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[gdb.git] / gdb / gdbserver / linux-low.c

/* Low level interface to ptrace, for the remote server for GDB.
   Copyright (C) 1995, 1996, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005,
   2006, 2007, 2008 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 "server.h"
#include "linux-low.h"

#include <sys/wait.h>
#include <stdio.h>
#include <sys/param.h>
#include <sys/dir.h>
#include <sys/ptrace.h>
#include <sys/user.h>
#include <signal.h>
#include <sys/ioctl.h>
#include <fcntl.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <sys/syscall.h>
#include <sched.h>

#ifndef PTRACE_GETSIGINFO
# define PTRACE_GETSIGINFO 0x4202
# define PTRACE_SETSIGINFO 0x4203
#endif

#ifndef O_LARGEFILE
#define O_LARGEFILE 0
#endif

/* If the system headers did not provide the constants, hard-code the normal
   values.  */
#ifndef PTRACE_EVENT_FORK

#define PTRACE_SETOPTIONS       0x4200
#define PTRACE_GETEVENTMSG      0x4201

/* options set using PTRACE_SETOPTIONS */
#define PTRACE_O_TRACESYSGOOD   0x00000001
#define PTRACE_O_TRACEFORK      0x00000002
#define PTRACE_O_TRACEVFORK     0x00000004
#define PTRACE_O_TRACECLONE     0x00000008
#define PTRACE_O_TRACEEXEC      0x00000010
#define PTRACE_O_TRACEVFORKDONE 0x00000020
#define PTRACE_O_TRACEEXIT      0x00000040

/* Wait extended result codes for the above trace options.  */
#define PTRACE_EVENT_FORK       1
#define PTRACE_EVENT_VFORK      2
#define PTRACE_EVENT_CLONE      3
#define PTRACE_EVENT_EXEC       4
#define PTRACE_EVENT_VFORK_DONE 5
#define PTRACE_EVENT_EXIT       6

#endif /* PTRACE_EVENT_FORK */

/* We can't always assume that this flag is available, but all systems
   with the ptrace event handlers also have __WALL, so it's safe to use
   in some contexts.  */
#ifndef __WALL
#define __WALL          0x40000000 /* Wait for any child.  */
#endif

#ifdef __UCLIBC__
#if !(defined(__UCLIBC_HAS_MMU__) || defined(__ARCH_HAS_MMU__))
#define HAS_NOMMU
#endif
#endif

/* ``all_threads'' is keyed by the LWP ID, which we use as the GDB protocol
   representation of the thread ID.

   ``all_processes'' is keyed by the process ID - which on Linux is (presently)
   the same as the LWP ID.  */

struct inferior_list all_processes;

/* A list of all unknown processes which receive stop signals.  Some other
   process will presumably claim each of these as forked children
   momentarily.  */

struct inferior_list stopped_pids;

/* FIXME this is a bit of a hack, and could be removed.  */
int stopping_threads;

/* FIXME make into a target method?  */
int using_threads = 1;
static int thread_db_active;

static int must_set_ptrace_flags;

static void linux_resume_one_process (struct inferior_list_entry *entry,
                                      int step, int signal, siginfo_t *info);
static void linux_resume (struct thread_resume *resume_info);
static void stop_all_processes (void);
static int linux_wait_for_event (struct thread_info *child);
static int check_removed_breakpoint (struct process_info *event_child);
static void *add_process (unsigned long pid);

struct pending_signals
{
  int signal;
  siginfo_t info;
  struct pending_signals *prev;
};

#define PTRACE_ARG3_TYPE long
#define PTRACE_XFER_TYPE long

#ifdef HAVE_LINUX_REGSETS
static int use_regsets_p = 1;
#endif

#define pid_of(proc) ((proc)->head.id)

/* FIXME: Delete eventually.  */
#define inferior_pid (pid_of (get_thread_process (current_inferior)))

static void
handle_extended_wait (struct process_info *event_child, int wstat)
{
  int event = wstat >> 16;
  struct process_info *new_process;

  if (event == PTRACE_EVENT_CLONE)
    {
      unsigned long new_pid;
      int ret, status;

      ptrace (PTRACE_GETEVENTMSG, inferior_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))
        {
          /* The new child has a pending SIGSTOP.  We can't affect it until it
             hits the SIGSTOP, but we're already attached.  */

          do {
            ret = waitpid (new_pid, &status, __WALL);
          } while (ret == -1 && errno == EINTR);

          if (ret == -1)
            perror_with_name ("waiting for new child");
          else if (ret != new_pid)
            warning ("wait returned unexpected PID %d", ret);
          else if (!WIFSTOPPED (status))
            warning ("wait returned unexpected status 0x%x", status);
        }

      ptrace (PTRACE_SETOPTIONS, new_pid, 0, PTRACE_O_TRACECLONE);

      new_process = (struct process_info *) add_process (new_pid);
      add_thread (new_pid, new_process, new_pid);
      new_thread_notify (thread_id_to_gdb_id (new_process->lwpid));

      /* Normally we will get the pending SIGSTOP.  But in some cases
         we might get another signal delivered to the group first.
         If we do, be sure not to lose it.  */
      if (WSTOPSIG (status) == SIGSTOP)
        {
          if (stopping_threads)
            new_process->stopped = 1;
          else
            ptrace (PTRACE_CONT, new_pid, 0, 0);
        }
      else
        {
          new_process->stop_expected = 1;
          if (stopping_threads)
            {
              new_process->stopped = 1;
              new_process->status_pending_p = 1;
              new_process->status_pending = status;
            }
          else
            /* Pass the signal on.  This is what GDB does - except
               shouldn't we really report it instead?  */
            ptrace (PTRACE_CONT, new_pid, 0, WSTOPSIG (status));
        }

      /* Always resume the current thread.  If we are stopping
         threads, it will have a pending SIGSTOP; we may as well
         collect it now.  */
      linux_resume_one_process (&event_child->head,
                                event_child->stepping, 0, NULL);
    }
}

/* This function should only be called if the process got a SIGTRAP.
   The SIGTRAP could mean several things.

   On i386, where decr_pc_after_break is non-zero:
   If we were single-stepping this process using PTRACE_SINGLESTEP,
   we will get only the one SIGTRAP (even if the instruction we
   stepped over was a breakpoint).  The value of $eip will be the
   next instruction.
   If we continue the process using PTRACE_CONT, we will get a
   SIGTRAP when we hit a breakpoint.  The value of $eip will be
   the instruction after the breakpoint (i.e. needs to be
   decremented).  If we report the SIGTRAP to GDB, we must also
   report the undecremented PC.  If we cancel the SIGTRAP, we
   must resume at the decremented PC.

