main.c

#include <arpa/inet.h>
#include <assert.h>
#include <errno.h>
#include <netinet/in.h>
#include <stdio.h>
#include <string.h>
#include <sys/epoll.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <unistd.h>

int master(int sh) {
  int s;
  int err;
  struct sockaddr_in addr;
  struct msghdr msg;
  struct cmsghdr* cmsg;
  struct iovec iov;
  char scratch[256];

  /* Create UDP socket */
  s = socket(AF_INET, SOCK_DGRAM, 0);
  assert(s != -1);

  /* Bind it to random port */
  memset(&addr, 0, sizeof(addr));
  addr.sin_family = AF_INET;
  addr.sin_port = 0;
  addr.sin_addr.s_addr = INADDR_ANY;
  err = bind(s, (struct sockaddr*) &addr, sizeof(addr));
  assert(err == 0);

  /* Send socket via SCM_RIGHTS */
  iov.iov_base = "X";
  iov.iov_len = 1;

  msg.msg_name = NULL;
  msg.msg_namelen = 0;
  msg.msg_iov = &iov;
  msg.msg_iovlen = 1;
  msg.msg_flags = 0;

  msg.msg_control = (void*) scratch;
  msg.msg_controllen = CMSG_LEN(sizeof(s));

  cmsg = CMSG_FIRSTHDR(&msg);
  cmsg->cmsg_level = SOL_SOCKET;
  cmsg->cmsg_type = SCM_RIGHTS;
  cmsg->cmsg_len = msg.msg_controllen;
  memcpy(CMSG_DATA(cmsg), &s, sizeof(s));

  do
    err = sendmsg(sh, &msg, 0);
  while (err < 0 && (errno == EINTR || errno == EAGAIN));
  assert(err == iov.iov_len);

  /* Uncomment next line to fix EPOLLOUT in worker */
  /* close(s); */
  for (;;) {}

  return 0;
}


int worker(int sh) {
  int ep;
  int err;
  int received;
  struct msghdr msg;
  struct cmsghdr* cmsg;
  struct iovec iov;
  struct epoll_event ev;
  char buf[256];
  char space[64];

  /* Receive socket via SCM_RIGHTS */
  iov.iov_base = buf;
  iov.iov_len = sizeof(buf);

  msg.msg_flags = 0;
  msg.msg_iov = &iov;
  msg.msg_iovlen = 1;
  msg.msg_name = NULL;
  msg.msg_namelen = 0;
  msg.msg_controllen = sizeof(space);
  msg.msg_control = space;

  do
    err = recvmsg(sh, &msg, 0);
  while (err < 0 && (errno == EINTR || errno == EAGAIN));

  cmsg = CMSG_FIRSTHDR(&msg);
  assert(cmsg != NULL);
  assert(cmsg->cmsg_type == SCM_RIGHTS);
  assert(cmsg->cmsg_len >= sizeof(received));
  received = *(int*) CMSG_DATA(cmsg);

  /* Initialize epoll */
  ep = epoll_create(1024);
  assert(ep != -1);

  /* Add received socket to epoll to poll for write events */
  memset(&ev, 0, sizeof(ev));
  ev.events = EPOLLOUT;
  ev.data.fd = received;
  err = epoll_ctl(ep, EPOLL_CTL_ADD, received, &ev);
  assert(err == 0);

  /* Close socket */
  close(received);

  /* Wait for events */
  err = epoll_wait(ep, &ev, 1, -1);
  assert(err == 1);

  fprintf(stdout, "Voila, got event on closed fd: %d\n", ev.data.fd);

  close(ep);

  return 0;
}


int main() {
  int err;
  int sh[2];

  err = socketpair(AF_UNIX, SOCK_STREAM, 0, sh);
  assert(err == 0);

  err = fork();
  assert(err != -1);

  if (err != 0) {
    close(sh[1]);
    err = master(sh[0]);
    close(sh[0]);
  } else {
    close(sh[0]);
    err = worker(sh[1]);
    close(sh[1]);
  }

  return err;
}

Makefile

CFLAGS ?=
CFLAGS += -g -Wall

all: main

%.o: %.c
	$(CC) $(CFLAGS) -c $< -o $@

main: main.o
	$(CC) $(CFLAGS) $< -o $@

.PHONY: all