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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 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.  (See additional notes below.)

       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 shouldn't make this request if its parent isn't 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 child's 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  child'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

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

              Copies the word data to offset addr in the  child's  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 child's general purpose or floating-point registers,
              respectively, to location data in the parent.  See  <sys/user.h>
              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 child's 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
                     with PTRACE_GETEVENTMSG.

              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
                     with PTRACE_GETEVENTMSG.

              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 child's 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 child's 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 nonzero 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 parent's 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.   The  data  argument  is  treated  as  for
              PTRACE_CONT.  (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 child's system calls.  The data argument is treated
              as for PTRACE_CONT.  (addr is  ignored;  not  supported  on  all

              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  child's 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.)

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

              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 parent's or child's memory, probably because the area wasn't
              mapped  or  accessible.   Unfortunately,  under Linux, different
              variations of this fault will return EIO or EFAULT more or  less

       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 parent's or child's 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); 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 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 3.27 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at

Linux                             2009-03-30                         PTRACE(2)


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