   (Presumably, not yet tested) On a non-decr_pc_after_break machine
   with hardware or kernel single-step:
   If we single-step over a breakpoint instruction, our PC will
   point at the following instruction.  If we continue and hit a
   breakpoint instruction, our PC will point at the breakpoint
   instruction.  */

static CORE_ADDR
get_stop_pc (void)
{
  CORE_ADDR stop_pc = (*the_low_target.get_pc) ();

  if (get_thread_process (current_inferior)->stepping)
    return stop_pc;
  else
    return stop_pc - the_low_target.decr_pc_after_break;
}

static void *
add_process (unsigned long pid)
{
  struct process_info *process;

  process = (struct process_info *) malloc (sizeof (*process));
  memset (process, 0, sizeof (*process));

  process->head.id = pid;
  process->lwpid = pid;

  add_inferior_to_list (&all_processes, &process->head);

  return process;
}

/* Start an inferior process and returns its pid.
   ALLARGS is a vector of program-name and args. */

static int
linux_create_inferior (char *program, char **allargs)
{
  void *new_process;
  int pid;

#if defined(__UCLIBC__) && defined(HAS_NOMMU)
  pid = vfork ();
#else
  pid = fork ();
#endif
  if (pid < 0)
    perror_with_name ("fork");

  if (pid == 0)
    {
      ptrace (PTRACE_TRACEME, 0, 0, 0);

      signal (__SIGRTMIN + 1, SIG_DFL);

      setpgid (0, 0);

      execv (program, allargs);
      if (errno == ENOENT)
        execvp (program, allargs);

      fprintf (stderr, "Cannot exec %s: %s.\n", program,
               strerror (errno));
      fflush (stderr);
      _exit (0177);
    }

  new_process = add_process (pid);
  add_thread (pid, new_process, pid);
  must_set_ptrace_flags = 1;

  return pid;
}

/* Attach to an inferior process.  */

void
linux_attach_lwp (unsigned long pid)
{
  struct process_info *new_process;

  if (ptrace (PTRACE_ATTACH, pid, 0, 0) != 0)
    {
      if (all_threads.head != NULL)
        {
          /* If we fail to attach to an LWP, just warn.  */
          fprintf (stderr, "Cannot attach to process %ld: %s (%d)\n", pid,
                   strerror (errno), errno);
          fflush (stderr);
          return;
        }
      else
        /* If we fail to attach to a process, report an error.  */
        error ("Cannot attach to process %ld: %s (%d)\n", pid,
               strerror (errno), errno);
    }

  ptrace (PTRACE_SETOPTIONS, pid, 0, PTRACE_O_TRACECLONE);

  new_process = (struct process_info *) add_process (pid);
  add_thread (pid, new_process, pid);
  new_thread_notify (thread_id_to_gdb_id (new_process->lwpid));

  /* The next time we wait for this LWP we'll see a SIGSTOP as PTRACE_ATTACH
     brings it to a halt.  We should ignore that SIGSTOP and resume the process
     (unless this is the first process, in which case the flag will be cleared
     in linux_attach).

     On the other hand, if we are currently trying to stop all threads, we
     should treat the new thread as if we had sent it a SIGSTOP.  This works
     because we are guaranteed that add_process added us to the end of the
     list, and so the new thread has not yet reached wait_for_sigstop (but
     will).  */
  if (! stopping_threads)
    new_process->stop_expected = 1;
}

int
linux_attach (unsigned long pid)
{
  struct process_info *process;

  linux_attach_lwp (pid);

  /* Don't ignore the initial SIGSTOP if we just attached to this process.
     It will be collected by wait shortly.  */
  process = (struct process_info *) find_inferior_id (&all_processes, pid);
  process->stop_expected = 0;

  return 0;
}

/* Kill the inferior process.  Make us have no inferior.  */

static void
linux_kill_one_process (struct inferior_list_entry *entry)
{
  struct thread_info *thread = (struct thread_info *) entry;
  struct process_info *process = get_thread_process (thread);
  int wstat;

  /* We avoid killing the first thread here, because of a Linux kernel (at
     least 2.6.0-test7 through 2.6.8-rc4) bug; if we kill the parent before
     the children get a chance to be reaped, it will remain a zombie
     forever.  */
  if (entry == all_threads.head)
    return;

  do
    {
      ptrace (PTRACE_KILL, pid_of (process), 0, 0);

      /* Make sure it died.  The loop is most likely unnecessary.  */
      wstat = linux_wait_for_event (thread);
    } while (WIFSTOPPED (wstat));
}

static void
linux_kill (void)
{
  struct thread_info *thread = (struct thread_info *) all_threads.head;
  struct process_info *process;
  int wstat;

  if (thread == NULL)
    return;

  for_each_inferior (&all_threads, linux_kill_one_process);

  /* See the comment in linux_kill_one_process.  We did not kill the first
     thread in the list, so do so now.  */
  process = get_thread_process (thread);
  do
    {
      ptrace (PTRACE_KILL, pid_of (process), 0, 0);

      /* Make sure it died.  The loop is most likely unnecessary.  */
      wstat = linux_wait_for_event (thread);
    } while (WIFSTOPPED (wstat));

  clear_inferiors ();
  free (all_processes.head);
  all_processes.head = all_processes.tail = NULL;
}

static void
linux_detach_one_process (struct inferior_list_entry *entry)
{
  struct thread_info *thread = (struct thread_info *) entry;
  struct process_info *process = get_thread_process (thread);

  /* Make sure the process isn't stopped at a breakpoint that's
     no longer there.  */
  check_removed_breakpoint (process);

  /* If this process is stopped but is expecting a SIGSTOP, then make
     sure we take care of that now.  This isn't absolutely guaranteed
     to collect the SIGSTOP, but is fairly likely to.  */
  if (process->stop_expected)
    {
      /* Clear stop_expected, so that the SIGSTOP will be reported.  */
      process->stop_expected = 0;
      if (process->stopped)
        linux_resume_one_process (&process->head, 0, 0, NULL);
      linux_wait_for_event (thread);
    }

  /* Flush any pending changes to the process's registers.  */
  regcache_invalidate_one ((struct inferior_list_entry *)
                           get_process_thread (process));

  /* Finally, let it resume.  */
  ptrace (PTRACE_DETACH, pid_of (process), 0, 0);
}

static int
linux_detach (void)
{
  delete_all_breakpoints ();
  for_each_inferior (&all_threads, linux_detach_one_process);
  clear_inferiors ();
  free (all_processes.head);
  all_processes.head = all_processes.tail = NULL;
  return 0;
}

static void
linux_join (void)
{
  extern unsigned long signal_pid;
  int status, ret;

  do {
    ret = waitpid (signal_pid, &status, 0);
    if (WIFEXITED (status) || WIFSIGNALED (status))
      break;
  } while (ret != -1 || errno != ECHILD);
}

/* Return nonzero if the given thread is still alive.  */
static int
linux_thread_alive (unsigned long lwpid)
{
  if (find_inferior_id (&all_threads, lwpid) != NULL)
    return 1;
  else
    return 0;
}

/* Return nonzero if this process stopped at a breakpoint which
   no longer appears to be inserted.  Also adjust the PC
   appropriately to resume where the breakpoint used to be.  */
static int
check_removed_breakpoint (struct process_info *event_child)
{
  CORE_ADDR stop_pc;
  struct thread_info *saved_inferior;

  if (event_child->pending_is_breakpoint == 0)
    return 0;

  if (debug_threads)
    fprintf (stderr, "Checking for breakpoint in process %ld.\n",
             event_child->lwpid);

  saved_inferior = current_inferior;
  current_inferior = get_process_thread (event_child);

  stop_pc = get_stop_pc ();

  /* If the PC has changed since we stopped, then we shouldn't do
     anything.  This happens if, for instance, GDB handled the
     decr_pc_after_break subtraction itself.  */
  if (stop_pc != event_child->pending_stop_pc)
    {
      if (debug_threads)
        fprintf (stderr, "Ignoring, PC was changed.  Old PC was 0x%08llx\n",
                 event_child->pending_stop_pc);

      event_child->pending_is_breakpoint = 0;
      current_inferior = saved_inferior;
      return 0;
    }

  /* If the breakpoint is still there, we will report hitting it.  */
  if ((*the_low_target.breakpoint_at) (stop_pc))
    {
      if (debug_threads)
        fprintf (stderr, "Ignoring, breakpoint is still present.\n");
      current_inferior = saved_inferior;
      return 0;
    }

  if (debug_threads)
    fprintf (stderr, "Removed breakpoint.\n");

  /* For decr_pc_after_break targets, here is where we perform the
     decrement.  We go immediately from this function to resuming,
     and can not safely call get_stop_pc () again.  */
  if (the_low_target.set_pc != NULL)
    (*the_low_target.set_pc) (stop_pc);

