Open Radar

 
If the .events field of a struct pollfd is POLLHUP (see rdar://37537852 for why this is needed) and the associated fd is readable but has not received hangup, poll will return with 0 if timeout is -1 and no other fds are ready.

See the following test case:

#include <sys/types.h>
#include <unistd.h>
#include <poll.h>
#include <stdio.h>
#include <sys/wait.h>
#include <stdlib.h>

int monitor_process(int fd) {
  struct pollfd spec = { .fd = fd, .events = POLLHUP };
  int res = poll(&spec, 1, -1);
  switch (res) {
  case -1:
    perror("waiting for events");
    return 1;
  case 0:
    fprintf(stderr, "poll timed out\n");
    return 1;
  default:
    return 0;
  }
}

int main(int argc, char ** argv) {
  int pipe_fds[2];
  if (pipe(pipe_fds) == -1) {
    perror("making a pipe");
    return 1;
  }

  pid_t child = fork();
  switch (child) {
  case -1:
    perror("forking");
    return 1;
  case 0:
    close(pipe_fds[1]);
    exit(monitor_process(pipe_fds[0]));
  }
  close(pipe_fds[0]);
  char c = 'c';
  write(pipe_fds[1], &c, 1);
  sleep(1);
  close(pipe_fds[1]);
  int status;
  waitpid(child, &status, 0);
  if (WIFSIGNALED(status)) {
    fprintf(stderr, "child process died with signal: %d", WTERMSIG(status));
    return 1;
  }
  return WEXITSTATUS(status);
}