Tcl_CreateInterp(), Tcl_DeleteInterp(), Tcl_InterpDeleted()

create and delete Tcl command interpreters 

Tcl Library Procedures


#include <tcl.h>

Tcl_Interp * Tcl_CreateInterp()


int Tcl_InterpDeleted(interp)


Tcl_Interp *interp (in) 

Token for interpreter to be destroyed.


Tcl_CreateInterp() creates a new interpreter structure and returns a token for it. The token is required in calls to most other Tcl procedures, such as Tcl_CreateCommand(), Tcl_Eval(), and Tcl_DeleteInterp(). Clients are only allowed to access a few of the fields of Tcl_Interp structures; see the Tcl_Interp() and Tcl_CreateCommand() reference pages for details. The new interpreter is initialized with no defined variables and only the built-in Tcl commands. To bind in additional commands, call Tcl_CreateCommand().

Tcl_DeleteInterp() marks an interpreter as deleted; the interpreter will eventually be deleted when all calls to Tcl_Preserve() for it have been matched by calls to Tcl_Release(). At that time, all of the resources associated with it, including variables, procedures, and application-specific command bindings, will be deleted. After Tcl_DeleteInterp() returns any attempt to use Tcl_Eval() on the interpreter will fail and return TCL_ERROR. After the call to Tcl_DeleteInterp() it is safe to examine the interpreter's result, query or set the values of variables, define, undefine or retrieve procedures, and examine the runtime evaluation stack. See below, in the section INTERPRETERS AND MEMORY MANAGEMENT for details.

Tcl_InterpDeleted() returns nonzero if Tcl_DeleteInterp() was called with interp as its argument; this indicates that the interpreter will eventually be deleted, when the last call to Tcl_Preserve() for it is matched by a call to Tcl_Release(). If nonzero is returned, further calls to Tcl_Eval() in this interpreter will return TCL_ERROR.

Tcl_InterpDeleted() is useful in deletion callbacks to distinguish between when only the memory the callback is responsible for is being deleted and when the whole interpreter is being deleted. In the former case the callback may recreate the data being deleted, but this would lead to an infinite loop if the interpreter were being deleted.


Tcl_DeleteInterp() can be called at any time on an interpreter that may be used by nested evaluations and C code in various extensions. Tcl implements a simple mechanism that allows callers to use interpreters without worrying about the interpreter being deleted in a nested call, and without requiring special code to protect the interpreter, in most cases. This mechanism ensures that nested uses of an interpreter can safely continue using it even after Tcl_DeleteInterp() is called.

The mechanism relies on matching up calls to Tcl_Preserve() with calls to Tcl_Release(). If Tcl_DeleteInterp() has been called, only when the last call to Tcl_Preserve() is matched by a call to Tcl_Release(), will the interpreter be freed. See the reference page for Tcl_Preserve() for a description of these functions.

The rules for when the user of an interpreter must call Tcl_Preserve() and Tcl_Release() are simple:

Interpreters Passed As Arguments 

Functions that are passed an interpreter as an argument can safely use the interpreter without any special protection. Thus, when you write an extension consisting of new Tcl commands, no special code is needed to protect interpreters received as arguments. This covers the majority of all uses.

Interpreter Creation And Deletion 

When a new interpreter is created and used in a call to Tcl_Eval(), Tcl_VarEval(), Tcl_GlobalEval(), Tcl_SetVar(), or Tcl_GetVar(), a pair of calls to Tcl_Preserve() and Tcl_Release() should be wrapped around all uses of the interpreter. Remember that it is unsafe to use the interpreter once Tcl_Release() has been called. To ensure that the interpreter is properly deleted when it is no longer needed, call Tcl_InterpDeleted() to test if some other code already called Tcl_DeleteInterp(); if not, call Tcl_DeleteInterp() before calling Tcl_Release() in your own code. Do not call Tcl_DeleteInterp() on an interpreter for which Tcl_InterpDeleted() returns nonzero.

Retrieving An Interpreter From A Data Structure 

When an interpreter is retrieved from a data structure (for example, the client data of a callback) for use in Tcl_Eval(), Tcl_VarEval(), Tcl_GlobalEval(), Tcl_SetVar(), or Tcl_GetVar(), a pair of calls to Tcl_Preserve() and Tcl_Release() should be wrapped around all uses of the interpreter; it is unsafe to reuse the interpreter once Tcl_Release() has been called. If an interpreter is stored inside a callback data structure, an appropriate deletion cleanup mechanism should be set up by the code that creates the data structure so that the interpreter is removed from the data structure (for example, by setting the field to NULL) when the interpreter is deleted. Otherwise, you may be using an interpreter that has been freed and whose memory may already have been reused.

All uses of interpreters in Tcl and Tk have already been protected. Extension writers should ensure that their code also properly protects any additional interpreters used, as described above.


Windows 8.1. Windows Server 2012 R2. Windows 10. Windows Server 2016. Windows Server 2019. Windows 11. Windows Server 2022.


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Tcl_Preserve(), Tcl_Release()

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