#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

       A tracee first needs to be attached to the tracer.  Attachment and sub-
       sequent 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 deliv-
       ered, 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

       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  pro-
       gram begins execution.

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

       The value of request determines the action to be performed:
       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,  return-
              ing 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

              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 modifica-
              tions 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_GETF-
              PREGS are not present on all architectures.

       PTRACE_GETREGSET (since Linux 2.6.34)
              Read  the  tracee's  registers.  addr specifies, in an architec-
              ture-dependent way, the type of registers to be read.  NT_PRSTA-
              TUS  (with numerical value 1) usually results in reading of gen-
              eral-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
              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 ignored.)

       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 use-
                     ful 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

                     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.

                     cation  is  done  after  the process is finished exiting.
                     Even though context is available, the tracer cannot  pre-
                     vent 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_O_TRACESYSGOOD (since Linux 2.4.6)
                     When delivering system call traps, set bit 7 in the  sig-
                     nal  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_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.

       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  sta-
              tus.        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.)

              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_LIS-
              TEN works only on tracees attached by PTRACE_SEIZE.

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

              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 inter-
              rupted and syscall-exit-stop is reported.  (The interrupted sys-
              tem 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  WSTOP-
              SIG(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
              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 affected.

       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  wait-
       pid(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 refus-
       ing ptrace requests.

       The tracer can't assume that the tracee always ends its life by report-
       ing  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.

       There are many kinds of states when  the  tracee  is  stopped,  and  in
       ptrace  discussions  they are often conflated.  Therefore, it is impor-
       tant 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

       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 functionality.

       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 sig-
       nal.  (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.
       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 pre-
       maturely.  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 sys-
       tem 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-deliv-
       ery-stop.  The tracer can't suppress this side effect (it can only sup-
       press signal injection, which only causes the SIGCONT handler to not be
       executed in the tracee, if such a handler is installed).  In fact, wak-
       ing up from group-stop may be followed by signal-delivery-stop for sig-
       nal(s) other than SIGCONT, if they were pending when SIGCONT was deliv-
       ered.   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.
       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

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

           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

       If tracee was attached using PTRACE_SEIZE, group-stop is  indicated  by
       PTRACE_EVENT_STOP: status>>16 == PTRACE_EVENT_STOP.  This allows detec-
       tion 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 prob-
       lem with transparent handling  of  stopping  signals.   If  the  tracer
       restarts  the  tracee  after  group-stop, the stopping signal is effec-
       tively ignored--the tracee doesn't remain stopped,  it  runs.   If  the
       tracer  doesn't  restart the tracee before entering into the next wait-
       pid(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
              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 ID.

              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

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

       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 inter-
       rupted by a  signal.   (That  is,  signal-delivery-stop  never  happens
       between  syscall-enter-stop  and  syscall-exit-stop;  it  happens after

       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
              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

       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

       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_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

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

           ptrace(cmd, pid, 0, sig);

       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 suppres-
       sion" 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
       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

       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  wait-
          pid(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

       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) later.

       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 below.
       per  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 occurred.

       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, dif-
              ferent 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  pro-
              cesses  that  they  cannot send signals to or those running set-
              user-ID/set-group-ID programs, for  obvious  reasons.   Alterna-
              tively,  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  proto-
       type  given,  glibc  currently declares ptrace() as a variadic function
       with only the request argument fixed.  It is recommended to always sup-
       ply  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 oper-
       ating-system- and architecture-specific.  The offset supplied, and  the
       data  returned,  might not entirely match with the definition of struct
       different  value than the one for 2.4.  This leads to applications com-
       piled with 2.6 kernel headers failing when run on  2.4  kernels.   This
       can  be  worked around by redefining PTRACE_SETOPTIONS to PTRACE_OLDSE-
       TOPTIONS, if that is defined.

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

       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  sub-
       sequent  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 oper-
       ation: 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

           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.)

Linux                             2013-07-11                         PTRACE(2)
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