Merge async and sync code paths some more

[deliverable/binutils-gdb.git] / gdb / linux-nat.c

/* GNU/Linux native-dependent code common to multiple platforms.

   Copyright (C) 2001-2015 Free Software Foundation, Inc.

   This file is part of GDB.

   This program is free software; you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation; either version 3 of the License, or
   (at your option) any later version.

   This program is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU General Public License for more details.

   You should have received a copy of the GNU General Public License
   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */

#include "defs.h"
#include "inferior.h"
#include "infrun.h"
#include "target.h"
#include "nat/linux-nat.h"
#include "nat/linux-waitpid.h"
#include "gdb_wait.h"
#ifdef HAVE_TKILL_SYSCALL
#include <unistd.h>
#include <sys/syscall.h>
#endif
#include "nat/gdb_ptrace.h"
#include "linux-nat.h"
#include "nat/linux-ptrace.h"
#include "nat/linux-procfs.h"
#include "nat/linux-personality.h"
#include "linux-fork.h"
#include "gdbthread.h"
#include "gdbcmd.h"
#include "regcache.h"
#include "regset.h"
#include "inf-child.h"
#include "inf-ptrace.h"
#include "auxv.h"
#include <sys/procfs.h>         /* for elf_gregset etc.  */
#include "elf-bfd.h"            /* for elfcore_write_* */
#include "gregset.h"            /* for gregset */
#include "gdbcore.h"            /* for get_exec_file */
#include <ctype.h>              /* for isdigit */
#include <sys/stat.h>           /* for struct stat */
#include <fcntl.h>              /* for O_RDONLY */
#include "inf-loop.h"
#include "event-loop.h"
#include "event-top.h"
#include <pwd.h>
#include <sys/types.h>
#include <dirent.h>
#include "xml-support.h"
#include <sys/vfs.h>
#include "solib.h"
#include "nat/linux-osdata.h"
#include "linux-tdep.h"
#include "symfile.h"
#include "agent.h"
#include "tracepoint.h"
#include "buffer.h"
#include "target-descriptions.h"
#include "filestuff.h"
#include "objfiles.h"
#include "nat/linux-namespaces.h"
#include "fileio.h"

#ifndef SPUFS_MAGIC
#define SPUFS_MAGIC 0x23c9b64e
#endif

/* This comment documents high-level logic of this file.

Waiting for events in sync mode
===============================

When waiting for an event in a specific thread, we just use waitpid, passing
the specific pid, and not passing WNOHANG.

When waiting for an event in all threads, waitpid is not quite good.  Prior to
version 2.4, Linux can either wait for event in main thread, or in secondary
threads.  (2.4 has the __WALL flag).  So, if we use blocking waitpid, we might
miss an event.  The solution is to use non-blocking waitpid, together with
sigsuspend.  First, we use non-blocking waitpid to get an event in the main 
process, if any.  Second, we use non-blocking waitpid with the __WCLONED
flag to check for events in cloned processes.  If nothing is found, we use
sigsuspend to wait for SIGCHLD.  When SIGCHLD arrives, it means something
happened to a child process -- and SIGCHLD will be delivered both for events
in main debugged process and in cloned processes.  As soon as we know there's
an event, we get back to calling nonblocking waitpid with and without 
__WCLONED.

Note that SIGCHLD should be blocked between waitpid and sigsuspend calls,
so that we don't miss a signal.  If SIGCHLD arrives in between, when it's
blocked, the signal becomes pending and sigsuspend immediately
notices it and returns.

Waiting for events in async mode
================================

In async mode, GDB should always be ready to handle both user input
and target events, so neither blocking waitpid nor sigsuspend are
viable options.  Instead, we should asynchronously notify the GDB main
event loop whenever there's an unprocessed event from the target.  We
detect asynchronous target events by handling SIGCHLD signals.  To
notify the event loop about target events, the self-pipe trick is used
--- a pipe is registered as waitable event source in the event loop,
the event loop select/poll's on the read end of this pipe (as well on
other event sources, e.g., stdin), and the SIGCHLD handler writes a
byte to this pipe.  This is more portable than relying on
pselect/ppoll, since on kernels that lack those syscalls, libc
emulates them with select/poll+sigprocmask, and that is racy
(a.k.a. plain broken).

Obviously, if we fail to notify the event loop if there's a target
event, it's bad.  OTOH, if we notify the event loop when there's no
event from the target, linux_nat_wait will detect that there's no real
event to report, and return event of type TARGET_WAITKIND_IGNORE.
This is mostly harmless, but it will waste time and is better avoided.

The main design point is that every time GDB is outside linux-nat.c,
we have a SIGCHLD handler installed that is called when something
happens to the target and notifies the GDB event loop.  Whenever GDB
core decides to handle the event, and calls into linux-nat.c, we
process things as in sync mode, except that the we never block in
sigsuspend.

While processing an event, we may end up momentarily blocked in
waitpid calls.  Those waitpid calls, while blocking, are guarantied to
return quickly.  E.g., in all-stop mode, before reporting to the core
that an LWP hit a breakpoint, all LWPs are stopped by sending them
SIGSTOP, and synchronously waiting for the SIGSTOP to be reported.
Note that this is different from blocking indefinitely waiting for the
next event --- here, we're already handling an event.

Use of signals
==============

We stop threads by sending a SIGSTOP.  The use of SIGSTOP instead of another
signal is not entirely significant; we just need for a signal to be delivered,
so that we can intercept it.  SIGSTOP's advantage is that it can not be
blocked.  A disadvantage is that it is not a real-time signal, so it can only
be queued once; we do not keep track of other sources of SIGSTOP.

Two other signals that can't be blocked are SIGCONT and SIGKILL.  But we can't
use them, because they have special behavior when the signal is generated -
not when it is delivered.  SIGCONT resumes the entire thread group and SIGKILL
kills the entire thread group.

A delivered SIGSTOP would stop the entire thread group, not just the thread we
tkill'd.  But we never let the SIGSTOP be delivered; we always intercept and 
cancel it (by PTRACE_CONT without passing SIGSTOP).

We could use a real-time signal instead.  This would solve those problems; we
could use PTRACE_GETSIGINFO to locate the specific stop signals sent by GDB.
But we would still have to have some support for SIGSTOP, since PTRACE_ATTACH
generates it, and there are races with trying to find a signal that is not
blocked.  */

#ifndef O_LARGEFILE
#define O_LARGEFILE 0
#endif

/* Does the current host support PTRACE_GETREGSET?  */
enum tribool have_ptrace_getregset = TRIBOOL_UNKNOWN;

/* The single-threaded native GNU/Linux target_ops.  We save a pointer for
   the use of the multi-threaded target.  */
static struct target_ops *linux_ops;
static struct target_ops linux_ops_saved;

/* The method to call, if any, when a new thread is attached.  */
static void (*linux_nat_new_thread) (struct lwp_info *);

/* The method to call, if any, when a new fork is attached.  */
static linux_nat_new_fork_ftype *linux_nat_new_fork;

/* The method to call, if any, when a process is no longer
   attached.  */
static linux_nat_forget_process_ftype *linux_nat_forget_process_hook;

/* Hook to call prior to resuming a thread.  */
static void (*linux_nat_prepare_to_resume) (struct lwp_info *);

/* The method to call, if any, when the siginfo object needs to be
   converted between the layout returned by ptrace, and the layout in
   the architecture of the inferior.  */
static int (*linux_nat_siginfo_fixup) (siginfo_t *,
                                       gdb_byte *,
                                       int);

/* The saved to_xfer_partial method, inherited from inf-ptrace.c.
   Called by our to_xfer_partial.  */
static target_xfer_partial_ftype *super_xfer_partial;

/* The saved to_close method, inherited from inf-ptrace.c.
   Called by our to_close.  */
static void (*super_close) (struct target_ops *);

static unsigned int debug_linux_nat;
static void
show_debug_linux_nat (struct ui_file *file, int from_tty,
                      struct cmd_list_element *c, const char *value)
{
  fprintf_filtered (file, _("Debugging of GNU/Linux lwp module is %s.\n"),
                    value);
}

struct simple_pid_list
{
  int pid;
  int status;
  struct simple_pid_list *next;
};
struct simple_pid_list *stopped_pids;

/* Async mode support.  */

/* The read/write ends of the pipe registered as waitable file in the
   event loop.  */
static int linux_nat_event_pipe[2] = { -1, -1 };

/* True if we're currently in async mode.  */
#define linux_is_async_p() (linux_nat_event_pipe[0] != -1)

/* Flush the event pipe.  */

static void
async_file_flush (void)
{
  int ret;
  char buf;

  do
    {
      ret = read (linux_nat_event_pipe[0], &buf, 1);
    }
  while (ret >= 0 || (ret == -1 && errno == EINTR));
}

/* Put something (anything, doesn't matter what, or how much) in event
   pipe, so that the select/poll in the event-loop realizes we have
   something to process.  */

static void
async_file_mark (void)
{
  int ret;

  /* It doesn't really matter what the pipe contains, as long we end
     up with something in it.  Might as well flush the previous
     left-overs.  */
  async_file_flush ();

  do
    {
      ret = write (linux_nat_event_pipe[1], "+", 1);
    }
  while (ret == -1 && errno == EINTR);

  /* Ignore EAGAIN.  If the pipe is full, the event loop will already
     be awakened anyway.  */
}

static int kill_lwp (int lwpid, int signo);

static int stop_callback (struct lwp_info *lp, void *data);
static int resume_stopped_resumed_lwps (struct lwp_info *lp, void *data);

static void block_child_signals (sigset_t *prev_mask);
static void restore_child_signals_mask (sigset_t *prev_mask);

struct lwp_info;
static struct lwp_info *add_lwp (ptid_t ptid);
static void purge_lwp_list (int pid);
static void delete_lwp (ptid_t ptid);
static struct lwp_info *find_lwp_pid (ptid_t ptid);

static int lwp_status_pending_p (struct lwp_info *lp);

static int check_stopped_by_breakpoint (struct lwp_info *lp);
static int sigtrap_is_event (int status);
static int (*linux_nat_status_is_event) (int status) = sigtrap_is_event;

\f
/* LWP accessors.  */

/* See nat/linux-nat.h.  */

ptid_t
ptid_of_lwp (struct lwp_info *lwp)
{
  return lwp->ptid;
}

/* See nat/linux-nat.h.  */

void
lwp_set_arch_private_info (struct lwp_info *lwp,
                           struct arch_lwp_info *info)
{
  lwp->arch_private = info;
}

/* See nat/linux-nat.h.  */

struct arch_lwp_info *
lwp_arch_private_info (struct lwp_info *lwp)
{
  return lwp->arch_private;
}

/* See nat/linux-nat.h.  */

int
lwp_is_stopped (struct lwp_info *lwp)
{
  return lwp->stopped;
}

/* See nat/linux-nat.h.  */

enum target_stop_reason
lwp_stop_reason (struct lwp_info *lwp)
{
  return lwp->stop_reason;
}

\f
/* Trivial list manipulation functions to keep track of a list of
   new stopped processes.  */
static void
add_to_pid_list (struct simple_pid_list **listp, int pid, int status)
{
  struct simple_pid_list *new_pid = XNEW (struct simple_pid_list);

  new_pid->pid = pid;
  new_pid->status = status;
  new_pid->next = *listp;
  *listp = new_pid;
}

static int
in_pid_list_p (struct simple_pid_list *list, int pid)
{
  struct simple_pid_list *p;

  for (p = list; p != NULL; p = p->next)
    if (p->pid == pid)
      return 1;
  return 0;
}

static int
pull_pid_from_list (struct simple_pid_list **listp, int pid, int *statusp)
{
  struct simple_pid_list **p;

  for (p = listp; *p != NULL; p = &(*p)->next)
    if ((*p)->pid == pid)
      {
        struct simple_pid_list *next = (*p)->next;

        *statusp = (*p)->status;
        xfree (*p);
        *p = next;
        return 1;
      }
  return 0;
}

/* Return the ptrace options that we want to try to enable.  */

static int
linux_nat_ptrace_options (int attached)
{
  int options = 0;

  if (!attached)
    options |= PTRACE_O_EXITKILL;

  options |= (PTRACE_O_TRACESYSGOOD
              | PTRACE_O_TRACEVFORKDONE
              | PTRACE_O_TRACEVFORK
              | PTRACE_O_TRACEFORK
              | PTRACE_O_TRACEEXEC);

  return options;
}

/* Initialize ptrace warnings and check for supported ptrace
   features given PID.

   ATTACHED should be nonzero iff we attached to the inferior.  */

static void
linux_init_ptrace (pid_t pid, int attached)
{
  int options = linux_nat_ptrace_options (attached);

  linux_enable_event_reporting (pid, options);
  linux_ptrace_init_warnings ();
}

static void
linux_child_post_attach (struct target_ops *self, int pid)
{
  linux_init_ptrace (pid, 1);
}

static void
linux_child_post_startup_inferior (struct target_ops *self, ptid_t ptid)
{
  linux_init_ptrace (ptid_get_pid (ptid), 0);
}

/* Return the number of known LWPs in the tgid given by PID.  */

static int
num_lwps (int pid)
{
  int count = 0;
  struct lwp_info *lp;

  for (lp = lwp_list; lp; lp = lp->next)
    if (ptid_get_pid (lp->ptid) == pid)
      count++;

  return count;
}

/* Call delete_lwp with prototype compatible for make_cleanup.  */

static void
delete_lwp_cleanup (void *lp_voidp)
{
  struct lwp_info *lp = lp_voidp;

  delete_lwp (lp->ptid);
}

/* Target hook for follow_fork.  On entry inferior_ptid must be the
   ptid of the followed inferior.  At return, inferior_ptid will be
   unchanged.  */

static int
linux_child_follow_fork (struct target_ops *ops, int follow_child,
                         int detach_fork)
{
  if (!follow_child)
    {
      struct lwp_info *child_lp = NULL;
      int status = W_STOPCODE (0);
      struct cleanup *old_chain;
      int has_vforked;
      ptid_t parent_ptid, child_ptid;
      int parent_pid, child_pid;

      has_vforked = (inferior_thread ()->pending_follow.kind
                     == TARGET_WAITKIND_VFORKED);
      parent_ptid = inferior_ptid;
      child_ptid = inferior_thread ()->pending_follow.value.related_pid;
      parent_pid = ptid_get_lwp (parent_ptid);
      child_pid = ptid_get_lwp (child_ptid);

      /* We're already attached to the parent, by default.  */
      old_chain = save_inferior_ptid ();
      inferior_ptid = child_ptid;
      child_lp = add_lwp (inferior_ptid);
      child_lp->stopped = 1;
      child_lp->last_resume_kind = resume_stop;

      /* Detach new forked process?  */
      if (detach_fork)
        {
          make_cleanup (delete_lwp_cleanup, child_lp);

          if (linux_nat_prepare_to_resume != NULL)
            linux_nat_prepare_to_resume (child_lp);

          /* When debugging an inferior in an architecture that supports
             hardware single stepping on a kernel without commit
             6580807da14c423f0d0a708108e6df6ebc8bc83d, the vfork child
             process starts with the TIF_SINGLESTEP/X86_EFLAGS_TF bits
             set if the parent process had them set.
             To work around this, single step the child process
             once before detaching to clear the flags.  */

          if (!gdbarch_software_single_step_p (target_thread_architecture
                                                   (child_lp->ptid)))
            {
              linux_disable_event_reporting (child_pid);
              if (ptrace (PTRACE_SINGLESTEP, child_pid, 0, 0) < 0)
                perror_with_name (_("Couldn't do single step"));
              if (my_waitpid (child_pid, &status, 0) < 0)
                perror_with_name (_("Couldn't wait vfork process"));
            }

          if (WIFSTOPPED (status))
            {
              int signo;

              signo = WSTOPSIG (status);
              if (signo != 0
                  && !signal_pass_state (gdb_signal_from_host (signo)))
                signo = 0;
              ptrace (PTRACE_DETACH, child_pid, 0, signo);
            }

          /* Resets value of inferior_ptid to parent ptid.  */
          do_cleanups (old_chain);
        }
      else
        {
          /* Let the thread_db layer learn about this new process.  */
          check_for_thread_db ();
        }

      do_cleanups (old_chain);

      if (has_vforked)
        {
          struct lwp_info *parent_lp;

          parent_lp = find_lwp_pid (parent_ptid);
          gdb_assert (linux_supports_tracefork () >= 0);

          if (linux_supports_tracevforkdone ())
            {
              if (debug_linux_nat)
                fprintf_unfiltered (gdb_stdlog,
                                    "LCFF: waiting for VFORK_DONE on %d\n",
                                    parent_pid);
              parent_lp->stopped = 1;

              /* We'll handle the VFORK_DONE event like any other
                 event, in target_wait.  */
            }
          else
            {
              /* We can't insert breakpoints until the child has
                 finished with the shared memory region.  We need to
                 wait until that happens.  Ideal would be to just
                 call:
                 - ptrace (PTRACE_SYSCALL, parent_pid, 0, 0);
                 - waitpid (parent_pid, &status, __WALL);
                 However, most architectures can't handle a syscall
                 being traced on the way out if it wasn't traced on
                 the way in.

                 We might also think to loop, continuing the child
                 until it exits or gets a SIGTRAP.  One problem is
                 that the child might call ptrace with PTRACE_TRACEME.

                 There's no simple and reliable way to figure out when
                 the vforked child will be done with its copy of the
                 shared memory.  We could step it out of the syscall,
                 two instructions, let it go, and then single-step the
                 parent once.  When we have hardware single-step, this
                 would work; with software single-step it could still
                 be made to work but we'd have to be able to insert
                 single-step breakpoints in the child, and we'd have
                 to insert -just- the single-step breakpoint in the
                 parent.  Very awkward.

                 In the end, the best we can do is to make sure it
                 runs for a little while.  Hopefully it will be out of
                 range of any breakpoints we reinsert.  Usually this
                 is only the single-step breakpoint at vfork's return
                 point.  */

              if (debug_linux_nat)
                fprintf_unfiltered (gdb_stdlog,
                                    "LCFF: no VFORK_DONE "
                                    "support, sleeping a bit\n");

              usleep (10000);

              /* Pretend we've seen a PTRACE_EVENT_VFORK_DONE event,
                 and leave it pending.  The next linux_nat_resume call
                 will notice a pending event, and bypasses actually
                 resuming the inferior.  */
              parent_lp->status = 0;
              parent_lp->waitstatus.kind = TARGET_WAITKIND_VFORK_DONE;
              parent_lp->stopped = 1;

              /* If we're in async mode, need to tell the event loop
                 there's something here to process.  */
              if (target_is_async_p ())
                async_file_mark ();
            }
        }
    }
  else
    {
      struct lwp_info *child_lp;

      child_lp = add_lwp (inferior_ptid);
      child_lp->stopped = 1;
      child_lp->last_resume_kind = resume_stop;

      /* Let the thread_db layer learn about this new process.  */
      check_for_thread_db ();
    }

  return 0;
}

\f
static int
linux_child_insert_fork_catchpoint (struct target_ops *self, int pid)
{
  return !linux_supports_tracefork ();
}

static int
linux_child_remove_fork_catchpoint (struct target_ops *self, int pid)
{
  return 0;
}

static int
linux_child_insert_vfork_catchpoint (struct target_ops *self, int pid)
{
  return !linux_supports_tracefork ();
}

static int
linux_child_remove_vfork_catchpoint (struct target_ops *self, int pid)
{
  return 0;
}

static int
linux_child_insert_exec_catchpoint (struct target_ops *self, int pid)
{
  return !linux_supports_tracefork ();
}

static int
linux_child_remove_exec_catchpoint (struct target_ops *self, int pid)
{
  return 0;
}

static int
linux_child_set_syscall_catchpoint (struct target_ops *self,
                                    int pid, int needed, int any_count,
                                    int table_size, int *table)
{
  if (!linux_supports_tracesysgood ())
    return 1;

  /* On GNU/Linux, we ignore the arguments.  It means that we only
     enable the syscall catchpoints, but do not disable them.

     Also, we do not use the `table' information because we do not
     filter system calls here.  We let GDB do the logic for us.  */
  return 0;
}

/* On GNU/Linux there are no real LWP's.  The closest thing to LWP's
   are processes sharing the same VM space.  A multi-threaded process
   is basically a group of such processes.  However, such a grouping
   is almost entirely a user-space issue; the kernel doesn't enforce
   such a grouping at all (this might change in the future).  In
   general, we'll rely on the threads library (i.e. the GNU/Linux
   Threads library) to provide such a grouping.

   It is perfectly well possible to write a multi-threaded application
   without the assistance of a threads library, by using the clone
   system call directly.  This module should be able to give some
   rudimentary support for debugging such applications if developers
   specify the CLONE_PTRACE flag in the clone system call, and are
   using the Linux kernel 2.4 or above.

