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POC: Writing a proper time-out
 

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  1. POC: Writing a proper time-out
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    <p>Upon closely scouring through resources, I'm still not entirely sure how to write a proper and usable timer function in C. I am not working with threads (or parallelizable code). I simply want to write a stopwatch function that I can use to trigger a bit of code after a small amount of time has passed.</p> <p>This is a very common use of a timer, in the situation of "time-out", where I have a client-server set up where the client is sending some data (UDP style with sendto(...) and recvfrom(...)). I have written my system so that the client sends a chunk of data in a packet struct I have defined, and the server processes it via CRC then sends back an acknowledgement packet (ACK) that the msg was received uncorrupted. However, I want to implement a time-out, where if the client does not receive an ACK in a certain period of time, the client resends the data chunk (of course the server is rigged to check for duplicates). I want to nest this bit of timer code in the client, and for some reason do not think this should be so difficult.</p> <p>I have dug up old signal handling code from work I had done long ago, as this seems to be the only way I commonly see mentioned as a solution, can someone please guide me as to how I can use the following signal handling code to not just receive a timed signal but trigger an action of some sort. Conceptually, I feel it would be: "send data, start timer, after timer expires execute a resend, reset timer...repeat until that ACK received". Better yet, would be an easier way of writing a timer function, but it doesn't look like there's much hope for that given C is a low-level language....</p> <pre><code>#include &lt;sys/time.h&gt; #include &lt;errno.h&gt; #include &lt;stdio.h&gt; #include &lt;signal.h&gt; extern char *strsignal(int sig); void timer_handler(int a) { // handle signal printf("&gt;&gt;&gt;&gt; signal caught\n"); printf("&gt;&gt;&gt;&gt; int parameter = %s\n", (char*) strsignal(a)); } int main(int argc, char* argv[]) { int retval; struct itimerval timerValue; struct itimerval oldTimerValue; struct sigaction action; action.sa_handler = &amp;timer_handler; action.sa_flags = SA_NODEFER; // initialize timer parameters: expires in 5 seconds timerValue.it_interval.tv_sec = 5; timerValue.it_interval.tv_usec = 0; timerValue.it_value.tv_sec = 5; timerValue.it_value.tv_usec = 0; // install signal handler to catch SIGALRM //signal(SIGALRM, timer_handler); sigaction(SIGALRM, &amp;action, NULL); retval = setitimer(ITIMER_REAL, &amp;timerValue, &amp;oldTimerValue); if (-1 == retval) perror("Could not set timer"); while(1); return 0; } </code></pre>
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    1. COYou should look at the [`select`](http://linux.die.net/man/2/select) function, which lets you wait for data to be received at a socket, as well as specifying a timeout value. Does that sound like what you want?
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    2. COThanks @Xymostech. Much appreciated, and I have credited you in my "what did I finally do" answer since you were the one to point out the use of "select".
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