[LC++]Question on pthreads

[David Filion]

Jan Pfeifer wrote:
> I think the easiest way would be to read user input from a separate 
> thread, and let the "main" thread only wait for the others. If your 
> polling thread finishes, the "main" thread could kill/cancel the thread 
> reading user input.
> 
> I was also wondering if getchar() is interrupted by signals, returning 
> "EINTR" ... If it is your exiting thread could signal the "main" thread. 
> Use something like SIGUSR and create some kind of flag to check for 
> that. If getchar() is not interrupted, try using read(2) directly.
> 
> Let me code the idea:
> * Disclaimer: havent' tested the code, nor considered race conditions, 
> such as the thread endind before the call to the function. *
> 
> bool ops = false;
> 
> void sigusr_handler( int i )
> { ops = true; }
> 
> char our_read_char()
> {
>      char c;
>      int ret;
>      while (true) {
>        ret = read( 0, &c, 1 );
>        if ( ret == 1 ) break;
>        if ( ret == -1 && errno = EINTR && ops )
>          {
>             ops = false;
>             throw thread_ended_exception();
>          }
>        if ( ret == -1 && errno != EINTR )
>             throw io_error_exception();
>      }
>    return c;
> }
> 
> best luck :)
> 
> jan
> 
> 
> Krishna Monian wrote:
> 
>> I still have a problem though. After creating the
>> thread in my main function, this is part of the code
>> that follows.
>>
>> while(getchar() != '\n') {}
>>
>> So basically I am waiting till the user hits the enter
>> key. As soon as the user does this, my program will
>> cleanup and quit. The thread that I spawn also does
>> some sort of polling operation.
>>
>> If I use pthread_join in main() then the program
>> doesn't terminate when the user hits return (obviously
>> it won't, since it has joined the other thread).
>>
>> How do I go about achieving this?
>>
>> Thanks
>> Krishna
>>
>>
>> --- Jack Lloyd <lloyd at acm.jhu.edu> wrote:
>>  
>>
>>> Yes. If the function created by pthread_create
>>> returns a value, then when you
>>> call pthread_join, it will return this status to
>>> you. In this way you can
>>> easily exit the thread and return a status code
>>> signaling the rest of your
>>> program to abend. That's why your thread start
>>> routines must return a void*,
>>> that's the status code.
>>>
>>> If you want to return an integer or some such,
>>> remember to dynamically allocate
>>> it and return the pointer -- if you return
>>> &status_code, it will go away once
>>> the thread exits. Obvious, but sometimes it's easy
>>> to forget (especially with
>>> pthreads). And, of course, free it in your main
>>> thread after you pthread_join
>>> the other thread.
>>>
>>> Since this when to the C++ list and not the C list,
>>> I'll also remind you that
>>> you can't (safely) throw an exception in a thread
>>> and have it be caught outside
>>> that thread. It could be done, but nobody requires
>>> it, and I'm sure it would be
>>> very difficult to implement, so nobody does. Just in
>>> case you were thinking
>>> about doing that to signal errors. :)
>>>
>>> -Jack
>>>
>>> On Wed, Jul 14, 2004 at 02:11:06PM -0700, Krishna
>>> Monian wrote:
>>>   
>>>
>>>> Hi, I have a pthread that is spawned from my main
>>>>     
>>>
>>> function
>>>   
>>>
>>>> and then the main function just waits until the
>>>>     
>>>
>>> user
>>>   
>>>
>>>> hits enter. Now I want the program to terminate if
>>>>     
>>>
>>> a
>>>   
>>>
>>>> something unexpected in the thread spawned. How
>>>> exactly do I go about doing this?
>>>>
>>>> Even better would be if the thread could return a
>>>> value to the main function. Is this possible?
>>>>
>>>> Thanks
>>>> Krishna
>>>>
>>>>
>>>>        
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What if you poll/select in your main instead of just waiting for a read? 
  When the poll/select times out, you can check for other conditions, 
such as a flag from a thread, then loop back to the poll/select and 
start the cycle over again.

David Filion