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

NAME

     ptrace -- process tracing and debugging

SYNOPSIS

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

     int
     ptrace(int request, pid_t pid, caddr_t 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 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 con-
		   fused 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 instruction of the new image.	Also, any setuid or setgid bits on the executable being executed will be ignored.

     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.

     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 indicate 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.

     PT_STEP	   The traced process continues execution for a single step.  The parameters are identical to those passed to PT_CONTINUE.

     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 otherwise 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 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.

     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.

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:

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

     [EINVAL]
	   o   A process attempted to use PT_ATTACH on itself.
	   o   The request was not one of the legal requests.
	   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.)

     [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 request.
	   o   A request (other than PT_ATTACH) specified a process that wasn't stopped.

     [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 violation of the requirements listed under PT_ATTACH above.

BSD
								 November 7, 1994							       BSD