ptrace - process trace

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

     int ptrace(int request, pid_t pid, long addr, long data)

     Note: This manual page has no  relation  to  Minix.   Someone  who  knows
     ptrace() has to check, or rewrite, this page.  (kjb)

     Ptrace provides a means  by  which  a  parent  process  may  control  the
     execution of a child process, and examine and change its core image.  Its
     primary use is for the implementation of breakpoint debugging.  There are
     four  arguments  whose  interpretation  depends  on  a  request argument.
     Generally, pid is the process ID of the traced process, which must  be  a
     child  (no  more  distant  descendant) of the tracing process.  A process
     being traced behaves normally until it  encounters  some  signal  whether
     internally  generated  like "illegal instruction" or externally generated
     like "interrupt".  See  sigaction(2)  for  the  list.   Then  the  traced
     process  enters  a  stopped state and its parent is notified via wait(2).
     When the child is in the stopped state, its core image  can  be  examined
     and  modified  using ptrace.  If desired, another ptrace request can then
     cause the child either to terminate or to continue, possibly ignoring the

     The value of the request argument determines the precise  action  of  the

         This request is the only one used by the child process;  it  declares
         that  the  process  is  to  be  traced  by its parent.  All the other
         arguments are ignored.  Peculiar results will  ensue  if  the  parent
         does not expect to trace the child.

         The word in the child process's address space at  addr  is  returned.
         If  I  and  D  space  are  separated (e.g. historically on a pdp-11),
         request PT_READ_I indicates I space, PT_READ_D D space.  Addr must be
         even on some machines.  The child must be stopped.  The input data is

         The word of the system's per-process data area corresponding to  addr
         is  returned.   Addr must be even on some machines and less than 512.
         This space contains the registers and  other  information  about  the
         process; its layout corresponds to the user structure in the system.

         The given data is written at the word in the process's address  space
         corresponding  to  addr,  which  must  be  even on some machines.  No
         useful value is returned.  If I and D space  are  separated,  request
         PT_WRITE_I  indicates I space, PT_WRITE_D D space.  Attempts to write
         in pure procedure fail if another process is executing the same file.

         The process's system data is written, as  it  is  read  with  request
         PT_READ_U.   Only  a  few  locations can be written in this way:  the
         general registers, the  floating  point  status  and  registers,  and
         certain bits of the processor status word.

         The data argument is  taken  as  a  signal  number  and  the  child's
         execution  continues  at  location  addr  as  if it had incurred that
         signal.  Normally the signal number will be either 0 to indicate that
         the  signal  that  caused  the  stop should be ignored, or that value
         fetched out of the process's image indicating which signal caused the
         stop.   If  addr  is  (int *)1 then execution continues from where it

         The traced process terminates.

         Execution continues as in request PT_CONTINUE; however,  as  soon  as
         possible after execution of at least one instruction, execution stops
         again.  The signal number from the stop is SIGTRAP.  (On  the  VAX-11
         the  T-bit  is  used  and just one instruction is executed.)  This is
         part of the mechanism for implementing breakpoints.

     As indicated, these calls (except for request PT_TRACE_ME)  can  be  used
     only  when  the  subject  process  has stopped.  The wait call is used to
     determine when a process stops; in such a case the  "termination"  status
     returned  by  wait  has  the  value 0177 to indicate stoppage rather than
     genuine termination.

     To forestall possible fraud, ptrace inhibits  the  set-user-id  and  set-
     group-id  facilities  on subsequent execve(2) calls.  If a traced process
     calls execve, it will stop before executing the first instruction of  the
     new image showing signal SIGTRAP.

     On a  VAX-11,  "word"  also  means  a  32-bit  integer,  but  the  "even"
     restriction does not apply.

     A 0 value is returned if the call succeeds.  If the call fails then a  -1
     is returned and the global variable errno is set to indicate the error.


     [EIO]          The request code is invalid.

     [ESRCH]        The specified process does not exist.

     [EIO]          The given signal number is invalid.

     [EIO]          The specified address is out of bounds.

     [EPERM]        The specified process cannot be traced.

     wait(2), sigaction(2), mdb(1).

     Ptrace is unique and arcane; it should be replaced with  a  special  file
     that  can  be  opened  and read and written.  The control functions could
     then be implemented with ioctl(2) calls on  this  file.   This  would  be
     simpler to understand and have much higher performance.

     The request PT_TRACE_ME call should be able to specify signals  that  are
     to  be  treated normally and not cause a stop.  In this way, for example,
     programs with simulated floating point (which use  "illegal  instruction"
     signals at a very high rate) could be efficiently debugged.

     The error indication, -1, is a legitimate  function  value;  errno,  (see
     intro(2)), can be used to disambiguate.

     It should be possible to stop a process on occurrence of a  system  call;
     in this way a completely controlled environment could be provided.