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NAME

       ptrace - process trace

SYNOPSIS

       #include <sys/ptrace.h>

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

DESCRIPTION

       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
       delivered, 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
       instead).

       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:

       PTRACE_TRACEME
              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
              SIGTRAP  to  be  sent  to it, giving the parent a chance to gain
              control before the new  program  begins  execution.   A  process
              probably  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.

       PTRACE_PEEKTEXT, PTRACE_PEEKDATA
              Reads  a  word  at  the  location  addr  in  the child’s memory,
              returning 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.)

       PTRACE_PEEKUSER
              Reads  a  word  at  offset  addr in the child’s USER area, which
              holds the registers and other information about the process (see
              <linux/user.h>  and  <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.)

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

       PTRACE_POKEUSER
              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.

       PTRACE_GETREGS, PTRACE_GETFPREGS
              Copies the child’s general purpose or floating-point  registers,
              respectively,   to   location   data   in   the   parent.    See
              <linux/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.)

       PTRACE_SETREGS, PTRACE_SETFPREGS
              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
              location 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
              specified 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_TRACESYSGOOD    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;
                     otherwise 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
                     context  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
              parent.  For PTRACE_EVENT_EXIT this is the child’s exit  status.
              For PTRACE_EVENT_FORK, PTRACE_EVENT_VFORK and PTRACE_EVENT_CLONE
              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.)

       PTRACE_CONT
              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.)

       PTRACE_SYSCALL, PTRACE_SINGLESTEP
              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
              system  call,  or  after  execution  of  a  single  instruction,
              respectively.   (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
              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.  (addr is ignored.)

       PTRACE_SYSEMU, PTRACE_SYSEMU_SINGLESTEP (since Linux 2.6.14)
              For  PTRACE_SYSEMU,  continue  and  stop  on  entry  to the next
              syscall,    which     will     not     be     executed.      For
              PTRACE_SYSEMU_SINGLESTEP, do the same but also singlestep if not
              a syscall.  This call is used by programs like User  Mode  Linux
              that  want  to  emulate all the child’s system calls.  (addr and
              data are ignored; not supported on all architectures.)

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

       PTRACE_ATTACH
              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.)

       PTRACE_DETACH
              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
              perhaps not intended, under Linux a traced child can be detached
              in this way regardless of which  method  was  used  to  initiate
              tracing.  (addr is ignored.)

RETURN VALUE

       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.

ERRORS

       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
              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 parent’s or child’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 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).

CONFORMING TO

       SVr4, 4.3BSD

NOTES

       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.   This  means that
       unneeded trailing 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
       processes.    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 simulate 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
       way.

BUGS

       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  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.

SEE ALSO

       gdb(1),  strace(1),  execve(2),  fork(2),  signal(2), wait(2), exec(3),
       capabilities(7)

COLOPHON

       This page is part of release 3.01 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/.