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PTRACE(2)		   Linux Programmers Manual		    PTRACE(2)

       ptrace - process trace


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

       The ptrace() system call provides a means by which a parent process may
       observe and control the execution of another process, and  examine  and
       change its core image and registers.  It is primarily used to implement
       breakpoint debugging and system call tracing.

       The parent can initiate a trace	by  calling  fork(2)  and  having  the
       resulting  child  do  a	PTRACE_TRACEME,  followed  (typically)	by  an
       exec(3).  Alternatively, the parent may commence trace of  an  existing
       process using PTRACE_ATTACH.

       While  being  traced,  the child will stop each time a signal is deliv
       ered, even if the signal is being ignored.  (The exception is  SIGKILL,
       which  has  its usual effect.)  The parent will be notified at its next
       wait(2) and may inspect and  modify  the  child	process  while	it  is
       stopped.   The  parent  then  causes  the child to continue, optionally
       ignoring the delivered signal (or even delivering  a  different	signal

       When  the  parent  is finished tracing, it can terminate the child with
       PTRACE_KILL or cause it to continue executing  in  a  normal,  untraced
       mode via PTRACE_DETACH.

       The value of request determines the action to be performed:

	      Indicates  that this process is to be traced by its parent.  Any
	      signal (except SIGKILL) delivered to this process will cause  it
	      to  stop	and  its parent to be notified via wait(2).  Also, all
	      subsequent calls to execve(2) by this process will cause a  SIG
	      TRAP  to	be sent to it, giving the parent a chance to gain con
	      trol before the new program begins execution.  A process	proba
	      bly shouldnt make this request if its parent isnt expecting to
	      trace it.  (pid, addr, and data are ignored.)

       The above request is used only by the child process; the rest are  used
       only by the parent.  In the following requests, pid specifies the child
       process to be acted on.	For requests other than PTRACE_KILL, the child
       process must be stopped.

	      Reads a word at the location addr in the childs memory, return
	      ing the word as the result of the ptrace() call.	Linux does not
	      have  separate text and data address spaces, so the two requests
	      are currently equivalent.  (The argument data is ignored.)

	      Reads a word at offset addr in  the  childs  USER  area,	which
	      holds the registers and other information about the process (see
	       and ).  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.)

	      Copies the word data to location addr in the childs memory.  As
	      above, the two requests are currently equivalent.

	      Copies the word data to offset addr in the  childs  USER	area.
	      As  above,  the offset must typically be word-aligned.  In order
	      to maintain the integrity of the kernel, some  modifications  to
	      the USER area are disallowed.

	      Copies  the childs general purpose or floating-point registers,
	      respectively,   to   location   data   in   the	parent.    See
	        for  information	on  the  format  of this data.
	      (addr is ignored.)

       PTRACE_GETSIGINFO (since Linux 2.3.99-pre6)
	      Retrieve information about the  signal  that  caused  the  stop.
	      Copies  a  siginfo_t structure (see sigaction(2)) from the child
	      to location data in the parent.  (addr is ignored.)

	      Copies the childs general purpose or floating-point  registers,
	      respectively,   from  location  data  in	the  parent.   As  for
	      PTRACE_POKEUSER, some general purpose register modifications may
	      be disallowed.  (addr is ignored.)

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

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

	      PTRACE_O_TRACESYSGOOD (since Linux 2.4.6)
		     When delivering syscall traps, set bit 7  in  the	signal
		     number (i.e., deliver (SIGTRAP | 0x80) This makes it easy
		     for the tracer to	tell  the  difference  between	normal
		     traps and those caused by a syscall.  (PTRACE_O_TRACESYS
		     GOOD may not work on all architectures.)

	      PTRACE_O_TRACEFORK (since Linux 2.5.46)
		     Stop the child at the next fork(2) call  with  SIGTRAP  |
		     PTRACE_EVENT_FORK << 8  and  automatically  start tracing
		     the  newly  forked  process,  which  will	start  with  a
		     SIGSTOP.	The  PID  for the new process can be retrieved

	      PTRACE_O_TRACEVFORK (since Linux 2.5.46)
		     Stop the child at the next vfork(2) call with  SIGTRAP  |
		     PTRACE_EVENT_VFORK << 8  and  automatically start tracing
		     the newly	vforked  process,  which  will	start  with  a
		     SIGSTOP.	The  PID  for the new process can be retrieved

	      PTRACE_O_TRACECLONE (since Linux 2.5.46)
		     Stop the child at the next clone(2) call with  SIGTRAP  |
		     PTRACE_EVENT_CLONE << 8  and  automatically start tracing
		     the  newly  cloned  process,  which  will	start  with  a
		     SIGSTOP.	The  PID  for the new process can be retrieved
		     with  PTRACE_GETEVENTMSG.	 This  option  may  not  catch
		     clone(2) calls in all cases.  If the child calls clone(2)
		     with the CLONE_VFORK  flag,  PTRACE_EVENT_VFORK  will  be
		     delivered	instead  if PTRACE_O_TRACEVFORK is set; other
		     wise if the child 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 child at the next execve(2) call with SIGTRAP  |
		     PTRACE_EVENT_EXEC << 8.

	      PTRACE_O_TRACEVFORKDONE (since Linux 2.5.60)
		     Stop  the	child  at  the completion of the next vfork(2)
		     call with SIGTRAP | PTRACE_EVENT_VFORK_DONE << 8.

