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       ptrace - process trace


       #include <sys/ptrace.h>

       long ptrace(enum __ptrace_request request, pid_t pid,
                   void *addr, void *data);


       The  ptrace() system call provides a means by which one process (the "tracer") may observe
       and control the execution of another process (the "tracee"), and examine  and  change  the
       tracee's memory and registers.  It is primarily used to implement breakpoint debugging and
       system call tracing.

       A tracee first needs to be attached to the tracer.  Attachment and subsequent commands are
       per  thread:  in  a  multithreaded process, every thread can be individually attached to a
       (potentially different) tracer, or left not attached and thus  not  debugged.   Therefore,
       "tracee"  always means "(one) thread", never "a (possibly multithreaded) process".  Ptrace
       commands are always sent to a specific tracee using a call of the form

           ptrace(PTRACE_foo, pid, ...)

       where pid is the thread ID of the corresponding Linux thread.

       (Note that in this page, a "multithreaded process" means  a  thread  group  consisting  of
       threads created using the clone(2) CLONE_THREAD flag.)

       A  process  can  initiate  a  trace by calling fork(2) and having the resulting child do a
       PTRACE_TRACEME, followed (typically) by an  execve(2).   Alternatively,  one  process  may
       commence tracing another process using PTRACE_ATTACH or PTRACE_SEIZE.

       While  being  traced,  the  tracee  will stop each time a signal is delivered, even if the
       signal is being ignored.  (An exception is SIGKILL, which  has  its  usual  effect.)   The
       tracer  will  be  notified  at  its  next call to waitpid(2) (or one of the related "wait"
       system calls); that call will return a status value containing information that  indicates
       the  cause  of  the  stop  in the tracee.  While the tracee is stopped, the tracer can use
       various ptrace requests to inspect and modify the tracee.   The  tracer  then  causes  the
       tracee  to  continue,  optionally  ignoring  the  delivered  signal  (or even delivering a
       different signal instead).

       If the PTRACE_O_TRACEEXEC option is not in effect, all successful calls  to  execve(2)  by
       the  traced  process will cause it to be sent a SIGTRAP signal, giving the parent a chance
       to gain control before the new program begins execution.

       When the tracer is finished tracing, it can cause the tracee to continue  executing  in  a
       normal, untraced mode via PTRACE_DETACH.

       The value of request determines the action to be performed:

              Indicate  that  this  process  is  to  be traced by its parent.  A process probably
              shouldn't make this request if its parent isn't expecting to trace it.  (pid, addr,
              and data are ignored.)

              Since  Ubuntu  10.10,  PTRACE_ATTACH  is not allowed against arbitrary matching-uid
              processes. The traced "child" must be a descendant  of  the  tracer  or  must  have
              called  prctl(2)  using  PR_SET_PTRACER,  with the pid of the tracer (or one of its
              ancestors).  For more details, see /etc/sysctl.d/10-ptrace.conf.

       The PTRACE_TRACEME request is used only by the tracee; the  remaining  requests  are  used
       only  by the tracer.  In the following requests, pid specifies the thread ID of the tracee
       to be acted on.  For requests other than PTRACE_ATTACH, PTRACE_SEIZE, PTRACE_INTERRUPT and
       PTRACE_KILL, the tracee must be stopped.

              Read  a  word at the address addr in the tracee's memory, returning the word as the
              result of the ptrace() call.  Linux does not have separate text  and  data  address
              spaces, so these two requests are currently equivalent.  (data is ignored.)

              Read a word at offset addr in the tracee's USER area, which holds the registers and
              other information about the process (see <sys/user.h>).  The word  is  returned  as
              the  result  of  the  ptrace()  call.   Typically, the offset must be word-aligned,
              though this might vary by architecture.  See NOTES.  (data is ignored.)

              Copy  the  word  data  to  the  address  addr  in  the  tracee's  memory.   As  for
              PTRACE_PEEKTEXT and PTRACE_PEEKDATA, these two requests are currently equivalent.

              Copy   the   word  data  to  offset  addr  in  the  tracee's  USER  area.   As  for
              PTRACE_PEEKUSER, the offset must typically be word-aligned.  In order  to  maintain
              the integrity of the kernel, some modifications to the USER area are disallowed.

              Copy the tracee's general-purpose or floating-point registers, respectively, to the
              address data in the tracer.  See <sys/user.h> for information on the format of this
              data.   (addr  is  ignored.)   Note that SPARC systems have the meaning of data and
              addr reversed; that is, data is ignored and the registers are copied to the address
              addr.  PTRACE_GETREGS and PTRACE_GETFPREGS are not present on all architectures.

       PTRACE_GETREGSET (since Linux 2.6.34)
              Read the tracee's registers.  addr specifies, in an architecture-dependent way, the
              type of registers to be read.  NT_PRSTATUS (with numerical value 1) usually results
              in  reading  of  general-purpose registers.  If the CPU has, for example, floating-
              point and/or vector registers, they  can  be  retrieved  by  setting  addr  to  the
              corresponding  NT_foo constant.  data points to a struct iovec, which describes the
              destination buffer's location and length.  On return, the kernel  modifies  iov.len
              to indicate the actual number of bytes returned.

              Modify the tracee's general-purpose or floating-point registers, respectively, from
              the address data in the  tracer.   As  for  PTRACE_POKEUSER,  some  general-purpose
              register  modifications  may  be  disallowed.   (addr is ignored.)  Note that SPARC
              systems have the meaning of data and addr reversed; that is, data  is  ignored  and
              the   registers   are   copied   from   the   address   addr.   PTRACE_SETREGS  and
              PTRACE_SETFPREGS are not present on all architectures.

       PTRACE_SETREGSET (since Linux 2.6.34)
              Modify the tracee's registers.  The meaning  of  addr  and  data  is  analogous  to

       PTRACE_GETSIGINFO (since Linux 2.3.99-pre6)
              Retrieve  information  about  the  signal  that  caused the stop.  Copy a siginfo_t
              structure (see sigaction(2)) from the tracee to the address  data  in  the  tracer.
              (addr is ignored.)