  /* We consumed the pending SIGTRAP.  */
  event_child->pending_is_breakpoint = 0;
  event_child->status_pending_p = 0;
  event_child->status_pending = 0;

  current_inferior = saved_inferior;
  return 1;
}

/* Return 1 if this process has an interesting status pending.  This function
   may silently resume an inferior process.  */
static int
status_pending_p (struct inferior_list_entry *entry, void *dummy)
{
  struct process_info *process = (struct process_info *) entry;

  if (process->status_pending_p)
    if (check_removed_breakpoint (process))
      {
        /* This thread was stopped at a breakpoint, and the breakpoint
           is now gone.  We were told to continue (or step...) all threads,
           so GDB isn't trying to single-step past this breakpoint.
           So instead of reporting the old SIGTRAP, pretend we got to
           the breakpoint just after it was removed instead of just
           before; resume the process.  */
        linux_resume_one_process (&process->head, 0, 0, NULL);
        return 0;
      }

  return process->status_pending_p;
}

static void
linux_wait_for_process (struct process_info **childp, int *wstatp)
{
  int ret;
  int to_wait_for = -1;

  if (*childp != NULL)
    to_wait_for = (*childp)->lwpid;

retry:
  while (1)
    {
      ret = waitpid (to_wait_for, wstatp, WNOHANG);

      if (ret == -1)
        {
          if (errno != ECHILD)
            perror_with_name ("waitpid");
        }
      else if (ret > 0)
        break;

      ret = waitpid (to_wait_for, wstatp, WNOHANG | __WCLONE);

      if (ret == -1)
        {
          if (errno != ECHILD)
            perror_with_name ("waitpid (WCLONE)");
        }
      else if (ret > 0)
        break;

      usleep (1000);
    }

  if (debug_threads
      && (!WIFSTOPPED (*wstatp)
          || (WSTOPSIG (*wstatp) != 32
              && WSTOPSIG (*wstatp) != 33)))
    fprintf (stderr, "Got an event from %d (%x)\n", ret, *wstatp);

  if (to_wait_for == -1)
    *childp = (struct process_info *) find_inferior_id (&all_processes, ret);

  /* If we didn't find a process, one of two things presumably happened:
     - A process we started and then detached from has exited.  Ignore it.
     - A process we are controlling has forked and the new child's stop
     was reported to us by the kernel.  Save its PID.  */
  if (*childp == NULL && WIFSTOPPED (*wstatp))
    {
      add_pid_to_list (&stopped_pids, ret);
      goto retry;
    }
  else if (*childp == NULL)
    goto retry;

  (*childp)->stopped = 1;
  (*childp)->pending_is_breakpoint = 0;

  (*childp)->last_status = *wstatp;

  if (debug_threads
      && WIFSTOPPED (*wstatp))
    {
      current_inferior = (struct thread_info *)
        find_inferior_id (&all_threads, (*childp)->lwpid);
      /* For testing only; i386_stop_pc prints out a diagnostic.  */
      if (the_low_target.get_pc != NULL)
        get_stop_pc ();
    }
}

static int
linux_wait_for_event (struct thread_info *child)
{
  CORE_ADDR stop_pc;
  struct process_info *event_child;
  int wstat;
  int bp_status;

  /* Check for a process with a pending status.  */
  /* It is possible that the user changed the pending task's registers since
     it stopped.  We correctly handle the change of PC if we hit a breakpoint
     (in check_removed_breakpoint); signals should be reported anyway.  */
  if (child == NULL)
    {
      event_child = (struct process_info *)
        find_inferior (&all_processes, status_pending_p, NULL);
      if (debug_threads && event_child)
        fprintf (stderr, "Got a pending child %ld\n", event_child->lwpid);
    }
  else
    {
      event_child = get_thread_process (child);
      if (event_child->status_pending_p
          && check_removed_breakpoint (event_child))
        event_child = NULL;
    }

  if (event_child != NULL)
    {
      if (event_child->status_pending_p)
        {
          if (debug_threads)
            fprintf (stderr, "Got an event from pending child %ld (%04x)\n",
                     event_child->lwpid, event_child->status_pending);
          wstat = event_child->status_pending;
          event_child->status_pending_p = 0;
          event_child->status_pending = 0;
          current_inferior = get_process_thread (event_child);
          return wstat;
        }
    }

  /* We only enter this loop if no process has a pending wait status.  Thus
     any action taken in response to a wait status inside this loop is
     responding as soon as we detect the status, not after any pending
     events.  */
  while (1)
    {
      if (child == NULL)
        event_child = NULL;
      else
        event_child = get_thread_process (child);

      linux_wait_for_process (&event_child, &wstat);

      if (event_child == NULL)
        error ("event from unknown child");

      current_inferior = (struct thread_info *)
        find_inferior_id (&all_threads, event_child->lwpid);

      /* Check for thread exit.  */
      if (! WIFSTOPPED (wstat))
        {
          if (debug_threads)
            fprintf (stderr, "LWP %ld exiting\n", event_child->head.id);

          /* If the last thread is exiting, just return.  */
          if (all_threads.head == all_threads.tail)
            return wstat;

          dead_thread_notify (thread_id_to_gdb_id (event_child->lwpid));

          remove_inferior (&all_processes, &event_child->head);
          free (event_child);
          remove_thread (current_inferior);
          current_inferior = (struct thread_info *) all_threads.head;

          /* If we were waiting for this particular child to do something...
             well, it did something.  */
          if (child != NULL)
            return wstat;

          /* Wait for a more interesting event.  */
          continue;
        }

      if (WIFSTOPPED (wstat)
          && WSTOPSIG (wstat) == SIGSTOP
          && event_child->stop_expected)
        {
          if (debug_threads)
            fprintf (stderr, "Expected stop.\n");
          event_child->stop_expected = 0;
          linux_resume_one_process (&event_child->head,
                                    event_child->stepping, 0, NULL);
          continue;
        }

      if (WIFSTOPPED (wstat) && WSTOPSIG (wstat) == SIGTRAP
          && wstat >> 16 != 0)
        {
          handle_extended_wait (event_child, wstat);
          continue;
        }

      /* If GDB is not interested in this signal, don't stop other
         threads, and don't report it to GDB.  Just resume the
         inferior right away.  We do this for threading-related
         signals as well as any that GDB specifically requested we
         ignore.  But never ignore SIGSTOP if we sent it ourselves,
         and do not ignore signals when stepping - they may require
         special handling to skip the signal handler.  */
      /* FIXME drow/2002-06-09: Get signal numbers from the inferior's
         thread library?  */
      if (WIFSTOPPED (wstat)
          && !event_child->stepping
          && (
#ifdef USE_THREAD_DB
              (thread_db_active && (WSTOPSIG (wstat) == __SIGRTMIN
                                    || WSTOPSIG (wstat) == __SIGRTMIN + 1))
              ||
#endif
              (pass_signals[target_signal_from_host (WSTOPSIG (wstat))]
               && (WSTOPSIG (wstat) != SIGSTOP || !stopping_threads))))
        {
          siginfo_t info, *info_p;

          if (debug_threads)
            fprintf (stderr, "Ignored signal %d for LWP %ld.\n",
                     WSTOPSIG (wstat), event_child->head.id);

          if (ptrace (PTRACE_GETSIGINFO, event_child->lwpid, 0, &info) == 0)
            info_p = &info;
          else
            info_p = NULL;
          linux_resume_one_process (&event_child->head,
                                    event_child->stepping,
                                    WSTOPSIG (wstat), info_p);
          continue;
        }

      /* If this event was not handled above, and is not a SIGTRAP, report
         it.  */
      if (!WIFSTOPPED (wstat) || WSTOPSIG (wstat) != SIGTRAP)
        return wstat;

