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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

       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:

              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 follow-
              ing 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; 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 NOTES.)

              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
              PTRACE_POKEUSER, some general-purpose register modifications may
              be disallowed.  (addr is ignored.)  Note that SPARC systems have
              the meaning of data and addr reversed; that is, data is  ignored
              and   the   registers   are   copied   from  the  address  addr.
              PTRACE_SETREGS and  PTRACE_SETFPREGS  are  not  present  on  all

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

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

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

       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.   siginfo_t
              structures  are  copied into the buffer pointed to by data.  The
              return value contains the number of copied signals  (zero  indi-
              cates  that  there  is  no signal corresponding to the specified
              ordinal position).  Within the returned siginfo structures,  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  signals 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., sizeof(sigset_t)).

       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.,

       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.

              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

                     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  notifi-
                     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.)

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

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

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

       PTRACE_LISTEN (since Linux 3.4)
              Restart  the stopped tracee, but prevent it from executing.  The
              resulting state of the tracee is similar to a process which  has
              been  stopped  by a SIGSTOP (or other stopping signal).  See the
              "group-stop" subsection for additional information.  PTRACE_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  user  space),
              it  stops  with  PTRACE_EVENT_STOP with WSTOPSIG(status) == SIG-
              TRAP.   PTRACE_INTERRUPT  only  works  on  tracees  attached  by

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

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

              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 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.

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

       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.
       If  the tracer doesn't suppress the signal, it passes the signal to the
       tracee in the next ptrace restart request.  This second step of  signal
       delivery  is called signal injection in this manual page.  Note that if
       the signal is blocked, signal-delivery-stop doesn't  happen  until  the
       signal  is  unblocked,  with  the usual exception that SIGSTOP can't be

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

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

           ptrace(PTRACE_restart, pid, 0, sig)

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

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

       Note that a suppressed signal still causes system calls to return  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 system calls which use a dif-
       ferent 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 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.
       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

       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

           (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 execu-
              tion (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 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
       tracer, then WSTOPSIG(status) will give the value (SIGTRAP | 0x80).

       Syscall-stops can be distinguished from signal-delivery-stop with  SIG-
       TRAP 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

       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_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

       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


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

       options are in 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  automati-
       cally  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

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

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

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

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

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

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

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

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

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

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

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

       On success, 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 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

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

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

   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 value.

       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  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.)

       gdb(1), strace(1),  clone(2),  execve(2),  fork(2),  gettid(2),  sigac-
       tion(2),  tgkill(2),  vfork(2),  waitpid(2),  exec(3), capabilities(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

Linux                             2014-08-19                         PTRACE(2)

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Created with the man page lookup class by Andrew Collington.
Based on a C man page viewer by Vadim Pavlov
Unicode soft-hyphen fix (as used by RedHat) by Dan Edwards
Some optimisations by Eli Argon
Caching idea and code contribution by James Richardson

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