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PTRACE(2)		    BSD	System Calls Manual		     PTRACE(2)

NAME
     ptrace -- process tracing and debugging

LIBRARY
     Standard C	Library	(libc, -lc)

SYNOPSIS
     #include <sys/types.h>
     #include <sys/ptrace.h>

     int
     ptrace(int	request, pid_t pid, void *addr,	int data);

DESCRIPTION
     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 par-
		   ent.	 All the other arguments are ignored.  (If the parent
		   process does	not expect to trace the	child, it will proba-
		   bly 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 instruc-
		   tion	of the new image.  Also, any setuid or setgid bits on
		   the executable being	executed will be ignored.

     PT_READ_I,	PT_READ_D
		   These requests read a single	int of data from the traced
		   process' address space.  Traditionally, ptrace() has	al-
		   lowed for machines with distinct address spaces for in-
		   struction 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
		   NetBSD 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 align-
		   ment	constraints.  The value	read is	returned as the	return
		   value from ptrace().

     PT_WRITE_I, PT_WRITE_D
		   These requests parallel PT_READ_I and PT_READ_D, except
		   that	they write rather than read.  The data argument	sup-
		   plies 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	indi-
		   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.  If a negative	value is supplied, that	is the nega-
		   tive	of the LWP ID of the thread to be resumed, and only
		   that	thread executes.

     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 other-
		   wise	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 re-
		   quest 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 ap-
		   ply.)  The tracing process will see the newly-traced
		   process stop	and may	then control it	as if it had been
		   traced all along.

		   Three other restrictions apply to all tracing processes,
		   even	those running as root.	First, no process may trace a
		   system process.  Second, no process may trace the process
		   running init(8).  Third, if a process has its root direc-
		   tory	set with chroot(2), it may not trace another process
		   unless that process's root directory	is at or below the
		   tracing process's root.

     PT_DETACH	   This	request	is like	PT_CONTINUE, except that after it suc-
		   ceeds, the traced process is	no longer traced and continues
		   execution normally.

     PT_IO	   This	request	is a more general interface that can be	used
		   instead of PT_READ_D, PT_WRITE_D, PT_READ_I,	and
		   PT_WRITE_I.	The I/O	request	is encoded in a	"struct
		   ptrace_io_desc" defined as:

			 struct	ptrace_io_desc {
				 int	 piod_op;
				 void	 *piod_offs;
				 void	 *piod_addr;
				 size_t	 piod_len;
			 };

		   where piod_offs is the offset within	the traced process
		   where the I/O operation should take place, piod_addr	is the
		   buffer in the tracing process, and piod_len is the length
		   of the I/O request.	The piod_op field specifies which type
		   of I/O operation to perform.	 Possible values are:

			 PIOD_READ_D
			 PIOD_WRITE_D
			 PIOD_READ_I
			 PIOD_WRITE_I

		   See the description of PT_READ_I for	the difference between
		   I and D spaces.  A pointer to the I/O descriptor is passed
		   in the addr argument	to ptrace().  On return, the piod_len
		   field in the	I/O descriptor will be updated with the	actual
		   number of bytes transferred.	 If the	requested I/O could
		   not be successfully performed, ptrace() will	return -1 and
		   set errno.

     PT_DUMPCORE   Makes the process specified in the pid pid generate a core
		   dump.  The addr argument should contain the name of the
		   core	file to	be generated and the data argument should con-
		   tain	the length of the core filename.  This ptrace call
		   currently does not stop the child process so	it can gener-
		   ate inconsistent data.

     PT_LWPINFO	   Returns information about a thread from the list of threads
		   for the process specified in	the pid	argument.  The addr
		   argument should contain a "struct ptrace_lwpinfo" defined
		   as:

			 struct	ptrace_lwpinfo {
				 lwpid_t pl_lwpid;
				 int pl_event;
			 };

		   where pl_lwpid contains a thread LWP	ID.  Information is
		   returned for	the thread following the one with the speci-
		   fied	ID in the process thread list, or for the first	thread
		   if pl_lwpid is 0.  Upon return pl_lwpid contains the	LWP ID
		   of the thread that was found, or 0 if there is no thread
		   after the one whose LWP ID was supplied in the call.
		   pl_event contains the event that stopped the	thread.	 Pos-
		   sible values	are:

			 PL_EVENT_NONE
			 PL_EVENT_SIGNAL

		   The data argument should contain "sizeof(struct
		   ptrace_lwpinfo)".

