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NRE(3)			    Tcl Library Procedures			NRE(3)

______________________________________________________________________________

NAME
       Tcl_NRCreateCommand,  Tcl_NRCallObjProc, Tcl_NREvalObj, Tcl_NREvalObjv,
       Tcl_NRCmdSwap, Tcl_NRAddCallback - Non-Recursive (stackless) evaluation
       of Tcl scripts.

SYNOPSIS
       #include <tcl.h>

       Tcl_Command
       Tcl_NRCreateCommand(interp, cmdName, proc, nreProc, clientData,
			   deleteProc)

       int
       Tcl_NRCallObjProc(interp, nreProc, clientData, objc, objv)

       int
       Tcl_NREvalObj(interp, objPtr, flags)

       int
       Tcl_NREvalObjv(interp, objc, objv, flags)

       int
       Tcl_NRCmdSwap(interp, cmd, objc, objv, flags)

       int
       Tcl_NRExprObj(interp, objPtr, resultPtr)

       void
       Tcl_NRAddCallback(interp, postProcPtr, data0, data1, data2, data3)

ARGUMENTS
       Tcl_Interp *interp (in)		       Interpreter  in which to create
					       or evaluate a command.

       char *cmdName (in)		       Name of a new command  to  cre‐
					       ate.

       Tcl_ObjCmdProc *proc (in)	       Implementation	of  a  command
					       that will  be  called  whenever
					       cmdName is invoked as a command
					       in the unoptimized way.

       Tcl_ObjCmdProc *nreProc (in)	       Implementation  of  a   command
					       that  will  be  called whenever
					       cmdName	  is	invoked	   and
					       requested  to  conserve	the  C
					       stack.

       ClientData clientData (in)	       Arbitrary one-word  value  that
					       will  be	 passed	 to proc, nre‐
					       Proc, deleteProc and objProc.

       Tcl_CmdDeleteProc *deleteProc (in/out)  Procedure to call  before  cmd‐
					       Name is deleted from the inter‐
					       preter.	This procedure	allows
					       for  command-specific  cleanup.
					       If deleteProc is NULL, then  no
					       procedure  is called before the
					       command is deleted.

       int objc (in)			       Count of parameters provided to
					       the  implementation  of	a com‐
					       mand.

       Tcl_Obj **objv (in)		       Pointer to an array of Tcl val‐
					       ues. Each value holds the value
					       of a single word in the command
					       to execute.

       Tcl_Obj *objPtr (in)		       Pointer	 to  a	Tcl_Obj	 whose
					       value is a script or expression
					       to execute.

       int flags (in)			       ORed  combination  of flag bits
					       that	specify	    additional
					       options.	   TCL_EVAL_GLOBAL  is
					       the only flag that is currently
					       supported.

       Tcl_Command cmd (in)		       Token  for a command that is to
					       be used	instead	 of  the  cur‐
					       rently executing command.

       Tcl_Obj *resultPtr (out)		       Pointer	to an unshared Tcl_Obj
					       where the result of  expression
					       evaluation is written.

       Tcl_NRPostProc *postProcPtr (in)	       Pointer to a function that will
					       be  invoked  when  the  command
					       currently   executing   in  the
					       interpreter    designated    by
					       interp completes.

       ClientData data0 (in)

       ClientData data1 (in)

       ClientData data2 (in)

       ClientData data3 (in)		       data0  through  data3  are four
					       one-word values	that  will  be
					       passed  to  the function desig‐
					       nated by postProcPtr when it is
					       invoked.
_________________________________________________________________

DESCRIPTION
       This  series of C functions provides an interface whereby commands that
       are implemented in C can be evaluated, and invoke Tcl commands  scripts
       and  scripts, without consuming space on the C stack. The non-recursive
       evaluation is done by installing a trampoline, a small  piece  of  code
       that  invokes  a command or script, and then executes a series of call‐
       backs when the command or script returns.

       The Tcl_NRCreateCommand function creates a Tcl command  in  the	inter‐
       preter  designated  by  interp  that is prepared to handle nonrecursive
       evaluation with a trampoline. The cmdName argument gives	 the  name  of
       the new command. If cmdName contains any namespace qualifiers, then the
       new command is added to the specified namespace; otherwise, it is added
       to  the	global namespace. proc gives the procedure that will be called
       when the interpreter wishes to evaluate the command in  an  unoptimized
       manner,	and  nreProc  is  the  procedure  that will be called when the
       interpreter  wishes  to	evaluate  the  command	using  a   trampoline.
       deleteProc  is  a  function  that  will be called before the command is
       deleted from the interpreter.  When  any	 of  the  three	 functions  is
       invoked, it is passed the clientData parameter.

