ocaml/otherlibs/threads/scheduler.c

/***********************************************************************/
/*                                                                     */
/*                           Objective Caml                            */
/*                                                                     */
/*            Xavier Leroy, projet Cristal, INRIA Rocquencourt         */
/*                                                                     */
/*  Copyright 1996 Institut National de Recherche en Informatique et   */
/*  en Automatique.  All rights reserved.  This file is distributed    */
/*  under the terms of the GNU Library General Public License.         */
/*                                                                     */
/***********************************************************************/

/* $Id$ */

/* The thread scheduler */

#include "callback.h"
#include "config.h"
#include "misc.h"
#include "mlvalues.h"
#include "stacks.h"
#include "fail.h"
#include "io.h"
#include "roots.h"
#include "alloc.h"
#include "memory.h"
#include "signals.h"
#include "sys.h"

#if ! (defined(HAS_SELECT) && \
       defined(HAS_SETITIMER) && \
       defined(HAS_GETTIMEOFDAY) && \
       (defined(HAS_WAITPID) || defined(HAS_WAIT4)))
#include "Cannot compile libthreads, system calls missing"
#endif

#ifdef __OpenBSD__
#include <string.h>
#endif

#include <errno.h>
#include <sys/time.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <sys/stat.h>
#include <fcntl.h>
#ifdef HAS_UNISTD
#include <unistd.h>
#endif
#ifdef HAS_SYS_SELECT_H
#include <sys/select.h>
#endif

#ifndef FD_ISSET
/* Assume old-style BSD 4.2 fd sets */
typedef int fd_set;
#define FD_SETSIZE (sizeof(int) * 8)
#define FD_SET(fd,fds) (*(fds) |= 1 << (fd))
#define FD_CLR(fd,fds) (*(fds) &= ~(1 << (fd)))
#define FD_ISSET(fd,fds) (*(fds) & (1 << (fd)))
#define FD_ZERO(fds) (*(fds) = 0)
#endif

#ifndef HAS_WAITPID
#define waitpid(pid,status,opts) wait4(pid,status,opts,NULL)
#endif

#ifndef O_NONBLOCK
#define O_NONBLOCK O_NDELAY
#endif

/* Configuration */

/* Initial size of stack when a thread is created (4 Ko) */
#define Thread_stack_size (Stack_size / 4)

/* Max computation time before rescheduling, in microseconds (50ms) */
#define Thread_timeout 50000

/* The thread descriptors */

struct thread_struct {
  value ident;                  /* Unique id (for equality comparisons) */
  struct thread_struct * next;  /* Double linking of threads */
  struct thread_struct * prev;
  value * stack_low;            /* The execution stack for this thread */
  value * stack_high;
  value * stack_threshold;
  value * sp;
  value * trapsp;
  value status;                 /* RUNNABLE, KILLED. etc (see below) */
  value fd;     /* File descriptor on which we're doing read or write */
  value readfds, writefds, exceptfds;
                /* Lists of file descriptors on which we're doing select() */
  value delay;                  /* Time until which this thread is blocked */
  value joining;                /* Thread we're trying to join */
  value waitpid;                /* PID of process we're waiting for */
  value retval;                 /* Value to return when thread resumes */
};

typedef struct thread_struct * thread_t;

#define RUNNABLE Val_int(0)
#define KILLED Val_int(1)
#define SUSPENDED Val_int(2)
#define BLOCKED_READ Val_int(4)
#define BLOCKED_WRITE Val_int(8)
#define BLOCKED_SELECT Val_int(16)
#define BLOCKED_DELAY Val_int(32)
#define BLOCKED_JOIN Val_int(64)
#define BLOCKED_WAIT Val_int(128)

#define RESUMED_WAKEUP Val_int(0)
#define RESUMED_DELAY Val_int(1)
#define RESUMED_JOIN Val_int(2)
#define RESUMED_IO Val_int(3)

#define TAG_RESUMED_SELECT 0
#define TAG_RESUMED_WAIT 1

#define NO_FDS Val_unit
#define NO_DELAY Val_unit
#define NO_JOINING Val_unit
#define NO_WAITPID Val_int(0)

