[SOLVED] In C, select() call does not wait

archieval · 07-22-2010, 01:56 AM

Hello everyone I have this program,

Code:

	int fd;
	fd_set rd_set;
        struct timeval to;
        char data[128];

        to.tv_sec = 5;
        to.tv_usec = 50;

        fd = open("/dev/misc/gpio", O_RDWR);

        while (1)
        {
            FD_ZERO(&rd_set);
            FD_SET(fd, &rd_set);

            s32ret = select(fd+1, &rd_set, NULL, NULL, &to);
            if (s32ret < 0)
            {
                printf("select() error\n");
		break;
            }
            else if (s32ret == 0)
            {
		printf("select() timeout\n");
                continue;
            }
            else
            {
                if (FD_ISSET(fd, &rd_set))
                {
	            /* do my stuff */
		}
	    }
        }

My problem is after the first 5 seconds of waiting, the timeout values does not take effect anymore on the select() call. There will be a tight loop on the else-if part. Sometimes the select() is successful but on other times it returns zero without even waiting. What could be the problem here?

jf.argentino · 07-22-2010, 02:57 AM

If you're under linux, "select" modify the timeout struct... And it's said in the manual (man select)

archieval · 07-22-2010, 03:32 AM

Yes in Linux. I didn't noticed that in the manpage. So timeout is undefined after a select() call. Thanks!

jf.argentino · 07-22-2010, 05:28 AM

Quote:

So timeout is undefined after a select() call

If I well remember it contains the remaining time, so 0 if timeout has expired, and "timeout - (T1 - T0)" where T0 is the date when you call "select", T1 the date when you get out of "select"