Question

I have this application which is a daemon program that detects and monitors devices. It accepts arguments which then prints what devices are available. e.g.

 ./udevmon -s //prints the devices that are connected to my server.

Sample Output

 Device: /dev/ttyUSB0  subsystem: tty

Now when I run it again to check what devices are available, typing ./udevmon -s again it creates a second instace of ./udevmon with different process id. When I type ./udevmon without arguments it creates a new instance again with different process id so a total of 3 processrs of ./udevmon now. In time this will make my system slow because I need to run ./udevmon many times.

How can I run my application so it only creates a single instance. e.g restarting it when i type ./udevmon -s or ./udevmon again?

Here’s sample code.

int main (int argc, char *argv[])
{    
    mon_init();      // initialize device monitor
    scan_init();     // initialize device scan

    //Fork the Parent Process
    pid = fork();
    if (pid < 0) { exit(EXIT_FAILURE); }

    //We got a good pid, Close the Parent Process
    if (pid > 0) { exit(EXIT_SUCCESS); }

    //Change File Mask
    umask(0);

    //Create a new Signature Id for our child
    sid = setsid();
    if (sid < 0) { exit(EXIT_FAILURE); }

    //Change Directory
    //If we cant find the directory we exit with failure.
    if ((chdir("/")) < 0) { exit(EXIT_FAILURE); }

    while(( c=getopt(argc, argv,"s")) != -1) {
        switch(c) {
            case 's': scan_run(); break;
            default: printf("wrong command\n");
        }
    }

    //Main Process
    while(1) {
       start_mon();
    }
    udev_unref(udev);
    return 0;       
}

Answer 1

Run your application under the following wrap instead:

killall -KILL udevmon &> /dev/null
./udevmon <ARG>

You may kill it softly using following script doing the same as above with some notifications:

#!/bin/bash

exist=`ps -e | grep udevmon | wc -l`

if [ "$exist" == "0" ] ; then
# there is no instance. run it
    echo "first run"
    ./udevmon -s
else
# kill old and run again
    pid=`ps -e | grep udevmon | awk '{print $1;}'`
    if [ "$pid" != "" ] ; then
        kill $pid
        echo "kill and run"
        ./udevmon -s
    else
        echo "unable to find pid!"
    fi
fi