altio/epoll_input.c

This is altio/epoll_input.c (Listing 63-5, page 1362), an example from the book, The Linux Programming Interface.

The source code file is copyright 2022, Michael Kerrisk, and is licensed under the GNU General Public License, version 3.

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/* epoll_input.c

   Example of the use of the Linux epoll API.

   Usage: epoll_input file...

   This program opens all of the files named in its command-line arguments
   and monitors the resulting file descriptors for input events.

   This program is Linux (2.6 and later) specific.
*/
#include <sys/epoll.h>
#include <fcntl.h>
#include "tlpi_hdr.h"

#define MAX_BUF     1000        /* Maximum bytes fetched by a single read() */
#define MAX_EVENTS     5        /* Maximum number of events to be returned from
                                   a single epoll_wait() call */
int
main(int argc, char *argv[])
{
    int epfd, ready, fd, s, j, numOpenFds;
    struct epoll_event ev;
    struct epoll_event evlist[MAX_EVENTS];
    char buf[MAX_BUF];

    if (argc < 2 || strcmp(argv[1], "--help") == 0)
        usageErr("%s file...\n", argv[0]);

    epfd = epoll_create(argc - 1);
    if (epfd == -1)
        errExit("epoll_create");

    /* Open each file on command line, and add it to the "interest
       list" for the epoll instance */

    for (j = 1; j < argc; j++) {
        fd = open(argv[j], O_RDONLY);
        if (fd == -1)
            errExit("open");
        printf("Opened \"%s\" on fd %d\n", argv[j], fd);

        ev.events = EPOLLIN;            /* Only interested in input events */
        ev.data.fd = fd;
        if (epoll_ctl(epfd, EPOLL_CTL_ADD, fd, &ev) == -1)
            errExit("epoll_ctl");
    }

    numOpenFds = argc - 1;

    while (numOpenFds > 0) {

        /* Fetch up to MAX_EVENTS items from the ready list of the
           epoll instance */

        printf("About to epoll_wait()\n");
        ready = epoll_wait(epfd, evlist, MAX_EVENTS, -1);
        if (ready == -1) {
            if (errno == EINTR)
                continue;               /* Restart if interrupted by signal */
            else
                errExit("epoll_wait");
        }

        printf("Ready: %d\n", ready);

        /* Deal with returned list of events */

        for (j = 0; j < ready; j++) {
            printf("  fd=%d; events: %s%s%s\n", evlist[j].data.fd,
                    (evlist[j].events & EPOLLIN)  ? "EPOLLIN "  : "",
                    (evlist[j].events & EPOLLHUP) ? "EPOLLHUP " : "",
                    (evlist[j].events & EPOLLERR) ? "EPOLLERR " : "");

            if (evlist[j].events & EPOLLIN) {
                s = read(evlist[j].data.fd, buf, MAX_BUF);
                if (s == -1)
                    errExit("read");
                printf("    read %d bytes: %.*s\n", s, s, buf);

            } else if (evlist[j].events & (EPOLLHUP | EPOLLERR)) {

                /* After the epoll_wait(), EPOLLIN and EPOLLHUP may both have
                   been set. But we'll only get here, and thus close the file
                   descriptor, if EPOLLIN was not set. This ensures that all
                   outstanding input (possibly more than MAX_BUF bytes) is
                   consumed (by further loop iterations) before the file
                   descriptor is closed. */

                printf("    closing fd %d\n", evlist[j].data.fd);
                if (close(evlist[j].data.fd) == -1)
                    errExit("close");
                numOpenFds--;
            }
        }
    }

    printf("All file descriptors closed; bye\n");
    exit(EXIT_SUCCESS);
}

 

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