   Note that there are some peculiarities in GNU/Linux that affect
   this code:

   - In general one should specify the __WCLONE flag to waitpid in
     order to make it report events for any of the cloned processes
     (and leave it out for the initial process).  However, if a cloned
     process has exited the exit status is only reported if the
     __WCLONE flag is absent.  Linux kernel 2.4 has a __WALL flag, but
     we cannot use it since GDB must work on older systems too.

   - When a traced, cloned process exits and is waited for by the
     debugger, the kernel reassigns it to the original parent and
     keeps it around as a "zombie".  Somehow, the GNU/Linux Threads
     library doesn't notice this, which leads to the "zombie problem":
     When debugged a multi-threaded process that spawns a lot of
     threads will run out of processes, even if the threads exit,
     because the "zombies" stay around.  */

/* List of known LWPs.  */
struct lwp_info *lwp_list;
\f

/* Original signal mask.  */
static sigset_t normal_mask;

/* Signal mask for use with sigsuspend in linux_nat_wait, initialized in
   _initialize_linux_nat.  */
static sigset_t suspend_mask;

/* Signals to block to make that sigsuspend work.  */
static sigset_t blocked_mask;

/* SIGCHLD action.  */
struct sigaction sigchld_action;

/* Block child signals (SIGCHLD and linux threads signals), and store
   the previous mask in PREV_MASK.  */

static void
block_child_signals (sigset_t *prev_mask)
{
  /* Make sure SIGCHLD is blocked.  */
  if (!sigismember (&blocked_mask, SIGCHLD))
    sigaddset (&blocked_mask, SIGCHLD);

  sigprocmask (SIG_BLOCK, &blocked_mask, prev_mask);
}

/* Restore child signals mask, previously returned by
   block_child_signals.  */

static void
restore_child_signals_mask (sigset_t *prev_mask)
{
  sigprocmask (SIG_SETMASK, prev_mask, NULL);
}

/* Mask of signals to pass directly to the inferior.  */
static sigset_t pass_mask;

/* Update signals to pass to the inferior.  */
static void
linux_nat_pass_signals (struct target_ops *self,
                        int numsigs, unsigned char *pass_signals)
{
  int signo;

  sigemptyset (&pass_mask);

  for (signo = 1; signo < NSIG; signo++)
    {
      int target_signo = gdb_signal_from_host (signo);
      if (target_signo < numsigs && pass_signals[target_signo])
        sigaddset (&pass_mask, signo);
    }
}

\f

/* Prototypes for local functions.  */
static int stop_wait_callback (struct lwp_info *lp, void *data);
static int linux_thread_alive (ptid_t ptid);
static char *linux_child_pid_to_exec_file (struct target_ops *self, int pid);
static int resume_stopped_resumed_lwps (struct lwp_info *lp, void *data);

\f

/* Destroy and free LP.  */

static void
lwp_free (struct lwp_info *lp)
{
  xfree (lp->arch_private);
  xfree (lp);
}

/* Remove all LWPs belong to PID from the lwp list.  */

static void
purge_lwp_list (int pid)
{
  struct lwp_info *lp, *lpprev, *lpnext;

  lpprev = NULL;

  for (lp = lwp_list; lp; lp = lpnext)
    {
      lpnext = lp->next;

      if (ptid_get_pid (lp->ptid) == pid)
        {
          if (lp == lwp_list)
            lwp_list = lp->next;
          else
            lpprev->next = lp->next;

          lwp_free (lp);
        }
      else
        lpprev = lp;
    }
}

/* Add the LWP specified by PTID to the list.  PTID is the first LWP
   in the process.  Return a pointer to the structure describing the
   new LWP.

   This differs from add_lwp in that we don't let the arch specific
   bits know about this new thread.  Current clients of this callback
   take the opportunity to install watchpoints in the new thread, and
   we shouldn't do that for the first thread.  If we're spawning a
   child ("run"), the thread executes the shell wrapper first, and we
   shouldn't touch it until it execs the program we want to debug.
   For "attach", it'd be okay to call the callback, but it's not
   necessary, because watchpoints can't yet have been inserted into
   the inferior.  */

static struct lwp_info *
add_initial_lwp (ptid_t ptid)
{
  struct lwp_info *lp;

  gdb_assert (ptid_lwp_p (ptid));

  lp = XNEW (struct lwp_info);

  memset (lp, 0, sizeof (struct lwp_info));

  lp->last_resume_kind = resume_continue;
  lp->waitstatus.kind = TARGET_WAITKIND_IGNORE;

  lp->ptid = ptid;
  lp->core = -1;

  lp->next = lwp_list;
  lwp_list = lp;

  return lp;
}

/* Add the LWP specified by PID to the list.  Return a pointer to the
   structure describing the new LWP.  The LWP should already be
   stopped.  */

static struct lwp_info *
add_lwp (ptid_t ptid)
{
  struct lwp_info *lp;

  lp = add_initial_lwp (ptid);

  /* Let the arch specific bits know about this new thread.  Current
     clients of this callback take the opportunity to install
     watchpoints in the new thread.  We don't do this for the first
     thread though.  See add_initial_lwp.  */
  if (linux_nat_new_thread != NULL)
    linux_nat_new_thread (lp);

  return lp;
}

/* Remove the LWP specified by PID from the list.  */

static void
delete_lwp (ptid_t ptid)
{
  struct lwp_info *lp, *lpprev;

  lpprev = NULL;

  for (lp = lwp_list; lp; lpprev = lp, lp = lp->next)
    if (ptid_equal (lp->ptid, ptid))
      break;

  if (!lp)
    return;

  if (lpprev)
    lpprev->next = lp->next;
  else
    lwp_list = lp->next;

  lwp_free (lp);
}

/* Return a pointer to the structure describing the LWP corresponding
   to PID.  If no corresponding LWP could be found, return NULL.  */

static struct lwp_info *
find_lwp_pid (ptid_t ptid)
{
  struct lwp_info *lp;
  int lwp;

  if (ptid_lwp_p (ptid))
    lwp = ptid_get_lwp (ptid);
  else
    lwp = ptid_get_pid (ptid);

  for (lp = lwp_list; lp; lp = lp->next)
    if (lwp == ptid_get_lwp (lp->ptid))
      return lp;

  return NULL;
}

/* See nat/linux-nat.h.  */

struct lwp_info *
iterate_over_lwps (ptid_t filter,
                   iterate_over_lwps_ftype callback,
                   void *data)
{
  struct lwp_info *lp, *lpnext;

  for (lp = lwp_list; lp; lp = lpnext)
    {
      lpnext = lp->next;

      if (ptid_match (lp->ptid, filter))
        {
          if ((*callback) (lp, data) != 0)
            return lp;
        }
    }

  return NULL;
}

/* Update our internal state when changing from one checkpoint to
   another indicated by NEW_PTID.  We can only switch single-threaded
   applications, so we only create one new LWP, and the previous list
   is discarded.  */

void
linux_nat_switch_fork (ptid_t new_ptid)
{
  struct lwp_info *lp;

  purge_lwp_list (ptid_get_pid (inferior_ptid));

  lp = add_lwp (new_ptid);
  lp->stopped = 1;

  /* This changes the thread's ptid while preserving the gdb thread
     num.  Also changes the inferior pid, while preserving the
     inferior num.  */
  thread_change_ptid (inferior_ptid, new_ptid);

  /* We've just told GDB core that the thread changed target id, but,
     in fact, it really is a different thread, with different register
     contents.  */
  registers_changed ();
}

/* Handle the exit of a single thread LP.  */

static void
exit_lwp (struct lwp_info *lp)
{
  struct thread_info *th = find_thread_ptid (lp->ptid);

  if (th)
    {
      if (print_thread_events)
        printf_unfiltered (_("[%s exited]\n"), target_pid_to_str (lp->ptid));

      delete_thread (lp->ptid);
    }

  delete_lwp (lp->ptid);
}

/* Wait for the LWP specified by LP, which we have just attached to.
   Returns a wait status for that LWP, to cache.  */

static int
linux_nat_post_attach_wait (ptid_t ptid, int first, int *cloned,
                            int *signalled)
{
  pid_t new_pid, pid = ptid_get_lwp (ptid);
  int status;

  if (linux_proc_pid_is_stopped (pid))
    {
      if (debug_linux_nat)
        fprintf_unfiltered (gdb_stdlog,
                            "LNPAW: Attaching to a stopped process\n");

      /* The process is definitely stopped.  It is in a job control
         stop, unless the kernel predates the TASK_STOPPED /
         TASK_TRACED distinction, in which case it might be in a
         ptrace stop.  Make sure it is in a ptrace stop; from there we
         can kill it, signal it, et cetera.

         First make sure there is a pending SIGSTOP.  Since we are
         already attached, the process can not transition from stopped
         to running without a PTRACE_CONT; so we know this signal will
         go into the queue.  The SIGSTOP generated by PTRACE_ATTACH is
         probably already in the queue (unless this kernel is old
         enough to use TASK_STOPPED for ptrace stops); but since SIGSTOP
         is not an RT signal, it can only be queued once.  */
      kill_lwp (pid, SIGSTOP);

      /* Finally, resume the stopped process.  This will deliver the SIGSTOP
         (or a higher priority signal, just like normal PTRACE_ATTACH).  */
      ptrace (PTRACE_CONT, pid, 0, 0);
    }

  /* Make sure the initial process is stopped.  The user-level threads
     layer might want to poke around in the inferior, and that won't
     work if things haven't stabilized yet.  */
  new_pid = my_waitpid (pid, &status, 0);
  if (new_pid == -1 && errno == ECHILD)
    {
      if (first)
        warning (_("%s is a cloned process"), target_pid_to_str (ptid));

      /* Try again with __WCLONE to check cloned processes.  */
      new_pid = my_waitpid (pid, &status, __WCLONE);
      *cloned = 1;
    }

  gdb_assert (pid == new_pid);

  if (!WIFSTOPPED (status))
    {
      /* The pid we tried to attach has apparently just exited.  */
      if (debug_linux_nat)
        fprintf_unfiltered (gdb_stdlog, "LNPAW: Failed to stop %d: %s",
                            pid, status_to_str (status));
      return status;
    }

  if (WSTOPSIG (status) != SIGSTOP)
    {
      *signalled = 1;
      if (debug_linux_nat)
        fprintf_unfiltered (gdb_stdlog,
                            "LNPAW: Received %s after attaching\n",
                            status_to_str (status));
    }

  return status;
}

/* Attach to the LWP specified by PID.  Return 0 if successful, -1 if
   the new LWP could not be attached, or 1 if we're already auto
   attached to this thread, but haven't processed the
   PTRACE_EVENT_CLONE event of its parent thread, so we just ignore
   its existance, without considering it an error.  */

int
lin_lwp_attach_lwp (ptid_t ptid)
{
  struct lwp_info *lp;
  int lwpid;

  gdb_assert (ptid_lwp_p (ptid));

  lp = find_lwp_pid (ptid);
  lwpid = ptid_get_lwp (ptid);

  /* We assume that we're already attached to any LWP that is already
     in our list of LWPs.  If we're not seeing exit events from threads
     and we've had PID wraparound since we last tried to stop all threads,
     this assumption might be wrong; fortunately, this is very unlikely
     to happen.  */
  if (lp == NULL)
    {
      int status, cloned = 0, signalled = 0;

      if (ptrace (PTRACE_ATTACH, lwpid, 0, 0) < 0)
        {
          if (linux_supports_tracefork ())
            {
              /* If we haven't stopped all threads when we get here,
                 we may have seen a thread listed in thread_db's list,
                 but not processed the PTRACE_EVENT_CLONE yet.  If
                 that's the case, ignore this new thread, and let
                 normal event handling discover it later.  */
              if (in_pid_list_p (stopped_pids, lwpid))
                {
                  /* We've already seen this thread stop, but we
                     haven't seen the PTRACE_EVENT_CLONE extended
                     event yet.  */
                  if (debug_linux_nat)
                    fprintf_unfiltered (gdb_stdlog,
                                        "LLAL: attach failed, but already seen "
                                        "this thread %s stop\n",
                                        target_pid_to_str (ptid));
                  return 1;
                }
              else
                {
                  int new_pid;
                  int status;

                  if (debug_linux_nat)
                    fprintf_unfiltered (gdb_stdlog,
                                        "LLAL: attach failed, and haven't seen "
                                        "this thread %s stop yet\n",
                                        target_pid_to_str (ptid));

                  /* We may or may not be attached to the LWP already.
                     Try waitpid on it.  If that errors, we're not
                     attached to the LWP yet.  Otherwise, we're
                     already attached.  */
                  gdb_assert (lwpid > 0);
                  new_pid = my_waitpid (lwpid, &status, WNOHANG);
                  if (new_pid == -1 && errno == ECHILD)
                    new_pid = my_waitpid (lwpid, &status, __WCLONE | WNOHANG);
                  if (new_pid != -1)
                    {
                      if (new_pid == 0)
                        {
                          /* The child hasn't stopped for its initial
                             SIGSTOP stop yet.  */
                          if (debug_linux_nat)
                            fprintf_unfiltered (gdb_stdlog,
                                                "LLAL: child hasn't "
                                                "stopped yet\n");
                        }
                      else if (WIFSTOPPED (status))
                        {
                          if (debug_linux_nat)
                            fprintf_unfiltered (gdb_stdlog,
                                                "LLAL: adding to stopped_pids\n");
                          add_to_pid_list (&stopped_pids, lwpid, status);
                        }
                      return 1;
                    }
                }
            }

          /* If we fail to attach to the thread, issue a warning,
             but continue.  One way this can happen is if thread
             creation is interrupted; as of Linux kernel 2.6.19, a
             bug may place threads in the thread list and then fail
             to create them.  */
          warning (_("Can't attach %s: %s"), target_pid_to_str (ptid),
                   safe_strerror (errno));
          return -1;
        }

      if (debug_linux_nat)
        fprintf_unfiltered (gdb_stdlog,
                            "LLAL: PTRACE_ATTACH %s, 0, 0 (OK)\n",
                            target_pid_to_str (ptid));

      status = linux_nat_post_attach_wait (ptid, 0, &cloned, &signalled);
      if (!WIFSTOPPED (status))
        return 1;

      lp = add_lwp (ptid);
      lp->stopped = 1;
      lp->last_resume_kind = resume_stop;
      lp->cloned = cloned;
      lp->signalled = signalled;
      if (WSTOPSIG (status) != SIGSTOP)
        {
          lp->resumed = 1;
          lp->status = status;
        }

      target_post_attach (ptid_get_lwp (lp->ptid));

      if (debug_linux_nat)
        {
          fprintf_unfiltered (gdb_stdlog,
                              "LLAL: waitpid %s received %s\n",
                              target_pid_to_str (ptid),
                              status_to_str (status));
        }
    }

  return 0;
}

static void
linux_nat_create_inferior (struct target_ops *ops, 
                           char *exec_file, char *allargs, char **env,
                           int from_tty)
{
  struct cleanup *restore_personality
    = maybe_disable_address_space_randomization (disable_randomization);

  /* The fork_child mechanism is synchronous and calls target_wait, so
     we have to mask the async mode.  */

  /* Make sure we report all signals during startup.  */
  linux_nat_pass_signals (ops, 0, NULL);

  linux_ops->to_create_inferior (ops, exec_file, allargs, env, from_tty);

  do_cleanups (restore_personality);
}

/* Callback for linux_proc_attach_tgid_threads.  Attach to PTID if not
   already attached.  Returns true if a new LWP is found, false
   otherwise.  */

static int
attach_proc_task_lwp_callback (ptid_t ptid)
{
  struct lwp_info *lp;

  /* Ignore LWPs we're already attached to.  */
  lp = find_lwp_pid (ptid);
  if (lp == NULL)
    {
      int lwpid = ptid_get_lwp (ptid);

      if (ptrace (PTRACE_ATTACH, lwpid, 0, 0) < 0)
        {
          int err = errno;

          /* Be quiet if we simply raced with the thread exiting.
             EPERM is returned if the thread's task still exists, and
             is marked as exited or zombie, as well as other
             conditions, so in that case, confirm the status in
             /proc/PID/status.  */
          if (err == ESRCH
              || (err == EPERM && linux_proc_pid_is_gone (lwpid)))
            {
              if (debug_linux_nat)
                {
                  fprintf_unfiltered (gdb_stdlog,
                                      "Cannot attach to lwp %d: "
                                      "thread is gone (%d: %s)\n",
                                      lwpid, err, safe_strerror (err));
                }
            }
          else
            {
              warning (_("Cannot attach to lwp %d: %s"),
                       lwpid,
                       linux_ptrace_attach_fail_reason_string (ptid,
                                                               err));
            }
        }
      else
        {
          if (debug_linux_nat)
            fprintf_unfiltered (gdb_stdlog,
                                "PTRACE_ATTACH %s, 0, 0 (OK)\n",
                                target_pid_to_str (ptid));

          lp = add_lwp (ptid);
          lp->cloned = 1;

          /* The next time we wait for this LWP we'll see a SIGSTOP as
             PTRACE_ATTACH brings it to a halt.  */
          lp->signalled = 1;

          /* We need to wait for a stop before being able to make the
             next ptrace call on this LWP.  */
          lp->must_set_ptrace_flags = 1;
        }

      return 1;
    }
  return 0;
}

static void
linux_nat_attach (struct target_ops *ops, const char *args, int from_tty)
{
  struct lwp_info *lp;
  int status;
  ptid_t ptid;

  /* Make sure we report all signals during attach.  */
  linux_nat_pass_signals (ops, 0, NULL);

  TRY
    {
      linux_ops->to_attach (ops, args, from_tty);
    }
  CATCH (ex, RETURN_MASK_ERROR)
    {
      pid_t pid = parse_pid_to_attach (args);
      struct buffer buffer;
      char *message, *buffer_s;

      message = xstrdup (ex.message);
      make_cleanup (xfree, message);

      buffer_init (&buffer);
      linux_ptrace_attach_fail_reason (pid, &buffer);

      buffer_grow_str0 (&buffer, "");
      buffer_s = buffer_finish (&buffer);
      make_cleanup (xfree, buffer_s);

      if (*buffer_s != '\0')
        throw_error (ex.error, "warning: %s\n%s", buffer_s, message);
      else
        throw_error (ex.error, "%s", message);
    }
  END_CATCH

  /* The ptrace base target adds the main thread with (pid,0,0)
     format.  Decorate it with lwp info.  */
  ptid = ptid_build (ptid_get_pid (inferior_ptid),
                     ptid_get_pid (inferior_ptid),
                     0);
  thread_change_ptid (inferior_ptid, ptid);

  /* Add the initial process as the first LWP to the list.  */
  lp = add_initial_lwp (ptid);

  status = linux_nat_post_attach_wait (lp->ptid, 1, &lp->cloned,
                                       &lp->signalled);
  if (!WIFSTOPPED (status))
    {
      if (WIFEXITED (status))
        {
          int exit_code = WEXITSTATUS (status);

          target_terminal_ours ();
          target_mourn_inferior ();
          if (exit_code == 0)
            error (_("Unable to attach: program exited normally."));
          else
            error (_("Unable to attach: program exited with code %d."),
                   exit_code);
        }
      else if (WIFSIGNALED (status))
        {
          enum gdb_signal signo;

          target_terminal_ours ();
          target_mourn_inferior ();

          signo = gdb_signal_from_host (WTERMSIG (status));
          error (_("Unable to attach: program terminated with signal "
                   "%s, %s."),
                 gdb_signal_to_name (signo),
                 gdb_signal_to_string (signo));
        }

      internal_error (__FILE__, __LINE__,
                      _("unexpected status %d for PID %ld"),
                      status, (long) ptid_get_lwp (ptid));
    }

  lp->stopped = 1;

  /* Save the wait status to report later.  */
  lp->resumed = 1;
  if (debug_linux_nat)
    fprintf_unfiltered (gdb_stdlog,
                        "LNA: waitpid %ld, saving status %s\n",
                        (long) ptid_get_pid (lp->ptid), status_to_str (status));

  lp->status = status;

  /* We must attach to every LWP.  If /proc is mounted, use that to
     find them now.  The inferior may be using raw clone instead of
     using pthreads.  But even if it is using pthreads, thread_db
     walks structures in the inferior's address space to find the list
     of threads/LWPs, and those structures may well be corrupted.
     Note that once thread_db is loaded, we'll still use it to list
     threads and associate pthread info with each LWP.  */
  linux_proc_attach_tgid_threads (ptid_get_pid (lp->ptid),
                                  attach_proc_task_lwp_callback);

  if (target_can_async_p ())
    target_async (1);
}

/* Get pending status of LP.  */
static int
get_pending_status (struct lwp_info *lp, int *status)
{
  enum gdb_signal signo = GDB_SIGNAL_0;

  /* If we paused threads momentarily, we may have stored pending
     events in lp->status or lp->waitstatus (see stop_wait_callback),
     and GDB core hasn't seen any signal for those threads.
     Otherwise, the last signal reported to the core is found in the
     thread object's stop_signal.