#define DELAY_INFTY 1E30        /* +infty, for this purpose */

/* The thread currently active */
static thread_t curr_thread = NULL;
/* Identifier for next thread creation */
static value next_ident = Val_int(0);

#define Assign(dst,src) modify((value *)&(dst), (value)(src))

/* Scan the stacks of the other threads */

static void (*prev_scan_roots_hook) (scanning_action);

static void thread_scan_roots(scanning_action action)
{
  thread_t th, start;

  /* Scan all active descriptors */
  start = curr_thread;
  (*action)((value) curr_thread, (value *) &curr_thread);
  /* Don't scan curr_thread->sp, this has already been done.
     Don't scan local roots either, for the same reason. */
  for (th = start->next; th != start; th = th->next) {
    do_local_roots(action, th->sp, th->stack_high, NULL);
  }
  /* Hook */
  if (prev_scan_roots_hook != NULL) (*prev_scan_roots_hook)(action);
}

/* Forward declarations for async I/O handling */

static int stdin_initial_status, stdout_initial_status, stderr_initial_status;
static void thread_restore_std_descr(void);

/* Initialize the thread machinery */

value thread_initialize(value unit)       /* ML */
{
  struct itimerval timer;
  /* Create a descriptor for the current thread */
  curr_thread =
    (thread_t) alloc_shr(sizeof(struct thread_struct) / sizeof(value), 0);
  curr_thread->ident = next_ident;
  next_ident = Val_int(Int_val(next_ident) + 1);
  curr_thread->next = curr_thread;
  curr_thread->prev = curr_thread;
  curr_thread->stack_low = stack_low;
  curr_thread->stack_high = stack_high;
  curr_thread->stack_threshold = stack_threshold;
  curr_thread->sp = extern_sp;
  curr_thread->trapsp = trapsp;
  curr_thread->status = RUNNABLE;
  curr_thread->fd = Val_int(0);
  curr_thread->readfds = NO_FDS;
  curr_thread->writefds = NO_FDS;
  curr_thread->exceptfds = NO_FDS;
  curr_thread->delay = NO_DELAY;
  curr_thread->joining = NO_JOINING;
  curr_thread->waitpid = NO_WAITPID;
  curr_thread->retval = Val_unit;
  /* Initialize GC */
  prev_scan_roots_hook = scan_roots_hook;
  scan_roots_hook = thread_scan_roots;
  /* Initialize interval timer */
  timer.it_interval.tv_sec = 0;
  timer.it_interval.tv_usec = Thread_timeout;
  timer.it_value = timer.it_interval;
  setitimer(ITIMER_VIRTUAL, &timer, NULL);
  /* Set standard file descriptors to non-blocking mode */
  stdin_initial_status = fcntl(0, F_GETFL);
  stdout_initial_status = fcntl(1, F_GETFL);
  stderr_initial_status = fcntl(2, F_GETFL);
  if (stdin_initial_status != -1)
    fcntl(0, F_SETFL, stdin_initial_status | O_NONBLOCK);
  if (stdout_initial_status != -1)
    fcntl(1, F_SETFL, stdout_initial_status | O_NONBLOCK);
  if (stderr_initial_status != -1)
    fcntl(2, F_SETFL, stderr_initial_status | O_NONBLOCK);
  /* Register an at-exit function to restore the standard file descriptors */
  atexit(thread_restore_std_descr);
  return Val_unit;
}