      /* If this target does not support breakpoints, we simply report the
         SIGTRAP; it's of no concern to us.  */
      if (the_low_target.get_pc == NULL)
        return wstat;

      stop_pc = get_stop_pc ();

      /* bp_reinsert will only be set if we were single-stepping.
         Notice that we will resume the process after hitting
         a gdbserver breakpoint; single-stepping to/over one
         is not supported (yet).  */
      if (event_child->bp_reinsert != 0)
        {
          if (debug_threads)
            fprintf (stderr, "Reinserted breakpoint.\n");
          reinsert_breakpoint (event_child->bp_reinsert);
          event_child->bp_reinsert = 0;

          /* Clear the single-stepping flag and SIGTRAP as we resume.  */
          linux_resume_one_process (&event_child->head, 0, 0, NULL);
          continue;
        }

      bp_status = check_breakpoints (stop_pc);

      if (bp_status != 0)
        {
          if (debug_threads)
            fprintf (stderr, "Hit a gdbserver breakpoint.\n");

          /* We hit one of our own breakpoints.  We mark it as a pending
             breakpoint, so that check_removed_breakpoint () will do the PC
             adjustment for us at the appropriate time.  */
          event_child->pending_is_breakpoint = 1;
          event_child->pending_stop_pc = stop_pc;

          /* We may need to put the breakpoint back.  We continue in the event
             loop instead of simply replacing the breakpoint right away,
             in order to not lose signals sent to the thread that hit the
             breakpoint.  Unfortunately this increases the window where another
             thread could sneak past the removed breakpoint.  For the current
             use of server-side breakpoints (thread creation) this is
             acceptable; but it needs to be considered before this breakpoint
             mechanism can be used in more general ways.  For some breakpoints
             it may be necessary to stop all other threads, but that should
             be avoided where possible.

             If breakpoint_reinsert_addr is NULL, that means that we can
             use PTRACE_SINGLESTEP on this platform.  Uninsert the breakpoint,
             mark it for reinsertion, and single-step.

             Otherwise, call the target function to figure out where we need
             our temporary breakpoint, create it, and continue executing this
             process.  */
          if (bp_status == 2)
            /* No need to reinsert.  */
            linux_resume_one_process (&event_child->head, 0, 0, NULL);
          else if (the_low_target.breakpoint_reinsert_addr == NULL)
            {
              event_child->bp_reinsert = stop_pc;
              uninsert_breakpoint (stop_pc);
              linux_resume_one_process (&event_child->head, 1, 0, NULL);
            }
          else
            {
              reinsert_breakpoint_by_bp
                (stop_pc, (*the_low_target.breakpoint_reinsert_addr) ());
              linux_resume_one_process (&event_child->head, 0, 0, NULL);
            }

          continue;
        }

      if (debug_threads)
        fprintf (stderr, "Hit a non-gdbserver breakpoint.\n");

      /* If we were single-stepping, we definitely want to report the
         SIGTRAP.  The single-step operation has completed, so also
         clear the stepping flag; in general this does not matter,
         because the SIGTRAP will be reported to the client, which
         will give us a new action for this thread, but clear it for
         consistency anyway.  It's safe to clear the stepping flag
         because the only consumer of get_stop_pc () after this point
         is check_removed_breakpoint, and pending_is_breakpoint is not
         set.  It might be wiser to use a step_completed flag instead.  */
      if (event_child->stepping)
        {
          event_child->stepping = 0;
          return wstat;
        }

      /* A SIGTRAP that we can't explain.  It may have been a breakpoint.
         Check if it is a breakpoint, and if so mark the process information
         accordingly.  This will handle both the necessary fiddling with the
         PC on decr_pc_after_break targets and suppressing extra threads
         hitting a breakpoint if two hit it at once and then GDB removes it
         after the first is reported.  Arguably it would be better to report
         multiple threads hitting breakpoints simultaneously, but the current
         remote protocol does not allow this.  */
      if ((*the_low_target.breakpoint_at) (stop_pc))
        {
          event_child->pending_is_breakpoint = 1;
          event_child->pending_stop_pc = stop_pc;
        }

      return wstat;
    }

  /* NOTREACHED */
  return 0;
}

/* Wait for process, returns status.  */

static unsigned char
linux_wait (char *status)
{
  int w;
  struct thread_info *child = NULL;

retry:
  /* If we were only supposed to resume one thread, only wait for
     that thread - if it's still alive.  If it died, however - which
     can happen if we're coming from the thread death case below -
     then we need to make sure we restart the other threads.  We could
     pick a thread at random or restart all; restarting all is less
     arbitrary.  */
  if (cont_thread != 0 && cont_thread != -1)
    {
      child = (struct thread_info *) find_inferior_id (&all_threads,
                                                       cont_thread);

      /* No stepping, no signal - unless one is pending already, of course.  */
      if (child == NULL)
        {
          struct thread_resume resume_info;
          resume_info.thread = -1;
          resume_info.step = resume_info.sig = resume_info.leave_stopped = 0;
          linux_resume (&resume_info);
        }
    }

  w = linux_wait_for_event (child);
  stop_all_processes ();

  if (must_set_ptrace_flags)
    {
      ptrace (PTRACE_SETOPTIONS, inferior_pid, 0, PTRACE_O_TRACECLONE);
      must_set_ptrace_flags = 0;
    }

  /* If we are waiting for a particular child, and it exited,
     linux_wait_for_event will return its exit status.  Similarly if
     the last child exited.  If this is not the last child, however,
     do not report it as exited until there is a 'thread exited' response
     available in the remote protocol.  Instead, just wait for another event.
     This should be safe, because if the thread crashed we will already
     have reported the termination signal to GDB; that should stop any
     in-progress stepping operations, etc.

     Report the exit status of the last thread to exit.  This matches
     LinuxThreads' behavior.  */

  if (all_threads.head == all_threads.tail)
    {
      if (WIFEXITED (w))
        {
          fprintf (stderr, "\nChild exited with retcode = %x \n", WEXITSTATUS (w));
          *status = 'W';
          clear_inferiors ();
          free (all_processes.head);
          all_processes.head = all_processes.tail = NULL;
          return WEXITSTATUS (w);
        }
      else if (!WIFSTOPPED (w))
        {
          fprintf (stderr, "\nChild terminated with signal = %x \n", WTERMSIG (w));
          *status = 'X';
          clear_inferiors ();
          free (all_processes.head);
          all_processes.head = all_processes.tail = NULL;
          return target_signal_from_host (WTERMSIG (w));
        }
    }
  else
    {
      if (!WIFSTOPPED (w))
        goto retry;
    }

  *status = 'T';
  return target_signal_from_host (WSTOPSIG (w));
}

/* Send a signal to an LWP.  For LinuxThreads, kill is enough; however, if
   thread groups are in use, we need to use tkill.  */

static int
kill_lwp (unsigned long lwpid, int signo)
{
  static int tkill_failed;

  errno = 0;

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

  return kill (lwpid, signo);
}

static void
send_sigstop (struct inferior_list_entry *entry)
{
  struct process_info *process = (struct process_info *) entry;

  if (process->stopped)
    return;

  /* If we already have a pending stop signal for this process, don't
     send another.  */
  if (process->stop_expected)
    {
      if (debug_threads)
        fprintf (stderr, "Have pending sigstop for process %ld\n",
                 process->lwpid);