     There's a corner case that isn't handled here at present.  Only
     if the thread stopped with a TARGET_WAITKIND_STOPPED does
     stop_signal make sense as a real signal to pass to the inferior.
     Some catchpoint related events, like
     TARGET_WAITKIND_(V)FORK|EXEC|SYSCALL, have their stop_signal set
     to GDB_SIGNAL_SIGTRAP when the catchpoint triggers.  But,
     those traps are debug API (ptrace in our case) related and
     induced; the inferior wouldn't see them if it wasn't being
     traced.  Hence, we should never pass them to the inferior, even
     when set to pass state.  Since this corner case isn't handled by
     infrun.c when proceeding with a signal, for consistency, neither
     do we handle it here (or elsewhere in the file we check for
     signal pass state).  Normally SIGTRAP isn't set to pass state, so
     this is really a corner case.  */

  if (lp->waitstatus.kind != TARGET_WAITKIND_IGNORE)
    signo = GDB_SIGNAL_0; /* a pending ptrace event, not a real signal.  */
  else if (lp->status)
    signo = gdb_signal_from_host (WSTOPSIG (lp->status));
  else if (target_is_non_stop_p () && !is_executing (lp->ptid))
    {
      struct thread_info *tp = find_thread_ptid (lp->ptid);

      signo = tp->suspend.stop_signal;
    }
  else if (!target_is_non_stop_p ())
    {
      struct target_waitstatus last;
      ptid_t last_ptid;

      get_last_target_status (&last_ptid, &last);

      if (ptid_get_lwp (lp->ptid) == ptid_get_lwp (last_ptid))
        {
          struct thread_info *tp = find_thread_ptid (lp->ptid);

          signo = tp->suspend.stop_signal;
        }
    }

  *status = 0;

  if (signo == GDB_SIGNAL_0)
    {
      if (debug_linux_nat)
        fprintf_unfiltered (gdb_stdlog,
                            "GPT: lwp %s has no pending signal\n",
                            target_pid_to_str (lp->ptid));
    }
  else if (!signal_pass_state (signo))
    {
      if (debug_linux_nat)
        fprintf_unfiltered (gdb_stdlog,
                            "GPT: lwp %s had signal %s, "
                            "but it is in no pass state\n",
                            target_pid_to_str (lp->ptid),
                            gdb_signal_to_string (signo));
    }
  else
    {
      *status = W_STOPCODE (gdb_signal_to_host (signo));

      if (debug_linux_nat)
        fprintf_unfiltered (gdb_stdlog,
                            "GPT: lwp %s has pending signal %s\n",
                            target_pid_to_str (lp->ptid),
                            gdb_signal_to_string (signo));
    }

  return 0;
}

static int
detach_callback (struct lwp_info *lp, void *data)
{
  gdb_assert (lp->status == 0 || WIFSTOPPED (lp->status));

  if (debug_linux_nat && lp->status)
    fprintf_unfiltered (gdb_stdlog, "DC:  Pending %s for %s on detach.\n",
                        strsignal (WSTOPSIG (lp->status)),
                        target_pid_to_str (lp->ptid));

  /* If there is a pending SIGSTOP, get rid of it.  */
  if (lp->signalled)
    {
      if (debug_linux_nat)
        fprintf_unfiltered (gdb_stdlog,
                            "DC: Sending SIGCONT to %s\n",
                            target_pid_to_str (lp->ptid));

      kill_lwp (ptid_get_lwp (lp->ptid), SIGCONT);
      lp->signalled = 0;
    }

  /* We don't actually detach from the LWP that has an id equal to the
     overall process id just yet.  */
  if (ptid_get_lwp (lp->ptid) != ptid_get_pid (lp->ptid))
    {
      int status = 0;

      /* Pass on any pending signal for this LWP.  */
      get_pending_status (lp, &status);

      if (linux_nat_prepare_to_resume != NULL)
        linux_nat_prepare_to_resume (lp);
      errno = 0;
      if (ptrace (PTRACE_DETACH, ptid_get_lwp (lp->ptid), 0,
                  WSTOPSIG (status)) < 0)
        error (_("Can't detach %s: %s"), target_pid_to_str (lp->ptid),
               safe_strerror (errno));

      if (debug_linux_nat)
        fprintf_unfiltered (gdb_stdlog,
                            "PTRACE_DETACH (%s, %s, 0) (OK)\n",
                            target_pid_to_str (lp->ptid),
                            strsignal (WSTOPSIG (status)));

      delete_lwp (lp->ptid);
    }

  return 0;
}

static void
linux_nat_detach (struct target_ops *ops, const char *args, int from_tty)
{
  int pid;
  int status;
  struct lwp_info *main_lwp;

  pid = ptid_get_pid (inferior_ptid);

  /* Don't unregister from the event loop, as there may be other
     inferiors running. */

  /* Stop all threads before detaching.  ptrace requires that the
     thread is stopped to sucessfully detach.  */
  iterate_over_lwps (pid_to_ptid (pid), stop_callback, NULL);
  /* ... and wait until all of them have reported back that
     they're no longer running.  */
  iterate_over_lwps (pid_to_ptid (pid), stop_wait_callback, NULL);

  iterate_over_lwps (pid_to_ptid (pid), detach_callback, NULL);

  /* Only the initial process should be left right now.  */
  gdb_assert (num_lwps (ptid_get_pid (inferior_ptid)) == 1);

  main_lwp = find_lwp_pid (pid_to_ptid (pid));

  /* Pass on any pending signal for the last LWP.  */
  if ((args == NULL || *args == '\0')
      && get_pending_status (main_lwp, &status) != -1
      && WIFSTOPPED (status))
    {
      char *tem;

      /* Put the signal number in ARGS so that inf_ptrace_detach will
         pass it along with PTRACE_DETACH.  */
      tem = alloca (8);
      xsnprintf (tem, 8, "%d", (int) WSTOPSIG (status));
      args = tem;
      if (debug_linux_nat)
        fprintf_unfiltered (gdb_stdlog,
                            "LND: Sending signal %s to %s\n",
                            args,
                            target_pid_to_str (main_lwp->ptid));
    }

  if (linux_nat_prepare_to_resume != NULL)
    linux_nat_prepare_to_resume (main_lwp);
  delete_lwp (main_lwp->ptid);

  if (forks_exist_p ())
    {
      /* Multi-fork case.  The current inferior_ptid is being detached
         from, but there are other viable forks to debug.  Detach from
         the current fork, and context-switch to the first
         available.  */
      linux_fork_detach (args, from_tty);
    }
  else
    linux_ops->to_detach (ops, args, from_tty);
}

/* Resume execution of the inferior process.  If STEP is nonzero,
   single-step it.  If SIGNAL is nonzero, give it that signal.  */

static void
linux_resume_one_lwp_throw (struct lwp_info *lp, int step,
                            enum gdb_signal signo)
{
  lp->step = step;

  /* stop_pc doubles as the PC the LWP had when it was last resumed.
     We only presently need that if the LWP is stepped though (to
     handle the case of stepping a breakpoint instruction).  */
  if (step)
    {
      struct regcache *regcache = get_thread_regcache (lp->ptid);

      lp->stop_pc = regcache_read_pc (regcache);
    }
  else
    lp->stop_pc = 0;

  if (linux_nat_prepare_to_resume != NULL)
    linux_nat_prepare_to_resume (lp);
  linux_ops->to_resume (linux_ops, lp->ptid, step, signo);

  /* Successfully resumed.  Clear state that no longer makes sense,
     and mark the LWP as running.  Must not do this before resuming
     otherwise if that fails other code will be confused.  E.g., we'd
     later try to stop the LWP and hang forever waiting for a stop
     status.  Note that we must not throw after this is cleared,
     otherwise handle_zombie_lwp_error would get confused.  */
  lp->stopped = 0;
  lp->stop_reason = TARGET_STOPPED_BY_NO_REASON;
  registers_changed_ptid (lp->ptid);
}

/* Called when we try to resume a stopped LWP and that errors out.  If
   the LWP is no longer in ptrace-stopped state (meaning it's zombie,
   or about to become), discard the error, clear any pending status
   the LWP may have, and return true (we'll collect the exit status
   soon enough).  Otherwise, return false.  */

static int
check_ptrace_stopped_lwp_gone (struct lwp_info *lp)
{
  /* If we get an error after resuming the LWP successfully, we'd
     confuse !T state for the LWP being gone.  */
  gdb_assert (lp->stopped);

  /* We can't just check whether the LWP is in 'Z (Zombie)' state,
     because even if ptrace failed with ESRCH, the tracee may be "not
     yet fully dead", but already refusing ptrace requests.  In that
     case the tracee has 'R (Running)' state for a little bit
     (observed in Linux 3.18).  See also the note on ESRCH in the
     ptrace(2) man page.  Instead, check whether the LWP has any state
     other than ptrace-stopped.  */

  /* Don't assume anything if /proc/PID/status can't be read.  */
  if (linux_proc_pid_is_trace_stopped_nowarn (ptid_get_lwp (lp->ptid)) == 0)
    {
      lp->stop_reason = TARGET_STOPPED_BY_NO_REASON;
      lp->status = 0;
      lp->waitstatus.kind = TARGET_WAITKIND_IGNORE;
      return 1;
    }
  return 0;
}

/* Like linux_resume_one_lwp_throw, but no error is thrown if the LWP
   disappears while we try to resume it.  */

static void
linux_resume_one_lwp (struct lwp_info *lp, int step, enum gdb_signal signo)
{
  TRY
    {
      linux_resume_one_lwp_throw (lp, step, signo);
    }
  CATCH (ex, RETURN_MASK_ERROR)
    {
      if (!check_ptrace_stopped_lwp_gone (lp))
        throw_exception (ex);
    }
  END_CATCH
}

/* Resume LP.  */

static void
resume_lwp (struct lwp_info *lp, int step, enum gdb_signal signo)
{
  if (lp->stopped)
    {
      struct inferior *inf = find_inferior_ptid (lp->ptid);

      if (inf->vfork_child != NULL)
        {
          if (debug_linux_nat)
            fprintf_unfiltered (gdb_stdlog,
                                "RC: Not resuming %s (vfork parent)\n",
                                target_pid_to_str (lp->ptid));
        }
      else if (!lwp_status_pending_p (lp))
        {
          if (debug_linux_nat)
            fprintf_unfiltered (gdb_stdlog,
                                "RC: Resuming sibling %s, %s, %s\n",
                                target_pid_to_str (lp->ptid),
                                (signo != GDB_SIGNAL_0
                                 ? strsignal (gdb_signal_to_host (signo))
                                 : "0"),
                                step ? "step" : "resume");

          linux_resume_one_lwp (lp, step, signo);
        }
      else
        {
          if (debug_linux_nat)
            fprintf_unfiltered (gdb_stdlog,
                                "RC: Not resuming sibling %s (has pending)\n",
                                target_pid_to_str (lp->ptid));
        }
    }
  else
    {
      if (debug_linux_nat)
        fprintf_unfiltered (gdb_stdlog,
                            "RC: Not resuming sibling %s (not stopped)\n",
                            target_pid_to_str (lp->ptid));
    }
}

/* Callback for iterate_over_lwps.  If LWP is EXCEPT, do nothing.
   Resume LWP with the last stop signal, if it is in pass state.  */

static int
linux_nat_resume_callback (struct lwp_info *lp, void *except)
{
  enum gdb_signal signo = GDB_SIGNAL_0;

  if (lp == except)
    return 0;

  if (lp->stopped)
    {
      struct thread_info *thread;

      thread = find_thread_ptid (lp->ptid);
      if (thread != NULL)
        {
          signo = thread->suspend.stop_signal;
          thread->suspend.stop_signal = GDB_SIGNAL_0;
        }
    }

  resume_lwp (lp, 0, signo);
  return 0;
}

static int
resume_clear_callback (struct lwp_info *lp, void *data)
{
  lp->resumed = 0;
  lp->last_resume_kind = resume_stop;
  return 0;
}

static int
resume_set_callback (struct lwp_info *lp, void *data)
{
  lp->resumed = 1;
  lp->last_resume_kind = resume_continue;
  return 0;
}

static void
linux_nat_resume (struct target_ops *ops,
                  ptid_t ptid, int step, enum gdb_signal signo)
{
  struct lwp_info *lp;
  int resume_many;

  if (debug_linux_nat)
    fprintf_unfiltered (gdb_stdlog,
                        "LLR: Preparing to %s %s, %s, inferior_ptid %s\n",
                        step ? "step" : "resume",
                        target_pid_to_str (ptid),
                        (signo != GDB_SIGNAL_0
                         ? strsignal (gdb_signal_to_host (signo)) : "0"),
                        target_pid_to_str (inferior_ptid));

  /* A specific PTID means `step only this process id'.  */
  resume_many = (ptid_equal (minus_one_ptid, ptid)
                 || ptid_is_pid (ptid));

  /* Mark the lwps we're resuming as resumed.  */
  iterate_over_lwps (ptid, resume_set_callback, NULL);

  /* See if it's the current inferior that should be handled
     specially.  */
  if (resume_many)
    lp = find_lwp_pid (inferior_ptid);
  else
    lp = find_lwp_pid (ptid);
  gdb_assert (lp != NULL);

  /* Remember if we're stepping.  */
  lp->last_resume_kind = step ? resume_step : resume_continue;

  /* If we have a pending wait status for this thread, there is no
     point in resuming the process.  But first make sure that
     linux_nat_wait won't preemptively handle the event - we
     should never take this short-circuit if we are going to
     leave LP running, since we have skipped resuming all the
     other threads.  This bit of code needs to be synchronized
     with linux_nat_wait.  */

  if (lp->status && WIFSTOPPED (lp->status))
    {
      if (!lp->step
          && WSTOPSIG (lp->status)
          && sigismember (&pass_mask, WSTOPSIG (lp->status)))
        {
          if (debug_linux_nat)
            fprintf_unfiltered (gdb_stdlog,
                                "LLR: Not short circuiting for ignored "
                                "status 0x%x\n", lp->status);

          /* FIXME: What should we do if we are supposed to continue
             this thread with a signal?  */
          gdb_assert (signo == GDB_SIGNAL_0);
          signo = gdb_signal_from_host (WSTOPSIG (lp->status));
          lp->status = 0;
        }
    }

  if (lwp_status_pending_p (lp))
    {
      /* FIXME: What should we do if we are supposed to continue
         this thread with a signal?  */
      gdb_assert (signo == GDB_SIGNAL_0);

      if (debug_linux_nat)
        fprintf_unfiltered (gdb_stdlog,
                            "LLR: Short circuiting for status 0x%x\n",
                            lp->status);

      if (target_can_async_p ())
        {
          target_async (1);
          /* Tell the event loop we have something to process.  */
          async_file_mark ();
        }
      return;
    }

  if (resume_many)
    iterate_over_lwps (ptid, linux_nat_resume_callback, lp);

  if (debug_linux_nat)
    fprintf_unfiltered (gdb_stdlog,
                        "LLR: %s %s, %s (resume event thread)\n",
                        step ? "PTRACE_SINGLESTEP" : "PTRACE_CONT",
                        target_pid_to_str (lp->ptid),
                        (signo != GDB_SIGNAL_0
                         ? strsignal (gdb_signal_to_host (signo)) : "0"));

  linux_resume_one_lwp (lp, step, signo);

  if (target_can_async_p ())
    target_async (1);
}

/* Send a signal to an LWP.  */

static int
kill_lwp (int lwpid, int signo)
{
  /* Use tkill, if possible, in case we are using nptl threads.  If tkill
     fails, then we are not using nptl threads and we should be using kill.  */

#ifdef HAVE_TKILL_SYSCALL
  {
    static int tkill_failed;

    if (!tkill_failed)
      {
        int ret;

        errno = 0;
        ret = syscall (__NR_tkill, lwpid, signo);
        if (errno != ENOSYS)
          return ret;
        tkill_failed = 1;
      }
  }
#endif

  return kill (lwpid, signo);
}

/* Handle a GNU/Linux syscall trap wait response.  If we see a syscall
   event, check if the core is interested in it: if not, ignore the
   event, and keep waiting; otherwise, we need to toggle the LWP's
   syscall entry/exit status, since the ptrace event itself doesn't
   indicate it, and report the trap to higher layers.  */

static int
linux_handle_syscall_trap (struct lwp_info *lp, int stopping)
{
  struct target_waitstatus *ourstatus = &lp->waitstatus;
  struct gdbarch *gdbarch = target_thread_architecture (lp->ptid);
  int syscall_number = (int) gdbarch_get_syscall_number (gdbarch, lp->ptid);

  if (stopping)
    {
      /* If we're stopping threads, there's a SIGSTOP pending, which
         makes it so that the LWP reports an immediate syscall return,
         followed by the SIGSTOP.  Skip seeing that "return" using
         PTRACE_CONT directly, and let stop_wait_callback collect the
         SIGSTOP.  Later when the thread is resumed, a new syscall
         entry event.  If we didn't do this (and returned 0), we'd
         leave a syscall entry pending, and our caller, by using
         PTRACE_CONT to collect the SIGSTOP, skips the syscall return
         itself.  Later, when the user re-resumes this LWP, we'd see
         another syscall entry event and we'd mistake it for a return.

         If stop_wait_callback didn't force the SIGSTOP out of the LWP
         (leaving immediately with LWP->signalled set, without issuing
         a PTRACE_CONT), it would still be problematic to leave this
         syscall enter pending, as later when the thread is resumed,
         it would then see the same syscall exit mentioned above,
         followed by the delayed SIGSTOP, while the syscall didn't
         actually get to execute.  It seems it would be even more
         confusing to the user.  */

      if (debug_linux_nat)
        fprintf_unfiltered (gdb_stdlog,
                            "LHST: ignoring syscall %d "
                            "for LWP %ld (stopping threads), "
                            "resuming with PTRACE_CONT for SIGSTOP\n",
                            syscall_number,
                            ptid_get_lwp (lp->ptid));

      lp->syscall_state = TARGET_WAITKIND_IGNORE;
      ptrace (PTRACE_CONT, ptid_get_lwp (lp->ptid), 0, 0);
      lp->stopped = 0;
      return 1;
    }

  if (catch_syscall_enabled ())
    {
      /* Always update the entry/return state, even if this particular
         syscall isn't interesting to the core now.  In async mode,
         the user could install a new catchpoint for this syscall
         between syscall enter/return, and we'll need to know to
         report a syscall return if that happens.  */
      lp->syscall_state = (lp->syscall_state == TARGET_WAITKIND_SYSCALL_ENTRY
                           ? TARGET_WAITKIND_SYSCALL_RETURN
                           : TARGET_WAITKIND_SYSCALL_ENTRY);

      if (catching_syscall_number (syscall_number))
        {
          /* Alright, an event to report.  */
          ourstatus->kind = lp->syscall_state;
          ourstatus->value.syscall_number = syscall_number;

          if (debug_linux_nat)
            fprintf_unfiltered (gdb_stdlog,
                                "LHST: stopping for %s of syscall %d"
                                " for LWP %ld\n",
                                lp->syscall_state
                                == TARGET_WAITKIND_SYSCALL_ENTRY
                                ? "entry" : "return",
                                syscall_number,
                                ptid_get_lwp (lp->ptid));
          return 0;
        }

      if (debug_linux_nat)
        fprintf_unfiltered (gdb_stdlog,
                            "LHST: ignoring %s of syscall %d "
                            "for LWP %ld\n",
                            lp->syscall_state == TARGET_WAITKIND_SYSCALL_ENTRY
                            ? "entry" : "return",
                            syscall_number,
                            ptid_get_lwp (lp->ptid));
    }
  else
    {
      /* If we had been syscall tracing, and hence used PT_SYSCALL
         before on this LWP, it could happen that the user removes all
         syscall catchpoints before we get to process this event.
         There are two noteworthy issues here:

         - When stopped at a syscall entry event, resuming with
           PT_STEP still resumes executing the syscall and reports a
           syscall return.

         - Only PT_SYSCALL catches syscall enters.  If we last
           single-stepped this thread, then this event can't be a
           syscall enter.  If we last single-stepped this thread, this
           has to be a syscall exit.