       PTRACE_SETSIGINFO (since Linux 2.3.99-pre6)
              Set  signal  information:  copy  a siginfo_t structure from the address data in the
              tracer to the tracee.  This  will  affect  only  signals  that  would  normally  be
              delivered to the tracee and were caught by the tracer.  It may be difficult to tell
              these normal signals from synthetic signals generated by ptrace() itself.  (addr is

       PTRACE_SETOPTIONS (since Linux 2.4.6; see BUGS for caveats)
              Set  ptrace  options  from  data.  (addr is ignored.)  data is interpreted as a bit
              mask of options, which are specified by the following flags:

              PTRACE_O_EXITKILL (since Linux 3.8)
                     If a tracer sets this flag, a SIGKILL signal will be sent to every tracee if
                     the  tracer  exits.   This  option is useful for ptrace jailers that want to
                     ensure that tracees can never escape the tracer's control.

              PTRACE_O_TRACECLONE (since Linux 2.5.46)
                     Stop the tracee at the next clone(2) and  automatically  start  tracing  the
                     newly  cloned process, which will start with a SIGSTOP, or PTRACE_EVENT_STOP
                     if PTRACE_SEIZE was used.  A waitpid(2) by the tracer will return  a  status
                     value such that

                       status>>8 == (SIGTRAP | (PTRACE_EVENT_CLONE<<8))

                     The PID of the new process can be retrieved with PTRACE_GETEVENTMSG.

                     This  option may not catch clone(2) calls in all cases.  If the tracee calls
                     clone(2) with the CLONE_VFORK flag,  PTRACE_EVENT_VFORK  will  be  delivered
                     instead  if  PTRACE_O_TRACEVFORK  is  set;  otherwise  if  the  tracee calls
                     clone(2) with the exit signal set  to  SIGCHLD,  PTRACE_EVENT_FORK  will  be
                     delivered if PTRACE_O_TRACEFORK is set.

              PTRACE_O_TRACEEXEC (since Linux 2.5.46)
                     Stop  the  tracee  at  the  next execve(2).  A waitpid(2) by the tracer will
                     return a status value such that

                       status>>8 == (SIGTRAP | (PTRACE_EVENT_EXEC<<8))

                     If the execing thread is not a thread group leader, the thread ID  is  reset
                     to  thread  group leader's ID before this stop.  Since Linux 3.0, the former
                     thread ID can be retrieved with PTRACE_GETEVENTMSG.

              PTRACE_O_TRACEEXIT (since Linux 2.5.60)
                     Stop the tracee at exit.  A waitpid(2) by the tracer will  return  a  status
                     value such that

                       status>>8 == (SIGTRAP | (PTRACE_EVENT_EXIT<<8))

                     The tracee's exit status can be retrieved with PTRACE_GETEVENTMSG.

                     The  tracee  is  stopped early during process exit, when registers are still
                     available, allowing the tracer to see where the exit occurred,  whereas  the
                     normal  exit  notification  is  done  after the process is finished exiting.
                     Even though context is available, the tracer cannot prevent  the  exit  from
                     happening at this point.

              PTRACE_O_TRACEFORK (since Linux 2.5.46)
                     Stop  the  tracee  at  the  next fork(2) and automatically start tracing the
                     newly forked process, which will start with a SIGSTOP, or  PTRACE_EVENT_STOP
                     if  PTRACE_SEIZE  was used.  A waitpid(2) by the tracer will return a status
                     value such that

                       status>>8 == (SIGTRAP | (PTRACE_EVENT_FORK<<8))

                     The PID of the new process can be retrieved with PTRACE_GETEVENTMSG.

              PTRACE_O_TRACESYSGOOD (since Linux 2.4.6)
                     When delivering system call traps, set bit 7 in  the  signal  number  (i.e.,
                     deliver  SIGTRAP|0x80).   This  makes  it easy for the tracer to distinguish
                     normal traps from those caused by a system call.  (PTRACE_O_TRACESYSGOOD may
                     not work on all architectures.)

              PTRACE_O_TRACEVFORK (since Linux 2.5.46)
                     Stop  the  tracee  at  the next vfork(2) and automatically start tracing the
                     newly vforked process, which will start with a SIGSTOP, or PTRACE_EVENT_STOP
                     if  PTRACE_SEIZE  was used.  A waitpid(2) by the tracer will return a status
                     value such that

                       status>>8 == (SIGTRAP | (PTRACE_EVENT_VFORK<<8))

                     The PID of the new process can be retrieved with PTRACE_GETEVENTMSG.

              PTRACE_O_TRACEVFORKDONE (since Linux 2.5.60)
                     Stop the tracee at the completion of the next vfork(2).  A waitpid(2) by the
                     tracer will return a status value such that

                       status>>8 == (SIGTRAP | (PTRACE_EVENT_VFORK_DONE<<8))

                     The  PID  of  the  new  process  can  (since Linux 2.6.18) be retrieved with

       PTRACE_GETEVENTMSG (since Linux 2.5.46)
              Retrieve a message (as an unsigned long) about the ptrace event that just happened,
              placing  it  at the address data in the tracer.  For PTRACE_EVENT_EXIT, this is the
              tracee's    exit    status.      For     PTRACE_EVENT_FORK,     PTRACE_EVENT_VFORK,
              PTRACE_EVENT_VFORK_DONE,  and  PTRACE_EVENT_CLONE,  this  is  the  PID  of  the new
              process.  (addr is ignored.)

              Restart the stopped tracee process.  If data is nonzero, it is interpreted  as  the
              number  of  a  signal  to  be  delivered  to  the  tracee;  otherwise, no signal is
              delivered.  Thus, for example, the tracer can control whether a signal sent to  the
              tracee is delivered or not.  (addr is ignored.)

              Restart  the  stopped  tracee  as for PTRACE_CONT, but arrange for the tracee to be
              stopped at the next entry to or exit from a system call, or after  execution  of  a
              single instruction, respectively.  (The tracee will also, as usual, be stopped upon
              receipt of a signal.)  From the tracer's perspective, the  tracee  will  appear  to
              have  been  stopped  by receipt of a SIGTRAP.  So, for PTRACE_SYSCALL, for example,
              the idea is to inspect the arguments to the system call at the first stop, then  do
              another  PTRACE_SYSCALL  and  inspect  the  return  value of the system call at the
              second stop.  The data argument is treated as for PTRACE_CONT.  (addr is ignored.)

