Embedded Linux poll() returns constantly

问题

I have a particular problem. Poll keeps returning when I know there is nothing to read.

So the setup it as follows, I have 2 File Descriptors which form part of a fd set that poll watches. One is for a Pin high to low change (GPIO). The other is for a proxy input. The problem occurs with the Proxy Input.

The order of processing is: start main functions; it will then poll; write data to proxy; poll will break; accept the data; send the data over SPI; receiving slave device, signals that is wants to send ack, by Dropping GPIO low; Poll() senses this drop and reacts; Infinite POLLINs :(

IF I have no timeout on the Poll function, the program works perfectly. The moment I include a timeout on the Poll. The Poll returns continuously. Not sure what I am doing wrong here.

     while(1)
         {
            memset((void*)fdset, 0, sizeof(fdset));
            fdset[0].fd = gpio_fd;
            fdset[0].events = POLLPRI; // POLLPRI - There is urgent data to read 

            fdset[1].fd = proxy_rx;
            fdset[1].events = POLLIN; // POLLIN   - There is data to read
            rc = poll(fdset, nfds, 1000);//POLL_TIMEOUT);

            if (rc < 0)     // Error
            {
                    printf("\npoll() failed/Interrupted!\n");
            }
            else if (rc == 0)       // Timeout occurred
            {
                  printf(" poll() timeout\n");
            }

            else {
                   if (fdset[1].revents & POLLIN)
                   {
                     printf("fdset[1].revents & POLLIN\n");
                    if( (resultR =read(fdset[1].fd,command_buf,10)<0) {                                   
                                    printf("Failed to read Data\n");

                            }
                    if (fdset[0].revents & POLLPRI)
                            //if( (gpio_fd != -1) && (FD_ISSET(gpio_fd, &err)))
                    {
                         lseek(fdset[0].fd, 0, SEEK_SET); // Read from the start of the file
                            len = read(fdset[0].fd, reader, 64);


                     }

So that is the gist of my code, sorry for untidiness, this interface gets some getting used to.

I have also used GDB and while debugging, I found that the GPIO descriptor was set with revents = 0x10 which means that and Error occured and that POLPRI also occured.

poll(2) doesn't empty the event queue In the above link something similar was addressed. But I do read all the time when ever I get POLLIN. It is a bit amazing, that this problem ONLY occurs when I include the timeout, if I replace the poll timeout with -1, it works perfectly.

Help Please.

回答1:

When poll fails (returning -1) you should do something with errno, perhaps thru perror; and your nfds (the second argument to poll) is not set, but it should be the constant 2.

Probably the GCC compiler would have given a warning, at least with all warnings enabled (-Wall), about nfds not being set.

^{(I'm guessing that nfds being uninitialized might be some "random" large value.... So the kernel might be polling other "random" file descriptors, those in your fdset after index 2...)}

BTW, you could strace your program. And using the fdset name is a bit confusing (it could refer to select(2)).

来源：https://stackoverflow.com/questions/16118668/embedded-linux-poll-returns-constantly

标签

Linux

embedded-linux