         The points above mean that the next resume, be it PT_STEP or
         PT_CONTINUE, can not trigger a syscall trace event.  */
      if (debug_linux_nat)
        fprintf_unfiltered (gdb_stdlog,
                            "LHST: caught syscall event "
                            "with no syscall catchpoints."
                            " %d for LWP %ld, ignoring\n",
                            syscall_number,
                            ptid_get_lwp (lp->ptid));
      lp->syscall_state = TARGET_WAITKIND_IGNORE;
    }

  /* The core isn't interested in this event.  For efficiency, avoid
     stopping all threads only to have the core resume them all again.
     Since we're not stopping threads, if we're still syscall tracing
     and not stepping, we can't use PTRACE_CONT here, as we'd miss any
     subsequent syscall.  Simply resume using the inf-ptrace layer,
     which knows when to use PT_SYSCALL or PT_CONTINUE.  */

  linux_resume_one_lwp (lp, lp->step, GDB_SIGNAL_0);
  return 1;
}

/* Handle a GNU/Linux extended wait response.  If we see a clone
   event, we need to add the new LWP to our list (and not report the
   trap to higher layers).  This function returns non-zero if the
   event should be ignored and we should wait again.  If STOPPING is
   true, the new LWP remains stopped, otherwise it is continued.  */

static int
linux_handle_extended_wait (struct lwp_info *lp, int status)
{
  int pid = ptid_get_lwp (lp->ptid);
  struct target_waitstatus *ourstatus = &lp->waitstatus;
  int event = linux_ptrace_get_extended_event (status);

  if (event == PTRACE_EVENT_FORK || event == PTRACE_EVENT_VFORK
      || event == PTRACE_EVENT_CLONE)
    {
      unsigned long new_pid;
      int ret;

      ptrace (PTRACE_GETEVENTMSG, pid, 0, &new_pid);

      /* If we haven't already seen the new PID stop, wait for it now.  */
      if (! pull_pid_from_list (&stopped_pids, new_pid, &status))
        {
          /* The new child has a pending SIGSTOP.  We can't affect it until it
             hits the SIGSTOP, but we're already attached.  */
          ret = my_waitpid (new_pid, &status,
                            (event == PTRACE_EVENT_CLONE) ? __WCLONE : 0);
          if (ret == -1)
            perror_with_name (_("waiting for new child"));
          else if (ret != new_pid)
            internal_error (__FILE__, __LINE__,
                            _("wait returned unexpected PID %d"), ret);
          else if (!WIFSTOPPED (status))
            internal_error (__FILE__, __LINE__,
                            _("wait returned unexpected status 0x%x"), status);
        }

      ourstatus->value.related_pid = ptid_build (new_pid, new_pid, 0);

      if (event == PTRACE_EVENT_FORK || event == PTRACE_EVENT_VFORK)
        {
          /* The arch-specific native code may need to know about new
             forks even if those end up never mapped to an
             inferior.  */
          if (linux_nat_new_fork != NULL)
            linux_nat_new_fork (lp, new_pid);
        }

      if (event == PTRACE_EVENT_FORK
          && linux_fork_checkpointing_p (ptid_get_pid (lp->ptid)))
        {
          /* Handle checkpointing by linux-fork.c here as a special
             case.  We don't want the follow-fork-mode or 'catch fork'
             to interfere with this.  */

          /* This won't actually modify the breakpoint list, but will
             physically remove the breakpoints from the child.  */
          detach_breakpoints (ptid_build (new_pid, new_pid, 0));

          /* Retain child fork in ptrace (stopped) state.  */
          if (!find_fork_pid (new_pid))
            add_fork (new_pid);

          /* Report as spurious, so that infrun doesn't want to follow
             this fork.  We're actually doing an infcall in
             linux-fork.c.  */
          ourstatus->kind = TARGET_WAITKIND_SPURIOUS;

          /* Report the stop to the core.  */
          return 0;
        }

      if (event == PTRACE_EVENT_FORK)
        ourstatus->kind = TARGET_WAITKIND_FORKED;
      else if (event == PTRACE_EVENT_VFORK)
        ourstatus->kind = TARGET_WAITKIND_VFORKED;
      else if (event == PTRACE_EVENT_CLONE)
        {
          struct lwp_info *new_lp;

          ourstatus->kind = TARGET_WAITKIND_IGNORE;

          if (debug_linux_nat)
            fprintf_unfiltered (gdb_stdlog,
                                "LHEW: Got clone event "
                                "from LWP %d, new child is LWP %ld\n",
                                pid, new_pid);

          new_lp = add_lwp (ptid_build (ptid_get_pid (lp->ptid), new_pid, 0));
          new_lp->cloned = 1;
          new_lp->stopped = 1;
          new_lp->resumed = 1;

          /* If the thread_db layer is active, let it record the user
             level thread id and status, and add the thread to GDB's
             list.  */
          if (!thread_db_notice_clone (lp->ptid, new_lp->ptid))
            {
              /* The process is not using thread_db.  Add the LWP to
                 GDB's list.  */
              target_post_attach (ptid_get_lwp (new_lp->ptid));
              add_thread (new_lp->ptid);
            }

          /* Even if we're stopping the thread for some reason
             internal to this module, from the perspective of infrun
             and the user/frontend, this new thread is running until
             it next reports a stop.  */
          set_running (new_lp->ptid, 1);
          set_executing (new_lp->ptid, 1);

          if (WSTOPSIG (status) != SIGSTOP)
            {
              /* This can happen if someone starts sending signals to
                 the new thread before it gets a chance to run, which
                 have a lower number than SIGSTOP (e.g. SIGUSR1).
                 This is an unlikely case, and harder to handle for
                 fork / vfork than for clone, so we do not try - but
                 we handle it for clone events here.  */

              new_lp->signalled = 1;

              /* We created NEW_LP so it cannot yet contain STATUS.  */
              gdb_assert (new_lp->status == 0);

              /* Save the wait status to report later.  */
              if (debug_linux_nat)
                fprintf_unfiltered (gdb_stdlog,
                                    "LHEW: waitpid of new LWP %ld, "
                                    "saving status %s\n",
                                    (long) ptid_get_lwp (new_lp->ptid),
                                    status_to_str (status));
              new_lp->status = status;
            }

          return 1;
        }

      return 0;
    }

  if (event == PTRACE_EVENT_EXEC)
    {
      if (debug_linux_nat)
        fprintf_unfiltered (gdb_stdlog,
                            "LHEW: Got exec event from LWP %ld\n",
                            ptid_get_lwp (lp->ptid));

      ourstatus->kind = TARGET_WAITKIND_EXECD;
      ourstatus->value.execd_pathname
        = xstrdup (linux_child_pid_to_exec_file (NULL, pid));

      /* The thread that execed must have been resumed, but, when a
         thread execs, it changes its tid to the tgid, and the old
         tgid thread might have not been resumed.  */
      lp->resumed = 1;
      return 0;
    }

  if (event == PTRACE_EVENT_VFORK_DONE)
    {
      if (current_inferior ()->waiting_for_vfork_done)
        {
          if (debug_linux_nat)
            fprintf_unfiltered (gdb_stdlog,
                                "LHEW: Got expected PTRACE_EVENT_"
                                "VFORK_DONE from LWP %ld: stopping\n",
                                ptid_get_lwp (lp->ptid));

          ourstatus->kind = TARGET_WAITKIND_VFORK_DONE;
          return 0;
        }

      if (debug_linux_nat)
        fprintf_unfiltered (gdb_stdlog,
                            "LHEW: Got PTRACE_EVENT_VFORK_DONE "
                            "from LWP %ld: ignoring\n",
                            ptid_get_lwp (lp->ptid));
      return 1;
    }

  internal_error (__FILE__, __LINE__,
                  _("unknown ptrace event %d"), event);
}

/* Wait for LP to stop.  Returns the wait status, or 0 if the LWP has
   exited.  */

static int
wait_lwp (struct lwp_info *lp)
{
  pid_t pid;
  int status = 0;
  int thread_dead = 0;
  sigset_t prev_mask;

  gdb_assert (!lp->stopped);
  gdb_assert (lp->status == 0);

  /* Make sure SIGCHLD is blocked for sigsuspend avoiding a race below.  */
  block_child_signals (&prev_mask);

  for (;;)
    {
      /* If my_waitpid returns 0 it means the __WCLONE vs. non-__WCLONE kind
         was right and we should just call sigsuspend.  */

      pid = my_waitpid (ptid_get_lwp (lp->ptid), &status, WNOHANG);
      if (pid == -1 && errno == ECHILD)
        pid = my_waitpid (ptid_get_lwp (lp->ptid), &status, __WCLONE | WNOHANG);
      if (pid == -1 && errno == ECHILD)
        {
          /* The thread has previously exited.  We need to delete it
             now because, for some vendor 2.4 kernels with NPTL
             support backported, there won't be an exit event unless
             it is the main thread.  2.6 kernels will report an exit
             event for each thread that exits, as expected.  */
          thread_dead = 1;
          if (debug_linux_nat)
            fprintf_unfiltered (gdb_stdlog, "WL: %s vanished.\n",
                                target_pid_to_str (lp->ptid));
        }
      if (pid != 0)
        break;

      /* Bugs 10970, 12702.
         Thread group leader may have exited in which case we'll lock up in
         waitpid if there are other threads, even if they are all zombies too.
         Basically, we're not supposed to use waitpid this way.
         __WCLONE is not applicable for the leader so we can't use that.
         LINUX_NAT_THREAD_ALIVE cannot be used here as it requires a STOPPED
         process; it gets ESRCH both for the zombie and for running processes.

         As a workaround, check if we're waiting for the thread group leader and
         if it's a zombie, and avoid calling waitpid if it is.

         This is racy, what if the tgl becomes a zombie right after we check?
         Therefore always use WNOHANG with sigsuspend - it is equivalent to
         waiting waitpid but linux_proc_pid_is_zombie is safe this way.  */

      if (ptid_get_pid (lp->ptid) == ptid_get_lwp (lp->ptid)
          && linux_proc_pid_is_zombie (ptid_get_lwp (lp->ptid)))
        {
          thread_dead = 1;
          if (debug_linux_nat)
            fprintf_unfiltered (gdb_stdlog,
                                "WL: Thread group leader %s vanished.\n",
                                target_pid_to_str (lp->ptid));
          break;
        }

      /* Wait for next SIGCHLD and try again.  This may let SIGCHLD handlers
         get invoked despite our caller had them intentionally blocked by
         block_child_signals.  This is sensitive only to the loop of
         linux_nat_wait_1 and there if we get called my_waitpid gets called
         again before it gets to sigsuspend so we can safely let the handlers
         get executed here.  */

      if (debug_linux_nat)
        fprintf_unfiltered (gdb_stdlog, "WL: about to sigsuspend\n");
      sigsuspend (&suspend_mask);
    }

  restore_child_signals_mask (&prev_mask);

  if (!thread_dead)
    {
      gdb_assert (pid == ptid_get_lwp (lp->ptid));

      if (debug_linux_nat)
        {
          fprintf_unfiltered (gdb_stdlog,
                              "WL: waitpid %s received %s\n",
                              target_pid_to_str (lp->ptid),
                              status_to_str (status));
        }

      /* Check if the thread has exited.  */
      if (WIFEXITED (status) || WIFSIGNALED (status))
        {
          if (ptid_get_pid (lp->ptid) == ptid_get_lwp (lp->ptid))
            {
              if (debug_linux_nat)
                fprintf_unfiltered (gdb_stdlog, "WL: Process %d exited.\n",
                                    ptid_get_pid (lp->ptid));

              /* This is the leader exiting, it means the whole
                 process is gone.  Store the status to report to the
                 core.  Store it in lp->waitstatus, because lp->status
                 would be ambiguous (W_EXITCODE(0,0) == 0).  */
              store_waitstatus (&lp->waitstatus, status);
              return 0;
            }

          thread_dead = 1;
          if (debug_linux_nat)
            fprintf_unfiltered (gdb_stdlog, "WL: %s exited.\n",
                                target_pid_to_str (lp->ptid));
        }
    }

  if (thread_dead)
    {
      exit_lwp (lp);
      return 0;
    }

  gdb_assert (WIFSTOPPED (status));
  lp->stopped = 1;

  if (lp->must_set_ptrace_flags)
    {
      struct inferior *inf = find_inferior_pid (ptid_get_pid (lp->ptid));
      int options = linux_nat_ptrace_options (inf->attach_flag);

      linux_enable_event_reporting (ptid_get_lwp (lp->ptid), options);
      lp->must_set_ptrace_flags = 0;
    }

  /* Handle GNU/Linux's syscall SIGTRAPs.  */
  if (WIFSTOPPED (status) && WSTOPSIG (status) == SYSCALL_SIGTRAP)
    {
      /* No longer need the sysgood bit.  The ptrace event ends up
         recorded in lp->waitstatus if we care for it.  We can carry
         on handling the event like a regular SIGTRAP from here
         on.  */
      status = W_STOPCODE (SIGTRAP);
      if (linux_handle_syscall_trap (lp, 1))
        return wait_lwp (lp);
    }

  /* Handle GNU/Linux's extended waitstatus for trace events.  */
  if (WIFSTOPPED (status) && WSTOPSIG (status) == SIGTRAP
      && linux_is_extended_waitstatus (status))
    {
      if (debug_linux_nat)
        fprintf_unfiltered (gdb_stdlog,
                            "WL: Handling extended status 0x%06x\n",
                            status);
      linux_handle_extended_wait (lp, status);
      return 0;
    }

  return status;
}

/* Send a SIGSTOP to LP.  */

static int
stop_callback (struct lwp_info *lp, void *data)
{
  if (!lp->stopped && !lp->signalled)
    {
      int ret;

      if (debug_linux_nat)
        {
          fprintf_unfiltered (gdb_stdlog,
                              "SC:  kill %s **<SIGSTOP>**\n",
                              target_pid_to_str (lp->ptid));
        }
      errno = 0;
      ret = kill_lwp (ptid_get_lwp (lp->ptid), SIGSTOP);
      if (debug_linux_nat)
        {
          fprintf_unfiltered (gdb_stdlog,
                              "SC:  lwp kill %d %s\n",
                              ret,
                              errno ? safe_strerror (errno) : "ERRNO-OK");
        }

      lp->signalled = 1;
      gdb_assert (lp->status == 0);
    }

  return 0;
}

/* Request a stop on LWP.  */

void
linux_stop_lwp (struct lwp_info *lwp)
{
  stop_callback (lwp, NULL);
}

/* See linux-nat.h  */

void
linux_stop_and_wait_all_lwps (void)
{
  /* Stop all LWP's ...  */
  iterate_over_lwps (minus_one_ptid, stop_callback, NULL);

  /* ... and wait until all of them have reported back that
     they're no longer running.  */
  iterate_over_lwps (minus_one_ptid, stop_wait_callback, NULL);
}

/* See linux-nat.h  */

void
linux_unstop_all_lwps (void)
{
  iterate_over_lwps (minus_one_ptid,
                     resume_stopped_resumed_lwps, &minus_one_ptid);
}

/* Return non-zero if LWP PID has a pending SIGINT.  */

static int
linux_nat_has_pending_sigint (int pid)
{
  sigset_t pending, blocked, ignored;

  linux_proc_pending_signals (pid, &pending, &blocked, &ignored);

  if (sigismember (&pending, SIGINT)
      && !sigismember (&ignored, SIGINT))
    return 1;

  return 0;
}

/* Set a flag in LP indicating that we should ignore its next SIGINT.  */

static int
set_ignore_sigint (struct lwp_info *lp, void *data)
{
  /* If a thread has a pending SIGINT, consume it; otherwise, set a
     flag to consume the next one.  */
  if (lp->stopped && lp->status != 0 && WIFSTOPPED (lp->status)
      && WSTOPSIG (lp->status) == SIGINT)
    lp->status = 0;
  else
    lp->ignore_sigint = 1;

  return 0;
}

/* If LP does not have a SIGINT pending, then clear the ignore_sigint flag.
   This function is called after we know the LWP has stopped; if the LWP
   stopped before the expected SIGINT was delivered, then it will never have
   arrived.  Also, if the signal was delivered to a shared queue and consumed
   by a different thread, it will never be delivered to this LWP.  */

static void
maybe_clear_ignore_sigint (struct lwp_info *lp)
{
  if (!lp->ignore_sigint)
    return;

  if (!linux_nat_has_pending_sigint (ptid_get_lwp (lp->ptid)))
    {
      if (debug_linux_nat)
        fprintf_unfiltered (gdb_stdlog,
                            "MCIS: Clearing bogus flag for %s\n",
                            target_pid_to_str (lp->ptid));
      lp->ignore_sigint = 0;
    }
}

/* Fetch the possible triggered data watchpoint info and store it in
   LP.

   On some archs, like x86, that use debug registers to set
   watchpoints, it's possible that the way to know which watched
   address trapped, is to check the register that is used to select
   which address to watch.  Problem is, between setting the watchpoint
   and reading back which data address trapped, the user may change
   the set of watchpoints, and, as a consequence, GDB changes the
   debug registers in the inferior.  To avoid reading back a stale
   stopped-data-address when that happens, we cache in LP the fact
   that a watchpoint trapped, and the corresponding data address, as
   soon as we see LP stop with a SIGTRAP.  If GDB changes the debug
   registers meanwhile, we have the cached data we can rely on.  */

static int
check_stopped_by_watchpoint (struct lwp_info *lp)
{
  struct cleanup *old_chain;

  if (linux_ops->to_stopped_by_watchpoint == NULL)
    return 0;

  old_chain = save_inferior_ptid ();
  inferior_ptid = lp->ptid;

  if (linux_ops->to_stopped_by_watchpoint (linux_ops))
    {
      lp->stop_reason = TARGET_STOPPED_BY_WATCHPOINT;

      if (linux_ops->to_stopped_data_address != NULL)
        lp->stopped_data_address_p =
          linux_ops->to_stopped_data_address (&current_target,
                                              &lp->stopped_data_address);
      else
        lp->stopped_data_address_p = 0;
    }

  do_cleanups (old_chain);

  return lp->stop_reason == TARGET_STOPPED_BY_WATCHPOINT;
}

/* Called when the LWP stopped for a trap that could be explained by a
   watchpoint or a breakpoint.  */

static void
save_sigtrap (struct lwp_info *lp)
{
  gdb_assert (lp->stop_reason == TARGET_STOPPED_BY_NO_REASON);
  gdb_assert (lp->status != 0);

  /* Check first if this was a SW/HW breakpoint before checking
     watchpoints, because at least s390 can't tell the data address of
     hardware watchpoint hits, and the kernel returns
     stopped-by-watchpoint as long as there's a watchpoint set.  */
  if (linux_nat_status_is_event (lp->status))
    check_stopped_by_breakpoint (lp);

  /* Note that TRAP_HWBKPT can indicate either a hardware breakpoint
     or hardware watchpoint.  Check which is which if we got
     TARGET_STOPPED_BY_HW_BREAKPOINT.  */
  if (lp->stop_reason == TARGET_STOPPED_BY_NO_REASON
      || lp->stop_reason == TARGET_STOPPED_BY_HW_BREAKPOINT)
    check_stopped_by_watchpoint (lp);
}

/* Returns true if the LWP had stopped for a watchpoint.  */

static int
linux_nat_stopped_by_watchpoint (struct target_ops *ops)
{
  struct lwp_info *lp = find_lwp_pid (inferior_ptid);

  gdb_assert (lp != NULL);

  return lp->stop_reason == TARGET_STOPPED_BY_WATCHPOINT;
}

static int
linux_nat_stopped_data_address (struct target_ops *ops, CORE_ADDR *addr_p)
{
  struct lwp_info *lp = find_lwp_pid (inferior_ptid);

  gdb_assert (lp != NULL);

  *addr_p = lp->stopped_data_address;

  return lp->stopped_data_address_p;
}

/* Commonly any breakpoint / watchpoint generate only SIGTRAP.  */

static int
sigtrap_is_event (int status)
{
  return WIFSTOPPED (status) && WSTOPSIG (status) == SIGTRAP;
}

/* Set alternative SIGTRAP-like events recognizer.  If
   breakpoint_inserted_here_p there then gdbarch_decr_pc_after_break will be
   applied.  */

void
linux_nat_set_status_is_event (struct target_ops *t,
                               int (*status_is_event) (int status))
{
  linux_nat_status_is_event = status_is_event;
}

/* Wait until LP is stopped.  */

static int
stop_wait_callback (struct lwp_info *lp, void *data)
{
  struct inferior *inf = find_inferior_ptid (lp->ptid);

  /* If this is a vfork parent, bail out, it is not going to report
     any SIGSTOP until the vfork is done with.  */
  if (inf->vfork_child != NULL)
    return 0;

  if (!lp->stopped)
    {
      int status;

      status = wait_lwp (lp);
      if (status == 0)
        return 0;

      if (lp->ignore_sigint && WIFSTOPPED (status)
          && WSTOPSIG (status) == SIGINT)
        {
          lp->ignore_sigint = 0;

          errno = 0;
          ptrace (PTRACE_CONT, ptid_get_lwp (lp->ptid), 0, 0);
          lp->stopped = 0;
          if (debug_linux_nat)
            fprintf_unfiltered (gdb_stdlog,
                                "PTRACE_CONT %s, 0, 0 (%s) "
                                "(discarding SIGINT)\n",
                                target_pid_to_str (lp->ptid),
                                errno ? safe_strerror (errno) : "OK");

          return stop_wait_callback (lp, NULL);
        }

      maybe_clear_ignore_sigint (lp);

      if (WSTOPSIG (status) != SIGSTOP)
        {
          /* The thread was stopped with a signal other than SIGSTOP.  */

          if (debug_linux_nat)
            fprintf_unfiltered (gdb_stdlog,
                                "SWC: Pending event %s in %s\n",
                                status_to_str ((int) status),
                                target_pid_to_str (lp->ptid));

