Re: dup2

From:

Jorgen Grahn <grahn+nntp@snipabacken.se>

Newsgroups:

comp.lang.c++

Date:

25 Apr 2014 06:33:56 GMT

Message-ID:

<slrnllk0ej.hu4.grahn+nntp@frailea.sa.invalid>

On Thu, 2014-04-24, David Brown wrote:

On 24/04/14 17:16, Jorgen Grahn wrote:

On Thu, 2014-04-24, David Brown wrote:

On 24/04/14 11:43, Jorgen Grahn wrote:

....

     void foo()
     {
         Lock lock(some_mutex);
          do_stuff_that_needs_locking();
     }

....

It is possible for things to be re-arranged, however. If you had this:

int protectedData;
static void do_stuff_that_needs_locking() {
    int x = 0;
    for (int i = 0; i < 1000; i++) {
        x += i;
    }
    protectedData = x;
}

Then the compiler /could/ generate this:

void foo() {
    int x = 0;
    for (int i = 0; i < 1000; i++) {
        x += i;
    }
    Lock lock(some_mutex);
    protectedData = x;
    // Destruct lock
}

Sure. My brain files all that under "as if", especially in C++98's
memory model.

....

Yes, I agree with you - and I agree it is all "as if". I really wanted
to point out that "as if" allows things to be split up and moved around
in the manner shown above - that catches many people out. (It will not
normally affect the correctness of your code, unless you are doing
something odd and haven't taken appropriate measures to control the
order of the code, but it can be a surprise during debugging or
examining the object code.)

True. And your example above /did/ frighten me for a few seconds.
Then I was relieved to see that it fit in my mental model after all,
which was nice.

/Jorgen

--
// Jorgen Grahn <grahn@ Oo o. . .
\X/ snipabacken.se> O o .