TIP 531: Static Tcl Interpreter Creation Function

Author:         Shannon Noe <shannon.noe@flightaware.com>
State:          Draft
Type:           Project
Vote:           Pending
Created:        14-Dec-2018
Keywords:       Tcl, stubs
Tcl-Version:    8.7


This TIP adds a helper function to the tclstubs library for creating an interpreter. The Tcl_CreateInterp function is dependent on the order defining USE_TCL_STUBS and inclusion of tcl.h. When users get this wrong the error is a null pointer exception (NPE) on the stubs table. For some TCLers/C users this takes a bit of digging to unravel the cause of the NPE.

Therefore, I propose a new function Tcl_CreateInterpWithStubs which is always fully independent of USE_TCL_STUBS. This function will be part of the libtclstub87.a library.


Users that are creating interpreters are often getting errors because they are getting the Tcl_CreateInterp macro. This error is a runtime null pointer exception which several users find confusing. The user as to be astute enough to follow instructions and see that USE_TCL_STUBS will replace a C function call with a macro. By providing a plain function, the users can have a easier time embedded interperters and using stubs.

/usr/local/include/tcl8.6/tclDecls.h:#define Tcl_CreateInterp \
/usr/local/include/tcl8.6/tclDecls.h- (tclStubsPtr->tcl_CreateInterp) /* 94 */


There will be a new function in libtclstub??.a that always performs interpreter initialization and stub table initialization. The signature is Tcl_CreateInterpWithStubs(const char *version, int exact). The version and exact parameters match Tcl_InitStubs.

On error NULL is returned instead of a TCL interpreter. Without this pointer there is no way to obtain extended error codes, but the trade off is that it makes the API much easier to use. (That could be an error, someone could clarify that for me.)

Example Implementation

/* must be compiled without stubs */

#include <tcl.h>

Tcl_Interp *
    const char *version,
    int exact)
    Tcl_Interp *interp = Tcl_CreateInterp();

    if (Tcl_Init(interp) == TCL_ERROR ||
            Tcl_InitStubs(interp, version, exact) == NULL ||
            Tcl_PkgRequire(interp, "Tcl", version, exact) == NULL) {
        return NULL;

    return interp;


Please refer to the tip-??? branch of the core Tcl repository.

Backwards Compatibility

This is a new function in the TCL C API.


This document has been placed in the public domain.