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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 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
		   allowed for machines with distinct address spaces for
		   instruction 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
		   OpenBSD 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.

     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
		   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.

     Additionally, machine-specific requests can exist.	 On the SPARC, these
     are:

     PT_GETREGS	   This request reads the traced process' 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' machine registers from the “struct reg”
		   (defined in ⟨machine/reg.h⟩) pointed to by addr.

     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.

     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.

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]
	   ·   A process attempted to use PT_ATTACH on itself.
	   ·   The request was not one of the legal requests.
	   ·   The signal number (in data) to PT_CONTINUE was neither 0 nor a
	       legal signal number.
	   ·   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]
	   ·   PT_ATTACH was attempted on a process that was already being
	       traced.
	   ·   A request attempted to manipulate a process that was being
	       traced by some process other than the one making the request.
	   ·   A request (other than PT_ATTACH) specified a process that
	       wasn't stopped.

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

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.

     Single-stepping is not available.

BSD			       November 7, 1994				   BSD
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