Tribblix: manual page: Tcl_NRCreateCommand.3

NRE(3) Tcl Library Procedures NRE(3)

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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

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.
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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.

KEYWORDS

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