[lug] [OT] Threads & Socket Programming

[Scott Herod]

Ok, I'll start interlacing my responses. :;-)

Mr Viggy wrote:

> Someone else may be able to correct me, but I thought sockets in C 
> (C++) were non-blocking. 

I believe that it depends on how the file ( pipe, socket, etc. ) is 
opened.  Typically a flag specified in the open or socket function call 
sets the behavior.

>
> David Morris wrote:

<snip>

>>
>> Everything works fine, except that if I write too many
>> packets to the socket too fast, I loose data packets (never
>> received by the client side).  I can solve this problem by
>> putting in a sleep(1) statement, but that kind of defeats
>> the purpose of having a thread to handle writing data to the
>> socket from the queue.
>
As an aside, it's my impression is that sleep and usleep are BAD THINGS 
in standard linux kernels (at least through 2.2.*) because the 
granularity of context switching is about 10 msec.  (This caused me all 
kinds of nightmares a year or so ago. )  I've not used any of the 
real-time or soft real-time extensions nor tested with any of the 2.4.* 
kernels.  BTW, in some code that I am looking at to recall this I have 
the following few lines.

    // select seems to work better with gdb than usleep
    // usleep( timeoutInMS/1000 );
    struct timeval tv;
    tv.tv_sec = 0; tv.tv_usec = timeoutInMS*1000;
    select( 1, NULL, NULL, NULL, &tv );

(No idea why I said that.)

>>
>> Anyone know why I would be having this problem?  Do I need
>> to call some function to poll the socket and find out if
>> data can be sent out yet, or if I have to wait (thought such
>> blocking was handled automatically).
>>
>> Thanks for any input you can give on solutions, or other
>> lists to ask.
>>
>> --David
>>
My impression, David, is that this is the reading code's problem.  While 
reads can be set to block or not, I almost always wrap a select call 
around one as well so that I can, in a sense, poll.  Check to see how 
the reading code is opening the socket and whether it is doing any 
polling.  If you want, reply off-line.  I've got a pretty good 
collection of sample code for reading/writing using various connections.

Scott
herod at dimensional.com