              For PTRACE_SYSEMU, continue and stop on entry to the next system call,  which  will
              not  be executed.  For PTRACE_SYSEMU_SINGLESTEP, do the same but also singlestep if
              not a system call.  This call is used by programs like User Mode Linux that want to
              emulate  all  the  tracee's  system  calls.   The  data  argument is treated as for
              PTRACE_CONT.  The addr argument is ignored.  These requests are currently supported
              only on x86.

       PTRACE_LISTEN (since Linux 3.4)
              Restart  the stopped tracee, but prevent it from executing.  The resulting state of
              the tracee is similar to a process which has been stopped by a  SIGSTOP  (or  other
              stopping  signal).   See  the  "group-stop"  subsection for additional information.
              PTRACE_LISTEN works only on tracees attached by PTRACE_SEIZE.

              Send the tracee a SIGKILL to terminate it.  (addr and data are ignored.)

              This operation is deprecated; do not use it!   Instead,  send  a  SIGKILL  directly
              using  kill(2)  or tgkill(2).  The problem with PTRACE_KILL is that it requires the
              tracee to be in signal-delivery-stop, otherwise it may not work (i.e., may complete
              successfully  but  won't kill the tracee).  By contrast, sending a SIGKILL directly
              has no such limitation.

       PTRACE_INTERRUPT (since Linux 3.4)
              Stop a tracee.   If  the  tracee  is  running  or  sleeping  in  kernel  space  and
              PTRACE_SYSCALL  is  in effect, the system call is interrupted and syscall-exit-stop
              is reported.  (The  interrupted  system  call  is  restarted  when  the  tracee  is
              restarted.)   If  the  tracee was already stopped by a signal and PTRACE_LISTEN was
              sent to it, the tracee stops with PTRACE_EVENT_STOP  and  WSTOPSIG(status)  returns
              the  stop  signal.   If  any  other  ptrace-stop is generated at the same time (for
              example, if a signal is sent to the tracee), this ptrace-stop happens.  If none  of
              the  above  applies  (for example, if the tracee is running in userspace), it stops
              with PTRACE_EVENT_STOP with WSTOPSIG(status)  ==  SIGTRAP.   PTRACE_INTERRUPT  only
              works on tracees attached by PTRACE_SEIZE.

              Attach  to the process specified in pid, making it a tracee of the calling process.
              The tracee is sent a  SIGSTOP,  but  will  not  necessarily  have  stopped  by  the
              completion  of  this  call; use waitpid(2) to wait for the tracee to stop.  See the
              "Attaching and detaching" subsection for additional information.   (addr  and  data
              are ignored.)

       PTRACE_SEIZE (since Linux 3.4)
              Attach  to the process specified in pid, making it a tracee of the calling process.
              Unlike PTRACE_ATTACH, PTRACE_SEIZE does not stop the process.  Only a PTRACE_SEIZEd
              process can accept PTRACE_INTERRUPT and PTRACE_LISTEN commands.  addr must be zero.
              data contains a bit mask of ptrace options to activate immediately.

              Restart the stopped tracee as for PTRACE_CONT, but first  detach  from  it.   Under
              Linux,  a tracee can be detached in this way regardless of which method was used to
              initiate tracing.  (addr is ignored.)

   Death under ptrace
       When a (possibly multithreaded) process receives a killing signal (one  whose  disposition
       is  set  to  SIG_DFL  and  whose default action is to kill the process), all threads exit.
       Tracees report their death to their tracer(s).  Notification of this  event  is  delivered
       via waitpid(2).

       Note  that  the killing signal will first cause signal-delivery-stop (on one tracee only),
       and only after it is injected by the tracer (or after it was dispatched to a thread  which
       isn't  traced),  will  death  from the signal happen on all tracees within a multithreaded
       process.  (The term "signal-delivery-stop" is explained below.)

       SIGKILL does not generate signal-delivery-stop and therefore the tracer can't suppress it.
       SIGKILL  kills even within system calls (syscall-exit-stop is not generated prior to death
       by SIGKILL).  The net effect is that SIGKILL always kills the process (all  its  threads),
       even if some threads of the process are ptraced.

       When the tracee calls _exit(2), it reports its death to its tracer.  Other threads are not

       When any thread executes exit_group(2), every tracee in its thread group reports its death
       to its tracer.

       If the PTRACE_O_TRACEEXIT option is on, PTRACE_EVENT_EXIT will happen before actual death.
       This applies to exits via exit(2), exit_group(2), and signal deaths (except SIGKILL),  and
       when threads are torn down on execve(2) in a multithreaded process.

       The  tracer cannot assume that the ptrace-stopped tracee exists.  There are many scenarios
       when the tracee may die while stopped (such as SIGKILL).  Therefore, the  tracer  must  be
       prepared  to handle an ESRCH error on any ptrace operation.  Unfortunately, the same error
       is returned if the tracee exists but is not ptrace-stopped (for commands which  require  a
       stopped  tracee), or if it is not traced by the process which issued the ptrace call.  The
       tracer needs to keep track of the stopped/running state of the tracee, and interpret ESRCH
       as  "tracee died unexpectedly" only if it knows that the tracee has been observed to enter
       ptrace-stop.  Note that there is no guarantee that waitpid(WNOHANG) will  reliably  report
       the  tracee's  death  status  if  a ptrace operation returned ESRCH.  waitpid(WNOHANG) may
       return 0 instead.  In other words, the tracee may be "not yet  fully  dead",  but  already
       refusing ptrace requests.

       The   tracer   can't   assume   that   the  tracee  always  ends  its  life  by  reporting
       WIFEXITED(status) or WIFSIGNALED(status); there are cases where this does not occur.   For
       example,  if a thread other than thread group leader does an execve(2), it disappears; its
       PID will never be seen again, and any subsequent ptrace stops will be reported  under  the
       thread group leader's PID.

   Stopped states
       A  tracee  can be in two states: running or stopped.  For the purposes of ptrace, a tracee
       which is blocked in a system call (such  as  read(2),  pause(2),  etc.)   is  nevertheless
       considered to be running, even if the tracee is blocked for a long time.  The state of the
       tracee after PTRACE_LISTEN is somewhat of a gray  area:  it  is  not  in  any  ptrace-stop
       (ptrace  commands  won't work on it, and it will deliver waitpid(2) notifications), but it
       also may be considered  "stopped"  because  it  is  not  executing  instructions  (is  not
       scheduled),  and  if  it  was  in  group-stop before PTRACE_LISTEN, it will not respond to
       signals until SIGCONT is received.