      /* We clear the stop_expected flag so that wait_for_sigstop
         will receive the SIGSTOP event (instead of silently resuming and
         waiting again).  It'll be reset below.  */
      process->stop_expected = 0;
      return;
    }

  if (debug_threads)
    fprintf (stderr, "Sending sigstop to process %ld\n", process->head.id);

  kill_lwp (process->head.id, SIGSTOP);
}

static void
wait_for_sigstop (struct inferior_list_entry *entry)
{
  struct process_info *process = (struct process_info *) entry;
  struct thread_info *saved_inferior, *thread;
  int wstat;
  unsigned long saved_tid;

  if (process->stopped)
    return;

  saved_inferior = current_inferior;
  saved_tid = ((struct inferior_list_entry *) saved_inferior)->id;
  thread = (struct thread_info *) find_inferior_id (&all_threads,
                                                    process->lwpid);
  wstat = linux_wait_for_event (thread);

  /* If we stopped with a non-SIGSTOP signal, save it for later
     and record the pending SIGSTOP.  If the process exited, just
     return.  */
  if (WIFSTOPPED (wstat)
      && WSTOPSIG (wstat) != SIGSTOP)
    {
      if (debug_threads)
        fprintf (stderr, "LWP %ld stopped with non-sigstop status %06x\n",
                 process->lwpid, wstat);
      process->status_pending_p = 1;
      process->status_pending = wstat;
      process->stop_expected = 1;
    }

  if (linux_thread_alive (saved_tid))
    current_inferior = saved_inferior;
  else
    {
      if (debug_threads)
        fprintf (stderr, "Previously current thread died.\n");

      /* Set a valid thread as current.  */
      set_desired_inferior (0);
    }
}

static void
stop_all_processes (void)
{
  stopping_threads = 1;
  for_each_inferior (&all_processes, send_sigstop);
  for_each_inferior (&all_processes, wait_for_sigstop);
  stopping_threads = 0;
}

/* 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_process (struct inferior_list_entry *entry,
                          int step, int signal, siginfo_t *info)
{
  struct process_info *process = (struct process_info *) entry;
  struct thread_info *saved_inferior;

  if (process->stopped == 0)
    return;

  /* If we have pending signals or status, and a new signal, enqueue the
     signal.  Also enqueue the signal if we are waiting to reinsert a
     breakpoint; it will be picked up again below.  */
  if (signal != 0
      && (process->status_pending_p || process->pending_signals != NULL
          || process->bp_reinsert != 0))
    {
      struct pending_signals *p_sig;
      p_sig = malloc (sizeof (*p_sig));
      p_sig->prev = process->pending_signals;
      p_sig->signal = signal;
      if (info == NULL)
        memset (&p_sig->info, 0, sizeof (siginfo_t));
      else
        memcpy (&p_sig->info, info, sizeof (siginfo_t));
      process->pending_signals = p_sig;
    }

  if (process->status_pending_p && !check_removed_breakpoint (process))
    return;

  saved_inferior = current_inferior;
  current_inferior = get_process_thread (process);

  if (debug_threads)
    fprintf (stderr, "Resuming process %ld (%s, signal %d, stop %s)\n", inferior_pid,
             step ? "step" : "continue", signal,
             process->stop_expected ? "expected" : "not expected");

  /* This bit needs some thinking about.  If we get a signal that
     we must report while a single-step reinsert is still pending,
     we often end up resuming the thread.  It might be better to
     (ew) allow a stack of pending events; then we could be sure that
     the reinsert happened right away and not lose any signals.

     Making this stack would also shrink the window in which breakpoints are
     uninserted (see comment in linux_wait_for_process) but not enough for
     complete correctness, so it won't solve that problem.  It may be
     worthwhile just to solve this one, however.  */
  if (process->bp_reinsert != 0)
    {
      if (debug_threads)
        fprintf (stderr, "  pending reinsert at %08lx", (long)process->bp_reinsert);
      if (step == 0)
        fprintf (stderr, "BAD - reinserting but not stepping.\n");
      step = 1;

      /* Postpone any pending signal.  It was enqueued above.  */
      signal = 0;
    }

  check_removed_breakpoint (process);

  if (debug_threads && the_low_target.get_pc != NULL)
    {
      fprintf (stderr, "  ");
      (*the_low_target.get_pc) ();
    }

  /* If we have pending signals, consume one unless we are trying to reinsert
     a breakpoint.  */
  if (process->pending_signals != NULL && process->bp_reinsert == 0)
    {
      struct pending_signals **p_sig;

      p_sig = &process->pending_signals;
      while ((*p_sig)->prev != NULL)
        p_sig = &(*p_sig)->prev;

      signal = (*p_sig)->signal;
      if ((*p_sig)->info.si_signo != 0)
        ptrace (PTRACE_SETSIGINFO, process->lwpid, 0, &(*p_sig)->info);

      free (*p_sig);
      *p_sig = NULL;
    }

  regcache_invalidate_one ((struct inferior_list_entry *)
                           get_process_thread (process));
  errno = 0;
  process->stopped = 0;
  process->stepping = step;
  ptrace (step ? PTRACE_SINGLESTEP : PTRACE_CONT, process->lwpid, 0, signal);

  current_inferior = saved_inferior;
  if (errno)
    perror_with_name ("ptrace");
}

static struct thread_resume *resume_ptr;

/* This function is called once per thread.  We look up the thread
   in RESUME_PTR, and mark the thread with a pointer to the appropriate
   resume request.

   This algorithm is O(threads * resume elements), but resume elements
   is small (and will remain small at least until GDB supports thread
   suspension).  */
static void
linux_set_resume_request (struct inferior_list_entry *entry)
{
  struct process_info *process;
  struct thread_info *thread;
  int ndx;

  thread = (struct thread_info *) entry;
  process = get_thread_process (thread);

  ndx = 0;
  while (resume_ptr[ndx].thread != -1 && resume_ptr[ndx].thread != entry->id)
    ndx++;

  process->resume = &resume_ptr[ndx];
}

/* This function is called once per thread.  We check the thread's resume
   request, which will tell us whether to resume, step, or leave the thread
   stopped; and what signal, if any, it should be sent.  For threads which
   we aren't explicitly told otherwise, we preserve the stepping flag; this
   is used for stepping over gdbserver-placed breakpoints.  */

static void
linux_continue_one_thread (struct inferior_list_entry *entry)
{
  struct process_info *process;
  struct thread_info *thread;
  int step;

  thread = (struct thread_info *) entry;
  process = get_thread_process (thread);

  if (process->resume->leave_stopped)
    return;

  if (process->resume->thread == -1)
    step = process->stepping || process->resume->step;
  else
    step = process->resume->step;

  linux_resume_one_process (&process->head, step, process->resume->sig, NULL);

  process->resume = NULL;
}

/* This function is called once per thread.  We check the thread's resume
   request, which will tell us whether to resume, step, or leave the thread
   stopped; and what signal, if any, it should be sent.  We queue any needed
   signals, since we won't actually resume.  We already have a pending event
   to report, so we don't need to preserve any step requests; they should
   be re-issued if necessary.  */

static void
linux_queue_one_thread (struct inferior_list_entry *entry)
{
  struct process_info *process;
  struct thread_info *thread;

  thread = (struct thread_info *) entry;
  process = get_thread_process (thread);

  if (process->resume->leave_stopped)
    return;

  /* If we have a new signal, enqueue the signal.  */
  if (process->resume->sig != 0)
    {
      struct pending_signals *p_sig;
      p_sig = malloc (sizeof (*p_sig));
      p_sig->prev = process->pending_signals;
      p_sig->signal = process->resume->sig;
      memset (&p_sig->info, 0, sizeof (siginfo_t));

      /* If this is the same signal we were previously stopped by,
         make sure to queue its siginfo.  We can ignore the return
         value of ptrace; if it fails, we'll skip
         PTRACE_SETSIGINFO.  */
      if (WIFSTOPPED (process->last_status)
          && WSTOPSIG (process->last_status) == process->resume->sig)
        ptrace (PTRACE_GETSIGINFO, process->lwpid, 0, &p_sig->info);

      process->pending_signals = p_sig;
    }

  process->resume = NULL;
}

/* Set DUMMY if this process has an interesting status pending.  */
static int
resume_status_pending_p (struct inferior_list_entry *entry, void *flag_p)
{
  struct process_info *process = (struct process_info *) entry;