     PT_SYSCALL	   Stops a process before and after executing each system
		   call.

     PT_SYSCALLEMU
		   Intercept and ignore	a system call before it	has been exe-
		   cuted, for use with PT_SYSCALL.

     Additionally, the following requests exist	but are	not available on all
     machine architectures.  The file <machine/ptrace.h> lists which requests
     exist on a	given machine.

     PT_STEP	   Execution continues as in request PT_CONTINUE; however as
		   soon	as possible after execution of at least	one instruc-
		   tion, execution stops again.	 If the	data argument is
		   greater than	0, it contains the LWP ID of the thread	to be
		   stepped, and	any other threads are continued.  If the data
		   argument is less than zero, it contains the negative	of the
		   LWP ID of the thread	to be stepped, and only	that thread
		   executes.

     PT_GETREGS	   This	request	reads the traced process' machine registers
		   into	the "struct reg" (defined in <machine/reg.h>) pointed
		   to by addr.	The data argument contains the LWP ID of the
		   thread whose	registers are to be read.  If zero is sup-
		   plied, the first thread of the process is read.

     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.  The data
		   argument contains the LWP ID	of the thread whose registers
		   are to be written.  If zero is supplied, the	first thread
		   of the process is written.

     PT_GETFPREGS  This	request	reads the traced process' floating-point reg-
		   isters into the "struct fpreg" (defined in <machine/reg.h>)
		   pointed to by addr.	The data argument contains the LWP ID
		   of the thread whose registers are to	be read.  If zero is
		   supplied, the first thread of the process is	read.

     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.
		   The data argument contains the LWP ID of the	thread whose
		   registers are to be written.	 If zero is supplied, the
		   first thread	of the process is written.

     PT_DUMPCORE   Cause the traced process to dump core.  If the addr argu-
		   ment	is not NULL it is taken	to be the pathname of the core
		   file	to be generated	and the	data argument should contain
		   the length of the pathname.	The pathname may contain %
		   patterns that are expanded as described in sysctl(8).  If
		   the data argument is	NULL, the default core file path gen-
		   eration rules are followed.

ERRORS
     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 after-
     wards.  The possible errors are:

     [EAGAIN]  Process is currently exec'ing and cannot	be traced.

     [EBUSY]
	       +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	re-
		   quest.
	       +o   A request (other than PT_ATTACH) specified a	process	that
		   wasn't stopped.

     [EINVAL]
	       +o   A process attempted to use PT_ATTACH	on itself.
	       +o   The request was not a legal request on this machine archi-
		   tecture.
	       +o   The signal number (in data) to PT_CONTINUE was neither 0
		   nor a legal signal number.
	       +o   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.)

     [EPERM]
	       +o   A request (other than PT_ATTACH) attempted to manipulate a
		   process that	wasn't being traced at all.
	       +o   An attempt was made to use PT_ATTACH	on a process in	viola-
		   tion	of the requirements listed under PT_ATTACH above.

     [ESRCH]   No process having the specified process ID exists.

SEE ALSO
     sigaction(2), signal(7)

BUGS
     On	the SPARC, the PC is set to the	provided PC value for PT_CONTINUE and
     similar calls, but	the NPC	is set willy-nilly to 4	greater	than the PC
     value.  Using PT_GETREGS and PT_SETREGS to	modify the PC, passing
     (caddr_t)1	to ptrace(), should be able to sidestep	this.

BSD				August 31, 2011				   BSD

NAME | LIBRARY | SYNOPSIS | DESCRIPTION | ERRORS | SEE ALSO | BUGS

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