          /* Save the sigtrap event.  */
          lp->status = status;
          gdb_assert (lp->signalled);
          save_sigtrap (lp);
        }
      else
        {
          /* We caught the SIGSTOP that we intended to catch, so
             there's no SIGSTOP pending.  */

          if (debug_linux_nat)
            fprintf_unfiltered (gdb_stdlog,
                                "SWC: Expected SIGSTOP caught for %s.\n",
                                target_pid_to_str (lp->ptid));

          /* Reset SIGNALLED only after the stop_wait_callback call
             above as it does gdb_assert on SIGNALLED.  */
          lp->signalled = 0;
        }
    }

  return 0;
}

/* Return non-zero if LP has a wait status pending.  Discard the
   pending event and resume the LWP if the event that originally
   caused the stop became uninteresting.  */

static int
status_callback (struct lwp_info *lp, void *data)
{
  /* Only report a pending wait status if we pretend that this has
     indeed been resumed.  */
  if (!lp->resumed)
    return 0;

  if (!lwp_status_pending_p (lp))
    return 0;

  if (lp->stop_reason == TARGET_STOPPED_BY_SW_BREAKPOINT
      || lp->stop_reason == TARGET_STOPPED_BY_HW_BREAKPOINT)
    {
      struct regcache *regcache = get_thread_regcache (lp->ptid);
      struct gdbarch *gdbarch = get_regcache_arch (regcache);
      CORE_ADDR pc;
      int discard = 0;

      pc = regcache_read_pc (regcache);

      if (pc != lp->stop_pc)
        {
          if (debug_linux_nat)
            fprintf_unfiltered (gdb_stdlog,
                                "SC: PC of %s changed.  was=%s, now=%s\n",
                                target_pid_to_str (lp->ptid),
                                paddress (target_gdbarch (), lp->stop_pc),
                                paddress (target_gdbarch (), pc));
          discard = 1;
        }

#if !USE_SIGTRAP_SIGINFO
      else if (!breakpoint_inserted_here_p (get_regcache_aspace (regcache), pc))
        {
          if (debug_linux_nat)
            fprintf_unfiltered (gdb_stdlog,
                                "SC: previous breakpoint of %s, at %s gone\n",
                                target_pid_to_str (lp->ptid),
                                paddress (target_gdbarch (), lp->stop_pc));

          discard = 1;
        }
#endif

      if (discard)
        {
          if (debug_linux_nat)
            fprintf_unfiltered (gdb_stdlog,
                                "SC: pending event of %s cancelled.\n",
                                target_pid_to_str (lp->ptid));

          lp->status = 0;
          linux_resume_one_lwp (lp, lp->step, GDB_SIGNAL_0);
          return 0;
        }
    }

  return 1;
}

/* Return non-zero if LP isn't stopped.  */

static int
running_callback (struct lwp_info *lp, void *data)
{
  return (!lp->stopped
          || (lwp_status_pending_p (lp) && lp->resumed));
}

/* Count the LWP's that have had events.  */

static int
count_events_callback (struct lwp_info *lp, void *data)
{
  int *count = data;

  gdb_assert (count != NULL);

  /* Select only resumed LWPs that have an event pending.  */
  if (lp->resumed && lwp_status_pending_p (lp))
    (*count)++;

  return 0;
}

/* Select the LWP (if any) that is currently being single-stepped.  */

static int
select_singlestep_lwp_callback (struct lwp_info *lp, void *data)
{
  if (lp->last_resume_kind == resume_step
      && lp->status != 0)
    return 1;
  else
    return 0;
}

/* Returns true if LP has a status pending.  */

static int
lwp_status_pending_p (struct lwp_info *lp)
{
  /* We check for lp->waitstatus in addition to lp->status, because we
     can have pending process exits recorded in lp->status and
     W_EXITCODE(0,0) happens to be 0.  */
  return lp->status != 0 || lp->waitstatus.kind != TARGET_WAITKIND_IGNORE;
}

/* Select the Nth LWP that has had an event.  */

static int
select_event_lwp_callback (struct lwp_info *lp, void *data)
{
  int *selector = data;

  gdb_assert (selector != NULL);

  /* Select only resumed LWPs that have an event pending.  */
  if (lp->resumed && lwp_status_pending_p (lp))
    if ((*selector)-- == 0)
      return 1;

  return 0;
}

/* Called when the LWP got a signal/trap that could be explained by a
   software or hardware breakpoint.  */

static int
check_stopped_by_breakpoint (struct lwp_info *lp)
{
  /* Arrange for a breakpoint to be hit again later.  We don't keep
     the SIGTRAP status and don't forward the SIGTRAP signal to the
     LWP.  We will handle the current event, eventually we will resume
     this LWP, and this breakpoint will trap again.

     If we do not do this, then we run the risk that the user will
     delete or disable the breakpoint, but the LWP will have already
     tripped on it.  */

  struct regcache *regcache = get_thread_regcache (lp->ptid);
  struct gdbarch *gdbarch = get_regcache_arch (regcache);
  CORE_ADDR pc;
  CORE_ADDR sw_bp_pc;
#if USE_SIGTRAP_SIGINFO
  siginfo_t siginfo;
#endif

  pc = regcache_read_pc (regcache);
  sw_bp_pc = pc - gdbarch_decr_pc_after_break (gdbarch);

#if USE_SIGTRAP_SIGINFO
  if (linux_nat_get_siginfo (lp->ptid, &siginfo))
    {
      if (siginfo.si_signo == SIGTRAP)
        {
          if (GDB_ARCH_IS_TRAP_BRKPT (siginfo.si_code))
            {
              if (debug_linux_nat)
                fprintf_unfiltered (gdb_stdlog,
                                    "CSBB: %s stopped by software "
                                    "breakpoint\n",
                                    target_pid_to_str (lp->ptid));

              /* Back up the PC if necessary.  */
              if (pc != sw_bp_pc)
                regcache_write_pc (regcache, sw_bp_pc);

              lp->stop_pc = sw_bp_pc;
              lp->stop_reason = TARGET_STOPPED_BY_SW_BREAKPOINT;
              return 1;
            }
          else if (siginfo.si_code == TRAP_HWBKPT)
            {
              if (debug_linux_nat)
                fprintf_unfiltered (gdb_stdlog,
                                    "CSBB: %s stopped by hardware "
                                    "breakpoint/watchpoint\n",
                                    target_pid_to_str (lp->ptid));

              lp->stop_pc = pc;
              lp->stop_reason = TARGET_STOPPED_BY_HW_BREAKPOINT;
              return 1;
            }
          else if (siginfo.si_code == TRAP_TRACE)
            {
              if (debug_linux_nat)
                fprintf_unfiltered (gdb_stdlog,
                                    "CSBB: %s stopped by trace\n",
                                    target_pid_to_str (lp->ptid));
            }
        }
    }
#else
  if ((!lp->step || lp->stop_pc == sw_bp_pc)
      && software_breakpoint_inserted_here_p (get_regcache_aspace (regcache),
                                              sw_bp_pc))
    {
      /* The LWP was either continued, or stepped a software
         breakpoint instruction.  */
      if (debug_linux_nat)
        fprintf_unfiltered (gdb_stdlog,
                            "CSBB: %s stopped by software breakpoint\n",
                            target_pid_to_str (lp->ptid));

      /* Back up the PC if necessary.  */
      if (pc != sw_bp_pc)
        regcache_write_pc (regcache, sw_bp_pc);

      lp->stop_pc = sw_bp_pc;
      lp->stop_reason = TARGET_STOPPED_BY_SW_BREAKPOINT;
      return 1;
    }

  if (hardware_breakpoint_inserted_here_p (get_regcache_aspace (regcache), pc))
    {
      if (debug_linux_nat)
        fprintf_unfiltered (gdb_stdlog,
                            "CSBB: stopped by hardware breakpoint %s\n",
                            target_pid_to_str (lp->ptid));

      lp->stop_pc = pc;
      lp->stop_reason = TARGET_STOPPED_BY_HW_BREAKPOINT;
      return 1;
    }
#endif

  return 0;
}


/* Returns true if the LWP had stopped for a software breakpoint.  */

static int
linux_nat_stopped_by_sw_breakpoint (struct target_ops *ops)
{
  struct lwp_info *lp = find_lwp_pid (inferior_ptid);

  gdb_assert (lp != NULL);

  return lp->stop_reason == TARGET_STOPPED_BY_SW_BREAKPOINT;
}

/* Implement the supports_stopped_by_sw_breakpoint method.  */

static int
linux_nat_supports_stopped_by_sw_breakpoint (struct target_ops *ops)
{
  return USE_SIGTRAP_SIGINFO;
}

/* Returns true if the LWP had stopped for a hardware
   breakpoint/watchpoint.  */

static int
linux_nat_stopped_by_hw_breakpoint (struct target_ops *ops)
{
  struct lwp_info *lp = find_lwp_pid (inferior_ptid);

  gdb_assert (lp != NULL);

  return lp->stop_reason == TARGET_STOPPED_BY_HW_BREAKPOINT;
}

/* Implement the supports_stopped_by_hw_breakpoint method.  */

static int
linux_nat_supports_stopped_by_hw_breakpoint (struct target_ops *ops)
{
  return USE_SIGTRAP_SIGINFO;
}

/* Select one LWP out of those that have events pending.  */

static void
select_event_lwp (ptid_t filter, struct lwp_info **orig_lp, int *status)
{
  int num_events = 0;
  int random_selector;
  struct lwp_info *event_lp = NULL;

  /* Record the wait status for the original LWP.  */
  (*orig_lp)->status = *status;

  /* In all-stop, give preference to the LWP that is being
     single-stepped.  There will be at most one, and it will be the
     LWP that the core is most interested in.  If we didn't do this,
     then we'd have to handle pending step SIGTRAPs somehow in case
     the core later continues the previously-stepped thread, as
     otherwise we'd report the pending SIGTRAP then, and the core, not
     having stepped the thread, wouldn't understand what the trap was
     for, and therefore would report it to the user as a random
     signal.  */
  if (!target_is_non_stop_p ())
    {
      event_lp = iterate_over_lwps (filter,
                                    select_singlestep_lwp_callback, NULL);
      if (event_lp != NULL)
        {
          if (debug_linux_nat)
            fprintf_unfiltered (gdb_stdlog,
                                "SEL: Select single-step %s\n",
                                target_pid_to_str (event_lp->ptid));
        }
    }

  if (event_lp == NULL)
    {
      /* Pick one at random, out of those which have had events.  */

      /* First see how many events we have.  */
      iterate_over_lwps (filter, count_events_callback, &num_events);
      gdb_assert (num_events > 0);

      /* Now randomly pick a LWP out of those that have had
         events.  */
      random_selector = (int)
        ((num_events * (double) rand ()) / (RAND_MAX + 1.0));

      if (debug_linux_nat && num_events > 1)
        fprintf_unfiltered (gdb_stdlog,
                            "SEL: Found %d events, selecting #%d\n",
                            num_events, random_selector);

      event_lp = iterate_over_lwps (filter,
                                    select_event_lwp_callback,
                                    &random_selector);
    }

  if (event_lp != NULL)
    {
      /* Switch the event LWP.  */
      *orig_lp = event_lp;
      *status = event_lp->status;
    }

  /* Flush the wait status for the event LWP.  */
  (*orig_lp)->status = 0;
}

/* Return non-zero if LP has been resumed.  */

static int
resumed_callback (struct lwp_info *lp, void *data)
{
  return lp->resumed;
}

/* Stop an active thread, verify it still exists, then resume it.  If
   the thread ends up with a pending status, then it is not resumed,
   and *DATA (really a pointer to int), is set.  */

static int
stop_and_resume_callback (struct lwp_info *lp, void *data)
{
  if (!lp->stopped)
    {
      ptid_t ptid = lp->ptid;

      stop_callback (lp, NULL);
      stop_wait_callback (lp, NULL);

      /* Resume if the lwp still exists, and the core wanted it
         running.  */
      lp = find_lwp_pid (ptid);
      if (lp != NULL)
        {
          if (lp->last_resume_kind == resume_stop
              && !lwp_status_pending_p (lp))
            {
              /* The core wanted the LWP to stop.  Even if it stopped
                 cleanly (with SIGSTOP), leave the event pending.  */
              if (debug_linux_nat)
                fprintf_unfiltered (gdb_stdlog,
                                    "SARC: core wanted LWP %ld stopped "
                                    "(leaving SIGSTOP pending)\n",
                                    ptid_get_lwp (lp->ptid));
              lp->status = W_STOPCODE (SIGSTOP);
            }

          if (!lwp_status_pending_p (lp))
            {
              if (debug_linux_nat)
                fprintf_unfiltered (gdb_stdlog,
                                    "SARC: re-resuming LWP %ld\n",
                                    ptid_get_lwp (lp->ptid));
              resume_lwp (lp, lp->step, GDB_SIGNAL_0);
            }
          else
            {
              if (debug_linux_nat)
                fprintf_unfiltered (gdb_stdlog,
                                    "SARC: not re-resuming LWP %ld "
                                    "(has pending)\n",
                                    ptid_get_lwp (lp->ptid));
            }
        }
    }
  return 0;
}

/* Check if we should go on and pass this event to common code.
   Return the affected lwp if we are, or NULL otherwise.  */

static struct lwp_info *
linux_nat_filter_event (int lwpid, int status)
{
  struct lwp_info *lp;
  int event = linux_ptrace_get_extended_event (status);

  lp = find_lwp_pid (pid_to_ptid (lwpid));

  /* Check for stop events reported by a process we didn't already
     know about - anything not already in our LWP list.

     If we're expecting to receive stopped processes after
     fork, vfork, and clone events, then we'll just add the
     new one to our list and go back to waiting for the event
     to be reported - the stopped process might be returned
     from waitpid before or after the event is.

     But note the case of a non-leader thread exec'ing after the
     leader having exited, and gone from our lists.  The non-leader
     thread changes its tid to the tgid.  */

  if (WIFSTOPPED (status) && lp == NULL
      && (WSTOPSIG (status) == SIGTRAP && event == PTRACE_EVENT_EXEC))
    {
      /* A multi-thread exec after we had seen the leader exiting.  */
      if (debug_linux_nat)
        fprintf_unfiltered (gdb_stdlog,
                            "LLW: Re-adding thread group leader LWP %d.\n",
                            lwpid);

      lp = add_lwp (ptid_build (lwpid, lwpid, 0));
      lp->stopped = 1;
      lp->resumed = 1;
      add_thread (lp->ptid);
    }

  if (WIFSTOPPED (status) && !lp)
    {
      if (debug_linux_nat)
        fprintf_unfiltered (gdb_stdlog,
                            "LHEW: saving LWP %ld status %s in stopped_pids list\n",
                            (long) lwpid, status_to_str (status));
      add_to_pid_list (&stopped_pids, lwpid, status);
      return NULL;
    }

  /* Make sure we don't report an event for the exit of an LWP not in
     our list, i.e. not part of the current process.  This can happen
     if we detach from a program we originally forked and then it
     exits.  */
  if (!WIFSTOPPED (status) && !lp)
    return NULL;

  /* This LWP is stopped now.  (And if dead, this prevents it from
     ever being continued.)  */
  lp->stopped = 1;

  if (WIFSTOPPED (status) && lp->must_set_ptrace_flags)
    {
      struct inferior *inf = find_inferior_pid (ptid_get_pid (lp->ptid));
      int options = linux_nat_ptrace_options (inf->attach_flag);

      linux_enable_event_reporting (ptid_get_lwp (lp->ptid), options);
      lp->must_set_ptrace_flags = 0;
    }

  /* Handle GNU/Linux's syscall SIGTRAPs.  */
  if (WIFSTOPPED (status) && WSTOPSIG (status) == SYSCALL_SIGTRAP)
    {
      /* No longer need the sysgood bit.  The ptrace event ends up
         recorded in lp->waitstatus if we care for it.  We can carry
         on handling the event like a regular SIGTRAP from here
         on.  */
      status = W_STOPCODE (SIGTRAP);
      if (linux_handle_syscall_trap (lp, 0))
        return NULL;
    }

  /* Handle GNU/Linux's extended waitstatus for trace events.  */
  if (WIFSTOPPED (status) && WSTOPSIG (status) == SIGTRAP
      && linux_is_extended_waitstatus (status))
    {
      if (debug_linux_nat)
        fprintf_unfiltered (gdb_stdlog,
                            "LLW: Handling extended status 0x%06x\n",
                            status);
      if (linux_handle_extended_wait (lp, status))
        return NULL;
    }

  /* Check if the thread has exited.  */
  if (WIFEXITED (status) || WIFSIGNALED (status))
    {
      if (num_lwps (ptid_get_pid (lp->ptid)) > 1)
        {
          /* If this is the main thread, we must stop all threads and
             verify if they are still alive.  This is because in the
             nptl thread model on Linux 2.4, there is no signal issued
             for exiting LWPs other than the main thread.  We only get
             the main thread exit signal once all child threads have
             already exited.  If we stop all the threads and use the
             stop_wait_callback to check if they have exited we can
             determine whether this signal should be ignored or
             whether it means the end of the debugged application,
             regardless of which threading model is being used.  */
          if (ptid_get_pid (lp->ptid) == ptid_get_lwp (lp->ptid))
            {
              iterate_over_lwps (pid_to_ptid (ptid_get_pid (lp->ptid)),
                                 stop_and_resume_callback, NULL);
            }

          if (debug_linux_nat)
            fprintf_unfiltered (gdb_stdlog,
                                "LLW: %s exited.\n",
                                target_pid_to_str (lp->ptid));

          if (num_lwps (ptid_get_pid (lp->ptid)) > 1)
            {
              /* If there is at least one more LWP, then the exit signal
                 was not the end of the debugged application and should be
                 ignored.  */
              exit_lwp (lp);
              return NULL;
            }
        }

      /* Note that even if the leader was ptrace-stopped, it can still
         exit, if e.g., some other thread brings down the whole
         process (calls `exit').  So don't assert that the lwp is
         resumed.  */
      if (debug_linux_nat)
        fprintf_unfiltered (gdb_stdlog,
                            "Process %ld exited (resumed=%d)\n",
                            ptid_get_lwp (lp->ptid), lp->resumed);

      /* This was the last lwp in the process.  Since events are
         serialized to GDB core, we may not be able report this one
         right now, but GDB core and the other target layers will want
         to be notified about the exit code/signal, leave the status
         pending for the next time we're able to report it.  */

      /* Dead LWP's aren't expected to reported a pending sigstop.  */
      lp->signalled = 0;

      /* Store the pending event in the waitstatus, because
         W_EXITCODE(0,0) == 0.  */
      store_waitstatus (&lp->waitstatus, status);
      return lp;
    }

  /* Check if the current LWP has previously exited.  In the nptl
     thread model, LWPs other than the main thread do not issue
     signals when they exit so we must check whenever the thread has
     stopped.  A similar check is made in stop_wait_callback().  */
  if (num_lwps (ptid_get_pid (lp->ptid)) > 1 && !linux_thread_alive (lp->ptid))
    {
      ptid_t ptid = pid_to_ptid (ptid_get_pid (lp->ptid));

      if (debug_linux_nat)
        fprintf_unfiltered (gdb_stdlog,
                            "LLW: %s exited.\n",
                            target_pid_to_str (lp->ptid));

      exit_lwp (lp);

      /* Make sure there is at least one thread running.  */
      gdb_assert (iterate_over_lwps (ptid, running_callback, NULL));

      /* Discard the event.  */
      return NULL;
    }

  /* Make sure we don't report a SIGSTOP that we sent ourselves in
     an attempt to stop an LWP.  */
  if (lp->signalled
      && WIFSTOPPED (status) && WSTOPSIG (status) == SIGSTOP)
    {
      lp->signalled = 0;

      if (lp->last_resume_kind == resume_stop)
        {
          if (debug_linux_nat)
            fprintf_unfiltered (gdb_stdlog,
                                "LLW: resume_stop SIGSTOP caught for %s.\n",
                                target_pid_to_str (lp->ptid));
        }
      else
        {
          /* This is a delayed SIGSTOP.  Filter out the event.  */

          if (debug_linux_nat)
            fprintf_unfiltered (gdb_stdlog,
                                "LLW: %s %s, 0, 0 (discard delayed SIGSTOP)\n",
                                lp->step ?
                                "PTRACE_SINGLESTEP" : "PTRACE_CONT",
                                target_pid_to_str (lp->ptid));

          linux_resume_one_lwp (lp, lp->step, GDB_SIGNAL_0);
          gdb_assert (lp->resumed);
          return NULL;
        }
    }

