## Tcl8.6.15/Tk8.6.15 Documentation > Tcl C API, version 8.6.15 > NRE

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  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
  DESCRIPTION
  EXAMPLE
  SEE ALSO
  KEYWORDS
  COPYRIGHT

### 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

Tcl_Interp *interp (in)
  The relevant Interpreter.

const char *cmdName (in)
  Name of the command to create.

Tcl_ObjCmdProc *proc (in)
  Called in order to evaluate a command. Is often just a small wrapper that uses Tcl_NRCallObjProc to call nreProc using a new trampoline. Behaves in the same way as the proc argument to Tcl_CreateObjCommand(3) (q.v.).

Tcl_ObjCmdProc *nreProc (in)
  Called instead of proc when a trampoline is already in use.

ClientData clientData (in)
  Arbitrary one-word value passed to proc, nreProc, deleteProc and objProc.

Tcl_CmdDeleteProc *deleteProc (in/out)
  Called before cmdName is deleted from the interpreter, allowing for command-specific cleanup. May be NULL.

int objc (in)
  Number of items in objv.

Tcl_Obj **objv (in)
  Words in the command.

Tcl_Obj *objPtr (in)
  A script or expression to evaluate.

int flags (in)
  As described for Tcl_EvalObjv.

Tcl_Command cmd (in)
  Token to use instead of one derived from the first word of objv in order to evaluate a command.

Tcl_Obj *resultPtr (out)
  Pointer to an unshared Tcl_Obj where the result of the evaluation is stored if the return code is TCL_OK.

Tcl_NRPostProc *postProcPtr (in)
  A function to push.

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 designated by postProcPtr when it is invoked.

### DESCRIPTION

These functions provide an interface to the function stack that an interpreter iterates through to evaluate commands. The routine behind a command is implemented by an initial function and any additional functions that the routine pushes onto the stack as it progresses. The interpreter itself pushes functions onto the stack to react to the end of a routine and to exercise other forms of control such as switching between in-progress stacks and the evaluation of other scripts at additional levels without adding frames to the C stack. To execute a routine, the initial function for the routine is called and then a small bit of code called a trampoline iteratively takes functions off the stack and calls them, using the value of the last call as the value of the routine.

Tcl_NRCallObjProc calls nreProc using a new trampoline.

Tcl_NRCreateCommand, an alternative to Tcl_CreateObjCommand, resolves cmdName, which may contain namespace qualifiers, relative to the current namespace, creates a command by that name, and returns a token for the command which may be used in subsequent calls to Tcl_GetCommandName. Except for a few cases noted below any existing command by the same name is first deleted. If interp is in the process of being deleted Tcl_NRCreateCommand does not create any command, does not delete any command, and returns NULL.

Tcl_NREvalObj pushes a function that is like Tcl_EvalObjEx but consumes no space on the C stack.

Tcl_NREvalObjv pushes a function that is like Tcl_EvalObjv but consumes no space on the C stack.

Tcl_NRCmdSwap is like Tcl_NREvalObjv, but uses cmd, a token previously returned by Tcl_CreateObjCommand or Tcl_GetCommandFromObj, instead of resolving the first word of objv.

Tcl_NRExprObj pushes a function that evaluates objPtr as an expression in the same manner as Tcl_ExprObj but without consuming space on the C stack.

All of the functions return TCL_OK if the evaluation of the script, command, or expression has been scheduled successfully. Otherwise (for example if the command name cannot be resolved), they return TCL_ERROR and store a message as the interpreter's result.

Tcl_NRAddCallback pushes postProcPtr. The signature for Tcl_NRPostProc is:

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

data is a pointer to an array containing data0 through data3. result is the value returned by the previous function implementing part the routine.

### EXAMPLE

The following command uses Tcl_EvalObjEx, which consumes space on the C stack, to evalute a script:

    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 avoid consuming space on the C stack, TheCmdOldObjProc is renamed to TheCmdNRObjProc and the postprocessing step is split into a separate function, TheCmdPostProc, which is pushed onto the function stack. Tcl_EvalObjEx is replaced with Tcl_NREvalObj, which uses a trampoline instead of consuming space on the C stack. A new version of TheCmdOldObjProc is just a a wrapper that uses Tcl_NRCallObjProc to call TheCmdNRObjProc:

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

    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);
    }

    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;
    }

Any function comprising a routine can push other functions, making it possible implement looping and sequencing constructs using the function stack.

### SEE ALSO

Tcl_CreateCommand, Tcl_CreateObjCommand, Tcl_EvalObjEx, Tcl_GetCommandFromObj, Tcl_ExprObj

### KEYWORDS

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

### COPYRIGHT

Copyright © 2008 Kevin B. Kenny. Copyright © 2018 Nathan Coulter.