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PTRACE(2)                 FreeBSD System Calls Manual                PTRACE(2)

     ptrace - process tracing and debugging

     Standard C Library (libc, -lc)

     #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 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 requests:
                   conceptually, PT_READ_I reads from the instruction space
                   and PT_READ_D reads from the data space.  In the current
                   FreeBSD 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
                   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_STEP       The traced process is single stepped one instruction.  addr
                   should be passed (caddr_t)1.  data is not used.

     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 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.

     Additionally, machine-specific requests can exist.  On the i386, these

     PT_GETREGS    This request reads the traced process' machine registers
                   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 addr.

     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.

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

     PT_SETDBREGS  This request is the converse of PT_GETDBREGS; it loads the
                   traced process' debug registers from the ``struct dbreg''
                   (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 can be set to 0 before the call and checked

     The ptrace() function may fail if:

                        +o   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,
                            PT_SETFPREGS, PT_GETDBREGS, or PT_SETDBREGS 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 traced.
                        +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
                        +o   An attempt was made to use PT_ATTACH on a process
                            in violation of the requirements listed under
                            PT_ATTACH above.

     execve(2), sigaction(2), wait(2), execv(3), i386_clr_watch(3),

     A ptrace() function call appeared in Version 7 AT&T UNIX.

FreeBSD 11.0-PRERELEASE        January 20, 1996        FreeBSD 11.0-PRERELEASE


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