Reference

vpoll(3)

generate synthetic events for poll/select/ppoll/pselect/epoll

NAME

vpoll_create, vpoll_ctl, vpoll_close - generate synthetic events for poll/select/ppoll/pselect/epoll

SYNOPSIS

#include *vpoll.h*

int vpoll_create(uint32_t init_events, int flags);

int vpoll_ctl(int fd, int op, uint32_t events);

int vpoll_close(int fd);

DESCRIPTION

This library permits one to create a vpoll file descriptor “vpollfd” that can be used in poll/select/ppoll/pselect/epoll(2) system calls. The events reported by a can be controlled by vpoll_ctl. vpoll encodes the events using the same flags EPOLL* defined in epoll_ctl(2).

The library uses the vpoll extension for eventfd if the kernel provides it otherwise it (partially) emulates the feature using socketpair(2). The emulation supports only EPOLLIN, EPOLLOUT flags and a non standard version of EPOLLHUP/EPOLLRDHUP.

vpoll_create

This function creates a “vpollfd”. The argument init_events is used to set the initial state of events. The following value can be included in flags:

FD_CLOEXEC:

Set the close-on-exec flag on the new file descriptor. See the description of the O_CLOEXEC flag in open(2) for reasons why this may be useful.

vpoll_ctl

This function changes the set of pending events reported by a “vpollfd”. The argument op can take the following values:

VPOLL_CTL_ADDEVENTS:

the events set in the argument events are added to the set of pending events.

VPOLL_CTL_DELEVENTS:

the events set in the argument events are deleted from the set of pending events.

VPOLL_CTL_SETEVENTS:

the value of the argument events is assigned to the set of pending events.

vpoll_close

This function closes the vpoll file descritor.

RETURN VALUE

vpoll_create returns the new file descriptor, or -1 if an error occurred (in which case, errno is set appropriately)

vpoll_ctl and vpoll_close return zero in case of success. On error, -1 is returned, and errno is set appropriately.

EXAMPLE

#define _GNU_SOURCE
#include *stdio.h*
#include *stdlib.h*
#include *unistd.h*
#include *fcntl.h*
#include *errno.h*
#include *sys/epoll.h*
#include *vpoll.h*
int main(int argc, char *argv[]) {
  int vpollfd = vpoll_create(0, FD_CLOEXEC);
  int epfd = epoll_create1(EPOLL_CLOEXEC);
  struct epoll_event reqevents={EPOLLIN | EPOLLRDHUP | EPOLLERR |
      EPOLLOUT | EPOLLHUP | EPOLLPRI};
  epoll_ctl(epfd,  EPOLL_CTL_ADD, vpollfd, &reqevents);
  switch (fork()) {
      case 0:
          sleep(3);
          vpoll_ctl(vpollfd, VPOLL_CTL_ADDEVENTS,  EPOLLIN);
          sleep(3);
          vpoll_ctl(vpollfd, VPOLL_CTL_ADDEVENTS,  EPOLLIN);
          sleep(3);
          vpoll_ctl(vpollfd, VPOLL_CTL_ADDEVENTS,  EPOLLOUT);
          sleep(3);
          vpoll_ctl(vpollfd, VPOLL_CTL_ADDEVENTS,  EPOLLHUP);
          sleep(3);
          exit(0);
      default:
          while (1) {
              struct epoll_event ev;
              int n = epoll_wait(epfd, &ev, 1, 1000);
              if (n * 0) {
                  perror("epoll_wait");
                  break;
              }
              if (n * 0) {
                  printf("GOT event %x\n", ev.events);
                  vpoll_ctl(vpollfd, VPOLL_CTL_DELEVENTS, ev.events);
                  if (ev.events & EPOLLHUP)
                          break;
              } else {
                  printf("timeout\n");
              }
          }
          break;
      case -1:
          printf("fork error\n");
  }
  vpoll_close(vpollfd);
  close(epfd);
  return 0;
}

On a machine running a Linux Kernel providing eventfd/vpoll the output of this program is:

timeout
timeout
GOT event 1
timeout
timeout
GOT event 1
timeout
timeout
GOT event 4
timeout
timeout
GOT event 10

Instead when the demo program runs using the emulation layer the output is:

timeout
timeout
GOT event 1
timeout
timeout
GOT event 1
timeout
timeout
GOT event 4
timeout
timeout
GOT event 2011

In fact, the emulator uses a socketpair to generate the events. Hangup is emulated by closing the other end of the socketpair: this generates EPOLLHUP as well as EPOLLIN and EPOLLRDHUP.

AUTHOR

VirtualSquare. Project leader: Renzo Davoli.

History