       Tcl_NRCreateCommand  deletes  any existing command name already associ‐
       ated with the interpreter (however see below for an exception where the
       existing	 command is not deleted).  It returns a token that may be used
       to refer to the command in subsequent calls to Tcl_GetCommandName.   If
       Tcl_NRCreateCommand is called for an interpreter that is in the process
       of being deleted, then it does not  create  a  new  command,  does  not
       delete any existing command of the same name, and returns NULL.

       The  proc and nreProc function are expected to conform to all the rules
       set forth for the proc argument to Tcl_CreateObjCommand(3) (q.v.).

       When a command that is written to cope with evaluation  via  trampoline
       is  invoked  without a trampoline on the stack, it will usually respond
       to the invocation by creating a trampoline and calling the  trampoline-
       enabled	implementation of the same command. This call is done by means
       of Tcl_NRCallObjProc. In the call  to  Tcl_NRCallObjProc,  the  interp,
       clientData,  objc and objv parameters should be the same ones that were
       passed to proc. The nreProc parameter should designate the  trampoline-
       enabled implementation of the command.

       Tcl_NREvalObj  arranges for the script contained in objPtr to be evalu‐
       ated in the interpreter designated by interp after the current  command
       (which must be trampoline-enabled) returns. It is the method by which a
       command may invoke a script without consuming space  on	the  C	stack.
       Similarly, Tcl_NREvalObjv arranges to invoke a single Tcl command whose
       words have already been separated and substituted. The  objc  and  objv
       parameters give the words of the command to be evaluated when execution
       reaches the trampoline.

       Tcl_NRCmdSwap allows for trampoline evaluation of a command whose reso‐
       lution  is  already known.  The cmd parameter gives a Tcl_Command token
       (returned from Tcl_CreateObjCommand or Tcl_GetCommandFromObj) identify‐
       ing  the	 command  to  be  invoked in the trampoline; this command must
       match the  word	in  objv[0].   The  remaining  arguments  are  as  for
       Tcl_NREvalObj.

       Tcl_NREvalObj,  Tcl_NREvalObjv  and  Tcl_NRCmdSwap  all	accept a flags
       parameter, which is an OR-ed-together set of bits  to  control  evalua‐
       tion. At the present time, the only supported flag available to callers
       is TCL_EVAL_GLOBAL.  If the TCL_EVAL_GLOBAL flag is set, the script  or
       command	is  evaluated in the global namespace. If it is not set, it is
       evaluated in the current namespace.

       Tcl_NRExprObj arranges for the expression contained  in	objPtr	to  be
       evaluated  in  the  interpreter	designated by interp after the current
       command (which must be trampoline-enabled) returns. It is the method by
       which  a	 command may evaluate a Tcl expression without consuming space
       on the C stack.	The argument resultPtr is a  pointer  to  an  unshared
       Tcl_Obj where the result of expression evaluation is to be written.  If
       expression evaluation returns any code other than TCL_OK, the resultPtr
       value is left untouched.

       All  of	the routines return TCL_OK if command or expression invocation
       has been scheduled successfully. If for any reason the scheduling  can‐
       not be completed (for example, if the interpreter is unable to find the
       requested command), they return TCL_ERROR with an  appropriate  message
       left in the interpreter's result.

       Tcl_NRAddCallback arranges to have a C function called when the current
       trampoline-enabled command in the Tcl interpreter designated by	interp
       returns.	  The  postProcPtr argument is a pointer to the callback func‐
       tion, which must have arguments and return value	 consistent  with  the
       Tcl_NRPostProc data type:

	      typedef int
	      Tcl_NRPostProc(
		      ClientData data[],
		      Tcl_Interp *interp,
		      int result);

       When  the  trampoline invokes the callback function, the data parameter
       will point to an array containing the  four  one-word  quantities  that
       were passed to Tcl_NRAddCallback in the data0 through data3 parameters.
       The Tcl interpreter will be designated by the interp parameter, and the
       result	parameter   will   contain   the  result  (TCL_OK,  TCL_ERROR,
       TCL_RETURN, TCL_BREAK or TCL_CONTINUE) that was returned by the command
       evaluation.  The	 callback  function  is	 expected,  in turn, either to
       return a result to control further evaluation.