/* Create a thread */

value thread_new(value clos)          /* ML */
{
  thread_t th;
  /* Allocate the thread and its stack */
  Begin_root(clos);
    th = (thread_t) alloc_shr(sizeof(struct thread_struct) / sizeof(value), 0);
  End_roots();
  th->ident = next_ident;
  next_ident = Val_int(Int_val(next_ident) + 1);
  th->stack_low = (value *) stat_alloc(Thread_stack_size);
  th->stack_high = th->stack_low + Thread_stack_size / sizeof(value);
  th->stack_threshold = th->stack_low + Stack_threshold / sizeof(value);
  th->sp = th->stack_high;
  th->trapsp = th->stack_high;
  /* Set up a return frame that pretends we're applying clos to ().
     This way, when this thread is activated, the RETURN will take us
     to the entry point of the closure. */
  th->sp -= 5;
  th->sp[0] = Val_unit;         /* dummy local to be popped by RETURN 1 */
  th->sp[1] = (value) Code_val(clos);
  th->sp[2] = clos;
  th->sp[3] = Val_long(0);      /* no extra args */
  th->sp[4] = Val_unit;         /* the () argument */
  /* Fake a C call frame */
  th->sp--;
  th->sp[0] = Val_unit;         /* a dummy environment */
  /* The thread is initially runnable */
  th->status = RUNNABLE;
  th->fd = Val_int(0);
  th->readfds = NO_FDS;
  th->writefds = NO_FDS;
  th->exceptfds = NO_FDS;
  th->delay = NO_DELAY;
  th->joining = NO_JOINING;
  th->waitpid = NO_WAITPID;
  th->retval = Val_unit;
  /* Insert thread in doubly linked list of threads */
  th->prev = curr_thread->prev;
  th->next = curr_thread;
  Assign(curr_thread->prev->next, th);
  Assign(curr_thread->prev, th);
  /* Return thread */
  return (value) th;
}

/* Return the thread identifier */

value thread_id(value th)             /* ML */
{
  return ((struct thread_struct *)th)->ident;
}

/* Return the current time as a floating-point number */

static double timeofday(void)
{
  struct timeval tv;
  gettimeofday(&tv, NULL);
  return (double) tv.tv_sec + (double) tv.tv_usec * 1e-6;
}

/* Find a runnable thread and activate it */

#define FOREACH_THREAD(x) x = curr_thread; do { x = x->next;
#define END_FOREACH(x) } while (x != curr_thread)

static value alloc_process_status(int pid, int status);
static void add_fdlist_to_set(value fdl, fd_set *set);
static value inter_fdlist_set(value fdl, fd_set *set, int *count);
static void find_bad_fd(int fd, fd_set *set);
static void find_bad_fds(value fdl, fd_set *set);

static value schedule_thread(void)
{
  thread_t run_thread, th;
  fd_set readfds, writefds, exceptfds;
  double delay, now;
  int need_select, need_wait;

  /* Don't allow preemption during a callback */
  if (callback_depth > 1) return curr_thread->retval;

  /* Save the status of the current thread */
  curr_thread->stack_low = stack_low;
  curr_thread->stack_high = stack_high;
  curr_thread->stack_threshold = stack_threshold;
  curr_thread->sp = extern_sp;
  curr_thread->trapsp = trapsp;

try_again:
  /* Find if a thread is runnable.
     Build fdsets and delay for select.
     See if some join or wait operations succeeded. */
  run_thread = NULL;
  FD_ZERO(&readfds);
  FD_ZERO(&writefds);
  FD_ZERO(&exceptfds);
  delay = DELAY_INFTY;
  now = -1.0;
  need_select = 0;
  need_wait = 0;

  FOREACH_THREAD(th)
    if (th->status <= SUSPENDED) continue;

    if (th->status & (BLOCKED_READ - 1)) {
      FD_SET(Int_val(th->fd), &readfds);
      need_select = 1;
    }
    if (th->status & (BLOCKED_WRITE - 1)) {
      FD_SET(Int_val(th->fd), &writefds);
      need_select = 1;
    }
    if (th->status & (BLOCKED_SELECT - 1)) {
      add_fdlist_to_set(th->readfds, &readfds);
      add_fdlist_to_set(th->writefds, &writefds);
      add_fdlist_to_set(th->exceptfds, &exceptfds);
      need_select = 1;
    }
    if (th->status & (BLOCKED_DELAY - 1)) {
      double th_delay;
      if (now < 0.0) now = timeofday();
      th_delay = Double_val(th->delay) - now;
      if (th_delay <= 0) {
        th->status = RUNNABLE;
        th->retval = RESUMED_DELAY;
      } else {
        if (th_delay < delay) delay = th_delay;
      }
    }
    if (th->status & (BLOCKED_JOIN - 1)) {
      if (((thread_t)(th->joining))->status == KILLED) {
        th->status = RUNNABLE;
        th->retval = RESUMED_JOIN;
      }
    }
    if (th->status & (BLOCKED_WAIT - 1)) {
      int status, pid;
      pid = waitpid(Int_val(th->waitpid), &status, WNOHANG);
      if (pid > 0) {
        th->status = RUNNABLE;
        Assign(th->retval, alloc_process_status(pid, status));
      } else {
        need_wait = 1;
      }
    }
  END_FOREACH(th);