  /* Make sure we don't report a SIGINT that we have already displayed
     for another thread.  */
  if (lp->ignore_sigint
      && WIFSTOPPED (status) && WSTOPSIG (status) == SIGINT)
    {
      if (debug_linux_nat)
        fprintf_unfiltered (gdb_stdlog,
                            "LLW: Delayed SIGINT caught for %s.\n",
                            target_pid_to_str (lp->ptid));

      /* This is a delayed SIGINT.  */
      lp->ignore_sigint = 0;

      linux_resume_one_lwp (lp, lp->step, GDB_SIGNAL_0);
      if (debug_linux_nat)
        fprintf_unfiltered (gdb_stdlog,
                            "LLW: %s %s, 0, 0 (discard SIGINT)\n",
                            lp->step ?
                            "PTRACE_SINGLESTEP" : "PTRACE_CONT",
                            target_pid_to_str (lp->ptid));
      gdb_assert (lp->resumed);

      /* Discard the event.  */
      return NULL;
    }

  /* Don't report signals that GDB isn't interested in, such as
     signals that are neither printed nor stopped upon.  Stopping all
     threads can be a bit time-consuming so if we want decent
     performance with heavily multi-threaded programs, especially when
     they're using a high frequency timer, we'd better avoid it if we
     can.  */
  if (WIFSTOPPED (status))
    {
      enum gdb_signal signo = gdb_signal_from_host (WSTOPSIG (status));

      if (!target_is_non_stop_p ())
        {
          /* Only do the below in all-stop, as we currently use SIGSTOP
             to implement target_stop (see linux_nat_stop) in
             non-stop.  */
          if (signo == GDB_SIGNAL_INT && signal_pass_state (signo) == 0)
            {
              /* If ^C/BREAK is typed at the tty/console, SIGINT gets
                 forwarded to the entire process group, that is, all LWPs
                 will receive it - unless they're using CLONE_THREAD to
                 share signals.  Since we only want to report it once, we
                 mark it as ignored for all LWPs except this one.  */
              iterate_over_lwps (pid_to_ptid (ptid_get_pid (lp->ptid)),
                                              set_ignore_sigint, NULL);
              lp->ignore_sigint = 0;
            }
          else
            maybe_clear_ignore_sigint (lp);
        }

      /* When using hardware single-step, we need to report every signal.
         Otherwise, signals in pass_mask may be short-circuited
         except signals that might be caused by a breakpoint.  */
      if (!lp->step
          && WSTOPSIG (status) && sigismember (&pass_mask, WSTOPSIG (status))
          && !linux_wstatus_maybe_breakpoint (status))
        {
          linux_resume_one_lwp (lp, lp->step, signo);
          if (debug_linux_nat)
            fprintf_unfiltered (gdb_stdlog,
                                "LLW: %s %s, %s (preempt 'handle')\n",
                                lp->step ?
                                "PTRACE_SINGLESTEP" : "PTRACE_CONT",
                                target_pid_to_str (lp->ptid),
                                (signo != GDB_SIGNAL_0
                                 ? strsignal (gdb_signal_to_host (signo))
                                 : "0"));
          return NULL;
        }
    }

  /* An interesting event.  */
  gdb_assert (lp);
  lp->status = status;
  save_sigtrap (lp);
  return lp;
}

/* Detect zombie thread group leaders, and "exit" them.  We can't reap
   their exits until all other threads in the group have exited.  */

static void
check_zombie_leaders (void)
{
  struct inferior *inf;

  ALL_INFERIORS (inf)
    {
      struct lwp_info *leader_lp;

      if (inf->pid == 0)
        continue;

      leader_lp = find_lwp_pid (pid_to_ptid (inf->pid));
      if (leader_lp != NULL
          /* Check if there are other threads in the group, as we may
             have raced with the inferior simply exiting.  */
          && num_lwps (inf->pid) > 1
          && linux_proc_pid_is_zombie (inf->pid))
        {
          if (debug_linux_nat)
            fprintf_unfiltered (gdb_stdlog,
                                "CZL: Thread group leader %d zombie "
                                "(it exited, or another thread execd).\n",
                                inf->pid);

          /* A leader zombie can mean one of two things:

             - It exited, and there's an exit status pending
             available, or only the leader exited (not the whole
             program).  In the latter case, we can't waitpid the
             leader's exit status until all other threads are gone.

             - There are 3 or more threads in the group, and a thread
             other than the leader exec'd.  On an exec, the Linux
             kernel destroys all other threads (except the execing
             one) in the thread group, and resets the execing thread's
             tid to the tgid.  No exit notification is sent for the
             execing thread -- from the ptracer's perspective, it
             appears as though the execing thread just vanishes.
             Until we reap all other threads except the leader and the
             execing thread, the leader will be zombie, and the
             execing thread will be in `D (disc sleep)'.  As soon as
             all other threads are reaped, the execing thread changes
             it's tid to the tgid, and the previous (zombie) leader
             vanishes, giving place to the "new" leader.  We could try
             distinguishing the exit and exec cases, by waiting once
             more, and seeing if something comes out, but it doesn't
             sound useful.  The previous leader _does_ go away, and
             we'll re-add the new one once we see the exec event
             (which is just the same as what would happen if the
             previous leader did exit voluntarily before some other
             thread execs).  */

          if (debug_linux_nat)
            fprintf_unfiltered (gdb_stdlog,
                                "CZL: Thread group leader %d vanished.\n",
                                inf->pid);
          exit_lwp (leader_lp);
        }
    }
}

static ptid_t
linux_nat_wait_1 (struct target_ops *ops,
                  ptid_t ptid, struct target_waitstatus *ourstatus,
                  int target_options)
{
  sigset_t prev_mask;
  enum resume_kind last_resume_kind;
  struct lwp_info *lp;
  int status;

  if (debug_linux_nat)
    fprintf_unfiltered (gdb_stdlog, "LLW: enter\n");

  /* The first time we get here after starting a new inferior, we may
     not have added it to the LWP list yet - this is the earliest
     moment at which we know its PID.  */
  if (ptid_is_pid (inferior_ptid))
    {
      /* Upgrade the main thread's ptid.  */
      thread_change_ptid (inferior_ptid,
                          ptid_build (ptid_get_pid (inferior_ptid),
                                      ptid_get_pid (inferior_ptid), 0));

      lp = add_initial_lwp (inferior_ptid);
      lp->resumed = 1;
    }

  /* Make sure SIGCHLD is blocked until the sigsuspend below.  */
  block_child_signals (&prev_mask);

  /* First check if there is a LWP with a wait status pending.  */
  lp = iterate_over_lwps (ptid, status_callback, NULL);
  if (lp != NULL)
    {
      if (debug_linux_nat)
        fprintf_unfiltered (gdb_stdlog,
                            "LLW: Using pending wait status %s for %s.\n",
                            status_to_str (lp->status),
                            target_pid_to_str (lp->ptid));
    }

  /* But if we don't find a pending event, we'll have to wait.  Always
     pull all events out of the kernel.  We'll randomly select an
     event LWP out of all that have events, to prevent starvation.  */

  while (lp == NULL)
    {
      pid_t lwpid;

      /* Always use -1 and WNOHANG, due to couple of a kernel/ptrace
         quirks:

         - If the thread group leader exits while other threads in the
           thread group still exist, waitpid(TGID, ...) hangs.  That
           waitpid won't return an exit status until the other threads
           in the group are reapped.

         - When a non-leader thread execs, that thread just vanishes
           without reporting an exit (so we'd hang if we waited for it
           explicitly in that case).  The exec event is reported to
           the TGID pid.  */

      errno = 0;
      lwpid = my_waitpid (-1, &status,  __WCLONE | WNOHANG);
      if (lwpid == 0 || (lwpid == -1 && errno == ECHILD))
        lwpid = my_waitpid (-1, &status, WNOHANG);

      if (debug_linux_nat)
        fprintf_unfiltered (gdb_stdlog,
                            "LNW: waitpid(-1, ...) returned %d, %s\n",
                            lwpid, errno ? safe_strerror (errno) : "ERRNO-OK");

      if (lwpid > 0)
        {
          if (debug_linux_nat)
            {
              fprintf_unfiltered (gdb_stdlog,
                                  "LLW: waitpid %ld received %s\n",
                                  (long) lwpid, status_to_str (status));
            }

          linux_nat_filter_event (lwpid, status);
          /* Retry until nothing comes out of waitpid.  A single
             SIGCHLD can indicate more than one child stopped.  */
          continue;
        }

      /* Now that we've pulled all events out of the kernel, resume
         LWPs that don't have an interesting event to report.  */
      iterate_over_lwps (minus_one_ptid,
                         resume_stopped_resumed_lwps, &minus_one_ptid);

      /* ... and find an LWP with a status to report to the core, if
         any.  */
      lp = iterate_over_lwps (ptid, status_callback, NULL);
      if (lp != NULL)
        break;

      /* Check for zombie thread group leaders.  Those can't be reaped
         until all other threads in the thread group are.  */
      check_zombie_leaders ();

      /* If there are no resumed children left, bail.  We'd be stuck
         forever in the sigsuspend call below otherwise.  */
      if (iterate_over_lwps (ptid, resumed_callback, NULL) == NULL)
        {
          if (debug_linux_nat)
            fprintf_unfiltered (gdb_stdlog, "LLW: exit (no resumed LWP)\n");

          ourstatus->kind = TARGET_WAITKIND_NO_RESUMED;

          restore_child_signals_mask (&prev_mask);
          return minus_one_ptid;
        }

      /* No interesting event to report to the core.  */

      if (target_options & TARGET_WNOHANG)
        {
          if (debug_linux_nat)
            fprintf_unfiltered (gdb_stdlog, "LLW: exit (ignore)\n");

          ourstatus->kind = TARGET_WAITKIND_IGNORE;
          restore_child_signals_mask (&prev_mask);
          return minus_one_ptid;
        }

      /* We shouldn't end up here unless we want to try again.  */
      gdb_assert (lp == NULL);

      /* Block until we get an event reported with SIGCHLD.  */
      if (debug_linux_nat)
        fprintf_unfiltered (gdb_stdlog, "LNW: about to sigsuspend\n");
      sigsuspend (&suspend_mask);
    }

  gdb_assert (lp);

  status = lp->status;
  lp->status = 0;

  if (!target_is_non_stop_p ())
    {
      /* Now stop all other LWP's ...  */
      iterate_over_lwps (minus_one_ptid, stop_callback, NULL);

      /* ... and wait until all of them have reported back that
         they're no longer running.  */
      iterate_over_lwps (minus_one_ptid, stop_wait_callback, NULL);
    }

  /* If we're not waiting for a specific LWP, choose an event LWP from
     among those that have had events.  Giving equal priority to all
     LWPs that have had events helps prevent starvation.  */
  if (ptid_equal (ptid, minus_one_ptid) || ptid_is_pid (ptid))
    select_event_lwp (ptid, &lp, &status);

  gdb_assert (lp != NULL);

  /* Now that we've selected our final event LWP, un-adjust its PC if
     it was a software breakpoint, and we can't reliably support the
     "stopped by software breakpoint" stop reason.  */
  if (lp->stop_reason == TARGET_STOPPED_BY_SW_BREAKPOINT
      && !USE_SIGTRAP_SIGINFO)
    {
      struct regcache *regcache = get_thread_regcache (lp->ptid);
      struct gdbarch *gdbarch = get_regcache_arch (regcache);
      int decr_pc = gdbarch_decr_pc_after_break (gdbarch);

      if (decr_pc != 0)
        {
          CORE_ADDR pc;

          pc = regcache_read_pc (regcache);
          regcache_write_pc (regcache, pc + decr_pc);
        }
    }

  /* We'll need this to determine whether to report a SIGSTOP as
     GDB_SIGNAL_0.  Need to take a copy because resume_clear_callback
     clears it.  */
  last_resume_kind = lp->last_resume_kind;

  if (!target_is_non_stop_p ())
    {
      /* In all-stop, from the core's perspective, all LWPs are now
         stopped until a new resume action is sent over.  */
      iterate_over_lwps (minus_one_ptid, resume_clear_callback, NULL);
    }
  else
    {
      resume_clear_callback (lp, NULL);
    }

  if (linux_nat_status_is_event (status))
    {
      if (debug_linux_nat)
        fprintf_unfiltered (gdb_stdlog,
                            "LLW: trap ptid is %s.\n",
                            target_pid_to_str (lp->ptid));
    }

  if (lp->waitstatus.kind != TARGET_WAITKIND_IGNORE)
    {
      *ourstatus = lp->waitstatus;
      lp->waitstatus.kind = TARGET_WAITKIND_IGNORE;
    }
  else
    store_waitstatus (ourstatus, status);

  if (debug_linux_nat)
    fprintf_unfiltered (gdb_stdlog, "LLW: exit\n");

  restore_child_signals_mask (&prev_mask);

  if (last_resume_kind == resume_stop
      && ourstatus->kind == TARGET_WAITKIND_STOPPED
      && WSTOPSIG (status) == SIGSTOP)
    {
      /* A thread that has been requested to stop by GDB with
         target_stop, and it stopped cleanly, so report as SIG0.  The
         use of SIGSTOP is an implementation detail.  */
      ourstatus->value.sig = GDB_SIGNAL_0;
    }

  if (ourstatus->kind == TARGET_WAITKIND_EXITED
      || ourstatus->kind == TARGET_WAITKIND_SIGNALLED)
    lp->core = -1;
  else
    lp->core = linux_common_core_of_thread (lp->ptid);

  return lp->ptid;
}

/* Resume LWPs that are currently stopped without any pending status
   to report, but are resumed from the core's perspective.  */

static int
resume_stopped_resumed_lwps (struct lwp_info *lp, void *data)
{
  ptid_t *wait_ptid_p = data;

  if (!lp->stopped)
    {
      if (debug_linux_nat)
        fprintf_unfiltered (gdb_stdlog,
                            "RSRL: NOT resuming LWP %s, not stopped\n",
                            target_pid_to_str (lp->ptid));
    }
  else if (!lp->resumed)
    {
      if (debug_linux_nat)
        fprintf_unfiltered (gdb_stdlog,
                            "RSRL: NOT resuming LWP %s, not resumed\n",
                            target_pid_to_str (lp->ptid));
    }
  else if (lwp_status_pending_p (lp))
    {
      if (debug_linux_nat)
        fprintf_unfiltered (gdb_stdlog,
                            "RSRL: NOT resuming LWP %s, has pending status\n",
                            target_pid_to_str (lp->ptid));
    }
  else
    {
      struct regcache *regcache = get_thread_regcache (lp->ptid);
      struct gdbarch *gdbarch = get_regcache_arch (regcache);

      TRY
        {
          CORE_ADDR pc = regcache_read_pc (regcache);
          int leave_stopped = 0;

          /* Don't bother if there's a breakpoint at PC that we'd hit
             immediately, and we're not waiting for this LWP.  */
          if (!ptid_match (lp->ptid, *wait_ptid_p))
            {
              if (breakpoint_inserted_here_p (get_regcache_aspace (regcache), pc))
                leave_stopped = 1;
            }

          if (!leave_stopped)
            {
              if (debug_linux_nat)
                fprintf_unfiltered (gdb_stdlog,
                                    "RSRL: resuming stopped-resumed LWP %s at "
                                    "%s: step=%d\n",
                                    target_pid_to_str (lp->ptid),
                                    paddress (gdbarch, pc),
                                    lp->step);

              linux_resume_one_lwp_throw (lp, lp->step, GDB_SIGNAL_0);
            }
        }
      CATCH (ex, RETURN_MASK_ERROR)
        {
          if (!check_ptrace_stopped_lwp_gone (lp))
            throw_exception (ex);
        }
      END_CATCH
    }

  return 0;
}

static ptid_t
linux_nat_wait (struct target_ops *ops,
                ptid_t ptid, struct target_waitstatus *ourstatus,
                int target_options)
{
  ptid_t event_ptid;

  if (debug_linux_nat)
    {
      char *options_string;

      options_string = target_options_to_string (target_options);
      fprintf_unfiltered (gdb_stdlog,
                          "linux_nat_wait: [%s], [%s]\n",
                          target_pid_to_str (ptid),
                          options_string);
      xfree (options_string);
    }

  /* Flush the async file first.  */
  if (target_is_async_p ())
    async_file_flush ();

  /* Resume LWPs that are currently stopped without any pending status
     to report, but are resumed from the core's perspective.  LWPs get
     in this state if we find them stopping at a time we're not
     interested in reporting the event (target_wait on a
     specific_process, for example, see linux_nat_wait_1), and
     meanwhile the event became uninteresting.  Don't bother resuming
     LWPs we're not going to wait for if they'd stop immediately.  */
  if (target_is_non_stop_p ())
    iterate_over_lwps (minus_one_ptid, resume_stopped_resumed_lwps, &ptid);

  event_ptid = linux_nat_wait_1 (ops, ptid, ourstatus, target_options);

  /* If we requested any event, and something came out, assume there
     may be more.  If we requested a specific lwp or process, also
     assume there may be more.  */
  if (target_is_async_p ()
      && ((ourstatus->kind != TARGET_WAITKIND_IGNORE
           && ourstatus->kind != TARGET_WAITKIND_NO_RESUMED)
          || !ptid_equal (ptid, minus_one_ptid)))
    async_file_mark ();

  return event_ptid;
}

static int
kill_callback (struct lwp_info *lp, void *data)
{
  /* PTRACE_KILL may resume the inferior.  Send SIGKILL first.  */

  errno = 0;
  kill_lwp (ptid_get_lwp (lp->ptid), SIGKILL);
  if (debug_linux_nat)
    {
      int save_errno = errno;

      fprintf_unfiltered (gdb_stdlog,
                          "KC:  kill (SIGKILL) %s, 0, 0 (%s)\n",
                          target_pid_to_str (lp->ptid),
                          save_errno ? safe_strerror (save_errno) : "OK");
    }

  /* Some kernels ignore even SIGKILL for processes under ptrace.  */

  errno = 0;
  ptrace (PTRACE_KILL, ptid_get_lwp (lp->ptid), 0, 0);
  if (debug_linux_nat)
    {
      int save_errno = errno;

      fprintf_unfiltered (gdb_stdlog,
                          "KC:  PTRACE_KILL %s, 0, 0 (%s)\n",
                          target_pid_to_str (lp->ptid),
                          save_errno ? safe_strerror (save_errno) : "OK");
    }

  return 0;
}

static int
kill_wait_callback (struct lwp_info *lp, void *data)
{
  pid_t pid;

  /* We must make sure that there are no pending events (delayed
     SIGSTOPs, pending SIGTRAPs, etc.) to make sure the current
     program doesn't interfere with any following debugging session.  */

  /* For cloned processes we must check both with __WCLONE and
     without, since the exit status of a cloned process isn't reported
     with __WCLONE.  */
  if (lp->cloned)
    {
      do
        {
          pid = my_waitpid (ptid_get_lwp (lp->ptid), NULL, __WCLONE);
          if (pid != (pid_t) -1)
            {
              if (debug_linux_nat)
                fprintf_unfiltered (gdb_stdlog,
                                    "KWC: wait %s received unknown.\n",
                                    target_pid_to_str (lp->ptid));
              /* The Linux kernel sometimes fails to kill a thread
                 completely after PTRACE_KILL; that goes from the stop
                 point in do_fork out to the one in
                 get_signal_to_deliever and waits again.  So kill it
                 again.  */
              kill_callback (lp, NULL);
            }
        }
      while (pid == ptid_get_lwp (lp->ptid));

      gdb_assert (pid == -1 && errno == ECHILD);
    }

  do
    {
      pid = my_waitpid (ptid_get_lwp (lp->ptid), NULL, 0);
      if (pid != (pid_t) -1)
        {
          if (debug_linux_nat)
            fprintf_unfiltered (gdb_stdlog,
                                "KWC: wait %s received unk.\n",
                                target_pid_to_str (lp->ptid));
          /* See the call to kill_callback above.  */
          kill_callback (lp, NULL);
        }
    }
  while (pid == ptid_get_lwp (lp->ptid));

  gdb_assert (pid == -1 && errno == ECHILD);
  return 0;
}

static void
linux_nat_kill (struct target_ops *ops)
{
  struct target_waitstatus last;
  ptid_t last_ptid;
  int status;

  /* If we're stopped while forking and we haven't followed yet,
     kill the other task.  We need to do this first because the
     parent will be sleeping if this is a vfork.  */

  get_last_target_status (&last_ptid, &last);

  if (last.kind == TARGET_WAITKIND_FORKED
      || last.kind == TARGET_WAITKIND_VFORKED)
    {
      ptrace (PT_KILL, ptid_get_pid (last.value.related_pid), 0, 0);
      wait (&status);

