threads and signals question

From: lparrab (lparrab_at_[hidden])
Date: 2002-08-28 17:39:57

Hi all,  
  
I guess my question is not specific to Boost.Thread, but to 
threading in general (I'm new at it)... still hope you can help me 
out, either directly or by redirecting me to the right place (links 
are very much appreciated) 
 
I'm starting a multi-threaded program using Boost.Thread library: 
it is a "server", in which a thread loops around a "select" to 
accept new connections and see when a socket has data to be read, in 
which case puts the fds in queue from which other threads are taking 
them out to read the data. 
The problem I have, is that when a "ReceiverThread" finishes reading 
the data from the socket, it puts the fd back in the "all 
connections list", but by this time the other ("main")thread already 
copied the fds from the list and got into the next "select" loop, 
which means the socket from which the "receiver" just finished 
reading, will not be "selected" (monitored) untill the next select 
loop, which will not be untill a) a new connection comes or b)one of 
the other connected sockets gets some data that takes makes the main 
thread return from select. 
 
reading the select man, I thought I could just use a signal to force 
select to return when the "receiver" has put the fd back in the "all 
connection list": 
 
the threads are now executing something like: 
 
main_thread 
{ 
*put all fds from "all sockets" list into a fdset 
*make a select on the fdset (block till something interesting 
happens) 
*take the fds with data out of "all sockets" and put them into    	
"sockets with data" 
*loop 
} 
 
receiver{ 
*get a fd from "sockets with data" (if nothing there blocks till the 
main_thread sends us a fd through the queue) 
*read from fd 
*put fd back into "all sockets" (problem: main_thread may be (is!) 
already into the select) 
*raise(SIGUSR1)  (to force select to return and enter the loop 
again, but this time with our fd) 
}  
 
the signal handler for SIGUSR1 does nothing, it just serves to force 
select to return, but 
the code is not really working as I expected, and I realized that 
the problem is that each thread has its own pid, so it seems raise 
is actually sending a signal to the wrong thread....  
 
as I said, I am new to mulithreading, but I thought all threads run 
in a process, which means, they all have the same pid. 
->if I first find out the "pid" of the main_thread and change  
raise(SIGUSR1) to 
kill(pid,SIGUSR1) it works ok. 
 
so my question: why does each thread have a own pid?? how can then I 
send signals from one thread to another?? 
I discovered there is a "pthread_kill" function to send signals to 
specific threads, the problem is I only have boost::thread objects, 
no pthread_t structures. 
 
or am I talking nonsense and shouldnt be using signals with 
Boost.Thread? if so, how do I solve the select problem?? 
 
thanx! 
 
luis