  /* Processes which will not be resumed are not interesting, because
     we might not wait for them next time through linux_wait.  */
  if (process->resume->leave_stopped)
    return 0;

  /* If this thread has a removed breakpoint, we won't have any
     events to report later, so check now.  check_removed_breakpoint
     may clear status_pending_p.  We avoid calling check_removed_breakpoint
     for any thread that we are not otherwise going to resume - this
     lets us preserve stopped status when two threads hit a breakpoint.
     GDB removes the breakpoint to single-step a particular thread
     past it, then re-inserts it and resumes all threads.  We want
     to report the second thread without resuming it in the interim.  */
  if (process->status_pending_p)
    check_removed_breakpoint (process);

  if (process->status_pending_p)
    * (int *) flag_p = 1;

  return 0;
}

static void
linux_resume (struct thread_resume *resume_info)
{
  int pending_flag;

  /* Yes, the use of a global here is rather ugly.  */
  resume_ptr = resume_info;

  for_each_inferior (&all_threads, linux_set_resume_request);

  /* If there is a thread which would otherwise be resumed, which
     has a pending status, then don't resume any threads - we can just
     report the pending status.  Make sure to queue any signals
     that would otherwise be sent.  */
  pending_flag = 0;
  find_inferior (&all_processes, resume_status_pending_p, &pending_flag);

  if (debug_threads)
    {
      if (pending_flag)
        fprintf (stderr, "Not resuming, pending status\n");
      else
        fprintf (stderr, "Resuming, no pending status\n");
    }

  if (pending_flag)
    for_each_inferior (&all_threads, linux_queue_one_thread);
  else
    for_each_inferior (&all_threads, linux_continue_one_thread);
}

#ifdef HAVE_LINUX_USRREGS

int
register_addr (int regnum)
{
  int addr;

  if (regnum < 0 || regnum >= the_low_target.num_regs)
    error ("Invalid register number %d.", regnum);

  addr = the_low_target.regmap[regnum];

  return addr;
}

/* Fetch one register.  */
static void
fetch_register (int regno)
{
  CORE_ADDR regaddr;
  int i, size;
  char *buf;

  if (regno >= the_low_target.num_regs)
    return;
  if ((*the_low_target.cannot_fetch_register) (regno))
    return;

  regaddr = register_addr (regno);
  if (regaddr == -1)
    return;
  size = (register_size (regno) + sizeof (PTRACE_XFER_TYPE) - 1)
         & - sizeof (PTRACE_XFER_TYPE);
  buf = alloca (size);
  for (i = 0; i < size; i += sizeof (PTRACE_XFER_TYPE))
    {
      errno = 0;
      *(PTRACE_XFER_TYPE *) (buf + i) =
        ptrace (PTRACE_PEEKUSER, inferior_pid, (PTRACE_ARG3_TYPE) regaddr, 0);
      regaddr += sizeof (PTRACE_XFER_TYPE);
      if (errno != 0)
        {
          /* Warning, not error, in case we are attached; sometimes the
             kernel doesn't let us at the registers.  */
          char *err = strerror (errno);
          char *msg = alloca (strlen (err) + 128);
          sprintf (msg, "reading register %d: %s", regno, err);
          error (msg);
          goto error_exit;
        }
    }
  if (the_low_target.left_pad_xfer
      && register_size (regno) < sizeof (PTRACE_XFER_TYPE))
    supply_register (regno, (buf + sizeof (PTRACE_XFER_TYPE)
                             - register_size (regno)));
  else
    supply_register (regno, buf);

error_exit:;
}

/* Fetch all registers, or just one, from the child process.  */
static void
usr_fetch_inferior_registers (int regno)
{
  if (regno == -1 || regno == 0)
    for (regno = 0; regno < the_low_target.num_regs; regno++)
      fetch_register (regno);
  else
    fetch_register (regno);
}

/* Store our register values back into the inferior.
   If REGNO is -1, do this for all registers.
   Otherwise, REGNO specifies which register (so we can save time).  */
static void
usr_store_inferior_registers (int regno)
{
  CORE_ADDR regaddr;
  int i, size;
  char *buf;

  if (regno >= 0)
    {
      if (regno >= the_low_target.num_regs)
        return;

      if ((*the_low_target.cannot_store_register) (regno) == 1)
        return;

      regaddr = register_addr (regno);
      if (regaddr == -1)
        return;
      errno = 0;
      size = (register_size (regno) + sizeof (PTRACE_XFER_TYPE) - 1)
             & - sizeof (PTRACE_XFER_TYPE);
      buf = alloca (size);
      memset (buf, 0, size);
      if (the_low_target.left_pad_xfer
          && register_size (regno) < sizeof (PTRACE_XFER_TYPE))
        collect_register (regno, (buf + sizeof (PTRACE_XFER_TYPE)
                                  - register_size (regno)));
      else
        collect_register (regno, buf);
      for (i = 0; i < size; i += sizeof (PTRACE_XFER_TYPE))
        {
          errno = 0;
          ptrace (PTRACE_POKEUSER, inferior_pid, (PTRACE_ARG3_TYPE) regaddr,
                  *(PTRACE_XFER_TYPE *) (buf + i));
          if (errno != 0)
            {
              if ((*the_low_target.cannot_store_register) (regno) == 0)
                {
                  char *err = strerror (errno);
                  char *msg = alloca (strlen (err) + 128);
                  sprintf (msg, "writing register %d: %s",
                           regno, err);
                  error (msg);
                  return;
                }
            }
          regaddr += sizeof (PTRACE_XFER_TYPE);
        }
    }
  else
    for (regno = 0; regno < the_low_target.num_regs; regno++)
      usr_store_inferior_registers (regno);
}
#endif /* HAVE_LINUX_USRREGS */



#ifdef HAVE_LINUX_REGSETS

static int
regsets_fetch_inferior_registers ()
{
  struct regset_info *regset;
  int saw_general_regs = 0;

  regset = target_regsets;

  while (regset->size >= 0)
    {
      void *buf;
      int res;

      if (regset->size == 0)
        {
          regset ++;
          continue;
        }

      buf = malloc (regset->size);
      res = ptrace (regset->get_request, inferior_pid, 0, buf);
      if (res < 0)
        {
          if (errno == EIO)
            {
              /* If we get EIO on the first regset, do not try regsets again.
                 If we get EIO on a later regset, disable that regset.  */
              if (regset == target_regsets)
                {
                  use_regsets_p = 0;
                  return -1;
                }
              else
                {
                  regset->size = 0;
                  continue;
                }
            }
          else
            {
              char s[256];
              sprintf (s, "ptrace(regsets_fetch_inferior_registers) PID=%ld",
                       inferior_pid);
              perror (s);
            }
        }
      else if (regset->type == GENERAL_REGS)
        saw_general_regs = 1;
      regset->store_function (buf);
      regset ++;
    }
  if (saw_general_regs)
    return 0;
  else
    return 1;
}

static int
regsets_store_inferior_registers ()
{
  struct regset_info *regset;
  int saw_general_regs = 0;

  regset = target_regsets;

  while (regset->size >= 0)
    {
      void *buf;
      int res;

      if (regset->size == 0)
        {
          regset ++;
          continue;
        }

      buf = malloc (regset->size);

      /* First fill the buffer with the current register set contents,
         in case there are any items in the kernel's regset that are
         not in gdbserver's regcache.  */
      res = ptrace (regset->get_request, inferior_pid, 0, buf);

      if (res == 0)
        {
          /* Then overlay our cached registers on that.  */
          regset->fill_function (buf);

