How to use threads with libev

Question

I'm making a webcrawler and want to spawn a number of threads each with its own event loop to monitor network activity. Here is my code so far:

static void my_cb(EV_P_ struct ev_io *w, int revents)
{
  GlobalInfo *g = (GlobalInfo *)w->data;
  i = 0;
  if (g->concurrent_connections < MAX_CONNECTIONS)
  {
    while (i < MAX_LOAD && i < MAX_CONNECTIONS - g->concurrent_connections)
    {
      add_url(g);
    }
  }
}

static int init(GlobalInfo *g)
{
  int fd;

  fd = open("myfile", O_RDWR | O_NONBLOCK, 0);
  if(fd == -1) {
    perror("open");
    exit(1);
  }

  g->input = fdopen(fd, "r");

  ev_io_init(&g->fifo_event, my_cb, fd, EV_READ);
  ev_io_start(g->loop, &g->fifo_event);
}

void *crawler(void *threadid)
{
  GlobalInfo g;

  long tid;
  tid = (long)threadid;
  printf("Initalised thread #%ld!\n", tid);

  memset(&g, 0, sizeof(GlobalInfo));
  g.loop = ev_loop_new(EVFLAG_AUTO);

  g.done = 0;
  g.downloaded = 0;
  g.head = 0;
  g.added = 0;
  g.concurrent_connections = 0;

  init(&g);
  g.multi = curl_multi_init();

  ev_timer_init(&g.timer_event, timer_cb, 0., 0.);
  g.timer_event.data = &g;
  g.fifo_event.data = &g;
  curl_multi_setopt(g.multi, CURLMOPT_SOCKETFUNCTION, sock_cb);
  curl_multi_setopt(g.multi, CURLMOPT_SOCKETDATA, &g);
  curl_multi_setopt(g.multi, CURLMOPT_TIMERFUNCTION, multi_timer_cb);
  curl_multi_setopt(g.multi, CURLMOPT_TIMERDATA, &g);

  /* we don't call any curl_multi_socket*() function yet as we have no handles
     added! */

  ev_loop(g.loop, 0);
  curl_multi_cleanup(g.multi);
  pthread_exit(NULL);
}

int main(int argc, char **argv)
{
  (void)argc;
  (void)argv;

  mysql_start();

  pthread_t threads[NUM_THREADS];
  int rc;
  long t;
  for(t=0; t<NUM_THREADS; t++){
    rc = pthread_create(&threads[t], NULL, crawler, (void *)t);
    if (rc){
      printf("ERROR; return code from pthread_create() is %d\n", rc);
      exit(-1);
    }
  }

  pthread_exit(NULL);

  mysql_stop();
  mysql_library_end();
  return 0;
}

As you can see I attempt to make a new event loop in each thread with ev_loop_new. However, the program aborts with the following backtrace:

#0  __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:50
#1  0x00007ffff75bb859 in __GI_abort () at abort.c:79
#2  0x00007ffff76263ee in __libc_message (action=action@entry=do_abort, fmt=fmt@entry=0x7ffff7750285 "%s\n") at ../sysdeps/posix/libc_fatal.c:155
#3  0x00007ffff762e47c in malloc_printerr (str=str@entry=0x7ffff7752690 "double free or corruption (!prev)") at malloc.c:5347
#4  0x00007ffff763012c in _int_free (av=0x7ffff7781b80 <main_arena>, p=0x5555569ff1e0, have_lock=<optimized out>) at malloc.c:4317
#5  0x00007ffff79518c4 in ?? () from /lib/x86_64-linux-gnu/libmysqlclient.so.21
#6  0x00007ffff795215a in ?? () from /lib/x86_64-linux-gnu/libmysqlclient.so.21
#7  0x00007ffff78ffd0e in ?? () from /lib/x86_64-linux-gnu/libmysqlclient.so.21
#8  0x00007ffff78fffa5 in ?? () from /lib/x86_64-linux-gnu/libmysqlclient.so.21
#9  0x00007ffff7900155 in ?? () from /lib/x86_64-linux-gnu/libmysqlclient.so.21
#10 0x00007ffff7903795 in ?? () from /lib/x86_64-linux-gnu/libmysqlclient.so.21
#11 0x00007ffff7905804 in mysql_real_query_nonblocking () from /lib/x86_64-linux-gnu/libmysqlclient.so.21
#12 0x0000555555559a4e in add_url (g=0x7ffff6e71e50) at threads.c:1202
#13 0x000055555555a69b in my_cb (loop=0x7ffff0000f70, w=0x7ffff6e71e58, revents=1) at threads.c:1457
#14 0x00007ffff7810bc3 in ev_invoke_pending () from /lib/x86_64-linux-gnu/libev.so.4
#15 0x00007ffff7814b93 in ev_run () from /lib/x86_64-linux-gnu/libev.so.4
#16 0x0000555555556a73 in ev_loop (loop=0x7ffff0000f70, flags=0) at /usr/include/ev.h:842
#17 0x000055555555a990 in crawler (threadid=0x0) at threads.c:1517
#18 0x00007ffff78b1609 in start_thread (arg=<optimized out>) at pthread_create.c:477
#19 0x00007ffff76b8293 in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95
(gdb) q

As you can see the offending line is ev_loop(g.loop, 0); which causes the abort. I thought I was doing the right thing with making a new event loop in each thread with ev_loop_new.

What am I doing wrong?