       There are many kinds of states when the tracee is stopped, and in ptrace discussions  they
       are often conflated.  Therefore, it is important to use precise terms.

       In  this  manual  page,  any  stopped  state in which the tracee is ready to accept ptrace
       commands from the tracer is called ptrace-stop.  Ptrace-stops can  be  further  subdivided
       into  signal-delivery-stop, group-stop, syscall-stop, and so on.  These stopped states are
       described in detail below.

       When the running tracee enters ptrace-stop, it notifies its tracer  using  waitpid(2)  (or
       one  of  the other "wait" system calls).  Most of this manual page assumes that the tracer
       waits with:

           pid = waitpid(pid_or_minus_1, &status, __WALL);

       Ptrace-stopped  tracees  are  reported  as  returns  with   pid   greater   than   0   and
       WIFSTOPPED(status) true.

       The  __WALL  flag  does  not  include  the  WSTOPPED  and WEXITED flags, but implies their

       Setting the WCONTINUED flag when calling waitpid(2) is not  recommended:  the  "continued"
       state is per-process and consuming it can confuse the real parent of the tracee.

       Use of the WNOHANG flag may cause waitpid(2) to return 0 ("no wait results available yet")
       even if the tracer knows there should be a notification.  Example:

           errno = 0;
           ptrace(PTRACE_CONT, pid, 0L, 0L);
           if (errno == ESRCH) {
               /* tracee is dead */
               r = waitpid(tracee, &status, __WALL | WNOHANG);
               /* r can still be 0 here! */

       The  following  kinds   of   ptrace-stops   exist:   signal-delivery-stops,   group-stops,
       PTRACE_EVENT   stops,   syscall-stops.    They   all   are  reported  by  waitpid(2)  with
       WIFSTOPPED(status) true.  They may be differentiated by examining the value status>>8, and
       if  there  is  ambiguity  in  that  value,  by  querying  PTRACE_GETSIGINFO.   (Note:  the
       WSTOPSIG(status) macro can't be used to perform this examination, because it  returns  the
       value (status>>8) & 0xff.)

       When  a  (possibly  multithreaded)  process receives any signal except SIGKILL, the kernel
       selects an arbitrary thread which handles the signal.  (If the signal  is  generated  with
       tgkill(2),  the  target thread can be explicitly selected by the caller.)  If the selected
       thread is traced, it enters signal-delivery-stop.  At this point, the signal  is  not  yet
       delivered  to  the  process,  and  can be suppressed by the tracer.  If the tracer doesn't
       suppress the signal, it passes the signal  to  the  tracee  in  the  next  ptrace  restart
       request.   This  second  step of signal delivery is called signal injection in this manual
       page.  Note that if the signal is blocked, signal-delivery-stop doesn't happen  until  the
       signal is unblocked, with the usual exception that SIGSTOP can't be blocked.

       Signal-delivery-stop   is   observed   by   the   tracer   as  waitpid(2)  returning  with
       WIFSTOPPED(status) true, with the signal returned by WSTOPSIG(status).  If the  signal  is
       SIGTRAP, this may be a different kind of ptrace-stop; see the "Syscall-stops" and "execve"
       sections below for details.  If WSTOPSIG(status) returns a stopping signal, this may be  a
       group-stop; see below.

   Signal injection and suppression
       After signal-delivery-stop is observed by the tracer, the tracer should restart the tracee
       with the call

           ptrace(PTRACE_restart, pid, 0, sig)

       where PTRACE_restart is one of the restarting ptrace requests.  If sig is 0, then a signal
       is  not  delivered.   Otherwise,  the  signal  sig is delivered.  This operation is called
       signal injection in this manual page, to distinguish it from signal-delivery-stop.

       The sig value may be different from the WSTOPSIG(status) value: the  tracer  can  cause  a
       different signal to be injected.

       Note  that  a  suppressed signal still causes system calls to return prematurely.  In this
       case system calls will be restarted: the tracer will observe the tracee to  reexecute  the
       interrupted  system call (or restart_syscall(2) system call for a few syscalls which use a
       different mechanism for restarting) if the tracer uses PTRACE_SYSCALL.  Even system  calls
       (such  as  poll(2))  which  are not restartable after signal are restarted after signal is
       suppressed; however, kernel bugs exist which cause some syscalls to fail with  EINTR  even
       though no observable signal is injected to the tracee.

       Restarting  ptrace commands issued in ptrace-stops other than signal-delivery-stop are not
       guaranteed to inject a signal, even if sig is nonzero.  No error is  reported;  a  nonzero
       sig may simply be ignored.  Ptrace users should not try to "create a new signal" this way:
       use tgkill(2) instead.

       The fact that signal injection requests may be ignored when restarting  the  tracee  after
       ptrace  stops  that  are  not  signal-delivery-stops  is a cause of confusion among ptrace
       users.  One typical scenario is that the  tracer  observes  group-stop,  mistakes  it  for
       signal-delivery-stop, restarts the tracee with

           ptrace(PTRACE_restart, pid, 0, stopsig)

       with the intention of injecting stopsig, but stopsig gets ignored and the tracee continues
       to run.

       The SIGCONT signal has a side effect  of  waking  up  (all  threads  of)  a  group-stopped
       process.  This side effect happens before signal-delivery-stop.  The tracer can't suppress
       this side effect (it can only suppress signal injection, which  only  causes  the  SIGCONT
       handler  to  not  be  executed  in  the tracee, if such a handler is installed).  In fact,
       waking up from group-stop may be followed by signal-delivery-stop for signal(s) other than
       SIGCONT,  if they were pending when SIGCONT was delivered.  In other words, SIGCONT may be
       not the first signal observed by the tracee after it was sent.

       Stopping signals cause (all threads of) a process to enter group-stop.  This  side  effect
       happens after signal injection, and therefore can be suppressed by the tracer.

       In Linux 2.4 and earlier, the SIGSTOP signal can't be injected.