  /* Find if a thread is runnable. */
  run_thread = NULL;
  FOREACH_THREAD(th)
    if (th->status == RUNNABLE) { run_thread = th; break; }
  END_FOREACH(th);

  /* Do the select if needed */
  if (need_select || run_thread == NULL) {
    struct timeval delay_tv, * delay_ptr;
    int retcode;
    /* If a thread is blocked on wait, don't block forever */
    if (need_wait && delay > Thread_timeout * 1e-6) {
      delay = Thread_timeout * 1e-6;
    }
    /* Convert delay to a timeval */
    /* If a thread is runnable, just poll */
    if (run_thread != NULL) {
      delay_tv.tv_sec = 0;
      delay_tv.tv_usec = 0;
      delay_ptr = &delay_tv;
    }
    else if (delay != DELAY_INFTY) {
      delay_tv.tv_sec = (unsigned int) delay;
      delay_tv.tv_usec = (delay - (double) delay_tv.tv_sec) * 1E6;
      delay_ptr = &delay_tv;
    }
    else {
      delay_ptr = NULL;
    }
    enter_blocking_section();
    retcode = select(FD_SETSIZE, &readfds, &writefds, &exceptfds, delay_ptr);
    leave_blocking_section();
    if (retcode == -1)
      switch (errno) {
      case EINTR:
        break;
      case EBADF:
        /* One of the descriptors in the sets was closed or is bad.
           Find it using fstat() and wake up the threads waiting on it
           so that they'll get an error when operating on it. */
        FOREACH_THREAD(th)
          if (th->status & (BLOCKED_READ - 1)) {
            find_bad_fd(Int_val(th->fd), &readfds);
          }
          if (th->status & (BLOCKED_WRITE - 1)) {
            find_bad_fd(Int_val(th->fd), &writefds);
          }
          if (th->status & (BLOCKED_SELECT - 1)) {
            find_bad_fds(th->readfds, &readfds);
            find_bad_fds(th->writefds, &writefds);
            find_bad_fds(th->exceptfds, &exceptfds);
          }
        END_FOREACH(th);
        retcode = FD_SETSIZE;
        break;
      default:
        sys_error(NO_ARG);
      }
    if (retcode > 0) {
      /* Some descriptors are ready.
         Mark the corresponding threads runnable. */
      FOREACH_THREAD(th)
        if (retcode <= 0) break;
        if ((th->status & (BLOCKED_READ - 1)) &&
            FD_ISSET(Int_val(th->fd), &readfds)) {
          th->retval = RESUMED_IO;
          th->status = RUNNABLE;
          if (run_thread == NULL) run_thread = th; /* Found one. */
          /* Wake up only one thread per fd */
          FD_CLR(Int_val(th->fd), &readfds);
          retcode--;
        }
        if ((th->status & (BLOCKED_WRITE - 1)) &&
            FD_ISSET(Int_val(th->fd), &writefds)) {
          th->retval = RESUMED_IO;
          th->status = RUNNABLE;
          if (run_thread == NULL) run_thread = th; /* Found one. */
          /* Wake up only one thread per fd */
          FD_CLR(Int_val(th->fd), &readfds);
          retcode--;
        }
        if (th->status & (BLOCKED_SELECT - 1)) {
          value r = Val_unit, w = Val_unit, e = Val_unit;
          Begin_roots3(r,w,e)
            r = inter_fdlist_set(th->readfds, &readfds, &retcode);
            w = inter_fdlist_set(th->writefds, &writefds, &retcode);
            e = inter_fdlist_set(th->exceptfds, &exceptfds, &retcode);
            if (r != NO_FDS || w != NO_FDS || e != NO_FDS) {
              value retval = alloc_small(3, TAG_RESUMED_SELECT);
              Field(retval, 0) = r;
              Field(retval, 1) = w;
              Field(retval, 2) = e;
              Assign(th->retval, retval);
              th->status = RUNNABLE;
              if (run_thread == NULL) run_thread = th; /* Found one. */
            }
          End_roots();
        }
      END_FOREACH(th);
    }
    /* If we get here with run_thread still NULL, one of the following
       may have happened:
       - a delay has expired
       - a wait() needs to be polled again
       - the select() failed (e.g. was interrupted)
       In these cases, we go through the loop once more to make the
       corresponding threads runnable. */
    if (run_thread == NULL &&
        (delay != DELAY_INFTY || need_wait || retcode == -1))
      goto try_again;
  }

