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.\"	$OpenBSD: ptrace.2,v 1.3 1996/10/08 01:20:12 michaels Exp $
.\"	$NetBSD: ptrace.2,v 1.3 1996/02/23 01:39:41 jtc Exp $
.\"
.\" This file is in the public domain.
.Dd March 25, 2015
.Dt PTRACE 2
.Os
.Sh NAME
.Nm ptrace
.Nd process tracing and debugging
.Sh SYNOPSIS
.Fd #include <sys/types.h>
.Fd #include <sys/ptrace.h>
.Ft int
.Fn ptrace "int request" "pid_t pid" "caddr_t addr" "int data"
.Sh DESCRIPTION
.Fn ptrace
provides tracing and debugging facilities.  It allows one process (the
.Em tracing
process) to control another (the
.Em traced
process).  Most of the time, the traced process runs normally, but when
it receives a signal
.Po
see
.Xr sigaction 2
.Pc ,
it stops.  The tracing process is expected to notice this via
.Xr wait 2
or the delivery of a
.Dv SIGCHLD
signal, examine the state of the stopped process, and cause it to
terminate or continue as appropriate.
.Fn ptrace
is the mechanism by which all this happens.
.Pp
The
.Fa 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
.Fn ptrace
calls are made by the tracing process, and the
.Fa pid
argument specifies the process ID of the traced process.
.Fa request
can be:
.Bl -tag -width 12n
.It Dv 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
.Eo \&
.Fn ptrace
.Ec \&.)
When a process has used this request and calls
.Xr execve 2
or any of the routines built on it
.Po
such as
.Xr execv 3
.Pc ,
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.
.It Dv PT_DENY_ATTACH
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 parent to
trace a process which has set this flag will result in a segmentation violation
in the parent.
.It Dv PT_CONTINUE
The traced process continues execution.
.Fa addr
is an address specifying the place where execution is to be resumed (a
new value for the program counter), or
.Li (caddr_t)1
to indicate that execution is to pick up where it left off.
.Fa 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.
.It Dv PT_STEP
The traced process continues execution for a single step.  The
parameters are identical to those passed to
.Dv PT_CONTINUE.
.It Dv PT_KILL
The traced process terminates, as if
.Dv PT_CONTINUE
had been used with
.Dv SIGKILL
given as the signal to be delivered.
.It Dv PT_ATTACH
This call has been replaced with PT_ATTACHEXC.
.It Dv PT_ATTACHEXC
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,
.Fa 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. Note that this call differs
from the prior call (
.Dv PT_ATTACH )
in that signals from the child are delivered to the parent as Mach exceptions (see EXC_SOFT_SIGNAL).
.It Dv 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.
.El
.Pp
.Sh ERRORS
Some requests can cause
.Fn ptrace
to return
.Li -1
as a non-error value; to disambiguate,
.Va errno
can be set to 0 before the call and checked afterwards.  The possible
errors are:
.Bl -tag -width 4n
.It Bq Er ESRCH
No process having the specified process ID exists.
.It Bq Er EINVAL
.Bl -bullet -compact
.It
A process attempted to use
.Dv PT_ATTACHEXC
on itself.
.It
The
.Fa request
was not one of the legal requests.
.It
The signal number (in
.Fa data )
to
.Dv PT_CONTINUE
was neither 0 nor a legal signal number.
.It
.Dv PT_GETREGS ,
.Dv PT_SETREGS ,
.Dv PT_GETFPREGS ,
or
.Dv PT_SETFPREGS
was attempted on a process with no valid register set.  (This is
normally true only of system processes.)
.El
.It Bq Er EBUSY
.Bl -bullet -compact
.It
.Dv PT_ATTACHEXC
was attempted on a process that was already being traced.
.It
A request attempted to manipulate a process that was being traced by
some process other than the one making the request.
.It
A request (other than
.Dv PT_ATTACHEXC )
specified a process that wasn't stopped.
.El
.It Bq Er EPERM
.Bl -bullet -compact
.It
A request (other than
.Dv PT_ATTACHEXC )
attempted to manipulate a process that wasn't being traced at all.
.It
An attempt was made to use
.Dv PT_ATTACHEXC
on a process in violation of the requirements listed under
.Dv PT_ATTACHEXC
above.
.El
.El