       PTRACE_GETSIGINFO  can  be used to retrieve a siginfo_t structure which corresponds to the
       delivered signal.  PTRACE_SETSIGINFO may be used to modify it.  If  PTRACE_SETSIGINFO  has
       been  used  to alter siginfo_t, the si_signo field and the sig parameter in the restarting
       command must match, otherwise the result is undefined.

       When a (possibly multithreaded) process receives a stopping signal, all threads stop.   If
       some  threads  are  traced,  they  enter a group-stop.  Note that the stopping signal will
       first cause signal-delivery-stop (on one tracee only), and only after it  is  injected  by
       the tracer (or after it was dispatched to a thread which isn't traced), will group-stop be
       initiated on all tracees within the multithreaded process.  As usual, every tracee reports
       its group-stop separately to the corresponding tracer.

       Group-stop is observed by the tracer as waitpid(2) returning with WIFSTOPPED(status) true,
       with the stopping signal available via WSTOPSIG(status).  The same result is  returned  by
       some  other  classes of ptrace-stops, therefore the recommended practice is to perform the

           ptrace(PTRACE_GETSIGINFO, pid, 0, &siginfo)

       The call can be avoided if the signal is not SIGSTOP, SIGTSTP, SIGTTIN, or  SIGTTOU;  only
       these four signals are stopping signals.  If the tracer sees something else, it can't be a
       group-stop.  Otherwise, the tracer needs to call PTRACE_GETSIGINFO.  If  PTRACE_GETSIGINFO
       fails with EINVAL, then it is definitely a group-stop.  (Other failure codes are possible,
       such as ESRCH ("no such process") if a SIGKILL killed the tracee.)

       If tracee was attached using PTRACE_SEIZE, group-stop is indicated  by  PTRACE_EVENT_STOP:
       status>>16  ==  PTRACE_EVENT_STOP.  This allows detection of group-stops without requiring
       an extra PTRACE_GETSIGINFO call.

       As of Linux 2.6.38, after the tracer sees the tracee ptrace-stop and until it restarts  or
       kills  it, the tracee will not run, and will not send notifications (except SIGKILL death)
       to the tracer, even if the tracer enters into another waitpid(2) call.

       The kernel behavior described in the previous paragraph causes a problem with  transparent
       handling  of  stopping  signals.   If the tracer restarts the tracee after group-stop, the
       stopping signal is effectively ignored—the tracee doesn't remain stopped, it runs.  If the
       tracer doesn't restart the tracee before entering into the next waitpid(2), future SIGCONT
       signals will not be reported to the tracer; this would cause the SIGCONT signals  to  have
       no effect on the tracee.

       Since  Linux  3.4,  there  is a method to overcome this problem: instead of PTRACE_CONT, a
       PTRACE_LISTEN command can be used to restart a tracee in a way where it does not  execute,
       but waits for a new event which it can report via waitpid(2) (such as when it is restarted
       by a SIGCONT).

       If the tracer sets PTRACE_O_TRACE_* options, the tracee  will  enter  ptrace-stops  called
       PTRACE_EVENT stops.

       PTRACE_EVENT   stops   are   observed   by   the   tracer  as  waitpid(2)  returning  with
       WIFSTOPPED(status), and WSTOPSIG(status) returns SIGTRAP.  An additional bit is set in the
       higher byte of the status word: the value status>>8 will be

           (SIGTRAP | PTRACE_EVENT_foo << 8).

       The following events exist:

              Stop  before  return from vfork(2) or clone(2) with the CLONE_VFORK flag.  When the
              tracee is continued after this stop, it will wait for  child  to  exit/exec  before
              continuing its execution (in other words, the usual behavior on vfork(2)).

              Stop before return from fork(2) or clone(2) with the exit signal set to SIGCHLD.

              Stop before return from clone(2).

              Stop  before  return from vfork(2) or clone(2) with the CLONE_VFORK flag, but after
              the child unblocked this tracee by exiting or execing.

       For all four stops described above, the stop occurs in the parent (i.e., the tracee),  not
       in  the newly created thread.  PTRACE_GETEVENTMSG can be used to retrieve the new thread's

              Stop before return from execve(2).  Since Linux 3.0, PTRACE_GETEVENTMSG returns the
              former thread ID.

              Stop before exit (including death from exit_group(2)), signal death, or exit caused
              by execve(2) in a  multithreaded  process.   PTRACE_GETEVENTMSG  returns  the  exit
              status.   Registers  can be examined (unlike when "real" exit happens).  The tracee
              is still alive; it needs to be PTRACE_CONTed or PTRACE_DETACHed to finish exiting.

              Stop induced by PTRACE_INTERRUPT command, or  group-stop,  or  initial  ptrace-stop
              when   a   new  child  is  attached  (only  if  attached  using  PTRACE_SEIZE),  or
              PTRACE_EVENT_STOP if PTRACE_SEIZE was used.

       PTRACE_GETSIGINFO on PTRACE_EVENT stops returns SIGTRAP in si_signo, with si_code  set  to
       (event<<8) | SIGTRAP.

       If  the  tracee was restarted by PTRACE_SYSCALL, the tracee enters syscall-enter-stop just
       prior to entering any system call.  If the tracer restarts the tracee with PTRACE_SYSCALL,
       the  tracee  enters  syscall-exit-stop  when  the  system  call  is  finished, or if it is
       interrupted by a signal.  (That is, signal-delivery-stop never  happens  between  syscall-
       enter-stop and syscall-exit-stop; it happens after syscall-exit-stop.)

       Other  possibilities  are  that  the  tracee  may stop in a PTRACE_EVENT stop, exit (if it
       entered _exit(2) or exit_group(2)), be killed by SIGKILL, or die  silently  (if  it  is  a
       thread  group  leader,  the  execve(2)  happened in another thread, and that thread is not
       traced by the same tracer; this situation is discussed later).

       Syscall-enter-stop  and  syscall-exit-stop  are  observed  by  the  tracer  as  waitpid(2)
       returning  with  WIFSTOPPED(status)  true,  and  WSTOPSIG(status)  giving SIGTRAP.  If the
       PTRACE_O_TRACESYSGOOD option was set by the tracer, then WSTOPSIG(status)  will  give  the
       value (SIGTRAP | 0x80).

