This is altio/epoll_flags_fork.c, an example to accompany the book, The Linux Programming Interface.

This file is not printed in the book; it is a supplementary file for Chapter 63.

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

In the listing below, the names of Linux system calls and C library functions are hyperlinked to manual pages from the Linux man-pages project, and the names of functions implemented in the book are hyperlinked to the implementations of those functions.


Download altio/epoll_flags_fork.c

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/* epoll_flags_fork.c

   A program to experiment with various epoll flags, in particular EPOLLET

   Usage: ./epoll_flags_fork [options] <FIFO> <num-children>

   Example usage, in this case to experiment with EPOLLEXCLUSIVE where five
   child processes each create their own epoll FD to which they all add the
   file descriptor for the read end of a FIFO:

        mkfifo p
        ./epoll_flags_fork -x p 5

   and then in another window, run the following command and type lines of

        cat > p

   To explore the difference when not using EPOLLEXCLUSIVE, repeat the
   above but run the program without the '-x' option:

        ./epoll_flags_fork p 5

   To explore the behavior where EPOLLET wakes up only one of multiple
   waiters on the same epoll FD, use the '-e' and '-s' options:

        ./epoll_flags_fork -es p 5
#include <sys/epoll.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#include <errno.h>
#include <unistd.h>
#include <string.h>

#define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); \
                        } while (0)

#define EPOLLEXCLUSIVE (1 << 28)
static void
usageError(char *pname)
    fprintf(stderr, "Usage: %s [-1eoprx] <FIFO> <num-children>\n",
    fprintf(stderr, "\t-s       Create one epoll FD before creating child "
    fprintf(stderr, "\t\t(By default, each child creates its own epoll FD "
            "after fork())\n");
    fprintf(stderr, "\t-e       Include EPOLLET flag\n");
    fprintf(stderr, "\t-x       Include EPOLLEXCLUSIVE flag\n");
    fprintf(stderr, "\t-o       Include EPOLLONESHOT flag\n");
    fprintf(stderr, "\t-p       Open FIFO individually in each child\n");
    fprintf(stderr, "\t\t(By default, each child inherits FD for FIFO opened "
            "by parent)\n");
    fprintf(stderr, "\t-r       Do a read() after epoll_wait() returns\n");
    fprintf(stderr, "\t-l       Children should loop, rather than "
            "calling epoll_wait() just once\n");
main(int argc, char *argv[])
    /* Parse command-line options and arguments */

    bool useOneEpollFD = false;
    bool readData = false;
    int eventsMask = EPOLLIN;
    bool openFifoInChild = false;
    bool useLoop = false;
    int opt;
    while ((opt = getopt(argc, argv, "eloprsx")) != -1) {
        switch (opt) {
        case 'e': eventsMask |= EPOLLET;        break;
        case 'o': eventsMask |= EPOLLONESHOT;   break;
        case 'x': eventsMask |= EPOLLEXCLUSIVE; break;
        case 'l': useLoop = true;               break;
        case 'p': openFifoInChild = true;       break;
        case 'r': readData = true;              break;
        case 's': useOneEpollFD = true;         break;
        default:  usageError(argv[0]);

    if (argc != optind + 2 || strcmp(argv[optind], "--help") == 0)

    char *fifoPath = argv[optind];
    int childMax = atoi(argv[optind + 1]);

    /* Either we open the FIFO once in the parent (and each child inherits
       the file descriptor from the parent, or each child opens the FIFO
       after fork() */

    int fd;
    if (!openFifoInChild) {
        fd = open(fifoPath, O_RDONLY | O_NONBLOCK);
        if (fd == -1)
        printf("Opened FIFO %s\n", fifoPath);

    /* Either we create the epoll FD once in the parent (and it is inherited by
       each child) and add the FIFO to the interest list of the epoll instance,
       or we perform these steps in each of the children after fork() */

    int epfd;
    struct epoll_event ev;
    if (useOneEpollFD) {
        printf("Creating single epoll FD and adding FIFO\n");
        epfd = epoll_create(2);
        if (epfd == -1)
            errExit("epoll_create"); = eventsMask;
        if (epoll_ctl(epfd, EPOLL_CTL_ADD, fd, &ev) == -1)


    /* Create child processes */

    for (int childNum = 0; childNum < childMax; childNum++) {
        switch (fork()) {
        case -1:

        case 0: /* Child */
            printf("Child %d: created\n", childNum);
            if (openFifoInChild) {
                fd = open(fifoPath, O_RDONLY | O_NONBLOCK);
                if (fd == -1)
                printf("Child %d: opened FIFO %s\n", childNum, fifoPath);

            if (!useOneEpollFD) {
                printf("Child %d: creating epoll FD and adding FIFO\n",
                epfd = epoll_create(2);
                if (epfd == -1)

       = eventsMask;
                if (epoll_ctl(epfd, EPOLL_CTL_ADD, fd, &ev) == -1)

            do {
                /* Wait on the epoll FD and print results */

                printf("Child %d: about to epoll_wait()\n", childNum);
                struct epoll_event rev;
                int numReady = epoll_wait(epfd, &rev, 1, -1);
                if (numReady == -1)
                printf("Child %d: epoll_wait() returned %d\n", childNum,

                /* If specified on command line, read data when the FIFO
                   becomes ready */

                if (readData) {
                    ssize_t nr;
                    char buf[50000];

                    nr = read(fd, buf, sizeof(buf));
                    if (nr == 0) {
                        printf("Child %d: read returned EOF\n", childNum);
                    } else if (nr > 0) {
                        printf("Child %d: read returned %zd bytes\n",
                                childNum, nr);
                    } else {
                        printf("Child %d: read failed: %s\n", childNum,
            } while (useLoop);

            printf("Child %d: terminating\n", childNum);



    for (int childNum = 0; childNum < childMax; childNum++)



Download altio/epoll_flags_fork.c

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