Here BLOCKING does not mean BUSYWAITING i.e. if any process waiting for an event on a file descriptor, it will go to sleep and will be added to wait queue by kernel. It will be interrupted by kernel if any event occur otherwise there will be timeout(passed while calling POLL(), -1 as timeout value means infinite waiting). POLL() is generally used in network programming and gives very good performance, as there are no of sockets to watch there. and we can watch them all by a single call without blocking to each other .POLL() and SELECT() work in same manner, but POLL() is easy to use.
Given below is the sample program describes how exactly POLL() works. Compile this program and execute, it will wait for some event on STDIN FD, try to enter some text and press enter to see o/p, if you done enter any data it will be time out.
#include <stdio.h>
#include <sys/time.h> #include <sys/types.h> #include <unistd.h> #include <poll.h>
int main(void) {
struct pollfd fds[1]; int timeout_msecs = 10000;/*10 secs*/ int ret;
fds[0].fd = 0; /*Adding fd(to be watched ) to FDLIST, here we are adding fd=0, i.e. STDIN to be monitored*/
fds[0].events = POLLIN; /*Adding an event to be watched for this added fd, here we are adding POLLIN event that means this fd will be monitored if it receive any data*/
ret = poll(fds, 1, -1);
if (fds[0].revents && POLLIN) { /*once poll() returns we need to check revent bit field for testing if any event occurred */ printf(“\n revents in fd[0] = 0x%x “, fds[0].revents); }
if (ret > 0) {
printf(“Data is available now.\n”); }
else if (ret == -1)
perror(“poll()”);
else
printf(“TIME OUT…No data within 10 seconds.\n”);
return 0;
}
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