on machine architecture and compiler
vagaries. It's easy to write toy examples and say, "Look, this seemed to
work when I ran it on my computer" (lots of MSDN examples exhibit that
quality), but it's another thing to write useful programs that work
reliably in the real world. While I would never promote code such as you've
presented, I will use shortcuts to demonstrate other points, e.g. see this

In that message's last paragraph, I explained why one should NOT write code
like the example I just presented. I only did it to demonstrate an
unrelated point.

I don't get the emphasis on "1 reader, 1 writer" thread.

There is no contention issue to deal with (or at least at levels the
OP should be concerned with).

Still don't get the emphasis.

Doug Harrison
Visual C++ MVP

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