Re: Qt on Mac (was: Getting ready to put out a 4.6 release candidate)

[Daniel J S. <dan...@ie...>]

On 01/05/2012 05:10 AM, Mojca Miklavec wrote:
> On Thu, Jan 5, 2012 at 08:35, Daniel J Sebald wrote:
>>
>>   If it is
>> the former, then perhaps Qt users should not be using fork, but QThread.
>
> As already said, I don't know anything about forking, but it is also
> not clear to me why forking or creating a new binary is needed at all.

The reason is (as from what I read about Qt and my limited 
understanding) QApplication has to be the only event loop in a process. 
  It cannot run as an event loop along with a process that is also 
running an event loop.  In other words, gnuplot (an event loop) can't 
coexist with QApplication GUI (an event loop because it is processing 
button/keyboard/other events).

A fork is simply a means of creating another process, at least for most 
high level applications.  (Low level, there are other aspects of the 
duplication from fork that can be utilized, but not important here.)


> Here is an example of communication between GUI and Threads:
>
> http://developer.qt.nokia.com/doc/qt-4.8/thread-basics.html#example-3-clock

Those are threads within the Qt application, which isn't exactly the 
same as the OS threads commonly thought of.  The example you show has a 
main() routine for which QApplication is the only event loop.  The other 
elements are QObjects and such.  The issue is:

1) QApplication must be the only event loop, i.e., being able to run a 
QApplication GUI and gnuplot simultaneously.  (Hence the need for 
fork...why Qt doesn't handle this is what I asked before.)

2) Dealing with the Mac OS X limitation that CoreFoundation APIs (which 
Qt uses) cannot appear in a child process any longer.  (Hence the need 
for fork + exec and a separate binary.)

Dan

>
> Mojca
>
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-- 

Dan Sebald
email: daniel(DOT)sebald(AT)ieee(DOT)org
URL: http://www(DOT)dansebald(DOT)com