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Full Discussion: Signals and semaphores
Top Forums Programming Signals and semaphores Post 302450696 by Corona688 on Friday 3rd of September 2010 01:05:02 PM
Old 09-03-2010
Quote:
Originally Posted by _thomas
Thank you for your reply. I think I have found quicker and safer solution for problem.

I've removed all my code from signal handler and added variable increment.

Code:
void timer_handler(int signum)
{
    tick++;    
}

This isn't threadsafe. Variables aren't atomic and/or threadsafe on their own, that's why we need things like mutexes and semaphores.

All your solution amounts to is exactly what I suggested anyway: doing sem_post in the interrupt handler(increment the semaphore), and sem_wait inside the thread(decrement the semaphore). Treat the semaphore like a thread-safe integer that blocks the thing trying to decrement it if it goes below zero. You don't have to post and wait in both, it's not a mutex.
Quote:
I'm using SA_RESTART flag so if sem_wait() or sem_post() are interrupted by signal, they will be restarted. Now I have semaphores inside infinite loop inside main program and in threads. I don't think I should experience similar problem again.

What do you think?
You didn't expect them to fail before, either. You can't account for everything. Always check the return value of an important system call, it's not optional!
Last edited by Corona688; 09-03-2010 at 02:12 PM..
 

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LEARN ABOUT NETBSD

wakeonlan

WAKEONLAN(8)						    BSD System Manager's Manual 					      WAKEONLAN(8)

NAME

     wakeonlan -- send Wake on LAN frames to hosts on a local Ethernet network

SYNOPSIS

     wakeonlan [interface] lladdr [lladdr ...]

DESCRIPTION

     The wakeonlan program is used to send Wake on LAN (WoL) frames over a local Ethernet network to one or more hosts using their link layer
     (hardware) addresses.  WoL functionality is generally enabled in a machine's BIOS and can be used to power on machines from a remote system
     without having physical access to them.

     interface is an Ethernet interface of the local machine and is used to send the Wake on LAN frames over it.  If there is only one Ethernet
     device available that is up and running, then the interface argument can be omitted.  lladdr is the link layer address of the remote machine.
     This can be specified as the actual hardware address (six hexadecimal numbers separated by colons) or as a hostname entry in /etc/ethers.
     wakeonlan accepts multiple lladdr addresses.  Link layer addresses can be determined and set using ifconfig(8).

FILES

     /etc/ethers  Ethernet host name data base.

SEE ALSO

     ethers(5), ifconfig(8)

AUTHORS

     wakeonlan was written by Marc Balmer <marc@msys.ch>.

BSD
								   May 25, 2012 							       BSD
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