       Syscall-stops  can  be  distinguished  from  signal-delivery-stop with SIGTRAP by querying
       PTRACE_GETSIGINFO for the following cases:

       si_code <= 0
              SIGTRAP was delivered as a result of a user-space action,  for  example,  a  system
              call  (tgkill(2),  kill(2), sigqueue(3), etc.), expiration of a POSIX timer, change
              of state on a POSIX message queue, or completion of an asynchronous I/O request.

       si_code == SI_KERNEL (0x80)
              SIGTRAP was sent by the kernel.

       si_code == SIGTRAP or si_code == (SIGTRAP|0x80)
              This is a syscall-stop.

       However,  syscall-stops  happen  very  often  (twice  per  system  call),  and  performing
       PTRACE_GETSIGINFO for every syscall-stop may be somewhat expensive.

       Some  architectures  allow  the  cases  to  be  distinguished by examining registers.  For
       example, on x86, rax == -ENOSYS in syscall-enter-stop.   Since  SIGTRAP  (like  any  other
       signal)  always  happens  after  syscall-exit-stop,  and  at  this  point rax almost never
       contains -ENOSYS, the SIGTRAP looks like "syscall-stop which is  not  syscall-enter-stop";
       in  other  words,  it looks like a "stray syscall-exit-stop" and can be detected this way.
       But such detection is fragile and is best avoided.

       Using the PTRACE_O_TRACESYSGOOD option is the recommended method to  distinguish  syscall-
       stops  from  other  kinds  of  ptrace-stops,  since  it  is  reliable and does not incur a
       performance penalty.

       Syscall-enter-stop and syscall-exit-stop are indistinguishable  from  each  other  by  the
       tracer.   The  tracer  needs to keep track of the sequence of ptrace-stops in order to not
       misinterpret syscall-enter-stop as syscall-exit-stop or vice  versa.   The  rule  is  that
       syscall-enter-stop  is  always  followed  by  syscall-exit-stop,  PTRACE_EVENT stop or the
       tracee's death; no other kinds of ptrace-stop can occur in between.

       If  after  syscall-enter-stop,  the  tracer  uses  a   restarting   command   other   than
       PTRACE_SYSCALL, syscall-exit-stop is not generated.

       PTRACE_GETSIGINFO  on  syscall-stops  returns  SIGTRAP  in  si_signo,  with si_code set to
       SIGTRAP or (SIGTRAP|0x80).

       [Details of these kinds of stops are yet to be documented.]

   Informational and restarting ptrace commands
       Most  ptrace   commands   (all   except   PTRACE_ATTACH,   PTRACE_SEIZE,   PTRACE_TRACEME,
       PTRACE_INTERRUPT,  and  PTRACE_KILL)  require the tracee to be in a ptrace-stop, otherwise
       they fail with ESRCH.

       When the tracee is in ptrace-stop, the tracer can read and write data to the tracee  using
       informational commands.  These commands leave the tracee in ptrace-stopped state:

           ptrace(PTRACE_PEEKTEXT/PEEKDATA/PEEKUSER, pid, addr, 0);
           ptrace(PTRACE_POKETEXT/POKEDATA/POKEUSER, pid, addr, long_val);
           ptrace(PTRACE_GETREGS/GETFPREGS, pid, 0, &struct);
           ptrace(PTRACE_SETREGS/SETFPREGS, pid, 0, &struct);
           ptrace(PTRACE_GETREGSET, pid, NT_foo, &iov);
           ptrace(PTRACE_SETREGSET, pid, NT_foo, &iov);
           ptrace(PTRACE_GETSIGINFO, pid, 0, &siginfo);
           ptrace(PTRACE_SETSIGINFO, pid, 0, &siginfo);
           ptrace(PTRACE_GETEVENTMSG, pid, 0, &long_var);
           ptrace(PTRACE_SETOPTIONS, pid, 0, PTRACE_O_flags);

       Note that some errors are not reported.  For example, setting signal information (siginfo)
       may have no effect in some ptrace-stops, yet the call may succeed (return 0  and  not  set
       errno);  querying  PTRACE_GETEVENTMSG  may succeed and return some random value if current
       ptrace-stop is not documented as returning a meaningful event message.

       The call

           ptrace(PTRACE_SETOPTIONS, pid, 0, PTRACE_O_flags);

       affects one tracee.  The tracee's current flags are replaced.  Flags are inherited by  new
       tracees created and "auto-attached" via active PTRACE_O_TRACEFORK, PTRACE_O_TRACEVFORK, or
       PTRACE_O_TRACECLONE options.

       Another group of commands makes the ptrace-stopped tracee run.  They have the form:

           ptrace(cmd, pid, 0, sig);

       PTRACE_SYSEMU, or PTRACE_SYSEMU_SINGLESTEP.  If the tracee is in signal-delivery-stop, sig
       is the signal to be injected (if it is nonzero).  Otherwise, sig may  be  ignored.   (When
       restarting  a  tracee  from  a  ptrace-stop  other  than signal-delivery-stop, recommended
       practice is to always pass 0 in sig.)

   Attaching and detaching
       A thread can be attached to the tracer using the call

           ptrace(PTRACE_ATTACH, pid, 0, 0);


           ptrace(PTRACE_SEIZE, pid, 0, PTRACE_O_flags);

       PTRACE_ATTACH sends SIGSTOP to this thread.  If the tracer wants this SIGSTOP to  have  no
       effect, it needs to suppress it.  Note that if other signals are concurrently sent to this
       thread during attach, the tracer may see the tracee enter signal-delivery-stop with  other
       signal(s)  first!   The usual practice is to reinject these signals until SIGSTOP is seen,
       then suppress SIGSTOP injection.  The design bug here  is  that  a  ptrace  attach  and  a
       concurrently delivered SIGSTOP may race and the concurrent SIGSTOP may be lost.

       Since attaching sends SIGSTOP and the tracer usually suppresses it, this may cause a stray
       EINTR return from the currently executing system call in the tracee, as described  in  the
       "Signal injection and suppression" section.

       Since Linux 3.4, PTRACE_SEIZE can be used instead of PTRACE_ATTACH.  PTRACE_SEIZE does not
       stop the attached process.  If you need to stop it after attach (or  at  any  other  time)
       without sending it any signals, use PTRACE_INTERRUPT command.

