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PTRACE(2)                 OpenBSD Programmer's 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 pro-
     cess (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 the only one 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 pro-
                   cess 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.

                   These requests read a single int of data from the traced
                   process' address space.  Traditionally, ptrace() has al-
                   lowed for machines with distinct address spaces for in-
                   struction and data, which is why there are two requests:
                   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 completely
                   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 align-
                   ment 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 sup-
                   plies 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 indi-
                   cate that execution is to pick up where it left off.  data
                   provides a signal number to be delivered to the traced pro-
                   cess 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 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 re-
                   quest 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 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.

     Additionally, machine-specific requests can exist.  On the SPARC, these

     PT_GETREGS    This request reads the traced process' machine registers
                   into the ``struct reg'' (defined in <machine/reg.h>) point-
                   ed 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 addr.

     PT_GETFPREGS  This request reads the traced process' floating-point reg-
                   isters 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.

     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 pos-
     sible 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.
               tempted on a process with no valid register set.  (This is nor-
               mally 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 pro-
               cess 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.

     On the SPARC, the PC is set to the provided PC value for PT_CONTINUE and
     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, passing
     (caddr_t)1 to ptrace(), should be able to sidestep this.

     Single-stepping is not available.

OpenBSD 2.8                    November 7, 1994                              3