  /* If we haven't something to run at that point, we're in big trouble. */
  if (run_thread == NULL) invalid_argument("Thread: deadlock");

  /* Free everything the thread was waiting on */
  Assign(run_thread->readfds, NO_FDS);
  Assign(run_thread->writefds, NO_FDS);
  Assign(run_thread->exceptfds, NO_FDS);
  Assign(run_thread->delay, NO_DELAY);
  Assign(run_thread->joining, NO_JOINING);
  run_thread->waitpid = NO_WAITPID;

  /* Activate the thread */
  curr_thread = run_thread;
  stack_low = curr_thread->stack_low;
  stack_high = curr_thread->stack_high;
  stack_threshold = curr_thread->stack_threshold;
  extern_sp = curr_thread->sp;
  trapsp = curr_thread->trapsp;
  return curr_thread->retval;
}

/* Since context switching is not allowed in callbacks, a thread that
   blocks during a callback is a deadlock. */

static void check_callback(void)
{
  if (callback_depth > 1)
    fatal_error("Thread: deadlock during callback");
}

/* Reschedule without suspending the current thread */

value thread_yield(value unit)        /* ML */
{
  Assert(curr_thread != NULL);
  curr_thread->retval = Val_unit;
  return schedule_thread();
}

/* Suspend the current thread */

value thread_sleep(value unit)        /* ML */
{
  Assert(curr_thread != NULL);
  check_callback();
  curr_thread->status = SUSPENDED;
  return schedule_thread();
}

/* Suspend the current thread on a read() or write() request */

static value thread_wait_rw(int kind, value fd)
{
  /* Don't do an error if we're not initialized yet
     (we can be called from thread-safe Pervasives before initialization),
     just return immediately. */
  if (curr_thread == NULL) return RESUMED_WAKEUP;
  /* As a special case, if we're in a callback, don't fail but block
     the whole process till I/O is possible */
  if (callback_depth > 1) {
    fd_set fds;
    FD_ZERO(&fds);
    FD_SET(Int_val(fd), &fds);
    switch(kind) {
      case BLOCKED_READ: select(FD_SETSIZE, &fds, NULL, NULL, NULL); break;
      case BLOCKED_WRITE: select(FD_SETSIZE, NULL, &fds, NULL, NULL); break;
    }
    return RESUMED_IO;
  } else {
    curr_thread->fd = fd;
    curr_thread->status = kind;
    return schedule_thread();
  }
}

value thread_wait_read(value fd)
{
  return thread_wait_rw(BLOCKED_READ, fd);
}

value thread_wait_write(value fd)
{
  return thread_wait_rw(BLOCKED_WRITE, fd);
}