       The request

           ptrace(PTRACE_TRACEME, 0, 0, 0);

       turns  the  calling  thread  into  a  tracee.   The thread continues to run (doesn't enter
       ptrace-stop).  A common practice is to follow the PTRACE_TRACEME with


       and allow the parent (which is our tracer now) to observe our signal-delivery-stop.

       effect,  then children created by, respectively, vfork(2) or clone(2) with the CLONE_VFORK
       flag, fork(2) or clone(2) with the  exit  signal  set  to  SIGCHLD,  and  other  kinds  of
       clone(2),  are  automatically  attached  to  the  same  tracer  which traced their parent.
       SIGSTOP is delivered to the children, causing them  to  enter  signal-delivery-stop  after
       they exit the system call which created them.

       Detaching of the tracee is performed by:

           ptrace(PTRACE_DETACH, pid, 0, sig);

       PTRACE_DETACH is a restarting operation; therefore it requires the tracee to be in ptrace-
       stop.  If the tracee is in signal-delivery-stop, a signal can be injected.  Otherwise, the
       sig parameter may be silently ignored.

       If the tracee is running when the tracer wants to detach it, the usual solution is to send
       SIGSTOP (using tgkill(2), to make sure it goes to the correct thread), wait for the tracee
       to  stop  in  signal-delivery-stop  for  SIGSTOP  and  then detach it (suppressing SIGSTOP
       injection).  A design bug is  that  this  can  race  with  concurrent  SIGSTOPs.   Another
       complication is that the tracee may enter other ptrace-stops and needs to be restarted and
       waited for again, until SIGSTOP is seen.  Yet another complication is to be sure that  the
       tracee  is  not already ptrace-stopped, because no signal delivery happens while it is—not
       even SIGSTOP.

       If the tracer dies, all tracees are automatically detached and restarted, unless they were
       in  group-stop.   Handling  of  restart  from  group-stop  is currently buggy, but the "as
       planned" behavior is to leave tracee stopped and waiting for SIGCONT.  If  the  tracee  is
       restarted from signal-delivery-stop, the pending signal is injected.

   execve(2) under ptrace
       When  one thread in a multithreaded process calls execve(2), the kernel destroys all other
       threads in the process, and resets the thread ID of the execing thread to the thread group
       ID  (process  ID).   (Or,  to put things another way, when a multithreaded process does an
       execve(2), at completion of the call, it appears as though the execve(2) occurred  in  the
       thread group leader, regardless of which thread did the execve(2).)  This resetting of the
       thread ID looks very confusing to tracers:

       *  All other threads stop in PTRACE_EVENT_EXIT stop, if the PTRACE_O_TRACEEXIT option  was
          turned  on.   Then  all other threads except the thread group leader report death as if
          they exited via _exit(2) with exit code 0.

       *  The execing tracee changes its thread ID while it  is  in  the  execve(2).   (Remember,
          under  ptrace,  the  "pid"  returned  from waitpid(2), or fed into ptrace calls, is the
          tracee's thread ID.)  That is, the tracee's thread ID is reset to be the  same  as  its
          process ID, which is the same as the thread group leader's thread ID.

       *  Then a PTRACE_EVENT_EXEC stop happens, if the PTRACE_O_TRACEEXEC option was turned on.

       *  If  the  thread  group  leader has reported its PTRACE_EVENT_EXIT stop by this time, it
          appears to the tracer that the dead thread leader "reappears from nowhere".  (Note: the
          thread group leader does not report death via WIFEXITED(status) until there is at least
          one other live thread.  This eliminates the possibility that the  tracer  will  see  it
          dying  and  then  reappearing.)   If  the  thread group leader was still alive, for the
          tracer this may look as if thread group leader returns from  a  different  system  call
          than  it  entered,  or  even "returned from a system call even though it was not in any
          system call".  If the thread group leader was not traced (or was traced by a  different
          tracer),  then  during  execve(2)  it  will  appear as if it has become a tracee of the
          tracer of the execing tracee.

       All of the above effects are the artifacts of the thread ID change in the tracee.

       The PTRACE_O_TRACEEXEC option is the recommended tool for  dealing  with  this  situation.
       First,  it enables PTRACE_EVENT_EXEC stop, which occurs before execve(2) returns.  In this
       stop, the tracer can use PTRACE_GETEVENTMSG to retrieve the  tracee's  former  thread  ID.
       (This  feature  was  introduced  in  Linux  3.0).   Second,  the PTRACE_O_TRACEEXEC option
       disables legacy SIGTRAP generation on execve(2).

       When the tracer receives PTRACE_EVENT_EXEC stop notification, it is guaranteed that except
       this tracee and the thread group leader, no other threads from the process are alive.

       On  receiving  the PTRACE_EVENT_EXEC stop notification, the tracer should clean up all its
       internal data structures describing the threads of this process, and retain only one  data
       structure—one which describes the single still running tracee, with

           thread ID == thread group ID == process ID.

       Example: two threads call execve(2) at the same time:

       *** we get syscall-enter-stop in thread 1: **
       PID1 execve("/bin/foo", "foo" <unfinished ...>
       *** we issue PTRACE_SYSCALL for thread 1 **
       *** we get syscall-enter-stop in thread 2: **
       PID2 execve("/bin/bar", "bar" <unfinished ...>
       *** we issue PTRACE_SYSCALL for thread 2 **
       *** we get PTRACE_EVENT_EXEC for PID0, we issue PTRACE_SYSCALL **
       *** we get syscall-exit-stop for PID0: **
       PID0 <... execve resumed> )             = 0

       If  the  PTRACE_O_TRACEEXEC  option  is  not  in effect for the execing tracee, the kernel
       delivers an extra SIGTRAP to the tracee after execve(2)  returns.   This  is  an  ordinary
       signal  (similar  to  one  which  can  be  generated by kill -TRAP), not a special kind of
       ptrace-stop.  Employing PTRACE_GETSIGINFO  for  this  signal  returns  si_code  set  to  0
       (SI_USER).   This  signal  may be blocked by signal mask, and thus may be delivered (much)

       Usually, the tracer (for example, strace(1)) would not want to show this extra post-execve
       SIGTRAP  signal  to the user, and would suppress its delivery to the tracee (if SIGTRAP is
       set to SIG_DFL, it is a killing signal).  However, determining which SIGTRAP  to  suppress
       is  not  easy.   Setting  the  PTRACE_O_TRACEEXEC  option  and thus suppressing this extra
       SIGTRAP is the recommended approach.

