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NAME    [Toc]    [Back]

     ptrace - process tracing and debugging

SYNOPSIS    [Toc]    [Back]

     #include <sys/types.h>
     #include <sys/ptrace.h>

     ptrace(int request, pid_t pid, caddr_t addr, int data);

DESCRIPTION    [Toc]    [Back]

     ptrace() provides tracing and debugging facilities.  It  allows one process
  (the  tracing  process) to control another (the traced
process).  Most
     of the time, the traced process runs normally, but  when  it
receives a
     signal (see sigaction(2)), it stops.  The tracing process is
expected to
     notice this via wait(2) or the delivery of a SIGCHLD signal,
examine the
     state  of  the stopped process, and cause it to terminate or
continue as
     appropriate.  ptrace() is the mechanism by  which  all  this

     The  request argument specifies what operation is being performed; the
     meaning of the rest of the arguments depends on  the  operation, but except
     for  one  special  case  noted below, all ptrace() calls are
made by the
     tracing process, and the pid argument specifies the  process
ID of the
     traced process.  request can be:

     PT_TRACE_ME    This  request  is  the  only  one used by the
traced process; it
                   declares that the process expects to be traced
by its parent.
   All  the  other  arguments are ignored.
(If the parent
                   process does not expect to trace the child, it
will probably
  be  rather  confused by the results; once
the traced process
 stops, it cannot be made to continue  except via
                   ptrace().)   When  a process has used this request and calls
                   execve(2) or any of the routines built  on  it
(such as
                   execv(3)),  it  will stop before executing the
first instruction
 of the new image.  Also,  any  setuid  or
setgid bits on
                   the executable being executed will be ignored.

                   These requests read a single int of data  from
the traced
                   process'    address   space.    Traditionally,
ptrace() has allowed
  for  machines  with  distinct   address
spaces for instruction
 and data, which is why there are two
                   conceptually, PT_READ_I  reads  from  the  instruction space
                   and  PT_READ_D  reads from the data space.  In
the current
                   OpenBSD implementation, these two requests are
                   identical.   The  addr  argument specifies the
address (in the
                   traced  process'  virtual  address  space)  at
which the read is
                   to  be  done.   This  address does not have to
meet any alignment
 constraints.  The value read is  returned
as the return
                   value from ptrace().

                   These    requests   parallel   PT_READ_I   and
PT_READ_D, except
                   that they write rather than  read.   The  data
argument supplies
 the value to be written.

     PT_CONTINUE    The traced process continues execution.  addr
is an address
                   specifying the place where execution is to  be
resumed (a
                   new  value  for the program counter), or (caddr_t)1 to indicate
 that execution is to  pick  up  where  it
left off.  data
                   provides  a  signal  number to be delivered to
the traced process
 as it resumes execution, or 0 if no  signal is to be

     PT_KILL        The  traced process terminates, as if PT_CONTINUE had been
                   used with SIGKILL given as the  signal  to  be

     PT_ATTACH      This request allows a process to gain control
of an otherwise
 unrelated process and begin  tracing  it.
It does not
                   need  any  cooperation  from  the to-be-traced
process.  In
                   this case, pid specifies the process ID of the
                   process,  and  the other two arguments are ignored.  This request
 requires that the  target  process  must
have the same
                   real  UID  as the tracing process, and that it
must not be
                   executing a set-user-ID or  set-group-ID  executable.  (If
                   the  tracing process is running as root, these
                   do not apply.)  The tracing process  will  see
the newly
                   traced process stop and may then control it as
if it had
                   been traced all along.

     PT_DETACH     This request is like PT_CONTINUE, except  that
it does not
                   allow specifying an alternate place to continue execution,
                   and after it succeeds, the traced  process  is
no longer
                   traced and continues execution normally.

     PT_IO          This request is a more general interface that
can be used
                   instead of  PT_READ_D,  PT_WRITE_D,  PT_READ_I
                   The  I/O  request  is  encoded  in  a ``struct
                   defined as:
                         struct ptrace_io_desc {
                                 int     piod_op;
                                 void    *piod_offs;
                                 void    *piod_addr;
                                 size_t  piod_len;
                   Where  piod_offs  is  the  offset  within  the
traced process
                   where   the  I/O  operation  should  be  made,
piod_addr is the
                   buffer in  the  parent  and  piod_len  is  the
length of the I/O
                   request.   The  piod_op  member specifies what
operation needs
                   to be done.  Possible values are:





                   See also the description of PT_READ_I for  the
                   between  D and I spaces.  A pointer to the descriptor is
                   passed in addr.  On return the piod_len  field
in the descriptor
  will be updated with the actual number of bytes
                   transferred.  If the requested I/O couldn't be
                   performed  ptrace()  will  return  -1  and set

     Additionally,  machine-specific  requests  can  exist.   All
OpenBSD platforms
     support the following requests:

     PT_GETREGS    This request reads the traced process' machine
                   into   the   ``struct   reg''   (defined    in
<machine/reg.h>) pointed
 to by addr.

     PT_SETREGS    This request is the converse of PT_GETREGS; it
loads the
                   traced process'  machine  registers  from  the
``struct reg''
                   (defined  in  <machine/reg.h>)  pointed  to by

     In addition, all platforms but vax support these  additional

     PT_GETFPREGS   This request reads the traced process' floating-point registers
 into the ``struct fpreg'' (defined in
                   <machine/reg.h>) pointed to by addr.

     PT_SETFPREGS  This request is the converse of  PT_GETFPREGS;
it loads the
                   traced  process' floating-point registers from
the ``struct
                   fpreg'' (defined in  <machine/reg.h>)  pointed
to by addr.

     Finally, the following request is available on sparc:

     PT_WCOOKIE    This request reads the traced process' `window
cookie' into
                   the int pointed to by addr.  The window cookie
needs to be
                   `XOR'ed' to stack-saved program counters.

ERRORS    [Toc]    [Back]

     Some requests can cause ptrace() to return -1 as a non-error
value; to
     disambiguate, errno is  set  to  zero  and  this  should  be
checked.  The possible
 errors are:

           No process having the specified process ID exists.

           +o   A process attempted to use PT_ATTACH on itself.
           +o   The request was not one of the legal requests.
           +o    The  signal  number  (in data) to PT_CONTINUE was
neither 0 nor a
               legal signal number.
           +o   PT_GETREGS, PT_SETREGS, PT_GETFPREGS, or  PT_SETFPREGS was attempted
  on  a process with no valid register set.
(This is normally
 true only of system processes.)

           +o   PT_ATTACH was attempted on a process that was  already being
           +o    A  request attempted to manipulate a process that
was being
               traced by some process other than the  one  making
the request.
           +o    A request (other than PT_ATTACH) specified a process that
               wasn't stopped.

           +o   A request (other than PT_ATTACH) attempted to  manipulate a process
 that wasn't being traced at all.
           +o    An attempt was made to use PT_ATTACH on a process
in violation
               of the requirements listed under PT_ATTACH  above.
           +o    An  attempt was made to use PT_ATTACH on a system

BUGS    [Toc]    [Back]

     On the SPARC, the PC is set to the  provided  PC  value  for
     similar  calls,  but the NPC is set willy-nilly to 4 greater
than the PC
     value.  Using PT_GETREGS and PT_SETREGS to  modify  the  PC,
     (caddr_t)1 to ptrace(), should be able to sidestep this.

     Single-stepping is not available.

OpenBSD      3.6                           March      2,     2004
[ Back ]
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