          /* Only now do we write the register set.  */
          res = ptrace (regset->set_request, inferior_pid, 0, buf);
        }

      if (res < 0)
        {
          if (errno == EIO)
            {
              /* If we get EIO on the first regset, do not try regsets again.
                 If we get EIO on a later regset, disable that regset.  */
              if (regset == target_regsets)
                {
                  use_regsets_p = 0;
                  return -1;
                }
              else
                {
                  regset->size = 0;
                  continue;
                }
            }
          else
            {
              perror ("Warning: ptrace(regsets_store_inferior_registers)");
            }
        }
      else if (regset->type == GENERAL_REGS)
        saw_general_regs = 1;
      regset ++;
      free (buf);
    }
  if (saw_general_regs)
    return 0;
  else
    return 1;
  return 0;
}

#endif /* HAVE_LINUX_REGSETS */


void
linux_fetch_registers (int regno)
{
#ifdef HAVE_LINUX_REGSETS
  if (use_regsets_p)
    {
      if (regsets_fetch_inferior_registers () == 0)
        return;
    }
#endif
#ifdef HAVE_LINUX_USRREGS
  usr_fetch_inferior_registers (regno);
#endif
}

void
linux_store_registers (int regno)
{
#ifdef HAVE_LINUX_REGSETS
  if (use_regsets_p)
    {
      if (regsets_store_inferior_registers () == 0)
        return;
    }
#endif
#ifdef HAVE_LINUX_USRREGS
  usr_store_inferior_registers (regno);
#endif
}


/* Copy LEN bytes from inferior's memory starting at MEMADDR
   to debugger memory starting at MYADDR.  */

static int
linux_read_memory (CORE_ADDR memaddr, unsigned char *myaddr, int len)
{
  register int i;
  /* Round starting address down to longword boundary.  */
  register CORE_ADDR addr = memaddr & -(CORE_ADDR) sizeof (PTRACE_XFER_TYPE);
  /* Round ending address up; get number of longwords that makes.  */
  register int count
    = (((memaddr + len) - addr) + sizeof (PTRACE_XFER_TYPE) - 1)
      / sizeof (PTRACE_XFER_TYPE);
  /* Allocate buffer of that many longwords.  */
  register PTRACE_XFER_TYPE *buffer
    = (PTRACE_XFER_TYPE *) alloca (count * sizeof (PTRACE_XFER_TYPE));
  int fd;
  char filename[64];

  /* Try using /proc.  Don't bother for one word.  */
  if (len >= 3 * sizeof (long))
    {
      /* We could keep this file open and cache it - possibly one per
         thread.  That requires some juggling, but is even faster.  */
      sprintf (filename, "/proc/%ld/mem", inferior_pid);
      fd = open (filename, O_RDONLY | O_LARGEFILE);
      if (fd == -1)
        goto no_proc;

      /* 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, myaddr, len, memaddr) != len)
#else
      if (lseek (fd, memaddr, SEEK_SET) == -1 || read (fd, memaddr, len) != len)
#endif
        {
          close (fd);
          goto no_proc;
        }

      close (fd);
      return 0;
    }

 no_proc:
  /* Read all the longwords */
  for (i = 0; i < count; i++, addr += sizeof (PTRACE_XFER_TYPE))
    {
      errno = 0;
      buffer[i] = ptrace (PTRACE_PEEKTEXT, inferior_pid, (PTRACE_ARG3_TYPE) addr, 0);
      if (errno)
        return errno;
    }

  /* Copy appropriate bytes out of the buffer.  */
  memcpy (myaddr, (char *) buffer + (memaddr & (sizeof (PTRACE_XFER_TYPE) - 1)), len);

  return 0;
}

/* Copy LEN bytes of data from debugger memory at MYADDR
   to inferior's memory at MEMADDR.
   On failure (cannot write the inferior)
   returns the value of errno.  */

static int
linux_write_memory (CORE_ADDR memaddr, const unsigned char *myaddr, int len)
{
  register int i;
  /* Round starting address down to longword boundary.  */
  register CORE_ADDR addr = memaddr & -(CORE_ADDR) sizeof (PTRACE_XFER_TYPE);
  /* Round ending address up; get number of longwords that makes.  */
  register int count
  = (((memaddr + len) - addr) + sizeof (PTRACE_XFER_TYPE) - 1) / sizeof (PTRACE_XFER_TYPE);
  /* Allocate buffer of that many longwords.  */
  register PTRACE_XFER_TYPE *buffer = (PTRACE_XFER_TYPE *) alloca (count * sizeof (PTRACE_XFER_TYPE));
  extern int errno;

  if (debug_threads)
    {
      fprintf (stderr, "Writing %02x to %08lx\n", (unsigned)myaddr[0], (long)memaddr);
    }

  /* Fill start and end extra bytes of buffer with existing memory data.  */

  buffer[0] = ptrace (PTRACE_PEEKTEXT, inferior_pid,
                      (PTRACE_ARG3_TYPE) addr, 0);

  if (count > 1)
    {
      buffer[count - 1]
        = ptrace (PTRACE_PEEKTEXT, inferior_pid,
                  (PTRACE_ARG3_TYPE) (addr + (count - 1)
                                      * sizeof (PTRACE_XFER_TYPE)),
                  0);
    }

  /* Copy data to be written over corresponding part of buffer */

  memcpy ((char *) buffer + (memaddr & (sizeof (PTRACE_XFER_TYPE) - 1)), myaddr, len);

  /* Write the entire buffer.  */

  for (i = 0; i < count; i++, addr += sizeof (PTRACE_XFER_TYPE))
    {
      errno = 0;
      ptrace (PTRACE_POKETEXT, inferior_pid, (PTRACE_ARG3_TYPE) addr, buffer[i]);
      if (errno)
        return errno;
    }

  return 0;
}

static int linux_supports_tracefork_flag;

/* Helper functions for linux_test_for_tracefork, called via clone ().  */

static int
linux_tracefork_grandchild (void *arg)
{
  _exit (0);
}

#define STACK_SIZE 4096

static int
linux_tracefork_child (void *arg)
{
  ptrace (PTRACE_TRACEME, 0, 0, 0);
  kill (getpid (), SIGSTOP);
#ifdef __ia64__
  __clone2 (linux_tracefork_grandchild, arg, STACK_SIZE,
            CLONE_VM | SIGCHLD, NULL);
#else
  clone (linux_tracefork_grandchild, arg + STACK_SIZE,
         CLONE_VM | SIGCHLD, NULL);
#endif
  _exit (0);
}

/* Wrapper function for waitpid which handles EINTR.  */

static int
my_waitpid (int pid, int *status, int flags)
{
  int ret;
  do
    {
      ret = waitpid (pid, status, flags);
    }
  while (ret == -1 && errno == EINTR);

  return ret;
}

/* Determine if PTRACE_O_TRACEFORK can be used to follow fork events.  Make
   sure that we can enable the option, and that it had the desired
   effect.  */

static void
linux_test_for_tracefork (void)
{
  int child_pid, ret, status;
  long second_pid;
  char *stack = malloc (STACK_SIZE * 4);

  linux_supports_tracefork_flag = 0;

