Communication with child process using pipes and epoll

问题

I'm writing an application which will start some processes (fork and exec) depending on users input and should inform user about every error in those processes (Print some internal ID + message written to stderr by the process). I'd also like to detect exiting processes. I'm facing the problem, that I cannot receive data after executing execl() command. Test data is received in epoll_wait loop, but process called by exec seems to not write anything to stderr nor even end (I don't know if I'm correct, but I expect pipe to be readable after it's writing end has been closed, when process exited). Some code reproducing my problem:

#include <unistd.h>
#include <sys/epoll.h>
#include <stdio.h>
#include <thread>
#include <stdlib.h>
#include <sys/wait.h>

thread_local int epoll;

int do_fork()
{
    int pip[2];
    pipe(pip);
    int pid = fork();
    if (pid == 0)
    {
        dup2(pip[1], STDERR_FILENO);
        close(pip[0]);  
        close(pip[1]);  
        write(STDERR_FILENO, "test data", 10);
        sleep(3);
        //exit(1);
        execl("tr", "tr", NULL); //this process will end immediately and write error on stderr
    }
    else
    {
        printf("PID %d\n", pid);
        close(pip[1]);
        epoll_event ev;
        ev.data.fd = pip[0];
        ev.events = EPOLLIN | EPOLLRDHUP | EPOLLHUP;
        printf("pipe %d\n", pip[0]);
        epoll_ctl(epoll, EPOLL_CTL_ADD, pip[0], &ev);
    }
}

int thread()
{
    epoll_event events[10];
    epoll = epoll_create1(0);

    epoll_event ev;
    ev.data.fd = STDIN_FILENO;
    ev.events = EPOLLIN;
    epoll_ctl(epoll, EPOLL_CTL_ADD, STDIN_FILENO, &ev);

    char buf[1000];
    while(true)
    {
        int r = epoll_wait(epoll, events, 10, -1);
        printf("r = %d\n", r);
        for (int i = 0; i < r; ++i)
        {
            if(events[i].data.fd == STDIN_FILENO)
            {
                int t = read(events[i].data.fd, buf, 1000);
                do_fork();
            }
            else
            {
                printf("event? %d\n", events[i].events);
                printf("pipe %d\n", events[i].data.fd);
                int t = read(events[i].data.fd, buf, 1000);
                printf("t == %d\n", t);
                if(t == -1)
                    printf("errno: %d\n", errno);
                if(events[i].events & EPOLLHUP || t == 0)
                {
                    epoll_ctl(epoll, EPOLL_CTL_DEL, events[i].data.fd, NULL);
                    if(close(events[i].data.fd) == -1)
                        printf("cannot close fd\n");
                }
            }
        }

    }
    return 0;
}

int main()
{
    std::thread t{thread};
    t.detach();
    while(true);
}

来源：https://stackoverflow.com/questions/37375047/communication-with-child-process-using-pipes-and-epoll