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Proper way of polling a device

Hi!
I have to read data from serial port, and so I have implement some kind for data polling rutene for the ttyS0. I know how to check if data has arrived in buffer, and I know how to read that data, but how do I write the data polling rutene to be as low CPU resource consuming as possible?

If I write a while loop and an ioctl in it to check if data has arrived, than this would probably consume almost all CPU resources. Plus I have no way of checking for a timeout ( in case the device isn't responding ). I could use the signal system call, but I don't know how it's suitable for data polling, although the man page says that it is.

Can anyone gime me some points how I should write my data polling rutene?

George

It's possible to have your program sleep completely (i.e. not using any CPU) until one of the control-lines of the serial port changes state from low-->high or high-->low (or a signal has arrived). So if you can use this in your program, you don't have to poll at all.

Assuming fd is a valid file descriptor that has /dev/ttyS0 (or another serial port) open for reading, you can do:

Code:

#include <stdio.h>

#include <stdlib.h>

#include <termios.h>

#include <sys/ioctl.h>

#include <sys/file.h>

#include <linux/serial.h>



/* .... */



int mask



/* We want to wait until either one of the following control lines changes state: */

mask = TIOCM_CAR | TIOCM_DSR | TIOCM_CTS | TIOCM_RNG;



/* The ioctl() call below sleeps and does not return until one of

 * the control lines metioned in mask changes state.

 * or a signal was received, or an error has ocurred.

 */

if (ioctl(fd, TIOCMIWAIT, mask) == -1) {

    if (errno == EINTR) {

        /*

        * Woken up by a signal, do something, maybe just

        * resume waiting again.

        */

    } else {

        /* Other error, report it and exit */

        perror("ioctl - TIOCMIWAIT");

        exit(1);

    }

}



/* Once we get here, one of the control lines in 'mask' has changed state */

/* Act accordingly from here */

Hope you can use this for your purpose.

Check out the functions cfmakeraw() and tcsetattr(). These should allow you to fine-tune the timing parameters of the serial line. I usually do serial communication in a subthread. This allows me to minimize scheduling latency in case the exact time of data arrival is important.

If you like to wait and than to apply the loop again ,so put the sleep() function somwere in the loop and it's ok.
In the brackets use a number to set the number of seconds until the loop continue/start again.
Example:

#include <stdio.h>
#include <stdlib.h>
#include <sys/time.h>
int main()
{
struct timeval tv;
gettimeofday(&tv,NULL);
tv.tv_usec=(tv.tv_usec+500)/1000;
printf ("%u%u\n",tv.tv_sec,tv.tv_usec);
sleep(3); /* In this line the program will stop for 3 seconds and than it wil continue*/
gettimeofday(&tv,NULL);
tv.tv_usec=(tv.tv_usec+500)/1000;
printf ("%u%u\n",tv.tv_sec,tv.tv_usec);
exit(0);
}

How to STOP the thing

I am working on a kde4-plasma applet that monitors a modem for telephony usage. So I have this very loop, lifted from xingd, waiting for rings. Works like a charm.

Problem is, when plasma or the test applet viewer want to close the program, this wait thread is still running. No very nice behavior indeed.

I tried placing a simple break in the EINTR code. Nothing changed.

xringd had set a lot of singals with handlers and such. Is this what I need to do? I do not need a specific handler as such if I can simply test the signal that triggered the EINTR. Question is was there such a signal when the program was closed, objects destroyed, or need I set it?