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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 November 7, 1994
	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 request allows a process to gain control of an otherwise unrelated
	98	process and begin tracing it. It does not need any cooperation from
	99	the to-be-traced process. In this case,
	100	.Fa pid
	101	specifies the process ID of the to-be-traced process, and the other two
	102	arguments are ignored. This request requires that the target process
	103	must have the same real UID as the tracing process, and that it must
	104	not be executing a setuid or setgid executable. (If the tracing
	105	process is running as root, these restrictions do not apply.) The
	106	tracing process will see the newly-traced process stop and may then
	107	control it as if it had been traced all along.
	108	.It Dv PT_DETACH
	109	This request is like PT_CONTINUE, except that it does not allow
	110	specifying an alternate place to continue execution, and after it
	111	succeeds, the traced process is no longer traced and continues
	112	execution normally.
	113	.El
	114	.Pp
	115	.Sh ERRORS
	116	Some requests can cause
	117	.Fn ptrace
	118	to return
	119	.Li -1
	120	as a non-error value; to disambiguate,
	121	.Va errno
	122	can be set to 0 before the call and checked afterwards. The possible
	123	errors are:
	124	.Bl -tag -width 4n
	125	.It Bq Er ESRCH
	126	No process having the specified process ID exists.
	127	.It Bq Er EINVAL
	128	.Bl -bullet -compact
	129	.It
	130	A process attempted to use
	131	.Dv PT_ATTACH
	132	on itself.
	133	.It
	134	The
	135	.Fa request
	136	was not one of the legal requests.
	137	.It
	138	The signal number (in
	139	.Fa data )
	140	to
	141	.Dv PT_CONTINUE
	142	was neither 0 nor a legal signal number.
	143	.It
	144	.Dv PT_GETREGS ,
	145	.Dv PT_SETREGS ,
	146	.Dv PT_GETFPREGS ,
	147	or
	148	.Dv PT_SETFPREGS
	149	was attempted on a process with no valid register set. (This is
	150	normally true only of system processes.)
	151	.El
	152	.It Bq Er EBUSY
	153	.Bl -bullet -compact
	154	.It
	155	.Dv PT_ATTACH
	156	was attempted on a process that was already being traced.
	157	.It
	158	A request attempted to manipulate a process that was being traced by
	159	some process other than the one making the request.
	160	.It
	161	A request (other than
	162	.Dv PT_ATTACH )
	163	specified a process that wasn't stopped.
	164	.El
	165	.It Bq Er EPERM
	166	.Bl -bullet -compact
	167	.It
	168	A request (other than
	169	.Dv PT_ATTACH )
	170	attempted to manipulate a process that wasn't being traced at all.
	171	.It
	172	An attempt was made to use
	173	.Dv PT_ATTACH
	174	on a process in violation of the requirements listed under
	175	.Dv PT_ATTACH
	176	above.
	177	.El
	178	.El