  /* Use CLONE_VM instead of fork, to support uClinux (no MMU).  */
#ifdef __ia64__
  child_pid = __clone2 (linux_tracefork_child, stack, STACK_SIZE,
                        CLONE_VM | SIGCHLD, stack + STACK_SIZE * 2);
#else
  child_pid = clone (linux_tracefork_child, stack + STACK_SIZE,
                     CLONE_VM | SIGCHLD, stack + STACK_SIZE * 2);
#endif
  if (child_pid == -1)
    perror_with_name ("clone");

  ret = my_waitpid (child_pid, &status, 0);
  if (ret == -1)
    perror_with_name ("waitpid");
  else if (ret != child_pid)
    error ("linux_test_for_tracefork: waitpid: unexpected result %d.", ret);
  if (! WIFSTOPPED (status))
    error ("linux_test_for_tracefork: waitpid: unexpected status %d.", status);

  ret = ptrace (PTRACE_SETOPTIONS, child_pid, 0, PTRACE_O_TRACEFORK);
  if (ret != 0)
    {
      ret = ptrace (PTRACE_KILL, child_pid, 0, 0);
      if (ret != 0)
        {
          warning ("linux_test_for_tracefork: failed to kill child");
          return;
        }

      ret = my_waitpid (child_pid, &status, 0);
      if (ret != child_pid)
        warning ("linux_test_for_tracefork: failed to wait for killed child");
      else if (!WIFSIGNALED (status))
        warning ("linux_test_for_tracefork: unexpected wait status 0x%x from "
                 "killed child", status);

      return;
    }

  ret = ptrace (PTRACE_CONT, child_pid, 0, 0);
  if (ret != 0)
    warning ("linux_test_for_tracefork: failed to resume child");

  ret = my_waitpid (child_pid, &status, 0);

  if (ret == child_pid && WIFSTOPPED (status)
      && status >> 16 == PTRACE_EVENT_FORK)
    {
      second_pid = 0;
      ret = ptrace (PTRACE_GETEVENTMSG, child_pid, 0, &second_pid);
      if (ret == 0 && second_pid != 0)
        {
          int second_status;

          linux_supports_tracefork_flag = 1;
          my_waitpid (second_pid, &second_status, 0);
          ret = ptrace (PTRACE_KILL, second_pid, 0, 0);
          if (ret != 0)
            warning ("linux_test_for_tracefork: failed to kill second child");
          my_waitpid (second_pid, &status, 0);
        }
    }
  else
    warning ("linux_test_for_tracefork: unexpected result from waitpid "
             "(%d, status 0x%x)", ret, status);

  do
    {
      ret = ptrace (PTRACE_KILL, child_pid, 0, 0);
      if (ret != 0)
        warning ("linux_test_for_tracefork: failed to kill child");
      my_waitpid (child_pid, &status, 0);
    }
  while (WIFSTOPPED (status));

  free (stack);
}


static void
linux_look_up_symbols (void)
{
#ifdef USE_THREAD_DB
  if (thread_db_active)
    return;

  thread_db_active = thread_db_init (!linux_supports_tracefork_flag);
#endif
}

static void
linux_request_interrupt (void)
{
  extern unsigned long signal_pid;

  if (cont_thread != 0 && cont_thread != -1)
    {
      struct process_info *process;

      process = get_thread_process (current_inferior);
      kill_lwp (process->lwpid, SIGINT);
    }
  else
    kill_lwp (signal_pid, SIGINT);
}

/* Copy LEN bytes from inferior's auxiliary vector starting at OFFSET
   to debugger memory starting at MYADDR.  */

static int
linux_read_auxv (CORE_ADDR offset, unsigned char *myaddr, unsigned int len)
{
  char filename[PATH_MAX];
  int fd, n;

  snprintf (filename, sizeof filename, "/proc/%ld/auxv", inferior_pid);

  fd = open (filename, O_RDONLY);
  if (fd < 0)
    return -1;

  if (offset != (CORE_ADDR) 0
      && lseek (fd, (off_t) offset, SEEK_SET) != (off_t) offset)
    n = -1;
  else
    n = read (fd, myaddr, len);

  close (fd);

  return n;
}

/* These watchpoint related wrapper functions simply pass on the function call
   if the target has registered a corresponding function.  */

static int
linux_insert_watchpoint (char type, CORE_ADDR addr, int len)
{
  if (the_low_target.insert_watchpoint != NULL)
    return the_low_target.insert_watchpoint (type, addr, len);
  else
    /* Unsupported (see target.h).  */
    return 1;
}

static int
linux_remove_watchpoint (char type, CORE_ADDR addr, int len)
{
  if (the_low_target.remove_watchpoint != NULL)
    return the_low_target.remove_watchpoint (type, addr, len);
  else
    /* Unsupported (see target.h).  */
    return 1;
}

static int
linux_stopped_by_watchpoint (void)
{
  if (the_low_target.stopped_by_watchpoint != NULL)
    return the_low_target.stopped_by_watchpoint ();
  else
    return 0;
}

static CORE_ADDR
linux_stopped_data_address (void)
{
  if (the_low_target.stopped_data_address != NULL)
    return the_low_target.stopped_data_address ();
  else
    return 0;
}

#if defined(__UCLIBC__) && defined(HAS_NOMMU)
#if defined(__mcoldfire__)
/* These should really be defined in the kernel's ptrace.h header.  */
#define PT_TEXT_ADDR 49*4
#define PT_DATA_ADDR 50*4
#define PT_TEXT_END_ADDR  51*4
#endif

/* Under uClinux, programs are loaded at non-zero offsets, which we need
   to tell gdb about.  */

static int
linux_read_offsets (CORE_ADDR *text_p, CORE_ADDR *data_p)
{
#if defined(PT_TEXT_ADDR) && defined(PT_DATA_ADDR) && defined(PT_TEXT_END_ADDR)
  unsigned long text, text_end, data;
  int pid = get_thread_process (current_inferior)->head.id;

  errno = 0;

  text = ptrace (PTRACE_PEEKUSER, pid, (long)PT_TEXT_ADDR, 0);
  text_end = ptrace (PTRACE_PEEKUSER, pid, (long)PT_TEXT_END_ADDR, 0);
  data = ptrace (PTRACE_PEEKUSER, pid, (long)PT_DATA_ADDR, 0);

  if (errno == 0)
    {
      /* Both text and data offsets produced at compile-time (and so
         used by gdb) are relative to the beginning of the program,
         with the data segment immediately following the text segment.
         However, the actual runtime layout in memory may put the data
         somewhere else, so when we send gdb a data base-address, we
         use the real data base address and subtract the compile-time
         data base-address from it (which is just the length of the
         text segment).  BSS immediately follows data in both
         cases.  */
      *text_p = text;
      *data_p = data - (text_end - text);
      
      return 1;
    }
#endif
 return 0;
}
#endif

static const char *
linux_arch_string (void)
{
  return the_low_target.arch_string;
}

static struct target_ops linux_target_ops = {
  linux_create_inferior,
  linux_attach,
  linux_kill,
  linux_detach,
  linux_join,
  linux_thread_alive,
  linux_resume,
  linux_wait,
  linux_fetch_registers,
  linux_store_registers,
  linux_read_memory,
  linux_write_memory,
  linux_look_up_symbols,
  linux_request_interrupt,
  linux_read_auxv,
  linux_insert_watchpoint,
  linux_remove_watchpoint,
  linux_stopped_by_watchpoint,
  linux_stopped_data_address,
#if defined(__UCLIBC__) && defined(HAS_NOMMU)
  linux_read_offsets,
#else
  NULL,
#endif
#ifdef USE_THREAD_DB
  thread_db_get_tls_address,
#else
  NULL,
#endif
  linux_arch_string,
  NULL,
  hostio_last_error_from_errno,
};

static void
linux_init_signals ()
{
  /* FIXME drow/2002-06-09: As above, we should check with LinuxThreads
     to find what the cancel signal actually is.  */
  signal (__SIGRTMIN+1, SIG_IGN);
}

void
initialize_low (void)
{
  thread_db_active = 0;
  set_target_ops (&linux_target_ops);
  set_breakpoint_data (the_low_target.breakpoint,
                       the_low_target.breakpoint_len);
  init_registers ();
  linux_init_signals ();
  linux_test_for_tracefork ();
}