tcl_createinterp(3)

NAME

Tcl_CreateInterp, Tcl_DeleteInterp, Tcl_InterpDeleted create and delete Tcl command interpreters

SYNOPSIS

#include <tcl.h>
Tcl_Interp *
Tcl_CreateInterp()
Tcl_DeleteInterp(interp)
int
Tcl_InterpDeleted(interp)

ARGUMENTS

Tcl_Interp *interp (in) Token for interpreter to: be destroyed.

DESCRIPTION

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_Cre ateCommand.

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 applicationspecific 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 inter preter'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 INTER
PRETERS 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 dis tinguish between when only the memory the callback is
responsible for is being deleted and when the whole inter
preter 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.

INTERPRETERS AND MEMORY MANAGEMENT

Tcl_DeleteInterp can be called at any time on an inter preter 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 exten
sion 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 struc
ture (e.g. the client data of a callback) for use
in Tcl_Eval, Tcl_VarEval, Tcl_GlobalEval, Tcl_Set Var, 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 mecha
nism 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. Other
wise, 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.

KEYWORDS

command, create, delete, interpreter

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See also