	      PTRACE_O_TRACEEXIT (since Linux 2.5.60)
		     Stop   the    child    at	  exit	  with	  SIGTRAP    |
		     PTRACE_EVENT_EXIT << 8.   The  childs exit status can be
		     retrieved with PTRACE_GETEVENTMSG.   This	stop  will  be
		     done  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  con
		     text  is  available,  the	tracer cannot prevent the exit
		     from happening at this point.

       PTRACE_GETEVENTMSG (since Linux 2.5.46)
	      Retrieve a message (as an unsigned long) about the ptrace  event
	      that  just happened, placing it in the location data in the par
	      ent.  For PTRACE_EVENT_EXIT this is  the	childs	exit  status.
	      this is the PID of the new process.  Since Linux 2.6.18, the PID
	      of     the     new     process	is    also    available    for
	      PTRACE_EVENT_VFORK_DONE.	(addr is ignored.)

	      Restarts the stopped child process.  If data is non-zero and not
	      SIGSTOP,	it  is	interpreted as a signal to be delivered to the
	      child; otherwise, no signal is delivered.   Thus,  for  example,
	      the  parent  can	control  whether a signal sent to the child is
	      delivered or not.  (addr is ignored.)

	      Restarts the stopped child as for PTRACE_CONT, but arranges  for
	      the child to be stopped at the next entry to or exit from a sys
	      tem call, or after execution of a  single  instruction,  respec
	      tively.  (The child will also, as usual, be stopped upon receipt
	      of a signal.)  From the parents  perspective,  the  child  will
	      appear  to  have	been stopped by receipt of a SIGTRAP.  So, for
	      PTRACE_SYSCALL, for example, the idea is to  inspect  the  argu
	      ments  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.  (addr is ignored.)

	      For  PTRACE_SYSEMU,  continue  and  stop	on  entry  to the next
	      syscall, which will not  be  executed.   For  PTRACE_SYSEMU_SIN
	      GLESTEP, do the same but also singlestep if not a syscall.  This
	      call is used by programs like User Mode Linux that want to  emu
	      late  all the childs system calls.  (addr and data are ignored;
	      not supported on all architectures.)

	      Sends the child a SIGKILL to terminate it.  (addr and  data  are

	      Attaches	to  the  process  specified in pid, making it a traced
	      "child" of the calling process; the behavior of the child is  as
	      if  it  had done a PTRACE_TRACEME.  The calling process actually
	      becomes the parent of the child process for most purposes (e.g.,
	      it  will	receive  notification  of  child events and appears in
	      ps(1) output as the childs parent), but  a  getppid(2)  by  the
	      child  will  still  return  the PID of the original parent.  The
	      child is sent a SIGSTOP, but will not necessarily  have  stopped
	      by  the  completion  of  this  call; use wait(2) to wait for the
	      child to stop.  (addr and data are ignored.)

	      Restarts	the  stopped  child  as  for  PTRACE_CONT,  but  first
	      detaches	from  the  process,  undoing the reparenting effect of
	      PTRACE_ATTACH, and the effects of PTRACE_TRACEME.  Although per
	      haps not intended, under Linux a traced child can be detached in
	      this way regardless of which method was used to  initiate  trac
	      ing.  (addr is ignored.)

       On  success,  PTRACE_PEEK*  requests  return  the requested data, 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 check errno after  such
       requests 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 parents or childs memory, probably because the area wasnt
	      mapped or accessible.   Unfortunately,  under  Linux,  different
	      variations  of this fault will return EIO or EFAULT more or less

       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 parents or childs memory, or there
	      was a word-alignment violation, or an invalid signal was	speci
	      fied during a restart request.

       EPERM  The  specified  process cannot be traced.  This could be because
	      the parent has insufficient privileges (the required  capability
	      is  CAP_SYS_PTRACE);  non-root  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 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 that).

       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.  This means that unneeded trail
       ing arguments may be omitted, though doing so makes use of undocumented
       gcc(1) behavior.

       init(8), the process with PID 1, may not be traced.

       The  layout  of	the contents of memory and the USER area are quite OS-
       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 OS variant (e.g., for 32-bit
       Linux it is 32 bits, etc.).

       Tracing causes a few subtle differences in the semantics of traced pro
       cesses.	 For  example, if a process is attached to with PTRACE_ATTACH,
       its original parent can no longer receive notification via wait(2) when
       it  stops,  and there is no way for the new parent to effectively simu
       late this notification.

       When the parent receives an event with PTRACE_EVENT_* set, the child is
       not  in	the normal signal delivery path.  This means the parent cannot
       do ptrace(PTRACE_CONT) with a signal or	ptrace(PTRACE_KILL).   kill(2)
       with  a	SIGKILL  signal  can be used instead to kill the child process
       after receiving one of these messages.

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

       The SunOS man page describes ptrace() as "unique and arcane", which  it
       is.  The proc-based debugging interface present in Solaris 2 implements
       a superset of ptrace() functionality in a  more	powerful  and  uniform

       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  such headers failing when run on 2.4 kernels.  This can be
       worked around by redefining PTRACE_SETOPTIONS to  PTRACE_OLDSETOPTIONS,
       if that is defined.

       gdb(1),	strace(1),  execve(2),	fork(2),  signal(2), wait(2), exec(3),

       This page is part of release 3.05 of the Linux  man-pages  project.   A
       description  of	the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.

Linux				  2008-05-21			     PTRACE(2)

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