/* Suspend the current thread on a read() or write() request with timeout */

static value thread_wait_timed_rw(int kind, value arg)
{
  double date;

  check_callback();
  curr_thread->fd = Field(arg, 0);
  date = timeofday() + Double_val(Field(arg, 1));
  Assign(curr_thread->delay, copy_double(date));
  curr_thread->status = kind | BLOCKED_DELAY;
  return schedule_thread();
}

value thread_wait_timed_read(value arg)
{
  return thread_wait_timed_rw(BLOCKED_READ, arg);
}

value thread_wait_timed_write(value arg)
{
  return thread_wait_timed_rw(BLOCKED_WRITE, arg);
}

/* Suspend the current thread on a select() request */

value thread_select(value arg)        /* ML */
{
  double date;
  check_callback();
  Assign(curr_thread->readfds, Field(arg, 0));
  Assign(curr_thread->writefds, Field(arg, 1));
  Assign(curr_thread->exceptfds, Field(arg, 2));
  date = Double_val(Field(arg, 3));
  if (date >= 0.0) {
    date += timeofday();
    Assign(curr_thread->delay, copy_double(date));
    curr_thread->status = BLOCKED_SELECT | BLOCKED_DELAY;
  } else {
    curr_thread->status = BLOCKED_SELECT;
  }
  return schedule_thread();
}

/* Primitives to implement suspension on buffered channels */

value thread_inchan_ready(value vchan) /* ML */
{
  struct channel * chan = Channel(vchan);
  return Val_bool(chan->curr < chan->max);
}

value thread_outchan_ready(value vchan, value vsize) /* ML */
{
  struct channel * chan = Channel(vchan);
  long size = Long_val(vsize);
  /* Negative size means we want to flush the buffer entirely */
  if (size < 0) {
    return Val_bool(chan->curr == chan->buff);
  } else {
    int free = chan->end - chan->curr;
    if (chan->curr == chan->buff)
      return Val_bool(size < free);
    else
      return Val_bool(size <= free);
  }
}

/* Suspend the current thread for some time */

value thread_delay(value time)          /* ML */
{
  double date = timeofday() + Double_val(time);
  Assert(curr_thread != NULL);
  check_callback();
  curr_thread->status = BLOCKED_DELAY;
  Assign(curr_thread->delay, copy_double(date));
  return schedule_thread();
}

/* Suspend the current thread until another thread terminates */

value thread_join(value th)          /* ML */
{
  check_callback();
  Assert(curr_thread != NULL);
  if (((thread_t)th)->status == KILLED) return Val_unit;
  curr_thread->status = BLOCKED_JOIN;
  Assign(curr_thread->joining, th);
  return schedule_thread();
}

/* Suspend the current thread until a Unix process exits */

value thread_wait_pid(value pid)          /* ML */
{
  Assert(curr_thread != NULL);
  check_callback();
  curr_thread->status = BLOCKED_WAIT;
  curr_thread->waitpid = pid;
  return schedule_thread();
}

/* Reactivate another thread */

value thread_wakeup(value thread)     /* ML */
{
  thread_t th = (thread_t) thread;
  switch (th->status) {
  case SUSPENDED:
    th->status = RUNNABLE;
    th->retval = RESUMED_WAKEUP;
    break;
  case KILLED:
    failwith("Thread.wakeup: killed thread");
  default:
    failwith("Thread.wakeup: thread not suspended");
  }
  return Val_unit;
}

/* Return the current thread */

value thread_self(value unit)         /* ML */
{
  Assert(curr_thread != NULL);
  return (value) curr_thread;
}

/* Kill a thread */

value thread_kill(value thread)       /* ML */
{
  value retval = Val_unit;
  thread_t th = (thread_t) thread;
  if (th->status == KILLED) failwith("Thread.kill: killed thread");
  /* Don't paint ourselves in a corner */
  if (th == th->next) failwith("Thread.kill: cannot kill the last thread");
  /* This thread is no longer waiting on anything */
  th->status = KILLED;
  /* If this is the current thread, activate another one */
  if (th == curr_thread) {
    Begin_root(thread);
    retval = schedule_thread();
    th = (thread_t) thread;
    End_roots();
  }
  /* Remove thread from the doubly-linked list */
  Assign(th->prev->next, th->next);
  Assign(th->next->prev, th->prev);
  /* Free its resources */
  stat_free((char *) th->stack_low);
  th->stack_low = NULL;
  th->stack_high = NULL;
  th->stack_threshold = NULL;
  th->sp = NULL;
  th->trapsp = NULL;
  return retval;
}