       Multiple Tcl_NRAddCallback invocations may request multiple  callbacks,
       which  may  be to the same or different callback functions. If multiple
       callbacks are requested, they are executed in last-in, first-out order,
       that is, the most recently requested callback is executed first.

EXAMPLE
       The  usual  pattern  for Tcl commands that invoke other Tcl commands is
       something like:

	      int
	      TheCmdObjProc(
		  ClientData clientData,
		  Tcl_Interp *interp,
		  int objc,
		  Tcl_Obj *const objv[])
	      {
		  int result;
		  Tcl_Obj *objPtr;

		  ... preparation ...

		  result = Tcl_EvalObjEx(interp, objPtr, 0);

		  ... postprocessing ...

		  return result;
	      }
	      Tcl_CreateObjCommand(interp, "theCommand",
		      TheCmdObjProc, clientData, TheCmdDeleteProc);

       To enable a command like this one for trampoline-based  evaluation,  it
       must be split into three pieces:

       ·      A	 non-trampoline	 implementation,  TheCmdNewObjProc, which will
	      simply create  a	trampoline  and	 invoke	 the  trampoline-based
	      implementation.

       ·      A	  trampoline-enabled  implementation,  TheCmdNRObjProc.	  This
	      function will perform the initialization, request that the tram‐
	      poline call the postprocessing routine after command evaluation,
	      and finally, request that the trampoline call the inner command.

       ·      A postprocessing routine,	 TheCmdPostProc.  This	function  will
	      perform  the  postprocessing formerly done after the return from
	      the inner command in TheCmdObjProc.

       The non-trampoline implementation is simple and stylized, containing  a
       single statement:

	      int
	      TheCmdNewObjProc(
		  ClientData clientData,
		  Tcl_Interp *interp,
		  int objc,
		  Tcl_Obj *const objv[])
	      {
		  return Tcl_NRCallObjProc(interp, name,
			  TheCmdNRObjProc, clientData, objc, objv);
	      }

       The  trampoline-enabled	implementation	requests  postprocessing,  and
       returns to the trampoline requesting command evaluation.

	      int
	      TheCmdNRObjProc
		  ClientData clientData,
		  Tcl_Interp *interp,
		  int objc,
		  Tcl_Obj *const objv[])
	      {
		  Tcl_Obj *objPtr;

		  ... preparation ...

		  Tcl_NRAddCallback(interp, TheCmdPostProc,
			  data0, data1, data2, data3);
		  /* data0 .. data3 are up to four one-word items to
		   * pass to the postprocessing procedure */

		  return Tcl_NREvalObj(interp, objPtr, 0);
	      }

       The postprocessing procedure does whatever  the	original  command  did
       upon return from the inner evaluation.

	      int
	      TheCmdNRPostProc(
		  ClientData data[],
		  Tcl_Interp *interp,
		  int result)
	      {
		  /* data[0] .. data[3] are the four words of data
		   * passed to Tcl_NRAddCallback */

		  ... postprocessing ...

		  return result;
	      }

       If theCommand is a command that results in multiple commands or scripts
       being evaluated, its postprocessing  routine  may  schedule  additional
       postprocessing  and then request another command evaluation by means of
       Tcl_NREvalObj or one of the  other  evaluation  routines.  Looping  and
       sequencing constructs may be implemented in this way.

       Finally,	 to  install  a trampoline-enabled command in the interpreter,
       Tcl_NRCreateCommand is  used  in	 place	of  Tcl_CreateObjCommand.   It
       accepts	two  command  procedures  instead of one. The first is for use
       when no trampoline is yet on the stack, and the second is for use  when
       there is already a trampoline in place.

	      Tcl_NRCreateCommand(interp, "theCommand",
		      TheCmdObjProc, TheCmdNRObjProc, clientData,
		      TheCmdDeleteProc);

SEE ALSO
       Tcl_CreateCommand(3),	 Tcl_CreateObjCommand(3),    Tcl_EvalObjEx(3),
       Tcl_GetCommandFromObj(3), Tcl_ExprObj(3)

KEYWORDS
       stackless,  nonrecursive,  execute,  command,  global,  value,  result,
       script

COPYRIGHT
       Copyright (c) 2008 by Kevin B. Kenny

Tcl				      8.6				NRE(3)
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