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PTRACE(2)		    BSD	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);

     The ptrace() system call provides tracing and debugging facilities.  It
     allows one	process	(the tracing process) to control another (the traced
     process).	The tracing process must first attach to the traced process,
     and then issue a series of	ptrace() system	calls to control the execution
     of	the process, as	well as	access process memory and register state.  For
     the duration of the tracing session, the traced process will be
     "re-parented", with its parent process ID (and resulting behavior)
     changed to	the tracing process.  It is permissible	for a tracing process
     to	attach to more than one	other process at a time.  When the tracing
     process has completed its work, it	must detach the	traced process;	if a
     tracing process exits without first detaching all processes it has	at-
     tached, those processes will be killed.

     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.  The signal may be	a normal process signal, generated as
     a result of traced	process	behavior, or use of the	kill(2)	system call;
     alternatively, it may be generated	by the tracing facility	as a result of
     attaching,	system calls, or stepping by the tracing process.  The tracing
     process may choose	to intercept the signal, using it to observe process
     behavior (such as SIGTRAP), or forward the	signal to the process if ap-
     propriate.	 The ptrace() system call is the mechanism by which all	this

     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.  The request argument 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.

		   These requests read a single	int of data from the traced
		   process's 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
		   FreeBSD implementation, these two requests are completely
		   identical.  The addr	argument specifies the address (in the
		   traced process's 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	sup-
		   plies the value to be written.

     PT_IO	   This	request	allows reading and writing arbitrary amounts
		   of data in the traced process's address space.  The addr
		   argument specifies a	pointer	to a struct ptrace_io_desc,
		   which is defined as follows:

		   struct ptrace_io_desc {
			   int	   piod_op;	   /* I/O operation */
			   void	   *piod_offs;	   /* child offset */
			   void	   *piod_addr;	   /* parent offset */
			   size_t  piod_len;	   /* request length */

		    * Operations in piod_op.
		   #define PIOD_READ_D	   1	   /* Read from	D space	*/
		   #define PIOD_WRITE_D	   2	   /* Write to D space */
		   #define PIOD_READ_I	   3	   /* Read from	I space	*/
		   #define PIOD_WRITE_I	   4	   /* Write to I space */

		   The data argument is	ignored.  The actual number of bytes
		   read	or written is stored in	piod_len upon return.

     PT_CONTINUE   The traced process continues	execution.  The	addr argument
		   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.	 The data argument provides a signal number to
		   be delivered	to the traced process as it resumes execution,
		   or 0	if no signal is	to be sent.

     PT_STEP	   The traced process is single	stepped	one instruction.  The
		   addr	argument should	be passed (caddr_t)1.  The data	argu-
		   ment	provides a signal number to be delivered to the	traced
		   process as it resumes execution, or 0 if no signal is to be

     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.

     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.

     PT_GETREGS	   This	request	reads the traced process's 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's machine registers from the "struct reg"
		   (defined in <machine/reg.h>)	pointed	to by addr.

     PT_GETFPREGS  This	request	reads the traced process's floating-point reg-
		   isters 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's floating-point registers from the "struct
		   fpreg" (defined in <machine/reg.h>) pointed to by addr.

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

     PT_LWPINFO	   This	request	can be used to obtain information about	the
		   kernel thread, also known as	light-weight process, that
		   caused the traced process to	stop.  The addr	argument spec-
		   ifies a pointer to a	struct ptrace_lwpinfo, which is	de-
		   fined as follows:

		   struct ptrace_lwpinfo {
			   lwpid_t pl_lwpid;	   /* LWP described. */
			   int	   pl_event;	   /* Event received. */

		   The data argument is	to be set to the size of the structure
		   known to the	caller.	 This allows the structure to grow
		   without affecting older programs.

     Additionally, machine-specific requests can exist.

     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-

     The ptrace() system call 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	argument was not one of	the legal re-
			o   The	signal number (in data)	to PT_CONTINUE was
			    neither 0 nor a legal signal number.
			    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 al-
			    ready 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 ma-
			    nipulate 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),

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

BSD				August 11, 2003				   BSD


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