   Real parent
       The ptrace API (ab)uses the standard UNIX parent/child signaling  over  waitpid(2).   This
       used to cause the real parent of the process to stop receiving several kinds of waitpid(2)
       notifications when the child process is traced by some other process.

       Many of these bugs have been fixed, but as of Linux 2.6.38 several still exist;  see  BUGS

       As of Linux 2.6.38, the following is believed to work correctly:

       *  exit/death  by  signal  is reported first to the tracer, then, when the tracer consumes
          the waitpid(2) result, to the real parent (to the  real  parent  only  when  the  whole
          multithreaded  process exits).  If the tracer and the real parent are the same process,
          the report is sent only once.


       On success, PTRACE_PEEK* requests return the requested data, while other  requests  return
       zero.   (On  Linux,  this  is  done in the libc wrapper around ptrace system call.  On the
       system call level, PTRACE_PEEK* requests have a different API: they store  the  result  at
       the address specified by data parameter, and return value is the error flag.)

       On  error,  all  requests  return  -1,  and  errno  is set appropriately.  Since the value
       returned by a successful PTRACE_PEEK* request may be  -1,  the  caller  must  clear  errno
       before  the  call,  and  then  check  it  afterward  to  determine whether or not an error


       EBUSY  (i386 only) There was an error with allocating or freeing a debug register.

       EFAULT There was an attempt to read from or write to an invalid area in  the  tracer's  or
              the  tracee's  memory,  probably  because  the  area  wasn't  mapped or accessible.
              Unfortunately, under Linux, different variations of this fault will return  EIO  or
              EFAULT more or less arbitrarily.

       EINVAL An attempt was made to set an invalid option.

       EIO    request is invalid, or an attempt was made to read from or write to an invalid area
              in the tracer's or the tracee's memory, or there was a word-alignment violation, or
              an invalid signal was specified during a restart request.

       EPERM  The  specified  process  cannot  be  traced.   This could be because the tracer has
              insufficient privileges (the required capability is  CAP_SYS_PTRACE);  unprivileged
              processes  cannot trace processes that they cannot send signals to or those running
              set-user-ID/set-group-ID programs, for obvious reasons.  Alternatively, the process
              may already be being traced, or (on kernels before 2.6.26) be init(8) (PID 1).

       ESRCH  The  specified  process  does  not  exist,  or is not currently being traced by the
              caller, or is not stopped (for requests that require a stopped tracee).


       SVr4, 4.3BSD.


       Although arguments to ptrace() are interpreted according to  the  prototype  given,  glibc
       currently  declares  ptrace() as a variadic function with only the request argument fixed.
       It is recommended to always supply four arguments, even if the  requested  operation  does
       not use them, setting unused/ignored arguments to 0L or (void *) 0.

       In Linux kernels before 2.6.26, init(8), the process with PID 1, may not be traced.

       The  layout  of  the  contents of memory and the USER area are quite operating-system- and
       architecture-specific.  The offset supplied, and the data  returned,  might  not  entirely
       match with the definition of struct user.

       The size of a "word" is determined by the operating-system variant (e.g., for 32-bit Linux
       it is 32 bits).

       This page documents the way the ptrace() call works  currently  in  Linux.   Its  behavior
       differs  noticeably  on  other  flavors  of  UNIX.  In any case, use of ptrace() is highly
       specific to the operating system and architecture.


       On hosts with 2.6 kernel headers, PTRACE_SETOPTIONS is declared  with  a  different  value
       than the one for 2.4.  This leads to applications compiled with 2.6 kernel headers failing
       when run on 2.4 kernels.  This can be worked around  by  redefining  PTRACE_SETOPTIONS  to
       PTRACE_OLDSETOPTIONS, if that is defined.

       Group-stop  notifications  are sent to the tracer, but not to real parent.  Last confirmed

       If a thread group leader is traced and exits by calling _exit(2), a PTRACE_EVENT_EXIT stop
       will  happen  for it (if requested), but the subsequent WIFEXITED notification will not be
       delivered until all other threads exit.  As explained above, if one of other threads calls
       execve(2),  the  death  of  the thread group leader will never be reported.  If the execed
       thread is not traced by this tracer, the tracer will never know that  execve(2)  happened.
       One  possible workaround is to PTRACE_DETACH the thread group leader instead of restarting
       it in this case.  Last confirmed on

       A SIGKILL signal may still cause a PTRACE_EVENT_EXIT  stop  before  actual  signal  death.
       This  may be changed in the future; SIGKILL is meant to always immediately kill tasks even
       under ptrace.  Last confirmed on

       Some system calls return with EINTR if a signal was sent to a  tracee,  but  delivery  was
       suppressed  by  the  tracer.   (This  is  very  typical  operation:  it is usually done by
       debuggers on every attach, in order to not introduce a bogus SIGSTOP).  As of Linux 3.2.9,
       the  following  system calls are affected (this list is likely incomplete): epoll_wait(2),
       and read(2) from an inotify(7) file descriptor.  The usual symptom of  this  bug  is  that
       when you attach to a quiescent process with the command

           strace -p <process-ID>

       then, instead of the usual and expected one-line output such as

           restart_syscall(<... resuming interrupted call ...>_


           select(6, [5], NULL, [5], NULL_

       ('_' denotes the cursor position), you observe more than one line.  For example:

           clock_gettime(CLOCK_MONOTONIC, {15370, 690928118}) = 0

       What is not visible here is that the process was blocked in epoll_wait(2) before strace(1)
       has attached to it.  Attaching caused epoll_wait(2) to return to user space with the error
       EINTR.   In  this  particular  case,  the program reacted to EINTR by checking the current
       time, and then executing epoll_wait(2) again.  (Programs which do not expect such  "stray"
       EINTR errors may behave in an unintended way upon an strace(1) attach.)


       gdb(1),  strace(1),  clone(2),  execve(2),  fork(2),  gettid(2),  sigaction(2), tgkill(2),
       vfork(2), waitpid(2), exec(3), capabilities(7), signal(7)


       This page is part of release 3.54 of the Linux man-pages project.  A  description  of  the
       project,     and    information    about    reporting    bugs,    can    be    found    at