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ptrace(2)                   BSD System Calls Manual                  ptrace(2)


     ptrace -- process tracing and debugging


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

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


     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 happens.

     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 one of two used by the traced process; it
                   declares that the process expects to be traced by its par-
                   ent.  All the other arguments are ignored.  (If the parent
                   process does not expect to trace the child, it will proba-
                   bly 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 instruc-
                   tion of the new image.  Also, any setuid or setgid bits on
                   the executable being executed will be ignored.

                   This request is the other operation used by the traced
                   process; it allows a process that is not currently being
                   traced to deny future traces by its parent.  All other
                   arguments are ignored.  If the process is currently being
                   traced, it will exit with the exit status of ENOTSUP; oth-
                   erwise, it sets a flag that denies future traces.  An
                   attempt by the parent to trace a process which has set this
                   flag will result in a segmentation violation in the parent.

     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 indi-
                   cate 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_STEP       The traced process continues execution for a single step.
                   The parameters are identical to those passed to

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

     PT_ATTACH     This request allows a process to gain control of an other-
                   wise 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 to-be-traced
                   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 setuid or setgid executable.  (If the tracing
                   process is running as root, these restrictions 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.


     Some requests can cause ptrace() to return -1 as a non-error value; to
     disambiguate, errno can be set to 0 before the call and checked after-
     wards.  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.
               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.

BSD                            November 7, 1994                            BSD

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