      /* Let the arch-specific native code know this process is
         gone.  */
      linux_nat_forget_process (ptid_get_pid (last.value.related_pid));
    }

  if (forks_exist_p ())
    linux_fork_killall ();
  else
    {
      ptid_t ptid = pid_to_ptid (ptid_get_pid (inferior_ptid));

      /* Stop all threads before killing them, since ptrace requires
         that the thread is stopped to sucessfully PTRACE_KILL.  */
      iterate_over_lwps (ptid, stop_callback, NULL);
      /* ... and wait until all of them have reported back that
         they're no longer running.  */
      iterate_over_lwps (ptid, stop_wait_callback, NULL);

      /* Kill all LWP's ...  */
      iterate_over_lwps (ptid, kill_callback, NULL);

      /* ... and wait until we've flushed all events.  */
      iterate_over_lwps (ptid, kill_wait_callback, NULL);
    }

  target_mourn_inferior ();
}

static void
linux_nat_mourn_inferior (struct target_ops *ops)
{
  int pid = ptid_get_pid (inferior_ptid);

  purge_lwp_list (pid);

  if (! forks_exist_p ())
    /* Normal case, no other forks available.  */
    linux_ops->to_mourn_inferior (ops);
  else
    /* Multi-fork case.  The current inferior_ptid has exited, but
       there are other viable forks to debug.  Delete the exiting
       one and context-switch to the first available.  */
    linux_fork_mourn_inferior ();

  /* Let the arch-specific native code know this process is gone.  */
  linux_nat_forget_process (pid);
}

/* Convert a native/host siginfo object, into/from the siginfo in the
   layout of the inferiors' architecture.  */

static void
siginfo_fixup (siginfo_t *siginfo, gdb_byte *inf_siginfo, int direction)
{
  int done = 0;

  if (linux_nat_siginfo_fixup != NULL)
    done = linux_nat_siginfo_fixup (siginfo, inf_siginfo, direction);

  /* If there was no callback, or the callback didn't do anything,
     then just do a straight memcpy.  */
  if (!done)
    {
      if (direction == 1)
        memcpy (siginfo, inf_siginfo, sizeof (siginfo_t));
      else
        memcpy (inf_siginfo, siginfo, sizeof (siginfo_t));
    }
}

static enum target_xfer_status
linux_xfer_siginfo (struct target_ops *ops, enum target_object object,
                    const char *annex, gdb_byte *readbuf,
                    const gdb_byte *writebuf, ULONGEST offset, ULONGEST len,
                    ULONGEST *xfered_len)
{
  int pid;
  siginfo_t siginfo;
  gdb_byte inf_siginfo[sizeof (siginfo_t)];

  gdb_assert (object == TARGET_OBJECT_SIGNAL_INFO);
  gdb_assert (readbuf || writebuf);

  pid = ptid_get_lwp (inferior_ptid);
  if (pid == 0)
    pid = ptid_get_pid (inferior_ptid);

  if (offset > sizeof (siginfo))
    return TARGET_XFER_E_IO;

  errno = 0;
  ptrace (PTRACE_GETSIGINFO, pid, (PTRACE_TYPE_ARG3) 0, &siginfo);
  if (errno != 0)
    return TARGET_XFER_E_IO;

  /* When GDB is built as a 64-bit application, ptrace writes into
     SIGINFO an object with 64-bit layout.  Since debugging a 32-bit
     inferior with a 64-bit GDB should look the same as debugging it
     with a 32-bit GDB, we need to convert it.  GDB core always sees
     the converted layout, so any read/write will have to be done
     post-conversion.  */
  siginfo_fixup (&siginfo, inf_siginfo, 0);

  if (offset + len > sizeof (siginfo))
    len = sizeof (siginfo) - offset;

  if (readbuf != NULL)
    memcpy (readbuf, inf_siginfo + offset, len);
  else
    {
      memcpy (inf_siginfo + offset, writebuf, len);

      /* Convert back to ptrace layout before flushing it out.  */
      siginfo_fixup (&siginfo, inf_siginfo, 1);

      errno = 0;
      ptrace (PTRACE_SETSIGINFO, pid, (PTRACE_TYPE_ARG3) 0, &siginfo);
      if (errno != 0)
        return TARGET_XFER_E_IO;
    }

  *xfered_len = len;
  return TARGET_XFER_OK;
}

static enum target_xfer_status
linux_nat_xfer_partial (struct target_ops *ops, enum target_object object,
                        const char *annex, gdb_byte *readbuf,
                        const gdb_byte *writebuf,
                        ULONGEST offset, ULONGEST len, ULONGEST *xfered_len)
{
  struct cleanup *old_chain;
  enum target_xfer_status xfer;

  if (object == TARGET_OBJECT_SIGNAL_INFO)
    return linux_xfer_siginfo (ops, object, annex, readbuf, writebuf,
                               offset, len, xfered_len);

  /* The target is connected but no live inferior is selected.  Pass
     this request down to a lower stratum (e.g., the executable
     file).  */
  if (object == TARGET_OBJECT_MEMORY && ptid_equal (inferior_ptid, null_ptid))
    return TARGET_XFER_EOF;

  old_chain = save_inferior_ptid ();

  if (ptid_lwp_p (inferior_ptid))
    inferior_ptid = pid_to_ptid (ptid_get_lwp (inferior_ptid));

  xfer = linux_ops->to_xfer_partial (ops, object, annex, readbuf, writebuf,
                                     offset, len, xfered_len);

  do_cleanups (old_chain);
  return xfer;
}

static int
linux_thread_alive (ptid_t ptid)
{
  int err, tmp_errno;

  gdb_assert (ptid_lwp_p (ptid));

  /* Send signal 0 instead of anything ptrace, because ptracing a
     running thread errors out claiming that the thread doesn't
     exist.  */
  err = kill_lwp (ptid_get_lwp (ptid), 0);
  tmp_errno = errno;
  if (debug_linux_nat)
    fprintf_unfiltered (gdb_stdlog,
                        "LLTA: KILL(SIG0) %s (%s)\n",
                        target_pid_to_str (ptid),
                        err ? safe_strerror (tmp_errno) : "OK");

  if (err != 0)
    return 0;

  return 1;
}

static int
linux_nat_thread_alive (struct target_ops *ops, ptid_t ptid)
{
  return linux_thread_alive (ptid);
}

/* Implement the to_update_thread_list target method for this
   target.  */

static void
linux_nat_update_thread_list (struct target_ops *ops)
{
  if (linux_supports_traceclone ())
    {
      /* With support for clone events, we add/delete threads from the
         list as clone/exit events are processed, so just try deleting
         exited threads still in the thread list.  */
      delete_exited_threads ();
    }
  else
    prune_threads ();
}

static char *
linux_nat_pid_to_str (struct target_ops *ops, ptid_t ptid)
{
  static char buf[64];

  if (ptid_lwp_p (ptid)
      && (ptid_get_pid (ptid) != ptid_get_lwp (ptid)
          || num_lwps (ptid_get_pid (ptid)) > 1))
    {
      snprintf (buf, sizeof (buf), "LWP %ld", ptid_get_lwp (ptid));
      return buf;
    }

  return normal_pid_to_str (ptid);
}

static char *
linux_nat_thread_name (struct target_ops *self, struct thread_info *thr)
{
  int pid = ptid_get_pid (thr->ptid);
  long lwp = ptid_get_lwp (thr->ptid);
#define FORMAT "/proc/%d/task/%ld/comm"
  char buf[sizeof (FORMAT) + 30];
  FILE *comm_file;
  char *result = NULL;

  snprintf (buf, sizeof (buf), FORMAT, pid, lwp);
  comm_file = gdb_fopen_cloexec (buf, "r");
  if (comm_file)
    {
      /* Not exported by the kernel, so we define it here.  */
#define COMM_LEN 16
      static char line[COMM_LEN + 1];

      if (fgets (line, sizeof (line), comm_file))
        {
          char *nl = strchr (line, '\n');

          if (nl)
            *nl = '\0';
          if (*line != '\0')
            result = line;
        }

      fclose (comm_file);
    }

#undef COMM_LEN
#undef FORMAT

  return result;
}

/* Accepts an integer PID; Returns a string representing a file that
   can be opened to get the symbols for the child process.  */

static char *
linux_child_pid_to_exec_file (struct target_ops *self, int pid)
{
  return linux_proc_pid_to_exec_file (pid);
}

/* Implement the to_xfer_partial interface for memory reads using the /proc
   filesystem.  Because we can use a single read() call for /proc, this
   can be much more efficient than banging away at PTRACE_PEEKTEXT,
   but it doesn't support writes.  */

static enum target_xfer_status
linux_proc_xfer_partial (struct target_ops *ops, enum target_object object,
                         const char *annex, gdb_byte *readbuf,
                         const gdb_byte *writebuf,
                         ULONGEST offset, LONGEST len, ULONGEST *xfered_len)
{
  LONGEST ret;
  int fd;
  char filename[64];

  if (object != TARGET_OBJECT_MEMORY || !readbuf)
    return TARGET_XFER_EOF;

  /* Don't bother for one word.  */
  if (len < 3 * sizeof (long))
    return TARGET_XFER_EOF;

  /* We could keep this file open and cache it - possibly one per
     thread.  That requires some juggling, but is even faster.  */
  xsnprintf (filename, sizeof filename, "/proc/%d/mem",
             ptid_get_pid (inferior_ptid));
  fd = gdb_open_cloexec (filename, O_RDONLY | O_LARGEFILE, 0);
  if (fd == -1)
    return TARGET_XFER_EOF;

  /* If pread64 is available, use it.  It's faster if the kernel
     supports it (only one syscall), and it's 64-bit safe even on
     32-bit platforms (for instance, SPARC debugging a SPARC64
     application).  */
#ifdef HAVE_PREAD64
  if (pread64 (fd, readbuf, len, offset) != len)
#else
  if (lseek (fd, offset, SEEK_SET) == -1 || read (fd, readbuf, len) != len)
#endif
    ret = 0;
  else
    ret = len;

  close (fd);

  if (ret == 0)
    return TARGET_XFER_EOF;
  else
    {
      *xfered_len = ret;
      return TARGET_XFER_OK;
    }
}


/* Enumerate spufs IDs for process PID.  */
static LONGEST
spu_enumerate_spu_ids (int pid, gdb_byte *buf, ULONGEST offset, ULONGEST len)
{
  enum bfd_endian byte_order = gdbarch_byte_order (target_gdbarch ());
  LONGEST pos = 0;
  LONGEST written = 0;
  char path[128];
  DIR *dir;
  struct dirent *entry;

  xsnprintf (path, sizeof path, "/proc/%d/fd", pid);
  dir = opendir (path);
  if (!dir)
    return -1;

  rewinddir (dir);
  while ((entry = readdir (dir)) != NULL)
    {
      struct stat st;
      struct statfs stfs;
      int fd;

      fd = atoi (entry->d_name);
      if (!fd)
        continue;

      xsnprintf (path, sizeof path, "/proc/%d/fd/%d", pid, fd);
      if (stat (path, &st) != 0)
        continue;
      if (!S_ISDIR (st.st_mode))
        continue;

      if (statfs (path, &stfs) != 0)
        continue;
      if (stfs.f_type != SPUFS_MAGIC)
        continue;

      if (pos >= offset && pos + 4 <= offset + len)
        {
          store_unsigned_integer (buf + pos - offset, 4, byte_order, fd);
          written += 4;
        }
      pos += 4;
    }

  closedir (dir);
  return written;
}

/* Implement the to_xfer_partial interface for the TARGET_OBJECT_SPU
   object type, using the /proc file system.  */

static enum target_xfer_status
linux_proc_xfer_spu (struct target_ops *ops, enum target_object object,
                     const char *annex, gdb_byte *readbuf,
                     const gdb_byte *writebuf,
                     ULONGEST offset, ULONGEST len, ULONGEST *xfered_len)
{
  char buf[128];
  int fd = 0;
  int ret = -1;
  int pid = ptid_get_pid (inferior_ptid);

  if (!annex)
    {
      if (!readbuf)
        return TARGET_XFER_E_IO;
      else
        {
          LONGEST l = spu_enumerate_spu_ids (pid, readbuf, offset, len);

          if (l < 0)
            return TARGET_XFER_E_IO;
          else if (l == 0)
            return TARGET_XFER_EOF;
          else
            {
              *xfered_len = (ULONGEST) l;
              return TARGET_XFER_OK;
            }
        }
    }

  xsnprintf (buf, sizeof buf, "/proc/%d/fd/%s", pid, annex);
  fd = gdb_open_cloexec (buf, writebuf? O_WRONLY : O_RDONLY, 0);
  if (fd <= 0)
    return TARGET_XFER_E_IO;

  if (offset != 0
      && lseek (fd, (off_t) offset, SEEK_SET) != (off_t) offset)
    {
      close (fd);
      return TARGET_XFER_EOF;
    }

  if (writebuf)
    ret = write (fd, writebuf, (size_t) len);
  else if (readbuf)
    ret = read (fd, readbuf, (size_t) len);

  close (fd);

  if (ret < 0)
    return TARGET_XFER_E_IO;
  else if (ret == 0)
    return TARGET_XFER_EOF;
  else
    {
      *xfered_len = (ULONGEST) ret;
      return TARGET_XFER_OK;
    }
}


/* Parse LINE as a signal set and add its set bits to SIGS.  */

static void
add_line_to_sigset (const char *line, sigset_t *sigs)
{
  int len = strlen (line) - 1;
  const char *p;
  int signum;

  if (line[len] != '\n')
    error (_("Could not parse signal set: %s"), line);

  p = line;
  signum = len * 4;
  while (len-- > 0)
    {
      int digit;

      if (*p >= '0' && *p <= '9')
        digit = *p - '0';
      else if (*p >= 'a' && *p <= 'f')
        digit = *p - 'a' + 10;
      else
        error (_("Could not parse signal set: %s"), line);

      signum -= 4;

      if (digit & 1)
        sigaddset (sigs, signum + 1);
      if (digit & 2)
        sigaddset (sigs, signum + 2);
      if (digit & 4)
        sigaddset (sigs, signum + 3);
      if (digit & 8)
        sigaddset (sigs, signum + 4);

      p++;
    }
}

/* Find process PID's pending signals from /proc/pid/status and set
   SIGS to match.  */

void
linux_proc_pending_signals (int pid, sigset_t *pending,
                            sigset_t *blocked, sigset_t *ignored)
{
  FILE *procfile;
  char buffer[PATH_MAX], fname[PATH_MAX];
  struct cleanup *cleanup;

  sigemptyset (pending);
  sigemptyset (blocked);
  sigemptyset (ignored);
  xsnprintf (fname, sizeof fname, "/proc/%d/status", pid);
  procfile = gdb_fopen_cloexec (fname, "r");
  if (procfile == NULL)
    error (_("Could not open %s"), fname);
  cleanup = make_cleanup_fclose (procfile);

  while (fgets (buffer, PATH_MAX, procfile) != NULL)
    {
      /* Normal queued signals are on the SigPnd line in the status
         file.  However, 2.6 kernels also have a "shared" pending
         queue for delivering signals to a thread group, so check for
         a ShdPnd line also.

         Unfortunately some Red Hat kernels include the shared pending
         queue but not the ShdPnd status field.  */

      if (startswith (buffer, "SigPnd:\t"))
        add_line_to_sigset (buffer + 8, pending);
      else if (startswith (buffer, "ShdPnd:\t"))
        add_line_to_sigset (buffer + 8, pending);
      else if (startswith (buffer, "SigBlk:\t"))
        add_line_to_sigset (buffer + 8, blocked);
      else if (startswith (buffer, "SigIgn:\t"))
        add_line_to_sigset (buffer + 8, ignored);
    }

  do_cleanups (cleanup);
}

static enum target_xfer_status
linux_nat_xfer_osdata (struct target_ops *ops, enum target_object object,
                       const char *annex, gdb_byte *readbuf,
                       const gdb_byte *writebuf, ULONGEST offset, ULONGEST len,
                       ULONGEST *xfered_len)
{
  gdb_assert (object == TARGET_OBJECT_OSDATA);

  *xfered_len = linux_common_xfer_osdata (annex, readbuf, offset, len);
  if (*xfered_len == 0)
    return TARGET_XFER_EOF;
  else
    return TARGET_XFER_OK;
}

static enum target_xfer_status
linux_xfer_partial (struct target_ops *ops, enum target_object object,
                    const char *annex, gdb_byte *readbuf,
                    const gdb_byte *writebuf, ULONGEST offset, ULONGEST len,
                    ULONGEST *xfered_len)
{
  enum target_xfer_status xfer;

  if (object == TARGET_OBJECT_AUXV)
    return memory_xfer_auxv (ops, object, annex, readbuf, writebuf,
                             offset, len, xfered_len);

  if (object == TARGET_OBJECT_OSDATA)
    return linux_nat_xfer_osdata (ops, object, annex, readbuf, writebuf,
                                  offset, len, xfered_len);

  if (object == TARGET_OBJECT_SPU)
    return linux_proc_xfer_spu (ops, object, annex, readbuf, writebuf,
                                offset, len, xfered_len);

  /* GDB calculates all the addresses in possibly larget width of the address.
     Address width needs to be masked before its final use - either by
     linux_proc_xfer_partial or inf_ptrace_xfer_partial.

     Compare ADDR_BIT first to avoid a compiler warning on shift overflow.  */

  if (object == TARGET_OBJECT_MEMORY)
    {
      int addr_bit = gdbarch_addr_bit (target_gdbarch ());

      if (addr_bit < (sizeof (ULONGEST) * HOST_CHAR_BIT))
        offset &= ((ULONGEST) 1 << addr_bit) - 1;
    }

  xfer = linux_proc_xfer_partial (ops, object, annex, readbuf, writebuf,
                                  offset, len, xfered_len);
  if (xfer != TARGET_XFER_EOF)
    return xfer;

  return super_xfer_partial (ops, object, annex, readbuf, writebuf,
                             offset, len, xfered_len);
}

static void
cleanup_target_stop (void *arg)
{
  ptid_t *ptid = (ptid_t *) arg;

  gdb_assert (arg != NULL);

  /* Unpause all */
  target_resume (*ptid, 0, GDB_SIGNAL_0);
}

static VEC(static_tracepoint_marker_p) *
linux_child_static_tracepoint_markers_by_strid (struct target_ops *self,
                                                const char *strid)
{
  char s[IPA_CMD_BUF_SIZE];
  struct cleanup *old_chain;
  int pid = ptid_get_pid (inferior_ptid);
  VEC(static_tracepoint_marker_p) *markers = NULL;
  struct static_tracepoint_marker *marker = NULL;
  char *p = s;
  ptid_t ptid = ptid_build (pid, 0, 0);

  /* Pause all */
  target_stop (ptid);

  memcpy (s, "qTfSTM", sizeof ("qTfSTM"));
  s[sizeof ("qTfSTM")] = 0;

  agent_run_command (pid, s, strlen (s) + 1);

  old_chain = make_cleanup (free_current_marker, &marker);
  make_cleanup (cleanup_target_stop, &ptid);

  while (*p++ == 'm')
    {
      if (marker == NULL)
        marker = XCNEW (struct static_tracepoint_marker);

      do
        {
          parse_static_tracepoint_marker_definition (p, &p, marker);

          if (strid == NULL || strcmp (strid, marker->str_id) == 0)
            {
              VEC_safe_push (static_tracepoint_marker_p,
                             markers, marker);
              marker = NULL;
            }
          else
            {
              release_static_tracepoint_marker (marker);
              memset (marker, 0, sizeof (*marker));
            }
        }
      while (*p++ == ',');      /* comma-separated list */

      memcpy (s, "qTsSTM", sizeof ("qTsSTM"));
      s[sizeof ("qTsSTM")] = 0;
      agent_run_command (pid, s, strlen (s) + 1);
      p = s;
    }

  do_cleanups (old_chain);

  return markers;
}

/* Create a prototype generic GNU/Linux target.  The client can override
   it with local methods.  */

static void
linux_target_install_ops (struct target_ops *t)
{
  t->to_insert_fork_catchpoint = linux_child_insert_fork_catchpoint;
  t->to_remove_fork_catchpoint = linux_child_remove_fork_catchpoint;
  t->to_insert_vfork_catchpoint = linux_child_insert_vfork_catchpoint;
  t->to_remove_vfork_catchpoint = linux_child_remove_vfork_catchpoint;
  t->to_insert_exec_catchpoint = linux_child_insert_exec_catchpoint;
  t->to_remove_exec_catchpoint = linux_child_remove_exec_catchpoint;
  t->to_set_syscall_catchpoint = linux_child_set_syscall_catchpoint;
  t->to_pid_to_exec_file = linux_child_pid_to_exec_file;
  t->to_post_startup_inferior = linux_child_post_startup_inferior;
  t->to_post_attach = linux_child_post_attach;
  t->to_follow_fork = linux_child_follow_fork;

  super_xfer_partial = t->to_xfer_partial;
  t->to_xfer_partial = linux_xfer_partial;

  t->to_static_tracepoint_markers_by_strid
    = linux_child_static_tracepoint_markers_by_strid;
}

struct target_ops *
linux_target (void)
{
  struct target_ops *t;

  t = inf_ptrace_target ();
  linux_target_install_ops (t);

  return t;
}

struct target_ops *
linux_trad_target (CORE_ADDR (*register_u_offset)(struct gdbarch *, int, int))
{
  struct target_ops *t;

  t = inf_ptrace_trad_target (register_u_offset);
  linux_target_install_ops (t);

  return t;
}

/* target_is_async_p implementation.  */

static int
linux_nat_is_async_p (struct target_ops *ops)
{
  return linux_is_async_p ();
}

/* target_can_async_p implementation.  */

static int
linux_nat_can_async_p (struct target_ops *ops)
{
  /* NOTE: palves 2008-03-21: We're only async when the user requests
     it explicitly with the "set target-async" command.
     Someday, linux will always be async.  */
  return target_async_permitted;
}

static int
linux_nat_supports_non_stop (struct target_ops *self)
{
  return 1;
}

/* to_always_non_stop_p implementation.  */

static int
linux_nat_always_non_stop_p (struct target_ops *self)
{
  if (linux_ops->to_always_non_stop_p != NULL)
    return linux_ops->to_always_non_stop_p (linux_ops);
  return 1;
}

/* True if we want to support multi-process.  To be removed when GDB
   supports multi-exec.  */

int linux_multi_process = 1;

static int
linux_nat_supports_multi_process (struct target_ops *self)
{
  return linux_multi_process;
}

static int
linux_nat_supports_disable_randomization (struct target_ops *self)
{
#ifdef HAVE_PERSONALITY
  return 1;
#else
  return 0;
#endif
}

static int async_terminal_is_ours = 1;

/* target_terminal_inferior implementation.

