第2个参数= 0的eventfd_write（）不起作用？

Question

when eventfd_write() is called with 2nd argument=0, epoll_wait() never returns, but when the argument is set to 1. epoll_wait() returns.

Here is how I reproduce: ./bug 0

It never returns.

./bug 1

It returns.

Here is the code:

#include <sys/epoll.h>
#include <sys/eventfd.h>
#include <assert.h>
#include <cstdlib>
#include <iostream>

int value = 0;
int efd = 0;

void* start(void* p) {
    std::cout << __PRETTY_FUNCTION__ << ": going to sleep for 5 sec" << std::endl;
    sleep(5);
    std::cout << __PRETTY_FUNCTION__ << ": going to call eventfd_write() with value=" << value << std::endl;
    const int rc = eventfd_write(efd, value);
    assert(0 == rc);
    return NULL;
}

int main(int argc, char** argv) {
    const int epFD = epoll_create1(0);
    assert(-1 != epFD);

    efd = eventfd(0, 0);
    assert(-1 != efd);

    struct epoll_event event;
    event.data.fd = efd;
    event.events = EPOLLIN;
    epoll_ctl(epFD, EPOLL_CTL_ADD, efd, &event);

    value = strtoul(argv[1], NULL, 10);

    const uint32_t nEvents = 2;
    struct epoll_event events[nEvents];

    pthread_t threadID;
    const int rc = pthread_create(&threadID, NULL, &start, NULL);
    assert(0 == rc);

    sleep(1);

    std::cout << __PRETTY_FUNCTION__ << ": going to wait for event" << std::endl;

    int n = epoll_wait(epFD, events, nEvents, -1);
    assert(n > 0);

    std::cout << "okay" << std::endl;

    return 0;
}

Here is how I compiled:

g++ -Wall bug.cpp -o bug -O3 -lpthread

Here is my glibc version:

$ rpm -qa | grep glibc
glibc-common-2.12-1.107.el6.x86_64
glibc-static-2.12-1.107.el6.x86_64
glibc-headers-2.12-1.107.el6.x86_64
glibc-2.12-1.107.el6.x86_64
glibc-devel-2.12-1.107.el6.x86_64

Here is my g++ version:

$ g++ --version
g++ (GCC) 4.6.2 20111027 (Red Hat 4.6.2-1)
Copyright (C) 2011 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

Any idea?

Answer 1

Short answer: Read the documentation

Longer answer: "A write(2) call adds the 8-byte integer value supplied in its buffer to the counter"

"The file descriptor is readable (the select(2) readfds argument; the poll(2) POLLIN flag) if the counter has a value greater than 0."

So, writing 0 doesn't add anything to the counter, and a value of 0 means that the file descriptor being waited on is not ready. Any non-zero value should work (except 0xffffffff which is reserved).