NRE(3)

NAME
Tcl_NRCreateCommand, Tcl_NRCallObjProc, Tcl_NREvalObj, Tcl_NREvalObjv, Tcl_NRCmdSwap, Tcl_NRExprObj, 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
Interpreter in which to create or evaluate a command. Name of a new command to create. Implementation of a command that will be called whenever cmdName is invoked as a command in the unoptimized way. Implementation of a command that will be called whenever cmdName is invoked and requested to conserve the C stack. Arbitrary one-word value that will be passed to proc, nreProc, deleteProc and objProc. Procedure to call before cmdName is deleted from the interpreter. This procedure allows for command-specific cleanup. If deleteProc is NULL, then no procedure is called before the command is deleted. Count of parameters provided to the implementation of a command. Pointer to an array of Tcl values. Each value holds the value of a single word in the command to execute. Pointer to a Tcl_Obj whose value is a script or expression to execute. ORed combination of flag bits that specify additional options. TCL_EVAL_GLOBAL is the only flag that is currently supported. Token for a command that is to be used instead of the currently executing command. Pointer to an unshared Tcl_Obj where the result of expression evaluation is written. Pointer to a function that will be invoked when the command currently executing in the interpreter designated by interp completes. data0 through data3 are four one-word values that will be passed to the function designated 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 callbacks when the command or script returns.

The Tcl_NRCreateCommand function creates a Tcl command in the interpreter 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 associated 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 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 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 resolution is already known. The cmd parameter gives a Tcl_Command token (returned from Tcl_CreateObjCommand or Tcl_GetCommandFromObj) identifying the command to be invoked in the trampoline; this command must match the word in objv[0]. The remaining arguments are as for Tcl_NREvalObjv.

Tcl_NREvalObj, Tcl_NREvalObjv and Tcl_NRCmdSwap all accept a flags parameter, which is an OR-ed-together set of bits to control evaluation. 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 cannot 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 function, 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
    TheCmdOldObjProc(
        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",
            TheCmdOldObjProc, 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 trampoline 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, 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",
            TheCmdNewObjProc, 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