   This is a wrapper around child_terminal_inferior to add async support.  */

static void
linux_nat_terminal_inferior (struct target_ops *self)
{
  child_terminal_inferior (self);

  /* Calls to target_terminal_*() are meant to be idempotent.  */
  if (!async_terminal_is_ours)
    return;

  delete_file_handler (input_fd);
  async_terminal_is_ours = 0;
  set_sigint_trap ();
}

/* target_terminal_ours implementation.

   This is a wrapper around child_terminal_ours to add async support (and
   implement the target_terminal_ours vs target_terminal_ours_for_output
   distinction).  child_terminal_ours is currently no different than
   child_terminal_ours_for_output.
   We leave target_terminal_ours_for_output alone, leaving it to
   child_terminal_ours_for_output.  */

static void
linux_nat_terminal_ours (struct target_ops *self)
{
  /* GDB should never give the terminal to the inferior if the
     inferior is running in the background (run&, continue&, etc.),
     but claiming it sure should.  */
  child_terminal_ours (self);

  if (async_terminal_is_ours)
    return;

  clear_sigint_trap ();
  add_file_handler (input_fd, stdin_event_handler, 0);
  async_terminal_is_ours = 1;
}

/* SIGCHLD handler that serves two purposes: In non-stop/async mode,
   so we notice when any child changes state, and notify the
   event-loop; it allows us to use sigsuspend in linux_nat_wait_1
   above to wait for the arrival of a SIGCHLD.  */

static void
sigchld_handler (int signo)
{
  int old_errno = errno;

  if (debug_linux_nat)
    ui_file_write_async_safe (gdb_stdlog,
                              "sigchld\n", sizeof ("sigchld\n") - 1);

  if (signo == SIGCHLD
      && linux_nat_event_pipe[0] != -1)
    async_file_mark (); /* Let the event loop know that there are
                           events to handle.  */

  errno = old_errno;
}

/* Callback registered with the target events file descriptor.  */

static void
handle_target_event (int error, gdb_client_data client_data)
{
  inferior_event_handler (INF_REG_EVENT, NULL);
}

/* Create/destroy the target events pipe.  Returns previous state.  */

static int
linux_async_pipe (int enable)
{
  int previous = linux_is_async_p ();

  if (previous != enable)
    {
      sigset_t prev_mask;

      /* Block child signals while we create/destroy the pipe, as
         their handler writes to it.  */
      block_child_signals (&prev_mask);

      if (enable)
        {
          if (gdb_pipe_cloexec (linux_nat_event_pipe) == -1)
            internal_error (__FILE__, __LINE__,
                            "creating event pipe failed.");

          fcntl (linux_nat_event_pipe[0], F_SETFL, O_NONBLOCK);
          fcntl (linux_nat_event_pipe[1], F_SETFL, O_NONBLOCK);
        }
      else
        {
          close (linux_nat_event_pipe[0]);
          close (linux_nat_event_pipe[1]);
          linux_nat_event_pipe[0] = -1;
          linux_nat_event_pipe[1] = -1;
        }

      restore_child_signals_mask (&prev_mask);
    }

  return previous;
}

/* target_async implementation.  */

static void
linux_nat_async (struct target_ops *ops, int enable)
{
  if (enable)
    {
      if (!linux_async_pipe (1))
        {
          add_file_handler (linux_nat_event_pipe[0],
                            handle_target_event, NULL);
          /* There may be pending events to handle.  Tell the event loop
             to poll them.  */
          async_file_mark ();
        }
    }
  else
    {
      delete_file_handler (linux_nat_event_pipe[0]);
      linux_async_pipe (0);
    }
  return;
}

/* Stop an LWP, and push a GDB_SIGNAL_0 stop status if no other
   event came out.  */

static int
linux_nat_stop_lwp (struct lwp_info *lwp, void *data)
{
  if (!lwp->stopped)
    {
      if (debug_linux_nat)
        fprintf_unfiltered (gdb_stdlog,
                            "LNSL: running -> suspending %s\n",
                            target_pid_to_str (lwp->ptid));


      if (lwp->last_resume_kind == resume_stop)
        {
          if (debug_linux_nat)
            fprintf_unfiltered (gdb_stdlog,
                                "linux-nat: already stopping LWP %ld at "
                                "GDB's request\n",
                                ptid_get_lwp (lwp->ptid));
          return 0;
        }

      stop_callback (lwp, NULL);
      lwp->last_resume_kind = resume_stop;
    }
  else
    {
      /* Already known to be stopped; do nothing.  */

      if (debug_linux_nat)
        {
          if (find_thread_ptid (lwp->ptid)->stop_requested)
            fprintf_unfiltered (gdb_stdlog,
                                "LNSL: already stopped/stop_requested %s\n",
                                target_pid_to_str (lwp->ptid));
          else
            fprintf_unfiltered (gdb_stdlog,
                                "LNSL: already stopped/no "
                                "stop_requested yet %s\n",
                                target_pid_to_str (lwp->ptid));
        }
    }
  return 0;
}

static void
linux_nat_stop (struct target_ops *self, ptid_t ptid)
{
  iterate_over_lwps (ptid, linux_nat_stop_lwp, NULL);
}

static void
linux_nat_interrupt (struct target_ops *self, ptid_t ptid)
{
  if (non_stop)
    iterate_over_lwps (ptid, linux_nat_stop_lwp, NULL);
  else
    linux_ops->to_interrupt (linux_ops, ptid);
}

static void
linux_nat_close (struct target_ops *self)
{
  /* Unregister from the event loop.  */
  if (linux_nat_is_async_p (self))
    linux_nat_async (self, 0);

  if (linux_ops->to_close)
    linux_ops->to_close (linux_ops);

  super_close (self);
}

/* When requests are passed down from the linux-nat layer to the
   single threaded inf-ptrace layer, ptids of (lwpid,0,0) form are
   used.  The address space pointer is stored in the inferior object,
   but the common code that is passed such ptid can't tell whether
   lwpid is a "main" process id or not (it assumes so).  We reverse
   look up the "main" process id from the lwp here.  */

static struct address_space *
linux_nat_thread_address_space (struct target_ops *t, ptid_t ptid)
{
  struct lwp_info *lwp;
  struct inferior *inf;
  int pid;

  if (ptid_get_lwp (ptid) == 0)
    {
      /* An (lwpid,0,0) ptid.  Look up the lwp object to get at the
         tgid.  */
      lwp = find_lwp_pid (ptid);
      pid = ptid_get_pid (lwp->ptid);
    }
  else
    {
      /* A (pid,lwpid,0) ptid.  */
      pid = ptid_get_pid (ptid);
    }

  inf = find_inferior_pid (pid);
  gdb_assert (inf != NULL);
  return inf->aspace;
}

/* Return the cached value of the processor core for thread PTID.  */

static int
linux_nat_core_of_thread (struct target_ops *ops, ptid_t ptid)
{
  struct lwp_info *info = find_lwp_pid (ptid);

  if (info)
    return info->core;
  return -1;
}

/* Implementation of to_filesystem_is_local.  */

static int
linux_nat_filesystem_is_local (struct target_ops *ops)
{
  struct inferior *inf = current_inferior ();

  if (inf->fake_pid_p || inf->pid == 0)
    return 1;

  return linux_ns_same (inf->pid, LINUX_NS_MNT);
}

/* Convert the INF argument passed to a to_fileio_* method
   to a process ID suitable for passing to its corresponding
   linux_mntns_* function.  If INF is non-NULL then the
   caller is requesting the filesystem seen by INF.  If INF
   is NULL then the caller is requesting the filesystem seen
   by the GDB.  We fall back to GDB's filesystem in the case
   that INF is non-NULL but its PID is unknown.  */

static pid_t
linux_nat_fileio_pid_of (struct inferior *inf)
{
  if (inf == NULL || inf->fake_pid_p || inf->pid == 0)
    return getpid ();
  else
    return inf->pid;
}

/* Implementation of to_fileio_open.  */

static int
linux_nat_fileio_open (struct target_ops *self,
                       struct inferior *inf, const char *filename,
                       int flags, int mode, int warn_if_slow,
                       int *target_errno)
{
  int nat_flags;
  mode_t nat_mode;
  int fd;

  if (fileio_to_host_openflags (flags, &nat_flags) == -1
      || fileio_to_host_mode (mode, &nat_mode) == -1)
    {
      *target_errno = FILEIO_EINVAL;
      return -1;
    }

  fd = linux_mntns_open_cloexec (linux_nat_fileio_pid_of (inf),
                                 filename, nat_flags, nat_mode);
  if (fd == -1)
    *target_errno = host_to_fileio_error (errno);

  return fd;
}

/* Implementation of to_fileio_readlink.  */

static char *
linux_nat_fileio_readlink (struct target_ops *self,
                           struct inferior *inf, const char *filename,
                           int *target_errno)
{
  char buf[PATH_MAX];
  int len;
  char *ret;

  len = linux_mntns_readlink (linux_nat_fileio_pid_of (inf),
                              filename, buf, sizeof (buf));
  if (len < 0)
    {
      *target_errno = host_to_fileio_error (errno);
      return NULL;
    }

  ret = xmalloc (len + 1);
  memcpy (ret, buf, len);
  ret[len] = '\0';
  return ret;
}

/* Implementation of to_fileio_unlink.  */

static int
linux_nat_fileio_unlink (struct target_ops *self,
                         struct inferior *inf, const char *filename,
                         int *target_errno)
{
  int ret;

  ret = linux_mntns_unlink (linux_nat_fileio_pid_of (inf),
                            filename);
  if (ret == -1)
    *target_errno = host_to_fileio_error (errno);

  return ret;
}

void
linux_nat_add_target (struct target_ops *t)
{
  /* Save the provided single-threaded target.  We save this in a separate
     variable because another target we've inherited from (e.g. inf-ptrace)
     may have saved a pointer to T; we want to use it for the final
     process stratum target.  */
  linux_ops_saved = *t;
  linux_ops = &linux_ops_saved;

  /* Override some methods for multithreading.  */
  t->to_create_inferior = linux_nat_create_inferior;
  t->to_attach = linux_nat_attach;
  t->to_detach = linux_nat_detach;
  t->to_resume = linux_nat_resume;
  t->to_wait = linux_nat_wait;
  t->to_pass_signals = linux_nat_pass_signals;
  t->to_xfer_partial = linux_nat_xfer_partial;
  t->to_kill = linux_nat_kill;
  t->to_mourn_inferior = linux_nat_mourn_inferior;
  t->to_thread_alive = linux_nat_thread_alive;
  t->to_update_thread_list = linux_nat_update_thread_list;
  t->to_pid_to_str = linux_nat_pid_to_str;
  t->to_thread_name = linux_nat_thread_name;
  t->to_has_thread_control = tc_schedlock;
  t->to_thread_address_space = linux_nat_thread_address_space;
  t->to_stopped_by_watchpoint = linux_nat_stopped_by_watchpoint;
  t->to_stopped_data_address = linux_nat_stopped_data_address;
  t->to_stopped_by_sw_breakpoint = linux_nat_stopped_by_sw_breakpoint;
  t->to_supports_stopped_by_sw_breakpoint = linux_nat_supports_stopped_by_sw_breakpoint;
  t->to_stopped_by_hw_breakpoint = linux_nat_stopped_by_hw_breakpoint;
  t->to_supports_stopped_by_hw_breakpoint = linux_nat_supports_stopped_by_hw_breakpoint;

  t->to_can_async_p = linux_nat_can_async_p;
  t->to_is_async_p = linux_nat_is_async_p;
  t->to_supports_non_stop = linux_nat_supports_non_stop;
  t->to_always_non_stop_p = linux_nat_always_non_stop_p;
  t->to_async = linux_nat_async;
  t->to_terminal_inferior = linux_nat_terminal_inferior;
  t->to_terminal_ours = linux_nat_terminal_ours;

  super_close = t->to_close;
  t->to_close = linux_nat_close;

  t->to_stop = linux_nat_stop;
  t->to_interrupt = linux_nat_interrupt;

  t->to_supports_multi_process = linux_nat_supports_multi_process;

  t->to_supports_disable_randomization
    = linux_nat_supports_disable_randomization;

  t->to_core_of_thread = linux_nat_core_of_thread;

  t->to_filesystem_is_local = linux_nat_filesystem_is_local;
  t->to_fileio_open = linux_nat_fileio_open;
  t->to_fileio_readlink = linux_nat_fileio_readlink;
  t->to_fileio_unlink = linux_nat_fileio_unlink;

  /* We don't change the stratum; this target will sit at
     process_stratum and thread_db will set at thread_stratum.  This
     is a little strange, since this is a multi-threaded-capable
     target, but we want to be on the stack below thread_db, and we
     also want to be used for single-threaded processes.  */

  add_target (t);
}

/* Register a method to call whenever a new thread is attached.  */
void
linux_nat_set_new_thread (struct target_ops *t,
                          void (*new_thread) (struct lwp_info *))
{
  /* Save the pointer.  We only support a single registered instance
     of the GNU/Linux native target, so we do not need to map this to
     T.  */
  linux_nat_new_thread = new_thread;
}

/* See declaration in linux-nat.h.  */

void
linux_nat_set_new_fork (struct target_ops *t,
                        linux_nat_new_fork_ftype *new_fork)
{
  /* Save the pointer.  */
  linux_nat_new_fork = new_fork;
}

/* See declaration in linux-nat.h.  */

void
linux_nat_set_forget_process (struct target_ops *t,
                              linux_nat_forget_process_ftype *fn)
{
  /* Save the pointer.  */
  linux_nat_forget_process_hook = fn;
}

/* See declaration in linux-nat.h.  */

void
linux_nat_forget_process (pid_t pid)
{
  if (linux_nat_forget_process_hook != NULL)
    linux_nat_forget_process_hook (pid);
}

/* Register a method that converts a siginfo object between the layout
   that ptrace returns, and the layout in the architecture of the
   inferior.  */
void
linux_nat_set_siginfo_fixup (struct target_ops *t,
                             int (*siginfo_fixup) (siginfo_t *,
                                                   gdb_byte *,
                                                   int))
{
  /* Save the pointer.  */
  linux_nat_siginfo_fixup = siginfo_fixup;
}

/* Register a method to call prior to resuming a thread.  */

void
linux_nat_set_prepare_to_resume (struct target_ops *t,
                                 void (*prepare_to_resume) (struct lwp_info *))
{
  /* Save the pointer.  */
  linux_nat_prepare_to_resume = prepare_to_resume;
}

/* See linux-nat.h.  */

int
linux_nat_get_siginfo (ptid_t ptid, siginfo_t *siginfo)
{
  int pid;

  pid = ptid_get_lwp (ptid);
  if (pid == 0)
    pid = ptid_get_pid (ptid);

  errno = 0;
  ptrace (PTRACE_GETSIGINFO, pid, (PTRACE_TYPE_ARG3) 0, siginfo);
  if (errno != 0)
    {
      memset (siginfo, 0, sizeof (*siginfo));
      return 0;
    }
  return 1;
}

/* See nat/linux-nat.h.  */

ptid_t
current_lwp_ptid (void)
{
  gdb_assert (ptid_lwp_p (inferior_ptid));
  return inferior_ptid;
}

/* Provide a prototype to silence -Wmissing-prototypes.  */
extern initialize_file_ftype _initialize_linux_nat;

void
_initialize_linux_nat (void)
{
  add_setshow_zuinteger_cmd ("lin-lwp", class_maintenance,
                             &debug_linux_nat, _("\
Set debugging of GNU/Linux lwp module."), _("\
Show debugging of GNU/Linux lwp module."), _("\
Enables printf debugging output."),
                             NULL,
                             show_debug_linux_nat,
                             &setdebuglist, &showdebuglist);

  add_setshow_boolean_cmd ("linux-namespaces", class_maintenance,
                           &debug_linux_namespaces, _("\
Set debugging of GNU/Linux namespaces module."), _("\
Show debugging of GNU/Linux namespaces module."), _("\
Enables printf debugging output."),
                           NULL,
                           NULL,
                           &setdebuglist, &showdebuglist);

  /* Save this mask as the default.  */
  sigprocmask (SIG_SETMASK, NULL, &normal_mask);

  /* Install a SIGCHLD handler.  */
  sigchld_action.sa_handler = sigchld_handler;
  sigemptyset (&sigchld_action.sa_mask);
  sigchld_action.sa_flags = SA_RESTART;

  /* Make it the default.  */
  sigaction (SIGCHLD, &sigchld_action, NULL);

  /* Make sure we don't block SIGCHLD during a sigsuspend.  */
  sigprocmask (SIG_SETMASK, NULL, &suspend_mask);
  sigdelset (&suspend_mask, SIGCHLD);

  sigemptyset (&blocked_mask);
}
\f

/* FIXME: kettenis/2000-08-26: The stuff on this page is specific to
   the GNU/Linux Threads library and therefore doesn't really belong
   here.  */

/* Read variable NAME in the target and return its value if found.
   Otherwise return zero.  It is assumed that the type of the variable
   is `int'.  */

static int
get_signo (const char *name)
{
  struct bound_minimal_symbol ms;
  int signo;

  ms = lookup_minimal_symbol (name, NULL, NULL);
  if (ms.minsym == NULL)
    return 0;

  if (target_read_memory (BMSYMBOL_VALUE_ADDRESS (ms), (gdb_byte *) &signo,
                          sizeof (signo)) != 0)
    return 0;

  return signo;
}

/* Return the set of signals used by the threads library in *SET.  */

void
lin_thread_get_thread_signals (sigset_t *set)
{
  struct sigaction action;
  int restart, cancel;

  sigemptyset (&blocked_mask);
  sigemptyset (set);

  restart = get_signo ("__pthread_sig_restart");
  cancel = get_signo ("__pthread_sig_cancel");

  /* LinuxThreads normally uses the first two RT signals, but in some legacy
     cases may use SIGUSR1/SIGUSR2.  NPTL always uses RT signals, but does
     not provide any way for the debugger to query the signal numbers -
     fortunately they don't change!  */

  if (restart == 0)
    restart = __SIGRTMIN;

  if (cancel == 0)
    cancel = __SIGRTMIN + 1;

  sigaddset (set, restart);
  sigaddset (set, cancel);

  /* The GNU/Linux Threads library makes terminating threads send a
     special "cancel" signal instead of SIGCHLD.  Make sure we catch
     those (to prevent them from terminating GDB itself, which is
     likely to be their default action) and treat them the same way as
     SIGCHLD.  */

  action.sa_handler = sigchld_handler;
  sigemptyset (&action.sa_mask);
  action.sa_flags = SA_RESTART;
  sigaction (cancel, &action, NULL);

  /* We block the "cancel" signal throughout this code ...  */
  sigaddset (&blocked_mask, cancel);
  sigprocmask (SIG_BLOCK, &blocked_mask, NULL);

  /* ... except during a sigsuspend.  */
  sigdelset (&suspend_mask, cancel);
}