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    <p>I posted this answer to illustrate a quite different approach to the "obvious" one, in the hope that someone discovers it to be exactly what they need. I didn't expect it to be selected as the best answer! Treat this solution with caution, because there are potential dangers and concurrency issues...</p> <p>You can use the <a href="http://linux.die.net/man/2/setitimer" rel="nofollow noreferrer">setitimer()</a> system call to have a SIGALRM (alarm signal) sent to your program after a specified number of milliseconds. Signals are asynchronous events (a bit like messages) that interrupt the executing program to let a signal handler run.</p> <p>A set of default signal handlers are installed by the OS when your program begins, but you can install a custom signal handler using <a href="http://linux.die.net/man/2/sigaction" rel="nofollow noreferrer">sigaction()</a>.</p> <p>So all you need is a single thread; use global variables so that the signal handler can access the necessary information and send off a new packet or repeat the last packet as appropriate.</p> <p>Here's an example for your benefit:</p> <pre><code>#include &lt;stdio.h&gt; #include &lt;signal.h&gt; #include &lt;sys/time.h&gt; int ticker = 0; void timerTick(int dummy) { printf("The value of ticker is: %d\n", ticker); } int main() { int i; struct sigaction action; struct itimerval time; //Here is where we specify the SIGALRM handler action.sa_handler = &amp;timerTick; sigemptyset(&amp;action.sa_mask); action.sa_flags = 0; //Register the handler for SIGALRM sigaction(SIGALRM, &amp;action, NULL); time.it_interval.tv_sec = 1; //Timing interval in seconds time.it_interval.tv_usec = 000000; //and microseconds time.it_value.tv_sec = 0; //Initial timer value in seconds time.it_value.tv_usec = 1; //and microseconds //Set off the timer setitimer(ITIMER_REAL, &amp;time, NULL); //Be busy while(1) for(ticker = 0; ticker &lt; 1000; ticker++) for(i = 0; i &lt; 60000000; i++) ; } </code></pre>
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    1. COI thought that the general advice was to keep signal handlers very simple? I/O (printf's and sending UDP packets) should certainly be avoided?
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      1. USThomas Padron-McCarthy
    2. COYes, because there can be strange interplay if the handler runs while the main program is doing I/O itself - and because of portability issues. But as long as it *works* on the target system, that's not a big concern.
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    3. COOh, and you can always temporarily block signals in the main program if you're about to do something risky.
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