Question

I’ve done quite a bit of programming on Windows but now I have to write my first Linux app.

I need to talk to a hardware device using UDP. I have to send 60 packets a second with a size of 40 bytes. If I send less than 60 packets within 1 second, bad things will happen. The data for the packets may take a while to generate. But if the data isn’t ready to send out on the wire, it’s ok to send the same data that was sent out last time. The computer is a command-line only setup and will only run this program.

I don’t know much about Linux so I was hoping to get a general idea how you might set up an app to meet these requirements. I was hoping for an answer like:

Make 2 threads, one for sending packets and the other for the calculations.

But I’m not sure it’s that simple (maybe it is). Maybe it would be more reliable to make some sort of daemon that just sent out packets from shared memory or something and then have another app do the calculations? If it is some multiple process solution, what communication mechanism would you recommend? Is there some way I can give my app more priority than normal or something similar?

PS: The more bulletproof the better!

Answer 1

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…

You can use the setitimer() 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.

A set of default signal handlers are installed by the OS when your program begins, but you can install a custom signal handler using sigaction().

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.

Here’s an example for your benefit:

#include <stdio.h> #include <signal.h> #include <sys/time.h> 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 = &timerTick;     sigemptyset(&action.sa_mask);     action.sa_flags = 0;      //Register the handler for SIGALRM     sigaction(SIGALRM, &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, &time, NULL);      //Be busy     while(1)         for(ticker = 0; ticker < 1000; ticker++)             for(i = 0; i < 60000000; i++)                 ; }