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PTRACE(2)                           Linux Programmer's Manual                           PTRACE(2)

       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 com‐
       mence 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 sig‐
       nal 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 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.)

              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; but  see

              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;  but  see

              Copy the word data to the address addr in the tracee's memory.  As for PTRACE_PEEK‐
              TEXT 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 corre‐
              sponding NT_foo constant.  data points to a struct iovec, which describes the  des‐
              tination  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 reg‐
              ister 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 regis‐
              ters 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 deliv‐
              ered 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_PEEKSIGINFO (since Linux 3.10)
              Retrieve siginfo_t structures without removing signals from a queue.   addr  points
              to  a  ptrace_peeksiginfo_args  structure  that specifies the ordinal position from
              which copying of signals should start, and the number of  signals  to  copy.   sig‐
              info_t  structures are copied into the buffer pointed to by data.  The return value
              contains the number of copied signals (zero indicates that there is no signal  cor‐
              responding  to the specified ordinal position).  Within the returned siginfo struc‐
              tures, the si_code field includes information (__SI_CHLD,  __SI_FAULT,  etc.)  that
              are not otherwise exposed to user space.

                 struct ptrace_peeksiginfo_args {
                     u64 off;    /* Ordinal position in queue at which
                                    to start copying signals */
                     u32 flags;  /* PTRACE_PEEKSIGINFO_SHARED or 0 */
                     s32 nr;     /* Number of signals to copy */

                 Currently,  there  is only one flag, PTRACE_PEEKSIGINFO_SHARED, for dumping sig‐
                 nals from the process-wide signal queue.  If this flag is not set,  signals  are
                 read from the per-thread queue of the specified thread.

       PTRACE_GETSIGMASK (since Linux 3.11)
              Place  a  copy  of  the  mask of blocked signals (see sigprocmask(2)) in the buffer
              pointed to by data, which should be a pointer to a buffer of  type  sigset_t.   The
              addr  argument  contains  the  size  of  the  buffer  pointed  to  by  data  (i.e.,

       PTRACE_SETSIGMASK (since Linux 3.11)
              Change the mask of blocked signals (see sigprocmask(2)) to the value  specified  in
              the  buffer  pointed  to  by  data,  which  should be a pointer to a buffer of type
              sigset_t.  The addr argument contains the size of the buffer  pointed  to  by  data
              (i.e., sizeof(sigset_t)).

       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 deliv‐
              ered.  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 sec‐
              ond 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 exam‐
              ple, 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 user space), 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  comple‐
              tion of this call; use waitpid(2) to wait for the tracee to stop.  See the "Attach‐
              ing and detaching" subsection for  additional  information.   (addr  and  data  are

       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(sta‐
       tus) 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  con‐
       sidered  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  sched‐
       uled),  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 com‐
       mands 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(sta‐
       tus) true.

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

       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 WIF‐
       STOPPED(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(sta‐
       tus) macro can't be used to perform this examination, because it returns the  value  (sta‐
       tus>>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 sup‐
       press 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  WIF‐
       STOPPED(status) true, with the signal returned by WSTOPSIG(status).  If the signal is SIG‐
       TRAP,  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 sig‐
       nal 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 system calls 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 system calls 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  sig‐
       nal-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, wak‐
       ing 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(sta‐
       tus), 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  sta‐
              tus.   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 inter‐
       rupted 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)  return‐
       ing   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 exam‐
       ple, 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 perfor‐
       mance 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 SIG‐
       TRAP 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 prac‐
       tice 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  con‐
       currently 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 injec‐
       tion).  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

       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 dis‐
       ables 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 sig‐
       nal  (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  SIG‐
       TRAP 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 mul‐
          tithreaded process exits).  If the tracer and the real parent are the same process, the
          report is sent only once.

       On  success,  the  PTRACE_PEEK*  requests return the requested data (but see NOTES), while
       other requests return zero.

       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.  Unfor‐
              tunately, 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 call‐
              er, 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 dif‐
       fers significantly on other flavors of UNIX.  In any case, use of ptrace() is highly  spe‐
       cific to the operating system and architecture.

   C library/kernel ABI differences
       At  the  system  call  level,  the  PTRACE_PEEKTEXT,  PTRACE_PEEKDATA, and PTRACE_PEEKUSER
       requests have a different API: they store the result at the address specified by the  data
       parameter,  and  the  return value is the error flag.  The glibc wrapper function provides
       the API given in DESCRIPTION above, with the result being returned via the function return

       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  debug‐
       gers  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 3.74 of the Linux man-pages project.  A description of the
       project, information about reporting bugs, and the latest version of  this  page,  can  be
       found at http://www.kernel.org/doc/man-pages/.

Linux                                       2014-08-19                                  PTRACE(2)

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