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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 argu-ments arguments
     ments 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 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 rou-tines routines
                   tines 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.

                   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; otherwise, it sets a flag that denies future traces.  An attempt by the par-ent parent
                   ent 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 indicate
                   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 sent.

     PT_STEP       The traced process continues execution for a single step.  The parameters are identical
                   to those passed to PT_CONTINUE.

     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 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 to-be-traced process, and the other two argu-ments arguments
                   ments 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 exe-cutable. executable.
                   cutable.  (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 afterwards.  The possible errors are:

           No process having the specified process ID exists.

           •   A process attempted to use PT_ATTACH on itself.
           •   The request was not one of the legal requests.
           •   The signal number (in data) to PT_CONTINUE was neither 0 nor a legal signal number.
           •   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.)

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

           •   A request (other than PT_ATTACH) attempted to manipulate a process that wasn't being traced
               at all.
           •   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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