#include <sys/event.h>
#include <sys/stat.h>
#include <string.h>
#include <unistd.h>
#include <fcntl.h>
#include <errno.h>
#include <stdio.h>
#include <err.h>
#include <sysexits.h>
#include <signal.h>

int main(void)
{
  // Check up front that stdout is a pipe. Otherwise we're going to
  // spew to the terminal or use up someone's disk.
  struct stat stat_buf;
  if ( fstat(1, &stat_buf) ) {
    err(EX_OSERR, "fstat");
  }
  if ( (stat_buf.st_mode & S_IFMT) != S_IFIFO ) {
    errx(EX_USAGE, "stdout is not a pipe");
  }

  // ignore SIGPIPE; set stdout nonblocking
  signal(SIGPIPE, SIG_IGN);
  int flags = fcntl(1, F_GETFL);
  if ( flags < 0 ) {
    err(EX_OSERR, "fcntl(F_GETFL)");
  }
  if ( fcntl(1, F_SETFL, flags | O_NONBLOCK) ) {
    err(EX_OSERR, "fcntl(F_SETFL)");
  }

  // Attempt to fill up the pipe that is stdout.
  char buf[4096];
  memset(buf, 'a', sizeof(buf));
  int got_eagain = 0;
  do {
    ssize_t nwritten = write(1, buf, sizeof(buf));
    if ( nwritten < 0 ) {
      if ( errno == EAGAIN ) {
        got_eagain = 1;
      } else {
        err(EX_OSERR, "write");
      }
    }
  } while ( !got_eagain );

  // wait for something interesting to happen on stdout.
  int kq = kqueue();
  if ( kq < 0 ) {
    errx(EX_OSERR, "kevent");
  }
  struct kevent kev;
  EV_SET(&kev, 1, EVFILT_WRITE, EV_ADD | EV_ENABLE, 0, 0, NULL);
  struct kevent result = { 0 };
  int wait_result = kevent(kq, &kev, 1, &result, 1, NULL);
  fprintf(stderr, "kevent returned %d\n", wait_result);
  if ( wait_result > 0 ) {
    fprintf(stderr, "\t%llu %d %d %d %lld %p\n", (unsigned long long)result.ident,
            (int)result.filter, (int)result.flags, (int)result.fflags,
            (long long)result.data, result.udata);
  }
  return 0;
}