/* Set a list of file descriptors in a fdset */

static void add_fdlist_to_set(value fdl, fd_set *set)
{
  for (/*nothing*/; fdl != NO_FDS; fdl = Field(fdl, 1)) {
    int fd = Int_val(Field(fdl, 0));
    /* Ignore funky file descriptors, which can cause crashes */
    if (fd >= 0 && fd < FD_SETSIZE) FD_SET(fd, set);
  }
}

/* Build the intersection of a list and a fdset (the list of file descriptors
   which are both in the list and in the fdset). */

static value inter_fdlist_set(value fdl, fd_set *set, int *count)
{
  value res = Val_unit;
  value cons;

  Begin_roots2(fdl, res);
    for (res = NO_FDS; fdl != NO_FDS; fdl = Field(fdl, 1)) {
      int fd = Int_val(Field(fdl, 0));
      if (FD_ISSET(fd, set)) {
        cons = alloc_small(2, 0);
        Field(cons, 0) = Val_int(fd);
        Field(cons, 1) = res;
        res = cons;
        FD_CLR(fd, set); /* wake up only one thread per fd ready */
        (*count)--;
      }
    }
  End_roots();
  return res;
}

/* Find closed file descriptors in a waiting list and set them to 1 in
   the given fdset */

static void find_bad_fd(int fd, fd_set *set)
{
  struct stat s;
  if (fd >= 0 && fd < FD_SETSIZE && fstat(fd, &s) == -1 && errno == EBADF)
    FD_SET(fd, set);
}

static void find_bad_fds(value fdl, fd_set *set)
{
  for (/*nothing*/; fdl != NO_FDS; fdl = Field(fdl, 1))
    find_bad_fd(Int_val(Field(fdl, 0)), set);
}

/* Auxiliary function for allocating the result of a waitpid() call */

#if !(defined(WIFEXITED) && defined(WEXITSTATUS) && defined(WIFSTOPPED) && \
      defined(WSTOPSIG) && defined(WTERMSIG))
/* Assume old-style V7 status word */
#define WIFEXITED(status) (((status) & 0xFF) == 0)
#define WEXITSTATUS(status) (((status) >> 8) & 0xFF)
#define WIFSTOPPED(status) (((status) & 0xFF) == 0xFF)
#define WSTOPSIG(status) (((status) >> 8) & 0xFF)
#define WTERMSIG(status) ((status) & 0x3F)
#endif

#define TAG_WEXITED 0
#define TAG_WSIGNALED 1
#define TAG_WSTOPPED 2

static value alloc_process_status(int pid, int status)
{
  value st, res;

  if (WIFEXITED(status)) {
    st = alloc_small(1, TAG_WEXITED);
    Field(st, 0) = Val_int(WEXITSTATUS(status));
  }
  else if (WIFSTOPPED(status)) {
    st = alloc_small(1, TAG_WSTOPPED);
    Field(st, 0) = Val_int(WSTOPSIG(status));
  }
  else {
    st = alloc_small(1, TAG_WSIGNALED);
    Field(st, 0) = Val_int(WTERMSIG(status));
  }
  Begin_root(st);
    res = alloc_small(2, TAG_RESUMED_WAIT);
    Field(res, 0) = Val_int(pid);
    Field(res, 1) = st;
  End_roots();
  return res;
}

/* Restore the standard file descriptors to their initial state */

static void thread_restore_std_descr(void)
{
  if (stdin_initial_status != -1) fcntl(0, F_SETFL, stdin_initial_status);
  if (stdout_initial_status != -1) fcntl(1, F_SETFL, stdout_initial_status);
  if (stderr_initial_status != -1) fcntl(2, F_SETFL, stderr_initial_status);
}