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	1	.\" $OpenBSD: ptrace.2,v 1.3 1996/10/08 01:20:12 michaels Exp $
	2	.\" $NetBSD: ptrace.2,v 1.3 1996/02/23 01:39:41 jtc Exp $
	3	.\"
	4	.\" This file is in the public domain.
	5	.Dd March 25, 2015
	6	.Dt PTRACE 2
	7	.Os
	8	.Sh NAME
	9	.Nm ptrace
	10	.Nd process tracing and debugging
	11	.Sh SYNOPSIS
	12	.Fd #include <sys/types.h>
	13	.Fd #include <sys/ptrace.h>
	14	.Ft int
	15	.Fn ptrace "int request" "pid_t pid" "caddr_t addr" "int data"
	16	.Sh DESCRIPTION
	17	.Fn ptrace
	18	provides tracing and debugging facilities. It allows one process (the
	19	.Em tracing
	20	process) to control another (the
	21	.Em traced
	22	process). Most of the time, the traced process runs normally, but when
	23	it receives a signal
	24	.Po
	25	see
	26	.Xr sigaction 2
	27	.Pc ,
	28	it stops. The tracing process is expected to notice this via
	29	.Xr wait 2
	30	or the delivery of a
	31	.Dv SIGCHLD
	32	signal, examine the state of the stopped process, and cause it to
	33	terminate or continue as appropriate.
	34	.Fn ptrace
	35	is the mechanism by which all this happens.
	36	.Pp
	37	The
	38	.Fa request
	39	argument specifies what operation is being performed; the meaning of
	40	the rest of the arguments depends on the operation, but except for one
	41	special case noted below, all
	42	.Fn ptrace
	43	calls are made by the tracing process, and the
	44	.Fa pid
	45	argument specifies the process ID of the traced process.
	46	.Fa request
	47	can be:
	48	.Bl -tag -width 12n
	49	.It Dv PT_TRACE_ME
	50	This request is one of two used by the traced process; it declares
	51	that the process expects to be traced by its parent. All the other
	52	arguments are ignored. (If the parent process does not expect to trace
	53	the child, it will probably be rather confused by the results; once the
	54	traced process stops, it cannot be made to continue except via
	55	.Eo \&
	56	.Fn ptrace
	57	.Ec \&.)
	58	When a process has used this request and calls
	59	.Xr execve 2
	60	or any of the routines built on it
	61	.Po
	62	such as
	63	.Xr execv 3
	64	.Pc ,
	65	it will stop before executing the first instruction of the new image.
	66	Also, any setuid or setgid bits on the executable being executed will
	67	be ignored.
	68	.It Dv PT_DENY_ATTACH
	69	This request is the other operation used by the traced process; it allows
	70	a process that is not currently being traced to deny future traces by its
	71	parent. All other arguments are ignored. If the process is currently
	72	being traced, it will exit with the exit status of ENOTSUP; otherwise,
	73	it sets a flag that denies future traces. An attempt by the parent to
	74	trace a process which has set this flag will result in a segmentation violation
	75	in the parent.
	76	.It Dv PT_CONTINUE
	77	The traced process continues execution.
	78	.Fa addr
	79	is an address specifying the place where execution is to be resumed (a
	80	new value for the program counter), or
	81	.Li (caddr_t)1
	82	to indicate that execution is to pick up where it left off.
	83	.Fa data
	84	provides a signal number to be delivered to the traced process as it
	85	resumes execution, or 0 if no signal is to be sent.
	86	.It Dv PT_STEP
	87	The traced process continues execution for a single step. The
	88	parameters are identical to those passed to
	89	.Dv PT_CONTINUE.
	90	.It Dv PT_KILL
	91	The traced process terminates, as if
	92	.Dv PT_CONTINUE
	93	had been used with
	94	.Dv SIGKILL
	95	given as the signal to be delivered.
	96	.It Dv PT_ATTACH
	97	This call has been replaced with PT_ATTACHEXC.
	98	.It Dv PT_ATTACHEXC
	99	This request allows a process to gain control of an otherwise unrelated
	100	process and begin tracing it. It does not need any cooperation from
	101	the to-be-traced process. In this case,
	102	.Fa pid
	103	specifies the process ID of the to-be-traced process, and the other two
	104	arguments are ignored. This request requires that the target process
	105	must have the same real UID as the tracing process, and that it must
	106	not be executing a setuid or setgid executable. (If the tracing
	107	process is running as root, these restrictions do not apply.) The
	108	tracing process will see the newly-traced process stop and may then
	109	control it as if it had been traced all along. Note that this call differs
	110	from the prior call (
	111	.Dv PT_ATTACH )
	112	in that signals from the child are delivered to the parent as Mach exceptions (see EXC_SOFT_SIGNAL).
	113	.It Dv PT_DETACH
	114	This request is like PT_CONTINUE, except that it does not allow
	115	specifying an alternate place to continue execution, and after it
	116	succeeds, the traced process is no longer traced and continues
	117	execution normally.
	118	.El
	119	.Pp
	120	.Sh ERRORS
	121	Some requests can cause
	122	.Fn ptrace
	123	to return
	124	.Li -1
	125	as a non-error value; to disambiguate,
	126	.Va errno
	127	can be set to 0 before the call and checked afterwards. The possible
	128	errors are:
	129	.Bl -tag -width 4n
	130	.It Bq Er ESRCH
	131	No process having the specified process ID exists.
	132	.It Bq Er EINVAL
	133	.Bl -bullet -compact
	134	.It
	135	A process attempted to use
	136	.Dv PT_ATTACHEXC
	137	on itself.
	138	.It
	139	The
	140	.Fa request
	141	was not one of the legal requests.
	142	.It
	143	The signal number (in
	144	.Fa data )
	145	to
	146	.Dv PT_CONTINUE
	147	was neither 0 nor a legal signal number.
	148	.It
	149	.Dv PT_GETREGS ,
	150	.Dv PT_SETREGS ,
	151	.Dv PT_GETFPREGS ,
	152	or
	153	.Dv PT_SETFPREGS
	154	was attempted on a process with no valid register set. (This is
	155	normally true only of system processes.)
	156	.El
	157	.It Bq Er EBUSY
	158	.Bl -bullet -compact
	159	.It
	160	.Dv PT_ATTACHEXC
	161	was attempted on a process that was already being traced.
	162	.It
	163	A request attempted to manipulate a process that was being traced by
	164	some process other than the one making the request.
	165	.It
	166	A request (other than
	167	.Dv PT_ATTACHEXC )
	168	specified a process that wasn't stopped.
	169	.El
	170	.It Bq Er EPERM
	171	.Bl -bullet -compact
	172	.It
	173	A request (other than
	174	.Dv PT_ATTACHEXC )
	175	attempted to manipulate a process that wasn't being traced at all.
	176	.It
	177	An attempt was made to use
	178	.Dv PT_ATTACHEXC
	179	on a process in violation of the requirements listed under
	180	.Dv PT_ATTACHEXC
	181	above.
	182	.El
	183	.El