Tcl_CreateInterp, Tcl_DeleteInterp, Tcl_InterpDeleted − create and delete Tcl command interpreters


#include <tcl.h>

Tcl_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 man pages for details. The new interpreter is initialized with the built-in Tcl commands and with the variables documented in tclvars(n). 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 manual entry 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.

Retrieving An Interpreter From A Data Structure

When an interpreter is retrieved from a data structure (e.g. 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 (e.g. 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.


Tcl_Preserve(3), Tcl_Release(3)


command, create, delete, interpreter


Personal Opportunity - Free software gives you access to billions of dollars of software at no cost. Use this software for your business, personal use or to develop a profitable skill. Access to source code provides access to a level of capabilities/information that companies protect though copyrights. Open source is a core component of the Internet and it is available to you. Leverage the billions of dollars in resources and capabilities to build a career, establish a business or change the world. The potential is endless for those who understand the opportunity.

Business Opportunity - Goldman Sachs, IBM and countless large corporations are leveraging open source to reduce costs, develop products and increase their bottom lines. Learn what these companies know about open source and how open source can give you the advantage.

Free Software

Free Software provides computer programs and capabilities at no cost but more importantly, it provides the freedom to run, edit, contribute to, and share the software. The importance of free software is a matter of access, not price. Software at no cost is a benefit but ownership rights to the software and source code is far more significant.

Free Office Software - The Libre Office suite provides top desktop productivity tools for free. This includes, a word processor, spreadsheet, presentation engine, drawing and flowcharting, database and math applications. Libre Office is available for Linux or Windows.

Free Books

The Free Books Library is a collection of thousands of the most popular public domain books in an online readable format. The collection includes great classical literature and more recent works where the U.S. copyright has expired. These books are yours to read and use without restrictions.

Source Code - Want to change a program or know how it works? Open Source provides the source code for its programs so that anyone can use, modify or learn how to write those programs themselves. Visit the GNU source code repositories to download the source.


Study at Harvard, Stanford or MIT - Open edX provides free online courses from Harvard, MIT, Columbia, UC Berkeley and other top Universities. Hundreds of courses for almost all major subjects and course levels. Open edx also offers some paid courses and selected certifications.

Linux Manual Pages - A man or manual page is a form of software documentation found on Linux/Unix operating systems. Topics covered include computer programs (including library and system calls), formal standards and conventions, and even abstract concepts.